The View Up Here

Random scribblings about kites, photography, machining, and anything else

Posts Tagged ‘Microphone’

Building the MS Alice Microphone – Part 2

Posted by Tom Benedict on 23/11/2016

This is the second half of a two-part article describing my build of the mid-side Alice microphone, following the Instructable written by Jules Ryckenbusch: Build the MS Alice Stereo Microphone. In Part 1 of this article I ran through how I was planning to build it (mostly following the same steps I used in another two-part series I wrote about another of Jules’s Instructables, Modify a Cheap LDC Condenser Microphone, namely: BM-800 Microphone Conversion Part 1 and Part 2.) I also covered my design for the saddle and post that holds the three capsules in the particular orientations required for Jules’s MS microphone build. (Jules used a different method, using PVC pipe, which you’ll see in his Instructable if you decide to build one of your own.)

Since writing Part 1 all the bits and pieces came in. I was eager to see how the 3D printed saddle and post turned out, and how well the TSB-165A capsules fit.

M-S Alice Capsule Saddle and Post - Unpopulated

I designed the cavities for the capsules at-size, meaning I didn’t leave any slop for fit. The plastic Shapeways uses to make their least expensive printed parts is described as “strong and flexible”. I took them up on that, figuring the part would flex enough to allow the capsules to snap into place. It worked like a charm.

M-S Alice Capsule Saddle and Post - Populated

The fit is snug, but not snug enough to hold the capsules in use. As with my first Alice, I glued the capsules into the saddle with E-6000 adhesive.

I’m a little disappointed with the handling noise on my first Alice mic. I chalk some of that up to the metal saddle and post, but some of it I chalk up to the relatively stiff wire I used to connect the capsule to the PCB. It was stiff enough that manipulating the wire wound up breaking off one of the ground tabs from the TSB-2555B capsule I used on that mic. Rather than repeat that experience, and in an effort to reduce conduction paths for handling noise, I gutted some of the Mogami cable I use for all my microphone projects and used the wires to connect the capsules. (NOTE: It didn’t actually affect handling noise that much. After thumping various bits of the mic, I’ve come to the conclusion the dominant frequency of the handling noise is driven by the resonant frequency of the mesh in the headbasket.)

I already had two Pimped Alice PCBs built, tuned, and ready to go for this project. The remaining steps were to screw one board onto each side of the mic frame, solder the capsule wires to the boards, solder the four 0.022uF capacitors between the ground pin (pin 1) and the remaining pins of the XLR connector (2, 3, 4, and 5), and to solder wires between the XLR and the PCBs.

M-S Alice Internals

Since I oriented the two capsules of the figure-eight mic side-by-side, they won’t fit inside the headbasket with the foam liner in place. So I stripped the foam out before closing up the mic.

The very last step was to build the 5-pin XLR to dual 3-pin XLR splitter cable. There are a number of ways I could’ve done this, but I followed (mostly) Jules’s build on the cable as well, using separate Mogami lavalier cables for each channel. This is a wonderfully floppy wire, and does an excellent job of reducing handling noise transmitted through the cable.

The one change I made to Jules’s design was to jacket the central eight feet of cable in a woven sleeve to keep it from tangling.

M-S Alice Patch Cord

I left the last foot and a half at each end loose, though, to take advantage of the wire’s floppiness. (Hey, that’s actually a word spellcheck recognizes!)

And at long long last I’m able to play with mid-side recording and compare it against my EM-172 based SASS.

SASS vs. M-S Comparison

Big big thanks to the following for making this all possible:

  • Jules Ryckenbusch – for writing the two Instructables that got me going on these microphones
  • Homero Leal – for coming up with the PCB layout for the Alice boards used in Jules’s Instructables
  • Scott Helmke – for designing the Alice circuit in the first place
  • Ricardo Lee and all of the above – for their endless patience with all of my questions and what-ifs
  • Dr. Ing – for designing the Schoeps CMC-5 in the first place, without which none of this would exist

For my own contribution, here’s the link to the MS Alice capsule saddle and post on Shapeways. I’ve listed these at-cost, with no mark up (meaning I don’t see a dime of the 5.35 USD price tag at the time of this writing – labor of love).

Have fun recording!

Tom

Advertisements

Posted in Audio, Engineering | Tagged: , , , , , , , , | 4 Comments »

Building the MS Alice Microphone – Part 1

Posted by Tom Benedict on 11/11/2016

This is a short pair of articles that glosses over most of the details of how I’m building a self-contained mid-side (MS) Alice microphone into a Neewer NW-800 microphone body. Part 1 covers most of the design and preparation, and Part 2 will cover the build.

The reason why this pair of articles is so brief is that most of the nitty-gritty was already covered in another pair of articles: BM-800 Microphone Conversion Part 1 and Part 2. The major differences between that microphone and this one are a change in capsules (Transsound TSB-165A instead of a Transsound TSB-2555B), the number of capsules (three instead of one), the number of Pimped Alice boards (two instead of one), and a change tof XLR connector (5-pin rather than 3-pin).

With the exception of how I’m planning to mount the capsules, all of this follows the Instructable written by Jules Ryckenbusch: Build the MS Alice Stereo Microphone. That’s the real reference for this build, so if you decide to build one of these yourself be sure to follow Jules’s notes.

The easy stuff first:

When I built my first Alice microphone I built three PCBs rather than just the one I needed, so I already have two Pimped Alice boards ready and waiting in the wings. I was on the fence whether to build a second mic around a TSB-2555B capsule or go straight to the MS Alice. After some recent field tests, I decided to commit the two boards to an MS Alice.

Jules pulled a neat trick for getting two signals out of a single XLR connector: use a different XLR connector! In his build he replaced the 3-pin XLR that came with his BM-800 microphone with a 5-pin. The two outputs share a common ground, but have independent signal pins. I’m following this part of his plan to the letter. (As a side note, this also gives me a spare 3-pin XLR connector to use when I finally build out my parabolic mic. New project in the works!)

The only things left to do were to order three TSB-165A capsules (done) and to figure out how to mount them.

Jules has a nice tutorial on how to build a 3-capsule saddle out of PVC pipe, but I had so much fun machining a custom saddle for my TSB-2555B capsule, I couldn’t pass up the opportunity to massively over-complicate life by designing a custom saddle for the MS Alice as well. Here’s what I came up with:

MS Alice TSB-165A Capsule Saddle

Which looks neat and all, but would be stupidly difficult to machine. It’s possible, provided you got rid of, or at least filleted the inside corner between the two side capsules, but it wouldn’t be fun. And since this is all about fun, I cheated. I sent it off to be 3D printed out of nylon. (If this pans out and there’s any interest, I’m happy to make the 3D model available for other people to print.)

So now I’m back to playing the waiting game. I’ve got parts coming in from Redco Audio (5-pin XLR to dual 3-pin XLR splitter cable), Mouser (smaller capacitors for the Alice boards to address the space constraint issue I ran into), Amazon (Switchcraft 5-pin XLR connector, NW-800 body, and associated doodads), JLI Electronics (three TSB-165A capsules), and finally Shapeways (the 3D printed mic saddle).

I’ll write the second half of this series once all the goodies show up.

