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Things I Like About the Zoom F8

Posted by Tom Benedict on 06/11/2018

I figure the progression of recording equipment I’ve used is pretty typical: I started with an inexpensive handheld, a Tascam DR-05, progressed to a four channel, four XLR input recorder, a Tascam DR-70D, and most recently bought a used Zoom F8 that I’ve been using for the better part of a year.

Most of the time when you change gear, there’s something you miss from the previous one. When I moved from the DR-05 to the DR-70D I added two inputs, added XLR jacks, added dual-level recording, etc. But I lost stereo-linking (the DR-05 only has one gain setting for both channels) and I lost deterministic gain (the DR-70D doesn’t tell you how much gain you’re applying, unlike the DR-05). I expected to face the same problem when I got the Zoom F8.

With the exception of the larger size and weight, though, I really didn’t lose anything. And what I gained was worth every penny. The more I dug through the menus, the more stuff I found that would make my life easier when I’m out recording. Here’s a fairly long laundry-list of the things I love about my Zoom F8 (with v.5 Firmware):

  • It has eight inputs. Every time I’m told I’ll never need that many inputs, I look back at the number of times I’ve plugged something into every single one. I’m constantly playing with different mics, different setups, different ways to record the world around me. Having eight inputs means I can plug in my SASS, a mid-side, an ORTF pair, and two spaced omnis all at the same time. It’s fun in a box.
  • It took the Tascam dual-level recording feature and made it better. When you set up dual recording on the F8, it splits the input and delivers it to two tracks. Every control is available on that second track. You can set it to a different gain level in case the first channel clips, but you can also set one up with a high pass filter and one without. Or one with a limiter and one without. One with a limiter and the other with -10dB gain as a “safety track”. Whatever you want, you can do it. It’s just another track.
  • You can set up four “groups” of gain control, so stereo or quad recordings all have their gains tied to a single knob. This was one I really missed when I moved to the DR-70D. Most of what I record is in stereo, so having the ability to link gains is massive. Even better, you can combine this with dual-level recording to have two pairs of stereo tracks from a single pair of inputs, each with their own gain knob.
  • The advanced look-ahead limiters that came with the v.5 firmware on the F8 work really really seamlessly well. I was surprised by how well these worked. I had the opportunity to use my F8 as an audio interface during a voice acting workshop (more on that in a sec), and during the live-directed battle emotes I was yelling loud enough to clip. But both of the directors told me it sounded fine. Even better, looking back at the tracks there was no sign of clipping. It’s smooth and seamless. (Quick side note: No matter how good your limiters are, you really don’t want to keep running into them. They’re insurance in case your gain was set too high, not a mode you want to operate in on a regular basis.)
  • The F8 can act as an audio interface. For a lot of people doing field recording, this won’t be a big deal, but I also do voice acting. It’s entirely possible to voice act into a recorder and load the tracks into a DAW afterward. I did that for some time with my DR-70D. But having the option to record through the F8 straight into a DAW, especially for a live-directed session, is awesome.
  • With the new v.5 firmware, the F8 can act as an interface while recording to SD cards! This came in handy for online workshops and would be great for online live-directed sessions. I could use my F8 as an interface to participate in the workshop, and I could record my own mic in one channel and the workshop as a whole in a second channel. I know it’s possible to make an interface talk to a DAW and to programs like Skype and Discord simultaneously, but this took all the risk out of it. I got native-quality recordings without the possibility of a software glitch costing me the session.
  • The input/output routing, combined with its ability to act as an interface, makes for a very powerful analytical tool. I’m beginning to get into measurement and characterization of the mics I’ve been building. The inputs and outputs on the F8 work great for injecting signals directly into a mic preamp, and when you combine them with an amplifier and speaker, can be used to measure the microphone as a whole. (That will be the topic of a future post.)
  • The F8 takes two SDXC cards. How you use them is up to you. You can record in PolyWAV on one card and individual WAV files for each track on the second, or you can record the ISO tracks on one and a stereo mix-down on the other, or WAV on one and MP3 on another, or any combination you can come up with. I use the second card as a backup and record PolyWAV to both. That way if I inadvertently erase a track or an entire day’s recording, I can recover from the second card.
  • The F8 can run off of eight AA batteries, but it also has a DC barrel jack and a four-pin Hirose for external power. The DC barrel connector is for an external AC supply (which is great when you’re using it as an interface) and the Hirose is for external batteries. I got a pair of Talentcell DC bricks that’ll each give me over 24 hours of recording time. I’m not a huge fan of the Hirose connector because I’ve had them fail at work, but if your gear is kept snug in a sound bag and not hanging off the back of a telescope, they’re pretty solid.
  • The preamps really are quite nice. When I went from the DR-05 to the DR-70D I got a 9dB (~8x) improvement in the equivalent input noise, which was huge. Going from the DR-70D to the F8 I got another 7dB (~5x) improvement. At -127dB EIN (unweighted), the preamps are really really quiet, which makes the +75dB of available gain on the preamps mostly useful without blowing up the noise floor.
  • But my favorite part of this whole thing is that you can save the state of the recorder as a configuration file that you can load again at some later date. No worries about missing a setting or forgetting to toggle something. I can plug in, turn on, and load the configuration of my choice. Ready to roll! Here’s the list of what I’ve got set up right now:
    • Mono Ch1 – Good for single sources, parabolic, voice acting
    • Mono Ch2 – Good for single sources, parabolic, voice acting
    • Mid-Side Ch12 – Mid-Side stereo mic
    • Mid-Side Ch34 – Mid-Side stereo mic
    • Stereo Ch12 – Left-Right stereo mic (SASS, ORTF, A/B, etc.)
    • Stereo Ch34 – Left-Right stereo mic (SASS, ORTF, A/B, etc.)
    • Stereo Ch1234 – For running dual stereo rigs
    • Quad Ch1234 – For running quad rigs like my spherical mic. All channels tied to one gain knob.
    • Dual Ch12-56 – Dual level recording setup for channels 1 and 2
    • Dual Ch34-78 –  Dual level recording setup for channels 3 and 4
    • Dual Ch1234-5678 – Dual level recording setup for running dual stereo rigs
    • Interface Ch12 – For when I’m using the F8 as an interface. My local mic goes on channel 1, the computer’s sound goes on channel 2

