The View Up Here

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

Posts Tagged ‘Cardioid’

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

Advertisements

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

Clippy EM184 Cardioid Mics and ORTF

Posted by Tom Benedict on 05/08/2016

I’d planned to write an article describing my trip to Edinburgh for SPIE 2016, but I got side-tracked. That article is yet to come.

I did some audio recording while I was there, but not nearly as much as I’d have liked. I wound up packing all of my sound gear, including my SASS, but the few times I pulled it out it rained. The one time I thought I’d get to use it for sure – poking it out of my hotel room window to record traffic sounds – I found it was too big to fit through the window. I wound up using spaced omnis to record traffic sounds, but the SASS didn’t get used even once. I found myself wishing I had other options.

A number of common stereo techniques require the use of cardioid microphones. Up until my trip to Scotland I only had omni microphones in my bag. There are still some stereo techniques that use omnis that I haven’t tried, but I’ve been wanting to play with cardioid mics for some time. Step one was to buy or make some cardioids.

The same circuit I used to make my EM172 omni mics can be used with other FET-enabled Primo capsules, including the EM184 cardioid capsule. FEL Communications (micboosters) sells these on their site either as individual caps or as matched pairs. I picked up a matched pair along with a pair of Clippy mic bodies, clips, and windscreens. I still had some Mogami cable and Neutrik connectors on hand, so I just drew from that stock to build out the new mics.

The Clippy mic bodies work nicely with the cardioid capsules, and the resulting mics have very little pickup at the back. It’s not zero, though, so you do have to be aware of everything that’s not directly in front of the mic. I’d been warned that cardioids are more sensitive to wind than omnis, and these mics bear that out. They’re stupid sensitive to wind. Even with the foam windscreens and some furries I got from Cat Ears, the slightest bit of wind kills them. I need to figure out some other solution for wind protection.

Step two was to come up with a way to hold the mics so they record a clean, well separated stereo image. There are plenty of choices for this, but the one I chose was ORTF, a technique designed around 1960 by Office de Radiodiffusion Télévision Française (ORTF) at Radio France. (See? Astronomers aren’t the only ones to recycle their acronyms!)

ORTF requires the microphones to be separated by 170mm and angled away from each other at a 110 degree angle. It’s a bit of a pain to set up in the field without some way to gauge the angle, so many people favor other setups such as NOS (Nederlandse Omroep Stichting) in which the mics are separated by 300mm and are angled out by 90 degrees. I wanted to play with ORTF, though, so I decided to solve the setup problems with a fixture.

Clippy ORTF Bar

Since the Clippy mic bodies register nicely with their lapel clips, I used the clips to orient the mics both in location and rotation. The clips have a tab on top that’s just over 6.2mm wide. I made 6.5mm wide slots at either end of a bar to receive the clips.

Clippy ORTF Bar With Mics

I wanted to keep things simple so I didn’t have to fuss with stuff in the field, and this lets me do that. With the clips fully seated in the slots the mics are angled out at a 110 degree angle and are 170mm apart. It takes more time to unroll the cables than it does to install the mics on the fixture. And the flat bar packs down a lot smaller than my SASS.

Clippy ORTF Bar Slot Detail

The bar I used was just over 4mm thick. I cut the slots to leave 2mm of material for the mic to clip to. This wound up being a little thin, but it made for a nice, deep slot to register the clip in.

Clippy ORTF Bar Velcro

The bare metal of the bar was too slick for the clip to get any real grip, so I put a tab of Industrial Velcro on the bottom of the bar under each of the slots so the clips would have something to grab onto.

I’m pleased with how easy it is to use this setup, and it’s tough to beat how compact it is. But I’m not 100% satisfied with how it works in the field just yet. I already mentioned the wind issue. Even with double protection the mics saturate when almost any amount of wind touches them. They’ll probably fare better inside  a Rycote or a Rode blimp, but for now I’ll have to save them for wind-free environments.

The sound is also significantly different from that of my SASS. (Sorry, no side-by-side comparisons yet.) The SASS picks up more reverberation than the ORTF setup, so there’s more of a sense of the space with the SASS than with the ORTF. But you don’t always want that sense of space. During an earlier test I had one of my omnis and one of the cardioids in a car. The omni picked up so much of the car noise, it was difficult to hear the people in the car speaking. The recording from the cardioids was much cleaner.

Needless to say there’s still plenty of testing to be done. Once I learn the strengths and weaknesses of this setup and have a better handle on wind protection, I’m sure it’ll see plenty of use.

Tom

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