Tom

Posted in Audio | Tagged: , , , , , , , , | 3 Comments »

Alice and Behringer Sitting In A Tree – Part 1

Posted by Tom Benedict on 03/11/2016

The field tests on my BM-800 Alice conversion will have to wait. Late last week I handed it over to a friend for tests I’m not equipped to make, including several mic comparisons. I’m eager to see (and hear!) his results.

Meanwhile my Behringer C-2 mics showed up. These are the ones I placed a bid on over at Ebay before I realized they were coming from Haifa, Israel. Despite the distance the shipping was actually less than FedEx charges to ship a letter-shaped package from the mainland US to Hawaii. (Go figure.) It still ramped the price of the mics up almost to market value, which on Amazon with its super saver free shipping basically means I could’ve ordered them new and had them weeks ago.

But they’re here. And they’re mine. And… to be honest they’re in pretty ratty shape. One of them had something loose in the capsule. If you pointed the mic up and shook it, it made a hellish noise and clipped constantly. Turn it upside down and shake, and you hear something rattling around. The other mic has something massively wrong with its circuit board. It sounds for all the world like a Huey is hovering overhead. Bup bup bup bup bup bup bup… It never ends.

So long story short, I don’t mind gutting these things and building something new. In the short term I put the good capsule on the good mic body so I have one working mic to play with. The other one I started taking apart.

Behringer C-2 Capsule Removed

These have interchangeable capsules, though I don’t know if Behringer (or anyone else) makes any other capsules for it. The one that came with my mics is a hypercardioid. (At least that’s what the icon on the side of the capsule looks like.) Given the size of the vents at the back, I can believe it.

Underneath the capsule is a white plastic plug with a pogo pin centered in it. Not much to look at. And no real clue how to open things up past there.

To gain access to the innards of the mic, peel back the the “Behringer Condenser Microphone” name tape at the base. This reveals a small set screw that should be familiar to anyone with Switchcraft XLR connectors. Screw the set screw all the way in. This releases the XLR connector from the body. Next, center the pad/high-pass filter switch and pull the switch button out with needle nose pliers. Finally, push on the white plastic plug to expose the circuit board.

Behringer C-2 Stereo Pair - Partially Disassembled

Here’s what’s inside:

Behringer C-2 PCB Top

Since one of my mics has a damaged board, rather than figure out how to tweak what’s already here, I went ahead and tried to figure out how to pack a Pimped Alice into the same board space. I started by taking measurements.

The board is 15.5mm wide x 52.35 mm long, and is 1mm thick. The thickness is important because the board slots into the white plastic insert. One nice thing about this method of mounting the board is that there are no screw holes, and except for the humongous XLR pins and the 2mm area that slots into the plastic insert the rest of the real estate on the board is free. The board is mounted just below the centerline of the mic, so there’s vertical room as well. Up to a point, anyway. Those capacitors are 6.5mm diameter x 8mm tall. Nothing bigger than that will fit, even centered on the board.

There’s really not enough room to use through-hole components everywhere, so I converted most of the Pimped Alice circuit to 0805 SMT components. The exceptions are the filter capacitors, the 1Gohm resistor, and the FET.

There seems to be some resistance to using surface mount technology for DIY mics, but SMT has been used for over a decade for DIY robotics and electronics. I’ve built AVR processor boards using SMT components, and figured this wouldn’t be much different. With the exception of the big filter caps and the FET, that’s how Behringer built the original board for the C-2, so I figured it was a safe way to go. As soon as I have a new PCB layout, I’ll send it out for fab.

Meanwhile I started taking apart the capsule. Just looking through the grille, it seems like the C-2 uses a Transsound capsule similar to the TSB-165A Scott Helmke used in the original Alice.

Behringer C-2 Capsule Front

I started by removing the back plate. This is just pressed into place, but it’s a bear to get out. I eventually removed it by gripping it by an inside edge with needle nose pliers (pushing outward), and spinning it out. It took a couple of attempts, but it came apart.

The rear side of the capsule has an open cell foam washer in it, presumably to provide wind protection and to act as a delay plate to shape the hypercardioid pickup pattern. With the washer removed, the back side of the capsule is visible, held in place by a brass retaining ring

Behringer C-2 Capsule Back - Baffle Removed

The holes in the ring are really tiny. My existing pin wrench didn’t work, so I used an old divider with dull points as a pin wrench. There’s a bit of red enamel to prevent the ring from backing out, which took a little force to crack. Once that was done, though, the ring backed out easily. (I’ll need to be sure to apply a fresh bit of enamel when I get the new capsule installed.)

Behringer C-2 Capsule Disassembled Back

Behringer C-2 Capsule Disassembled Front

I was hoping the capsule was a Transsound TSB-165A, the same one Scott Helmke used in his original Alice microphone. Unfortunately it’s not. The capsule in the C-2 is 16mm in diameter x 6mm thick. The TSB-165A is 16.5mm x 8mm. But after some poking around on the JLI Electronics web site I think I found a match: the TSB-160A. The specs are almost identical to the TSB-165A, so it should play nicely with the Alice circuit (yay!), but the form factor matches what’s in the C-2. I’ll order a pair of these when I place the order for the 165A capsules for my MS Alice.

Behringer C-2 Capsule Mesh Outside

Another concern with the C-2 capsule holder is how the capsule is recessed, and how close the edges of the holder come to the input ports on the capsule. From my experiences with my first rev of mic bodies, I know that can color the sound enough to hear it. I’d like to open this up, if possible. It’s a simple enough job on a lathe as long as I can get the grill out.

Behringer C-2 Capsule Mesh Inside

The grill looks like it’s a two-layer mesh that’s either glued or soldered into the capsule holder. That should be easy enough to remove with heat, one way or the other. I might even be able to re-use it if I’m not too rough getting it out.

The grill serves two purposes. First and foremost, it’s an RF shield to keep stray electromagnetic radiation from getting into the signal path. Second, it helps to keep the capsule free of debris. Third, some manufacturers will stick enough mesh in front of the capsule to act as a rudimentary pop filter, and at least reduce the effect of wind. The problem with that third purpose is that you need a lot of tight mesh to pull that off. Enough so that it colors the sound of the mic. Not surprisingly, one of the more obvious mic mods is to remove a layer of mesh from the capsule housing.

But given how open the outer mesh is, I’m afraid it will make the mic prone to RF interference. For now I’ll leave it alone.

The next steps are to finalize the design of the new board, send it out for fab, and source all the components and capsules. But before I can finalize the board design I want to see if I can add in one of the features of the C-2: The switch on the side of the mic lets you select a high pass filter or a -10dB pad. If I can find the real estate on the board to accommodate the switch and the components necessary to add these into the Alice circuit, I will.

Tom

Posted in Audio, Electronics, Engineering | Tagged: , , , , , , , , , , , , , , | Leave a Comment »

BM-800 Microphone Conversion Part 2

Posted by Tom Benedict on 22/10/2016

This is the second half of a two-part article describing my conversion of a BM-800 microphone to an Alice microphone using a Transsound TSB-2555B cardioid capsule. All of this is based off of a pair of Instructables written by Jules Ryckebusch: Modify a cheap LDC Condenser microphone and Build the MS Alice Stereo Microphone.