I’ve completely ignored the other half of the F8: that you can use it as a location mixer. Some of the mixer features of the F8 make it a really solid tool for doing location sound. But since that’s not really my forte, I can’t give you my take on it. As a field recorder, it’s excellent.

UPDATE: And just this morning, Samuel Greene from Zoom announced there’s a firmware update for the Zoom F8 (v.5.10E). And that’s one more thing I love about my F8: Zoom’s excellent customer support.

 

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Inexpensive “Stealth” Bag

Posted by Tom Benedict on 25/06/2018

Almost a year ago, I made a drop bag according to Andrew Jones’s instructions on Wavreport.com. This featured in the photo set for my DIY Parabolic Mic that I wrote about back at the beginning of the year.

Recently I found another use for it.

The bulk of my field recording is nature recording. I hike out somewhere remote, set up mics, and make stereo recordings in an effort to allow the listener to feel like they’re really there.

But sometimes I record the sounds of people: crowds, traffic, the hustle and bustle of life. For the most part, the birds and the beasties don’t care what my mics look like. But people do. Even when I’m out recording the sounds of nature, people will spot my mics, walk over, and ask what I’m doing. (Which, if you’re into the obvious, means they show up in my recording. So much for pristine nature tracks.) Sometimes it’s advantageous to conceal the equipment in order to make a “clean” recording.

Recently, I had the opportunity to record in three cities: Austin, Texas, Washington DC, and New York City. (To be fair, I’m heading to New York tomorrow, then back to DC later in the week.) I knew I wanted to be able to record street traffic, the New York Subway, and crowd sounds. I turned to Andrew’s Drop Bag.

"Stealth" Recording Bag - Closed

I didn’t follow all the instructions from Andrew’s build. Most notably, I didn’t modify the top of the bag. It makes the bag a bit of a pain to use when tweaking the settings on the recorder, but it means it can be zipped shut with no one the wiser as to what’s going on inside.

"Stealth" Recording Bag - Open

What is going on is a Zoom F8 recorder, a Talentcell lithium battery modified according to another of Andrew’s articles, two EM-172 omni mics clipped to the outside of the bag, and a set of earbuds for monitoring.

The bag has a pair of loops on either side of the front compartment that you can clip lavalier mics to.  I stuck the lav mic case in the front pouch to give it a little depth, and voila, it’s a partially baffled stereo array with almost the same mic spacing as a human head. Not a bad setup for making stereo recordings, and unless you’re looking for the mics they blend right into the bag.

The Talentcell does an amazing job of keeping the recorder powered. Even after recording aggressively for several days, I have yet to reach the limit of the Talentcell battery.

Sound pressure levels in cities vary wildly, from the hushed tones of a museum crowd to the scream of a train entering a subway station. One of the nice features of the Zoom F8 (and of the Tascam DR-70D I used before) is the dual recording feature: Set the inputs for the mics up for the quiet sources and the backup tracks for the loud sources, and pick the appropriate track when curating the sounds in post.

At some point, I’ll order a second one of these and do Andrew’s full build on it. But I’m leaving this bag as it is. It’s just too handy for this sort of thing.

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34mm LDC Alice

Posted by Tom Benedict on 22/04/2018

I built a new vocal microphone!

Up until now all the vocal mics I’ve built have been based on the Schoeps CMC 5 circuitry and a Transound electret capsule. The first was back in September, 2016, when I made my first Alice, based on Jules Ryckebusch’s Instructable. Almost a year later I built three more along similar lines using BM-700 donor bodies and the Transound TSB-25AX capsule. That first one I built is still my main vocal mic, but I’ve been using all four in one form or another ever since.

This time I wanted to try something different. I’ve wanted to build a 34mm non-electret condenser ever since I started dabbling in voice acting. A combination of events finally let that happen.

The first was finding some relatively inexpensive 34mm capsules on Ali Express back in December, 2017. I wound up getting one with the plan of developing a charge pump board to go with the Alice boards I’d used on the last set of mics. Before I could do that, though, a long-term project at work took my full attention and I had to set the capsule aside.

Around the same time the work project began ramping up, Homero Leal posted a new version of his Pimped Alice board to the Yahoo! micbuilder forum. This one included circuitry to bias a non-electret condenser capsule. (Yahoo!!) I ordered ten. But until the project at work ramped down enough for me to have a real life again, the capsule and boards sat in a box, waiting for me to get back to them.

Just when I started to worry I’d never build this thing, the project reached a good pause point and the rest of the pieces came together. I ordered a donor body, designed a capsule mount, and finally finally got busy. (Considering this is coming only six months after my TSB-25AX Alice trio instead of the year it took me between the previous two sets, I’m doing better this time! I really can’t complain.)

34mm LDC Alice Open>

Everything fit nicely inside the donor body, including the capsule and mount. I was concerned that the thicker capsules wouldn’t fit inside the head basket without requiring modifications, but it juuust barely fit. (The big exposed diaphragm on this capsule gives me the heebiejeebies every time I take the head basket off, though!)

Capsule Mount

Unlike the other capsule mounts I’ve designed, this one uses four M1.6x6mm screws to hold the capsule in place. (Normal for an LDC capsule.) It uses the same shock mount system as the previous mounts, though, and still has the tunnel down the center for routing the capsule’s wires to the PCB. (Version 2 of this mount will have holes on both sides of the post so it can work with dual-diaphragm capsules.)

34mm LCD Alice

Once the microphone is all closed up, it’s ready to go!

I’ve only just begun testing it against the other Alice mics I’ve built, but so far I’m pleased as can be. The 34mm capsule has a little more low-end punch (which is great on my voice!) and a subtly different high end. I haven’t decided which microphone I prefer for higher frequencies, but for my voice the LDC Alice is a clear winner.

Three Generations of Alice

And here’s the multi-generational line-up of the Alice mics I’ve built, starting with my very first one on the left, retrofitted with a shock-isolated capsule mount. The TSB-25AX Alice is in the middle, featuring the first shock-isolated capsule mount I designed. And the 34mm LCD Alice is on the right, featuring Homero’s board and the new capsule mount.

For anyone else itching to build one of these, here are the links to the version 1.0 capsule mounts, both for those who don’t have their own 3D printers:

34mm LDC Capsule Mount on Shapeways

And for those who do:

34mm LDC Capsule Mount on Thingiverse

(Mounts for the internal switch for switching the HF EQ in and out can be found on the same sites.)

Happy mic building!

Tom

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DIY Stereo Parabolic Microphone

Posted by Tom Benedict on 21/12/2017

I’ve had a parabolic dish I bought off Ebay sitting around for the better part of a year. The whole idea was to build a parabolic mic out of it based on the dishes from Telinga, but for one reason or another I got stuck in the design-and-abandon phase and never got to the just-cut-metal phase. So the dish sat, collected dust, and didn’t do much except confuse my cats and annoy my family.

At some point in there I picked up eight EM-172 capsules in two matched sets of four from FEL Communications aka Mic Booster along with a bunch of yoga blocks. My plan with these was to make a pair of quad capsule foam SASS arrays similar to Vicki Powys’s for recording surround sound. I got stuck in design-and-abandon on this project, too.

In the end, though, it all worked out for the best because of yet another project. Only, this one actually reached completion, mostly because I was doing it for someone else. (There’s nothing like a deadline for getting a designer off top dead center.) The project involved designing and 3D printing some mating threaded parts. I modeled the threads in CAD, put the clearance into the model, and printed the parts. The threads meshed first try. W00t!