Part 1 of this article showed pretty pictures of the donor mic (a Neewer NW-800 with an excess of bling), a description of the cable that came with the mic (which I don’t intend to use), photos of the mic in various stages of disassembly, and a CAD drawing of the salient features inside the microphone to help others lay out circuit boards for their own conversions.

Since writing part 1, all of the bits and pieces I ordered to do the conversion arrived: enough electronics to build three Alice boards, and a TSB-2555B capsule to put in the first one.

Everything for an Alice Conversion

Before populating the boards I did a test fit to make sure they would actually fit. I was pleased to see how well the screw holes lined up, and I came pretty close with the taper.

NW-800 With Alice PCB

The next step was to populate the boards. Opinions differ on how to wire the high-impedance (high-Z) end of the board, so I started with all of the low-Z components.

The circuit used in Jules’s first article had zener diodes on the output stage to protect it against over-voltage on the XLR pins. The circuit as-built in his second article omits the zeners since the 2N5087 transistors are rated for more than the 48V likely to be seen on an XLR connector. I ordered the zeners, but left them out for now.

Alice Trio with Low-Z Components

After I’d already wired all the boards I installed one in the mic and ran into my first problem: With the board installed right-side-up, the 47uF capacitor pokes up high enough that it interferes with the body tube. For my first mic I’m planning to install the board up-side-down to give the capacitor more room. But if I wind up building the MS mic from Jules’s second Instructable, I’ll need to install new capacitors that lay flat against the circuit board.

BM-800 Alice Board Placement

The reason for the difference of opinions on the high-Z end of the circuit is that it’s sensitive to contamination: leftover solder flux, dirt, dirt combined with humidity, oxidization, etc. on the high-Z end can all cause unwanted noise in the mic. Jules soldered his components to the board without issue. Others have used Teflon standoffs to float that part of the circuit above the PCB. Homero Leal built his Charis mic by point-to-point soldering the high-Z components, letting them float above the board without standoffs. Scott Helmke, the original designer of the Alice circuit, solders the high-Z components directly to the back of the mic capsule. For my first pass at this I soldered the low-Z legs of the FET to the board, but floated the high-Z circuit without stand-offs, similar to Homero’s Charis mic. I can always change my mind later and re-wire them.

High-Z Components Air-Floated

With the board built, the next step was to add 22nF capacitors between pins 1 and 3 and pins 1 and 2 on the XLR connector to provide additional RF noise filtering. After that I installed the modified connector and the board in the mic body.

Alice Board and XLR with RF Filter Caps

The rest of the action takes place inside the headbasket.

It’s possible to cut away the original mic capsule to leave a saddle for mounting the TSB-2555B, but I wanted to make an entirely new saddle. Chalk some of this up to not wanting to make a modification I can’t back out. Chalk some of it up to my wanting a machining project to go along with the electronics project. Either way it needlessly complicates an otherwise pretty simple project.

Space inside the headbasket is tight, so rather than run into more interference issues I fleshed out the 2D CAD drawing and turned it into a 3D model. The space constraints almost entirely dictated the shape of the new saddle and post. The mic frame is drilled and tapped for M2.5 screws on a 10mmx15mm rectangular pattern, only two of which are used on the original saddle. I chose to use all four. The mic wires pass through holes spaced 20mm apart, centered on the long axis of the bolt pattern. In the CAD model I indicated these with 3.13mm holes, but in the final part I cut them as slots to make installing and removing the capsule easier.

Mic Saddle - CAD vs. As-Built

I attached the TSB-2555B capsule to the saddle with E-6000 silicone adhesive. A better method for the saddle shape I used would’ve been a polyurethane adhesive like Gorilla Glue, but I wanted to be able to remove the capsule in case I decide to add shock isolation inside the mic to cut down on handling noise. As-built the capsule can be removed by passing a fine wire between the capsule and the saddle, cutting the silicone bond.

EDIT: The first time through, I missed an important step: One of the charms of the Pimped Alice circuit is the potentiometer next to the 1Gohm resistor. It allows you to bias the FET properly, regardless of which FET you use. The catch is that by definition, if you don’t do anything with the potentiometer it will not be properly biased! In all ignorance I soldered everything up, closed up the mic, and went testing. Even with an improperly biased FET it still performed beautifully. I did go back and do a proper job of it, though.

In Jules’s first Instructable, toward the end, there’s a nice write-up for how to bias the FET. The catch is that this step must be done before the capsule is soldered to the board.

With the FET properly biased and the capsule attached to the saddle and post, all that was left was to put it all together and close it up.

Finished BM-800 / TSB-2555B Alice

I did a quick side-by-side against one of my Primo-EM184 cardioid mics. The Alice runs a little hotter, but not by too much. I’m reserving further judgement on the new mic until I have a chance to get it out in the field and try it on some quiet sources.

Tom

Posted in Audio, Electronics, Engineering | Tagged: , , , , , , , , , | 3 Comments »

BM-800 Microphone Conversion Part 1

Posted by Tom Benedict on 09/10/2016

Several posts ago I mentioned a plan to build an MS mic by following an Instructable written by Jules Ryckebusch. Jules used a BM-800 microphone as a donor mic and replaced its guts with two Pimped Alice circuits and three cardioid capsules. After several Ebay vendors whose listings indicated they would ship to Hawaii later changed their story and said they wouldn’t, I finally picked up a BM-800 microphone off of Amazon. The one I got is a Neewer NW-800. (I liked its shock mount better than the other one I found.) It arrived, and I started poking and prodding at it.

Along the way I discovered another reason to use a windscreen on a microphone. This thing is bling central. To be fair some of the other BM-800 mics I found on Ebay didn’t have nearly as much… presence… but this is the one I could get.

Neewer NW-800 Bling

Unless I’m recording birds that are drawn to shiny objects the windscreen will probably become a permanent fixture on this mic, just to keep me from going blind.

Neewer NW-800 Windscreen

Before tearing into the thing I decided to try it as-is. On the face of it it’s a phantom powered cardioid condenser mic. This means plugging it into a device that doesn’t provide some kind of power (aka my laptop, my phone, even my kids’ desktop computer) won’t work.

The mic has a male XLR jack at the back, and came with an XLR-to-3.5mm cable. 3.5mm inputs that provide power typically provide plug-in-power (2.3V to 5V, depending on the device). XLR inputs provide phantom power (typically 12V, 24V, or 48V). That discrepancy made me a little leery of just plugging this into whatever and cranking volts through it. I started by ringing out the cable to see what it was actually doing.

XLR to 3.5mm Cable

Up to this point all the XLR plugs I’ve dealt with have been for balanced signals. That is to say that one pin of the 3-pin XLR is ground (pin 1), another is the positive signal (pin 2), and the third is the inverse or negative signal (pin 3).

3.5mm inputs typically use a TRS connector and unbalanced signals. In the case of the 3.5mm stereo input on my recorders the tip is the left positive channel, the ring is the right positive channel, and the sleeve is ground.