That was reason enough to celebrate, but it was also reason to scratch my chin and go, “Hmmm!” Being able to 3D print threads solved one of my issues with the parabolic mic: how to attach the handle to the dish. While reading this article from Avesrares on a DIY parabolic dish I had my second epiphany: Their stereo parabolic mic used four matched EM-172 capsules… just like the ones I had sitting in an envelope. Thank goodness I hadn’t built those SASS arrays, either!

So with technique and components in hand, I set about making my own stereo parabolic mic.

The Dish

One difference between my DIY parabolic mic project and many others is that I made no attempt to fabricate my own dish. There’s an Ebay seller who makes 22″ Lexan dishes for under $50USD. Shipping to Hawaii was expensive, but the dish itself was pretty affordable. This was where the whole project started.

The Handle

I like how the Telinga handles attach to their mics. The Avesrares article mentioned using auxiliary drill handles in a similar fashion, so I searched Ebay and found one of a suitable size for under $10US. The next step was to model a corresponding threaded tube and two screw plates to attach the handle to the dish.

DIY Parabolic - Clamp Tube and Collars Render

I wanted to be able to build interchangeable microphones, so I sized the inside diameter to take 3/4″ schedule 40 PVC pipe. The screw plates are 4″ in diameter – about the maximum my 3D printer can handle. The screw pitch is 14 TPI (sorry, metric system), and was sized to fit the outside diameter of the tube. If I remember right the threads left 10% flats top and bottom, and I left 0.020″ clearance between the tube and the screw plates. (They meshed great, too. W00t!)

Since these are sandwiching a parabolic form, the inner surfaces of the screw plates are curved to match. I added a layer of craft foam inside and out in an effort to provide some damping for handling noise (which it didn’t) and to make a nice snug fit (which it did.) Everything went together fine.

DIY Parabolic - Clamp Tube and Collars

The Microphone

In an article written by Klas Strandberg from Telinga Microphones, he explains that the focus of an acoustic paraboloid isn’t a precise affair. The mathematical focus of the paraboloid will be the point of best focus, but the size of the region of focus is frequency-dependent. I’d try to paraphrase here, but the article puts it so well there’s really no point. If you’re interested, read his article.

The upshot is that you can, to some degree, adjust the EQ of a parabolic mic by shifting where the capsules are with respect to the point of best focus. I knew I wanted some latitude, but I didn’t know how much range I might need. So I cut the 3/4″ PVC pipe a little long, reasoning I could always cut it down later.

One bonus to using 3/4″ PVC pipe is that Neutrik XLR connectors are a nice snug press fit into the tube. Since I was planning to make a stereo microphone, and since I’d already built an XLR-5 to twin XLR-3 splitter cable for my MS Alice microphone, I used an XLR-5 connector for this mic as well. While not the most elegant setup for this connector, it was an expedient solution that let me get on with the business of building the rest of the microphone. I can always revisit this later.

(As a quick aside, “I can revisit this later” is designer code for “You’re going to see this in the final version.”)

DIY Parabolic - Handle, XLR5 Connector, and Reflex Sight

My first design for the business end of the microphone followed the logic in the Avesrares article: a baffle that splits the dish left-from-right and a second baffle facing the dish. Unfortunately my dish has a shorter focal length than its depth, so adding the second baffle would’ve cut off about half the surface area of the dish.

Instead I went with a baffle similar to the one used on the Telinga microphones, except that instead of using the microphone capsules as pressure zone transducers (PZT), I stuck to familiar territory and arranged them as boundary mics. I’m not 100% sure this was an ideal approach since it moves the diaphragms of the capsules away from the axis of the dish, limiting how close they can get to the point of best focus in the radial direction. Still, it worked.

DIY Parabolic - The Giant Lollipop - Render

Originally I’d intended to make solid baffle plates on the 3D printer, but these would’ve been costly in terms of material and time. Instead I took a tip from the SASS I built and made them only 1mm thick. This made for a very thin, unfortunately flexible plate with a high resonant frequency. I backed it with Dynamat, which does an excellent job of damping vibrations, and sandwiched dense latex foam between the two plates to provide further damping. The whole stack wound up being acoustically dead to handling noise. (But more on the handling noise later.)

Unfortunately, before I took any pictures of the finished parts I stuck open celled foam on either plate to provide wind protection. So the only photos I have look like a big foam lollipop.