The cable supplied with the BM-800 ties XLR pins 1 and 3 together and routes them to the sleeve of the 3.5mm plug, and routes XLR pin 2 to both the tip and ring of the 3.5mm plug. This effectively turns the balanced output of the mic into two channel mono unbalanced output on the 3.5mm plug, meaning it should be able to be plugged into any 3.5mm stereo input and drive both left and right channels with the same signal. Neat!

What this also means is that as long as the mic can run on a wide range of voltages, the plug-in-power on any recorder should be able to drive this thing. So should the battery box I got from Church Audio. Or by removing the XLR-to-3.5mm cable and plugging in an XLR-to-XLR cable, I should be able to power it with phantom power (12V or 48v – the only two options on my recorder) and use it as a single channel balanced input.

Still leery of running such a wide range of voltages through it, I tried all three configurations anyway. I’m planning to gut this mic, after all, so if I burned it out the loss would be minimal. To my surprise all three worked! The plug-in-power on my DR-70D puts out a little under 3V, and my battery box from Church Audio puts out a little over 9V with a fresh battery. The 48V phantom power on the XLR inputs on the DR-70D put out right around 48V. I noticed a gain difference between the PiP and battery box, but because of the different gains on the XLR vs. 3.5mm inputs on the DR-70D I wasn’t able to tell if the additional voltage was doing anything to the mic itself. (My guess is it doesn’t. To survive that wide a range of voltages I’m guessing the mic has a voltage regulator on board. Past a certain point it’s just dissipating as heat.)

So how does it sound?

Um…

How to put this…

I’ve seen the shock mount it came with listed for more than what I paid for the mic. I don’t think this is too far out of line with how it sounds. It’s not bad, mind you. It’s just not anything I’d write home about. A little creative EQing would probably make it a decent podcast microphone. But as for making ambient nature recordings? Mmmm… no.

So without further ado I tore into it to see what I was going to have to deal with.

Neewer NW-800 Disassembly: Assembled

The first step in disassembling the microphone is to unscrew the butt cap. This also releases the shell, which simply slips off to expose the circuit board. The shell is keyed to a tab just under the headbasket which fixes the orientation of the logo on the mic. This is important since the mic is a side-entry rather than end-entry, meaning sound must enter from the side and not the end. Added to that, it’s a directional microphone so it’s only sensitive on one side. Can you guess which side? (Answer: The one with the logo.)

Neewer NW-800 Disassembly: Shell Removed

Some nice features on the inside of the thing: First, there’s a ton of room. Second, there’s a nice frame with mounting holes tapped for M2.5 screws. (More about those in a sec.) The only weird part is the taper on the frame and the circuit board. I like the look of the tapered board, so I decided to taper the boards for my Alice conversion, too, and put mounting holes in the boards to make use of the holes in the NW-800 frame.

Neewer NW-800 Disassembly: Headbasket Removed

Two M2.5 flat head Phillips screws hold the headbasket in place. They’re located just under the headbasket, above the circuit board. Once the screws are removed the headbasket lifts off, exposing the capsule.

Despite the appearance, the capsule in this mic is the same size as the EM-172 and the EM-184 capsules from Primo: 10mm diameter. At this point I was sorely tempted to gut the mic, drop an EM-184 capsule in the mic saddle, and call it quits. But the whole purpose of this exercise is to move beyond Primo all-in-one capsules and try my hand at building more complicated (and better performing!) microphones.

Neewer NW-800 Disassembly: Circuit Board Closeup

All of this starts with the circuit board.

Simple stuff first: The screws are M2.5, spaced 30mm apart. They’re biased a couple of millimeters above the centerline of the cavity. If you’re planning to make a rectangular circuit board to fit inside this mic, that’s probably all you’ll need. (The tube with the logo has vertical walls, so a rectangular board will fit fine.)

Since I wanted to make a tapered board I measured the whole cavity and threw it into CAD. At some point I’ll draw it in 3D, but for now a 2D representation is plenty for me to design the new board outlines. I’m building the Alice boards using through-hole components, so I needed a little more real estate than the original board provided. The 2D drawing of the cavity and the new board outline looks like this:

2D CAD - NW-800 Cavity and Board Outline

I sent the boards out for fab and ordered enough components from Mouser to build out three of them. One is destined to receive the TSB-2555B capsule I ordered from JLI. The other two will eventually be used to build a copy of Jules’s MS mic using three TSB-165A capsules, but that’s a project for another time. Once all the bits arrive I’ll write the second half of this article, which will cover the construction of the TSB-2555B mic.

Tom

Posted in Audio, Electronics, Engineering | Tagged: , , , , , , , , | 5 Comments »

Building New Mics

Posted by Tom Benedict on 17/09/2016

When you’re faced with a dilemma like choosing the next step to improve your recording gear, instead of finding the right answer to the question, sometimes it’s more fun to dodge the issue completely and go off on a tangent.

So I went off on a tangent! Building new microphones!

I’ve currently got two projects in the works. A parabolic mic and a self-contained mid-side mic.

The Parabolic

I’m basing my parabolic mic off of the family of parabolic mics  from  Telinga Microphones. The mics from Telinga offer all kinds of neat options. One contains a cardioid mic facing the parabola and an omni facing away from the parabola. This lets you record a distant sound and the ambient sound field at the same time on two different channels. Another contains two omnis on either side of a baffle plate so you can record a distant sound in stereo.

But at its simplest, a parabolic microphone uses a parabolic reflector to direct pressure waves at a single microphone located at or near its focus. That’s where I’m starting.

I picked up a 22″ parabolic dish from sdill471 on Ebay. He sells them for around 50USD and ships all over the place, including Hawaii (yay!), using USPS shipping.

The microphone for this project is the first EM-172 lavalier mic I built back when I started building external mics for my DR-05. It’s since been converted to XLR and received the full shielded treatment the rest of my EM-172 mics got when I did that conversion.

The rest of the project will be to make all the mechanical bits to place the EM-172 mic at the focus of the dish. I’m drawing a good bit of inspiration from WW Knapp’s Homemade Parabolic Mic page, though I’m making two big departures Knapp’s design: The first is to think more in terms of parts I can make in a machine shop rather than what I can find at the hardware store. (This departure is called “needlessly complicating a good, simple design”.) The other is to take a tip from Klas Strandberg at Telinga: You don’t always want the mic to be at the exact focus of the paraboloid. Having the ability to rack the microphone through focus gives you some much needed flexibility in the field to widen or narrow the pickup pattern of the mic, or even to tune which frequencies are focused on the mic by the dish.

I’ll post the design and build articles once I’ve finished the mic.

Mid-Side Microphone

The entire idea for the self-contained mid-side microphone comes from an Instructables article written by Jules Ryckebusch. Jules took a BM-800 mic – about ~20USD off of Ebay depending on the seller – gutted it, and replaced its innards with two Pimped Alice amplifier boards and three TSB-165 capsules. The really clever part is how he did it, but for any of that to make sense it helps to understand how mid-side microphones work.

The easiest way to understand mid-side recording is to read a really good article about it. What I wrote below won’t be nearly as good, so I urge you to follow that link. That being said, here’s my take on mid-side:

Back when recording was in its infancy no one even thought in terms of stereo recordings, quadrophonic, 5.1, 7.1, or any of the other immersive formats we’ve since come up with. Mid-side was one of the earliest stereo techniques, patented by Alan Blumlein in 1933.

Mid-side uses two microphones: one to pick up the center part of the sound field (the “mid” mic) and another to pick up the sound on either side (the “side” mic). In most cases the mid microphone is a cardioid, which preferentially picks up sound in front of the mic. In all cases the “side” mic is a figure-eight – a microphone that picks up sound in two opposite directions, but nowhere else.

To create what we consider a conventional Left-Right stereo image from a Mid-Side (M/S) recording requires a little math. The equations look like this:

Left = Mid + (+Side)

Right = Mid + (-Side)

In the equations the Mid channel is taken as-is. The Side channel is used twice: first it’s used as-is (+Side) and the second time it’s used inverted (-Side).

As wonky as that sounds, and as convoluted as the post-processing sounds, it offers some distinct advantages when mixing the tracks afterward. Want a wider stereo sound? Mix in a little more of the Side channel and a little less Mid. Want to focus the listener’s attention on the bird in front of the mic and down-play the forest full of frogs chirping in the background? Bump up the Mid and turn down the Side. Want to mix a mono track to go with an accompanying video on Youtube? Use only the Mid channel for clean mono without any phasing issues. The real strength of mid-side is the flexibility and versatility it offers after the fact.

The one catch with mid-side, as with all stereo techniques, is that it requires two distinct microphones. ORTF requires two cardioid mics and a bar to mount them on. A/B requires two widely spaced omnis. Even my SASS consists of two omni mics mounted in an admittedly rather large baffle. M/S is no different, requiring a cardioid and a figure-eight.

What makes M/S special is that you want the microphones to be as close to each other as you can get them. By its very design it’s inherently physically compact. (Side note: This is true of X/Y as well, which uses two cardioids pointing 90 degrees to each other, and of the Blumlein arrangement, which replaces the cardioids with figure-eights.)

Which leads us back to Jules’s M/S microphone, which takes “compact” to a new level by cramming multiple microphones into just one mic body. That makes for a light, portable recording kit that’s quick to set up and tear down; perfect for traveling, or for recording subjects that require substantial hiking to reach.

So why three capsules instead of two? Jules realized that if he took two of the TSB-165 cardioid capsules, faced them in opposite directions, and wired them 180 degrees out of phase with each other in series, they act like a single microphone with a figure-eight pickup pattern. Add a third TSB-165 capsule in the center and you have all the makings of a well matched mid-side microphone.

Where Things Stand Now

My parabolic reflector arrived last week. The mic for the parabolic project is already in-hand, though I may have to (yet again) cut it out of its housing and install it in a new one. I’m in the process of designing the mechanical bits, and should be able to start making them in the next couple of weeks.

I ordered the BM-800 donor mic for my mid-side mic just this morning. Jules posted a link to download the Pimped Alice PCB files that Homero Leal designed based off of Scott Helmke’s original Alice design. Once I have the board mounting hole pattern off of the BM-800 microphone, I’ll add those to Homero’s PCB layout and send the files off to OSH Park for fab.

Work on both of these is contingent on my getting a number of other gotta-do’s off my plate, but I hope to make some progress on both in the next couple of weeks.

Tom

Posted in Audio, Engineering | Tagged: , , , , , , , , , , , , , , , | 1 Comment »

An Inexpensive Shock Isolation Mount

Posted by Tom Benedict on 17/06/2016

One of the problems with building a funky microphone setup is that off-the-shelf gear won’t always work with it. It’s pretty straightforward to find wind protection for a shotgun mic or for a single omni. No one makes wind protection for do-it-yourself SASS arrays. (And no, that’s not what this post is about. That’s still a work in progress.)

Up until now I’ve run my DIY-SASS without shock isolation. It’s worked after a fashion, but any time I position my gear in foliage I wind up with tap-tap-tap noises of branches or long grass touching the tripod legs. More than one person has pointed out that even rudimentary shock isolation would get rid of most of that.

Unlike wind protection, it’s possible to adapt other shock isolation mounts to my DIY-SASS. Any of the lyre-style mounts for handheld recorders would work fine. But most of these are relatively tall. I wanted something more compact. And cheaper, if I could swing it. Here’s what I came up with:

Microphone Shock Mount Top

It’s adapted from an anti-vibration camera mount for a multi-rotor. As I received it, the mount consisted of two carbon fiber plates with four vibration damping balls (yes, that’s the real term). The balls are replaceable, and can be swapped out for harder or softer ones. The mount had 1/4″ clearance holes top and bottom. I wanted this to fit between a tripod and my DIY-SASS, or between my DR-70D and my DIY-SASS, so I needed a threaded hole on the bottom and a threaded thumbscrew on top.

Microphone Shock Mount Bottom

Adding a threaded hole to the bottom was relatively straightforward. This would’ve been prettier with a round piece of metal, but I had the plate stock in-hand, and it was almost the right size. I squared it up, transferred the hole pattern from the carbon fiber plate to the aluminum, and added a 1/4″-20 threaded hole in the middle.

Adding the thumbscrew to the top was a little more involved. I had some 2″ 6061 aluminum round on-hand, so I knurled it at that diameter, faced off the front to leave an 0.250″ diameter x 0.375″ long boss, and threaded it with a 1/4″-20 die.

Normally you’d want to single point thread a boss like that to avoid all the normal ills of die cut threads: drunken threads, off-axis starts, offset threads, etc. But since this only had to screw into a 1/4″ T-nut to hold my microphones in place, a die cut job was fine. I parted the thumbscrew off the bar, flipped it around, and faced off the other side.

The damping balls that came with the mount turned out to be a pretty good match for my DIY-SASS. I’d have to swap them out for softer ones if I used it with my DR-05 handheld recorder. But since this is probably going to be a permanent addition to my DIY-SASS, it’s fine as-is.

I finally had the opportunity to test this in a systematic way. I put two contact mics on my tripod legs and tapped the center column while adjusting the gains on those channels until they both read the same. Then I moved one of them to the top of my SASS and tapped the center column to see how much attenuation the isolator provided. I recorded both configurations so I could compare in Audacity. The isolator very consistently provided 21dB of attenuation. I don’t know how that compares to a commercial isolator like one of the lyre mounts I mentioned earlier, but it’s a darned sight better than the zero dB attenuation I’ve had up to this point.

I always feel a little weird posting a DIY that requires the use of a machine tool. In this case it involved both a lathe and a mill. But the core idea of this is to adapt a multirotor camera mount to microphones for field recording. There are other ways to get that threaded hole and thumbscrew. Imagination and ingenuity are powerful tools of their own.

Have fun!

Tom

Posted in Audio, Engineering | Tagged: , , , , | 3 Comments »

DIY Microphone: EM172 Capsule and XLR Plug

Posted by Tom Benedict on 05/03/2016

This is the last in a four part series about powering the Primo EM172 microphone capsule. Part 1 outlined the problem of how to provide 5-10v to the capsule and predicted some results. Part 2 shared some results and pointed out that the gain differences between inputs on my recorder invalidated my predictions. Part 3 discussed my reasons for going with XLR connectors on all my microphones, and some of the details of that. This last part puts it all together into a step-by-step DIY for building microphones with Primo EM172 capsules, powered by 48v phantom power on an XLR plug.