DIY Parabolic - The Giant Lollipop

I wired the capsules using the same pattern as Vicki Powys’s SASS, except instead of wiring them into a 3.5mm TRS plug, I wired them into the XLR-5 connector using David McGriffy’s and Ricardo Lee’s Simple P48 circuit. Tuning the resistor for the Simple P48 circuit took some experimentation since each circuit was now driving two capsules instead of one. I’d quote the value of the resistors I wound up using, but there’s really no point. If you choose to build one of these, best to tune the resistors to get the bias voltage you want for your capsules.

With everything closed up, the microphone slides into the clamp tube on the dish and locks into place by twisting the drill handle. Voila, the DIY stereo parabolic microphone.

DIY Parabolic - Finished and In the Field

The Bling:

With such a narrow pattern microphone, I erroneously thought I’d need some way to aim it. (I’ve since used it in the field enough to know you aim these things by ear and not by eye.) One of the remnants of a previous project of mine, the camera helmet, was the small reflex sight I’d used for aiming the cameras. I mounted a short length of Weaver rail to the drill handle using a 3D printed adapter, and clamped the reflex sight to the rail. Voila, a neat looking, but ultimately useless addition to the parabolic mic.

This was rendered even more useless by the next bit of bling.

DIY Parabolic - Useless Reflex Sight
Wind Protection:

After some initial testing in the field, I found I wanted the option of another layer of wind protection. The foam on the lollipop was nice, but it wasn’t really enough to stop the kind of wind you get outdoors. (Foam is never enough to stop real wind. Like ever. Eventually I will learn.)

DIY Parabolic - Wind Protection

Chris Owens wrote a really good article describing the wind protection he made for his Telinga parabolic mic. I followed his directions to the letter and wound up with a very serviceable (and machine-washable!) cover for my mic.

DIY Parabolic - Wind Protection Detail

He used his dish to measure a circle of cloth, and rolled a hem at the edge that holds a length of shock cord. Once the hem is finished, the shock cord is pulled tight(ish) and is tied off. Chris Owens cautioned not to pull the shock cord too tight or the dish will warp. I made mine just snug enough to make sure the cloth stays on unless I’m ready to pull it off.

 

Post Script:

Now that I’ve had the chance to use my dish in the field I’ve found a number of shortcomings I’m planning to address in the future:

  • Anything you can’t put a shoulder strap on is lame – My dish is incredibly awkward to carry on a trail. Klas at Telinga solved this by making 1mm thick dishes that can be rolled up and carried in a shoulder bag. I’m sticking with my dish, but I have to add a shoulder strap. This is irritating.
  • Handles that can twist bother me – The drill handle I chose tightens (and loosens!) by twisting the handle. I’ve had it try to come undone in the field a couple of times when turning the dish. I’m planning to replace mine with a Bosch handle that uses a thumbscrew.
  • Handling noise is lame – I’ve been chasing handling noise on the BM-*00 conversion mics I’ve been building with some success, but this thing has it twice as bad. You can’t just shock-isolate the capsules. You have to shock-isolate the dish as well because it basically acts like a great big sounding board. Anything that’ll conduct handling noise to the dish shows up in the recording. A cushioned handle like the ones on the Telinga dishes would be a step in the right direction. For now I’m wearing gloves any time I use it.
  • I’m not 100% happy with the mic setup – The first prototype mock-up I did with this dish involved mounting my Alice M-S at the focal point of the dish. This worked way better than I suspected, and provided a better stereo image and a better mono image than the mic I’ve got in there right now. I’m thinking of recovering my four EM-172 capsules so I can go ahead and build that SASS surround mic, and building a dedicated mid-side mic to live in the dish.

 

And now for the rest…

First, A Sob Story:

I like to include audio samples in my build articles. Unfortunately I can’t for this one. Not yet, anyway. A little over a week ago I got some kind of outer ear infection in both ears. The tinnitus I’ve experienced for the last twelve years or so is about a bazillion times worse at the moment, and my right ear has lost about half its frequency range. I tried to edit a recording I’d made of a katydid and realized I couldn’t even hear the katydid in that ear. (And it’s not like they’re subtle or anything.) Even worse, my two ears currently hear broad-spectrum noise as two completely different sounds. Until this infection clears up, my ears are screwed. I can record, but I can’t edit. Sorry.

Second, A Note on the Photos and a Thank You:

No, that’s not me holding the mic in the field. I was the one tripping the shutter. (I know which end of the camera I like to stay on.)

About half the pictures in this set involve wet gear. This is one of the hazards of recording in Hawaii: it rains. Rain is not ideal for recording with a parabolic mic. Each rain drop on the dish was picked up by the mic, and there were plenty of rain drops. I did make some recordings during the photo session, but because of the rain none of them really showed off the mic very well. Everything survived fine, though.