If you need to build a microphone based around the EM172 capsule that plugs into the 1/8″ mic jack on your recorder, or a laptop, tablet, whatever, there are already several excellent tutorials out there. Rather than adapt this one to your needs, refer to one of the existing tutorials. The two I used when I first started building EM172 microphones were the ones on Zach Poff’s page and the one on Wild Mountain Echoes.

In this DIY I’m going to assume you already have a plan for making a mic body. I made mine out of Delrin bar stock on a lathe. Others have used Sharpie pen caps, which also provide a nice clip for clipping the mic to things (see the tutorial on Wild Mountain Echoes), PVC pipe, brass tubing, etc. When mounting the mic in the mic body, make sure the front of the capsule is flush with or slightly proud of the mic body. Don’t recess it. I made that mistake with my first set of mics and wound up with mics that sounded like they were inside a sewer pipe. If in doubt experiment by wiring up the mic completely, plugging it in, and listening to it as you slide it in and out of the mic body you plan to use. After all, this is DIY. Experimentation is part of the deal.

Primo BT-EM172 to P48 XLR Wiring

Credit for the circuit goes entirely to David McGriffy, and credit for the component choice goes entirely to David McGriffy and Ricardo Lee. Ricardo Lee’s writeup, SimpleP48wm61, goes into the theory of the circuit and the reasons for the component choices in depth. It’s the real reference for this. (In order to use that link to download Ricardo’s file, you may need to be a member of the micbuilders group on Yahoo!. If you’re doing this DIY you’re a mic builder, so it’s not a stretch.)

EDIT: A couple of weeks ago Akira So brought to my attention that I had the capacitor poloarity reversed from how David McGriffy and Ricardo Lee have it in SimpleP48. I’ve since corrected the schematic here. Credit where credit’s due.

EDIT: Akira also pointed out that my value for R (120k) resulted in something like 1.3-1.5V at the capsule. I experimented with a number of resistors to see what value of R would produce 7.5V at the capsule on my recorder, and for a Tascam DR-70D, R=40k produces just over 7.5V. When you do this build, you will have to find what works best for your equipment.

EDIT: I also swapped the supplier for the EM172 from Frogloggers to Micbooster (FEL Communications). I haven’t heard from Gene at Frogloggers in a while. Hoping he’s doing ok.

For my build I used the following:

I also used some metal tape (copper in my case, from the local gardening center), heat shrink of various sizes, and the solder I found on the bench in the lab. (My Alphametals solder I’ve been using for the past 20 years isn’t ROHS certified, so I can’t say “use this stuff, it’s great!”)

Not including the tools necessary to fabricate the mic bodies, you’ll also need:

  • Soldering iron (temperature regulated if possible)
  • Source of heat for heat shrink (heat gun, lighter, etc.)
  • Assortment of wire cutters, strippers, fine tip pliers, etc.

Since most of the bodies people use for these require the mic to slide in  from the front end of the housing, we’ll start with the mic capsule.

EM172 Back End

The first step is to strip one end of the cable, trim back the red and white wires to a workable length, and still leave plenty of shield exposed. The red and white wires are then soldered onto the appropriate pads on the capsule.

Warning: The EM172 capsule is sensitive to heat. These two photos were made with a capsule I’d killed using an unregulated soldering iron, which is why the capsule looks a little ugly. If you have access to a regulated iron set your iron no higher than 735C and don’t hold the iron on a pad for more than a few seconds. If you don’t have access to a regulated soldering iron, be sure to get EM172 capsules with stub leads already soldered in place. The tutorial on Wild Mountain Echoes uses capsules with stub leads, so you can see how she did it. Do all your work on the stub leads. Don’t fry your microphones!

EM172 With Wires

Now we build the shielding around the capsule itself. Insulate the sides and back of the capsule with some heat shrink.

Capsule Isolated

Be sure to account for every strand in the shield as you bring it up and over the heat shrink. Wrap with foil tape and trim back the shield so no wires protrude. Be sure no wires cross over the heat shrink and touch the front of the capsule.

Making a Shield

Apply a second layer of heat shrink over the foil tape. I like to apply a short length of colored heat shrink to help me identify which mic is which when I’m running wires and plugging things in out in the field.

Heat Shrunk Ready To Go

At this point go ahead and run the mic cable through your mic body, but don’t mount the capsule just yet. Once you’ve soldered the connector end of the cable, it’s a good idea to test everything to make sure you didn’t make any soldering mistakes, and to make sure the capsule didn’t get damaged during soldering. Strip the other end of the cable, leaving a little more wire to work with than on the capsule end. Thread the wire through the end cap for the XLR connector and set it aside. Since the XLR connector provides its own shield you don’t have to do any metal tape trickery on this end. Gather the wires from the cable’s shield, twist into a bundle, and cover with heat shrink tubing. This is also a good time to apply a length of colored heat shrink to match the capsule end of the cable.

Cable Prepped With Shell

Grab the XLR connector body in a vise or some other holding fixture. If you don’t have a vise, a set of vise-grip pliers with tape over the serrated part of the jaw works well. Just don’t grab it so hard that the connector body is damaged or distorted. Another way to hold these connectors that works great is to have the mating connector screwed into a board. Plug the connector you’re working on into its counterpart and solder to your heart’s content. (I used a vise.)

Trim back the leads on the capacitor and resistor to something reasonable that’ll fit inside the XLR connector. Save the snipped off bits of the leads. One of these works well to bridge from pin 1 to the ground tab.

Resistor and Capacitor

Solder a leftover component lead from pin 1 to the ground tab. Next, solder one end of the resistor to the ground tab as well. Next, solder the (+) end of the capacitor to pin 2. Finally, tie the two free ends of the capacitor and resistor together.

XLR Plug with McGriffy Components

All that’s left is to solder the cable onto the plug. Red goes to pin 3, white goes to the (+) lead of the capacitor as well as the free end of the resistor, and the cable’s shield is soldered to the ground tab. (In this photo the connector is rotated 180 degrees from how it’s drawn in the schematic, but that’s how the solder cups are oriented. Flip it around in your mind and it’ll make sense.)

XLR Plug with Cable

At this point your microphone’s electronics are finished. Put the connector together and screw things tight.

This is a good time to test the mic to make sure nothing went wrong. Plug it into your recorder, turn on phantom 48v power, and dial up the gain. If all went well you should have a low noise microphone ready to be installed in its mic body. If not, go back and check each step to find out what went wrong.

Finished Mic

Have fun recording!

Tom

Posted in Audio, Electronics, Engineering | Tagged: , , , , , , , , , , | 88 Comments »

Powering the EM172 Capsule

Posted by Tom Benedict on 16/01/2016

Chris Hass wrote a very nice article on building microphones around the EM172 capsules on her site, Wild Mountain Echoes. In it she mentions the issue of power. The datasheet for the EM172 specifies a supply voltage of 5-10v, but most handheld recorders supply something considerably lower than that. Chris and I compared notes, and her Sony PCM-M10 and both of my Tascams supply something closer to 2.3-2.7v. My question to her was how this affects performance, and what my options are for doing something about it.

Chris suggested bypassing the recorder’s own built-in power and using an external battery box to supply a higher voltage to the mic. She pointed me toward the boxes made by Church Audio. I followed her advice and bought a Bat 2B from them. It should be here in a couple of weeks.

Since my 70D has XLR inputs I decided to pursue another possibility as well. Most recorders can supply 24V or 48V phantom power on their XLR inputs. The only trick left is to drop that down to the 5-10V the microphones want. I ran across a thread on the Yahoo! micbuilder forum that referenced a circuit by David McGriffy called Simple P48 WM61 (referring to a simple circuit to power the Panasonic WM61 microphone from 48v phantom power). Richard Lee uploaded a document to the forum describing McGriffy’s circuit, along with modifications for using it with (you guessed it) the EM172 capsule. I still have a bunch of Mogami cable left over from building my earlier mics, so I ordered the remaining parts for McGriffy’s circuit from Mouser Electronics. he parts should be here in a couple of weeks as well.

In the meantime I figured it would be a good mental exercise to try to predict what each of these approaches would buy me in terms of performance. All of this ties back to a set of graphs on the micbuilder forum. It’s in Files/EM172/Primo EM172 Sens Noise vs RL VL.pdf. The graphs show the performance of the EM172 capsule as a function of supply voltage and input impedance. Using a battery box or McGriffy’s XLR circuit will let me change the supply voltage, but the input impedance is a function of the recorder. Here are some cases:

 

Tascam DR-05 and DR-70D 1/8″ Inputs:

Both the Tascams supply just under 3v for plug-in-power. The input impedance on the DR-05 is 25k ohms, and the DR-70D is 10k ohms. The graphs only go up to 10k ohms, so I’m using that number for both cases. Bumping the supply voltage from 3v to 9v should have the following effect:

Sensitivity: -38.6dB -> -36.7dB (smaller negative numbers are better)
Noise Floor: -112.7dB -> -116.1dB (bigger negative numbers are better)
S/N: 74.1 -> 79.4dB (bigger numbers are better)

In reality the DR-05 should get a bigger bump since its baseline performance will be lower than at 10k ohms, judging by the trend in the graphs. But the preamps on the DR-05 are noisier than those on the DR-70D, so I may not be able to hear the improvement.

 

Tascam DR-70D XLR Input:

The DR-70D’s XLR inputs have an input impedance of 2k ohms. Since I’m starting at 5V it should have the following performance:

Sensitivity: -38.3dB
Noise Floor: -116.8dB
S/N: 78.5dB

The noise floor is better than on the 1/8″ input, but the sensitivity won’t be quite as high. If I re-sized the resistor in the McGriffy circuit to provide something closer to 10v I’d get the following results:

Sensitivity: -37.7dB
Noise Floor: -116.8dB
S/N: 79.1dB

No change in the noise floor, but the sensitivity would improve by another 0.6dB. I’m not sure I can hear that, so it’s probably not worth dinking with.

 

Sony PCM-M10:

I also ran the numbers for Chris’s recorder. The Sony has an impedance of 3.9k ohms. Bumping from 3V to 9V should have the following effect:

Sensitivity: -37.8dB -> -37.2dB
Noise Floor: -115.1dB -> -116.5dB
S/N: 77.3dB -> 79.3dB

Almost 1.5dB improvement in noise floor, and 2dB overall improvement in signal to noise.

 

Sony PCM-D100:

The input impedance of the higher-end companion to the M10, the PCM-D100, is 22k ohms. It should see a similar performance bump to the Tascam DR-05, but since the preamps on the D100 are so much better than the DR-05, this will likely make for an audible improvement in the performance of the mic.

 

From the standpoint of mic performance, both approaches provide a clear gain. Whether my ear is sensitive enough to tell the difference remains to be seen (or heard!) From the standpoint of convenience, additional gear complexity, etc. each one has its pluses and minuses.

On the up side, the Church Audio battery box supplies 9V and will work with any recorder with a 1/8″ input, so I can use it on both of my recorders. Another up side for me, personally, is that so far I’ve built all my EM172 mics with 1/8″ plugs, so it requires the least re-work in order to test. On the down side it means I have to add a 9v battery, battery box, and cable to my setup. Velcro will go a long way toward making this a non-issue (mostly), but I wish this kind of thing could be designed in from the get-go. (Recorder manufacturers take heed! Being able to dial in a particular plug-in-power voltage would be nifty!)

The up side with the XLR approach is that from the standpoint of gear it amounts to changing the plug at the end of the cable. All of the circuitry fits inside the XLR plug. As an added bonus I’ll be able to plug EM172 mics into all four XLR inputs on my 70D, which is pretty darned cool. (The 70D only has one 1/8″ plug, which is tied to channels 1 and 2 only. Up until now I’ve only been able to do two channel recording on my four channel recorder.) The down side is that the 48v phantom supply on the 70D is a battery hog. So even if it works it means I’ll have to pack extra batteries or an external battery pack.

Good news is neither approach was all that expensive, and even with the Bat 2B or the external battery to compensate for the extra load from the 48v phantom power, neither adds too much bulk to my bag. For the moment I’m looking at it as having more options rather than having to choose between one approach or the other. In the extreme case it would give me the ability to plug two mics into my DR-05 with the Bat 2B, and another four into my DR-70D using XLR plugs. Six channels at once!

Now all I need is a subject that actually needs six channel audio. But that’s for another day.

Tom

Posted in Audio | Tagged: , , , , , , , , , , , , , , | 4 Comments »

Pseudo-SASS Array

Posted by Tom Benedict on 03/09/2015

Following the successful testing of the new mics, I turned my attention toward building a pseudo SASS array. My prototype design was a copy of a copy of a copy of… well… a copy. This becomes important later, because it’s lessons learned from one of those copies that drove part of my final design. First, a bit of history, mostly gleaned from Vicki Powys’s web site:

The SASS, or Stereo Ambient Sampling System, was designed by Michael Billingsley in 1987 for Crown International. It was (and still is) aimed at recording music performances, audiences, and other subjects that lend themselves to stereo recording. It offers relatively strong stereo separation in a small package. Crown sold the SASS with microphones built into the system, which worked well for its intended purpose. But the original microphones exhibited a high noise floor, and weren’t well suited for quiet subjects. Lang Elliott and Michael Billingsley modified a Crown SASS to work with higher-end microphones, and Walter Knapp took that concept and offered re-tooled Crown SASS units that would take, among other mics, the Sennheiser MKH20. This made it a viable choice for recording quiet ambient sounds and field recording.

More variations tailored to field recording continue to be made. Rob Danielson’s PBB2N, built out of wood and PVC pipe along similar lines to the Crown SASS, takes the same range of microphones as the units made by Walter Knapp, and offers better bass response. Vicki Powys, concerned about the weight of a wood array, took that concept and made her own version out of dense closed-cell foam (pool floaties) using Primo BT-EM172 capusles. After building her foam SASS, she did side-by-side tests with it against a Crown SASS with MKH20 microphones. The performance of the two were remarkably close.