The black audio bag worn by the recordist in the photos is the brainchild of Andrew Jones, described in an article he wrote on WAV.REPORT, Tutorial: “DIY” Audio Drop Bag. Mine’s only about half finished (it still zips shut) but it’s a really good article and an unbeatable bag for the price. Thanks, Andrew!

Third, An Apology:

Normally at the end of my build articles I like to link to all the necessary bits and pieces, including any parts I designed. I’m not 100% comfortable doing that for this project because it’s still such a work in progress and there’s so much work still to be done to make it what I want it to be. But if you’re dead set on building one of these, feel free to contact me.

 

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Useful Little Clampy Things

Posted by Tom Benedict on 20/12/2017

A while back I needed the ability to put a 1/4″-20 threaded stud… Somewhere. Anywhere. Everywhere. I tried some things on the 3D printer, but realized none of them would be as robust as I needed these to be. So…

I turned to Ebay!

A Useful Clamp

Turns out you can get insanely cheap clamps by searching on “super clamp”. Some come with T-bar handles, some with offset handles like this one. Some come with articulated arms (which I used for mounting cameras on our vacuum chamber for work). And some just come with 1/4″-20 and 3/8″-16 threaded holes. Mine came with threaded holes.

I added the 1/4″-20 threaded stud by chopping the heads off some bolts and cleaning them up with a grinder and a die. The thumb screws came off of a camera hotshoe to 1/4″-20 adapter (also courtesy of Ebay.)

All in all these things came together for less than $6 apiece.

A Useful Clamp with Shock Mount

My original purpose for these was to be able to space mics anywhere I wanted across a bar for trying different stereo recording techniques. To do that I needed to be able to mount any of my self-contained stereo setups, all of which take a 1/4″-20 thread, or to mount individual shock mounts like this one.

A Useful Clamp with Ballhead

But there’s no reason to stop there. Pop a small ballhead on one of these and you can mount a camera, a handheld sound recorder, an off-camera strobe, or whatever else strikes your fancy.

The only problem I’ve found with these is that the handles like to come off, leaving the clamp firmly attached to whatever you cranked it onto. If you decide to get any of these, be sure to test them first, and if any handles come off, re-install them using red or green Loctite. (I had to re-install three of mine!)

I wound up building five of these, but now I wish I’d ordered more.

P.S. Since writing this, I’ve been told this is all pretty standard fare for lighting and grip gear on a film set, and that re-purposing lighting gear is a cheap and easy way to build solid recording setups. My take-away: 1 – There’s nothing new under the sun with this article. 2 – My butt’s gonna go numb the next time I sit down with the B&H catalog or start surfing their web site! 3 – Neither of these is a bad thing.

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Clean Clean Clean!

Posted by Tom Benedict on 13/12/2017

I’ve spent the past year or so at work helping prepare for the arrival of a new instrument. One of the requirements is for the room to meet ISO 8 standards with a goal to reach ISO 7.

The reason for the requirement is that the instrument is one big cryostat – about 2m in diameter and about 3m long – and will be bolted to the floor. Once it’s installed, the only way to service it is to open it up wherever it is. It can’t go in the clean room, so we brought the clean room to it.

ISO 8 roughly translates to Class 100,000. It allows for 29,300 >=5µm particles per cubic meter, 832,000 >=1µm, and 3,520,000 >=0.5µm particles. ISO 7 , our goal, knocks that down by a factor of ten (2,9300 >=5µm, 83,200 >=1µm, 352,000 >=0.5µm, Class 10,000).

 

Coude Room Particle Counts 2017-12-13

As of this week we’re hitting under 175,000 for 0.5µm, 75,000 for 1µm, and under 3,500 for 5µm with upwards of ten people in the room, two of them installing weather stripping on the doors. That’s well inside ISO 7 with a shot at ISO 6 once we finish all the work, provided we can demonstrate counts at 0.3µm and smaller. (That instrument is in use in another room at the moment.)

When no one is in the room, the particle counter doesn’t register any particles at all. W00t!!

Just a happy point in a long work schedule. Details of the work on the room will be presented in a paper at the SPIE Astronomical Telescopes + Instrumentation conference in Austin, Texas in June, 2018. The instrument itself will show up at the end of January, 2018, so we’ll have plenty of time to put the room through its paces during integration and commissioning.

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Clippy ORTF / X-Y Bar

Posted by Tom Benedict on 25/11/2017

One of the real pains about setting up some stereo mic arrangements is positioning everything juuuust right. The ORTF setup is the classic example. It uses two cardioid microphones positioned so the diaphragms are 170mm apart and angled outward at 110 degrees. Possible to do with a stereo bar? Sure. Easy to do? Not so much. Not without some way to verify the angle and spacing.

My first cardioid mics were a pair of Primo EM-184 cardioid capsules mounted in Clippy lavalier bodies from FEL Communications (Micbooster.com). Take all the normal complications of trying to set things up for ORTF and complicate them by doing everything with lapel clips. Possible to do? Sure. Easy to do? You gotta be kidding me.

Back in May of 2016 I wrote about a nifty little bar I made in the shop to hold everything in just the right place. I machined it out of a chunk from the scrap box that wound up being 7075 aluminum. Massive overkill, all things considering, but it worked. I profiled the bar and machined slots in it so the clips would hold the mics juuuust right. It worked, and it made the job of setting up for ORTF a snap.

Since then I machined another set of slots and cut-outs in the bar so I could also set up for X-Y. (Almost X-Y… the capsules aren’t quite coincident, but they’re quite close.) This also worked great, though I still have problems remembering to swap the mics. (ORTF mics face outward, X-Y mics face inward, so the left and right mics are on opposite sides for the two setups.)

My plan was always to make this design available for others trying to do stereo on a budget. I dropped the ball and never got around to putting the design out there, but I finally fixed that.

I put the STL file up on Thingiverse for those who want to grow their own, and on Shapeways for those who want to get one pre-made (or whose printers don’t have enough volume to print the whole bar (like mine!))

 

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BigMic With a Side of Cheese Sticks

Posted by Tom Benedict on 15/11/2017

Some projects just never stop being projects. And sometimes the resulting feature creep isn’t necessarily a bad thing.

A couple of months ago I was trying to find a good acoustic space for doing voice acting, and after pulling my hair out trying to find a quiet spot in my house (which happens to be within twenty feet of a highway) I finally settled on using my Civic. It’s mobile, it’s sound-proofed…ish, and it’s actually a pretty comfy place to sit. I added some acoustic treatment to reduce the remaining standing waves in the car and moved on to the next problem: The electronics.

To make the Civic work as a mobile sound booth I needed a way to mount my scissor arm while still letting it act like a scissor arm. So I machined a bar that would clamp to the headrest uprights on one of the front seats. The bar had a row of threaded holes so I could get coarse left-right adjustment to center the pivot point of the scissor arm, and an upright with a 1/2″ bore for the scissor arm to socket into.

"Cheese Stick" with scissor arm and vocal mic

While I had the thing on the mill I added some 1/4″-20 threaded holes through the top of bar so I could stick a ball head on the opposite side from the scissor arm socket. This let me flip the bar over to use as a mic mount for doing in-car recordings. It’s shown here with my SASS bolted on top.

(As a side note, this isn’t actually the best setup unless you’re trying to record the sound of the car itself. The SASS uses omni capsules, so it picks up every sound in the car.)

"Cheese Stick" with SASS for recording in-car stereo sound

Since the threaded holes were so useful I went back and had a mad drilling session on the mill to pepper the thing with threaded and clear holes for doing… whatever…

That’s when I found out there’s a name for this thing. For the drilled and tapped end of this thing, anyway: It’s a “cheese bar”.

But when I looked up “cheese bar” on Google all I got were these specialty cheese shops that are set up like bars. Go figure. (It’s worth searching on “cheese bar” just to see what these places look like!) Searching on “video cheese bar” got me closer to the mark.

Since mine started life as something else entirely, I hesitate to call it a cheese bar. There’s the headrest clamp at one end and the scissor arm socket at the other. Rather than add confusion to confusion, I decided to call mine something a lot more descriptive: the “cheese stick”.

It’s probably just as bad a term as “cheese bar” when it comes to Google searches, but now I can truthfully say “I’ll have a big mic with a side of cheese sticks!”

Har…

I’ve used the cheesy side of the thing a number of times, now. While doing EQ testing on a bunch of microphones I clamped it to a stand and lined the mics up across the top. Over the last weekend I used it to build something a little more ambitious:

Double-MS setup using the "cheese stick" for fixturing

This is a double-MS setup entirely built using Alice microphones. The center mic is my self-contained MS Alice, but in this case I’m only using the figure-8 for the side channel. The other two mics are a pair of the TSB-25AX Alice mics I built, set up as the front and rear cardioid channels of the double-MS.

I haven’t taken it out in the field yet, but it’s a pretty straightforward setup to put together. I’m hoping to do field testing with it in the next couple of weeks.

Meanwhile I took the test files I made in my house and tested the post processing toolchain I’m planning to use. I started off doing the mid-side decoding by hand, but in the end ran them through the Schoeps Double-MS VST plugin in Reaper, which did an excellent job of generating 5.0 surround sound. If I add an omni on the fourth input of my recorder, I can use it for the LF necessary to do full-blown 5.1 surround.