The idea behind the Crown SASS, Rob Danielson’s PBB2N, and Vicki Powys’s SASS-LN2, is to baffle the microphones so that the array acts as a pseudo-binaural stereo pair similar to the human head. The wings act as boundary surfaces, and the foam baffle in the center partially absorbs sound from unwanted angles. The end result is a higher gain than a bare microphone, and considerably better separation than two microphones arranged as an X-Y pair. This photo of a Crown Audio SASS-P MkII was a later model that used PZM mics, but the overall shape didn’t really change much from the original SASS:

I built my prototype based off of measurements I took from a photo of an original Crown SASS, scaled to the overall size of Vicki Powys’s SASS-LN2. I wanted to test the idea before leaping in and building an airborne unit, so I built the prototype out of 1/2″ foam core board. The unit provided quite good stereo separation, and had considerably higher gain than the built-in mics on the Tascam DR-05, but it lacked bass punch. I wanted to figure out why before going from prototype to final design.

One clue came from another of Rob Danielson’s designs: PBMB2. His design calls for significantly larger wings than the original Crown SASS. The larger wings provide a larger boundary surface for the microphones to work with, and therefore provide gain at lower frequencies than the original from Crown. Since I’m planning to use this in the air, cross-section is an important design consideration I need to take into account: larger objects are more easily affected by the wind than smaller ones. Rather than using the larger wings from Rob Danielson’s PBB2N array, I stuck with the dimensions of the Crown SASS.

Another clue came from a set of posts on the micbuilders Yahoo group: Electrets mics need to be mounted flush with the end of their enclosures. Mine aren’t. They’re recessed several millimeters into their tubes and hide behind a layer of stainless mesh. Recessing the mics this way colors the sound they pick up.

Mono Mic Assembled

Unfortunately I learned this late in the game, after I’d already built all five of my mic enclosures as well as my airborne pseudo-SASS. Before going out and designing all new enclosures I decided to test this for myself. I disassembled one of my mics and tried sliding the mic deeper into the tube. I found that the more recessed it was, the more mid-range gain I got, and the less bass. Finally I pulled it out entirely, bare to the world, and tried it that way. I could easily tell the difference. There was a lot more bass, and the mic sounded a lot less tinny. (Hey, if I can tell the difference, it’s HUGE!) Time to design all new mic enclosures!

The last clue came from the folks at DIY Boundary Mics. They ran some tests on the array built by Vicki Powys and the modified Crown SASS with Sennheiser MKH20 microphones. Rob D. (Rob Danielson?) from DIY Boundary Mics noted that Vicki’s foam array lacked some of the lower frequency response the Crown SASS / MKH20 combination had. He attributed it to the soft nature of the boundary surface (foam). Paul Jacobson at DIY Boundary Mics ran a comparison between Vicki Powys’s array and a similar one made of untreated wood, similar to Rob Danielson’s array. The untreated wood array recovered some of the bass lost in the foam array. This agrees with Rob D’s conclusions about the hardness of the boundary surface.

Which leads me back to the prototype I built out of foam core. The outer surface of the foam core is relatively hard, but the foam itself is acoustically thin, and the foam core board has a high natural frequency. I’m guessing that some of the lack of bass punch in my prototype can be traced to the material I used to build the array and the lack of damping material in the array’s inner cavity. I needed something better.

Years ago I made a kite aerial 4×5 film camera out of birch plywood. I never was completely happy with the photos it produced, but it turned out super pretty. Since Rob Danielson was making boundary array mics out of wood, and since the wood SASS had better bass performance than Vicki’s foam one, I figured I could build mine out of wood as well.

I already had some 0.200″ baltic birch plywood left over from the 4×5 camera, so that’s what I used for the array body. The woodwork came together relatively quickly, but I couldn’t finish sealing up the box until I had the damping material glued in place. Here’s one problem with living on an island: no one sells acoustic materials. Rob Danielson used carpet padding in his PBB2N, so I went that route. Here’s another problem with living on an island: stores that sell carpet padding don’t like breaking up rolls! I finally wound up at Home Depot. I waited patiently in the flooring department until someone could help me. I’d already been to several stores, and had received more than my fair share of blank stares when I asked for one foot of carpet padding. I wasn’t expecting much.

The folks at Home Depot surprised me! When I asked for such a small amount, the guy in flooring said, “You building a speaker box or something?” “A microphone array, actually, but it’s the same idea.” “Cool!” He was super helpful, and sent me home with my one foot roll.

Eventually my DIY-SASS came together. It’s shown here with my original mic enclosures, but in the next few weeks I’m planning to swap them out with flush mounts:

DIY-SASS

It uses the same Primo BT-EM172 capsules as Vicki Powys’s array, though she used four and I only used two. The covering for the baffle gave me fits until I finally bent to common wisdom and used sheet metal. (I’d wanted to make it out of the same plywood I’d used for the rest of the SASS for cosmetic reasons, but I ran into structural issues.) The hardware store sold 6″x12″ aluminum for almost the same cost as 6″x12″ polished stainless, so I went with the stainless. But I had to bead blast the outside of it to keep myself from going blind when I took it out in the sun.

DIY-SASS Front

There are a lot of screw holes on this, both to hold the baffle cover in place and to attach the array to a tripod (or a KAS rig!) I’ve seen too many wood screws strip out over time, so I epoxied T-nuts into each screw hole to provide machine threads. Since  I do a lot of my KAP along the coast and plan to record sound in that environment as well, I went with as much stainless hardware as I could. Even so, I’m going to have to open the unit periodically to check the wiring for salt contamination. (One more reason to be glad I used T-nuts!)

DIY-SASS Bottom

Since I’m planning to use this on the ground as well as in the air I didn’t want to wire in a dedicated cable. I’ll only need 2-3′ for aerial work, but on the ground there’s good reason to put some distance between a microphone and the recordist. Having a way to swap cables seemed like a good idea, so I wired it with a 1/8″ TRS jack so I can use the cable of choice, depending on what I’m doing.

I’ve now used my pseudo-SASS in the field several times. I was pleased to find that the heavier construction worked, and that I got back a lot of the bass punch I’d lost with the foam core prototype array. I’m looking forward to trying it with the flush mounted microphone enclosures to see how much more bass I can recover.

Meanwhile I’m facing yet another design problem. Like any microphone, my pseudo-SASS array suffers from wind noise. I learned this the hard way while trying to record the sound of waves crashing on rocks.

In Dire Need of a Windjammer

The wind buffeting was more than the mics could handle, so I wrapped the whole thing with my folded up t-shirt. Even that wasn’t enough to cut the wind, so none of the files were usable. Bummer!

Unfortunately the wind there was nothing compared to the wind I’ll get when I hang this thing from a kite line. And since Rycote and Rode don’t make windjammers for DIY mic projects I’ll have to build my own. My last act of the weekend was to order a yard of 2″ pile 100% polyester artificial fur with the loosest backing I could find. As I finished checking out I couldn’t help thinking yet again, “You’re getting in deep, man.” I fear I’ll learn how to sew fake fur before I learn how to make my own kites.

– Tom

Posted in Audio, Engineering | Tagged: , , , , , , | 3 Comments »