W00t!

And all because of the feature-creepy cheese stick!

Tom

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Contact Mics – Four Channels Good to Go

Posted by Tom Benedict on 05/11/2017

I finally finished installing the other two channels in the impedance-matching preamp box I built back in July for plugging contact mics into my recorder. The amplifiers I used came from Stompville in the UK. They’re compact, low noise, and let plenty of the low frequencies through.

Contact Mic Preamp Box

Side note: I just checked the Stompville web site and they’re running out of the JFETs they used in this design. They’re going to offer a new product using a new JFET, so if you build something like this you may want to order all of yours at once rather than piecemeal the way I did. Otherwise there’s no guarantee of a match between channels.

I also finally packaged my contact mics so they’re not just copper tape wrapped piezo discs like the one I used for my kite line recording. (I eventually broke this contact mic through rough handling.)

Contact Mic on Line

Coming up with a housing for the mics took some thought. You don’t want to do anything that changes the sensitivity or frequency response (an impossible task, but you try to minimize those effects) but you also want a housing that works with how you intend to use the mics.

Most of the time I’ve attached my contacts using double-sided tape or Blue Tak (thanks to Tim Prebble and his excellent post, The First Rule of Contact Mic Club for that tip!) Other times I’ve used big honkin’ magnets (thanks to Richard M and his post on his Megalithia site for that tip!) But sometimes the easiest way to attach a contact mic to a thing is to clamp it.

In all the testing I’ve done with piezo contact mics, the biggest gotcha I’ve seen is that they really don’t like to have pressure applied directly to the top of the piezo element. It introduces a big DC offset that scales with pressure. What this means is that clamping directly to the center of the disc is not just bad for the piezo disc. It’s also bad for anything it’s plugged into. So clamping directly to the disc is a big no-no. (That’s also how I cracked my tape-wrapped contact mic!)

I wanted to find a housing for my contact mics that didn’t change the characteristics of the piezo more than strictly necessary, was made of something a magnet would stick to in case I wanted to use a magnet mount, and let me use spring clamps to attach the mic without applying any pressure to the piezo disc. Here’s what I made:

I started with the piezo disc recommended on the Stompville site, the Murata 7BB-35-3. I got mine from Mouser, but you can get these from a number of sources. They’re unfortunately quite large, with the outside diameter a whopping 35mm.

Murata 7BB-35-3 Size

After a couple of months of poking around and not coming up with much, I started searching on metal cans just to see what was out there. I found that the 1/2 oz screw-top tins used for lip balm were a perfect fit. Rather than caking on the lip balm for a couple of months to get empties, I found a supplier who sold them in bulk for DIY folks like me. (Though I think they meant them for people who make their own lip balm rather than people trying to cram electronics in them.) The Murata piezos fit fine.

It Fits!

One of the things I like about the Stompville preamp is that it uses three wires for the piezo: positive, negative, and ground. The ground is intended to be used as a shield. Since the lip balm tins are actually made of steel, this provided a nice way to provide a continuous shield all the way from the piezo element to the recorder. This seriously helps minimize RF interference. The only catch is that the tins are coated to keep them from rusting when used for lip balm, so I had to sand some clear patches for soldering the shield as well as for making good contact between the container and the lid.

The good thing about this coating is that it keeps the brass disc from shorting out against the shield. No special treatment is required. I glued the discs using the same E6000 silicone I use for mounting microphone capsules to their posts. This probably loses me some high frequencies, and epoxy would probably be a better choice, but I kept the layer as thin as possible so the effect should be minimal.

As it turns out, the same servo grommets I used for providing isolation on the shock-mounted capsule posts for the Alice mics are a really good fit on Mogami lavalier cable. I drilled a hole in the side of each tin to take the cable, installed the grommet, and glued it in place with E6000 as well.

Contact Mic in a Tin

With everything installed, all that was left was to solder all the wires and close it up.

Four Contact Mics Ready to Run

Not quite true. All that was left was to solder all the wires, close it up, and go play.

So why four channels?

Initially I knew I wanted four because my recorder has four inputs. I hate having inputs I can’t use simply because I didn’t plan ahead. But after playing with the two channel version for a while I realized something about contact mics: Depending on the medium, sound may not travel all that far. Having widely spaced contact mics may mean that an activator traveling across the object will move out of the range of one mic before it really starts getting picked up by another. With four contact mics you have all the same choices available that you would setting up the mics for a concert or for field recording. In this case, setting up a mic for hard left, center left, center right, and hard right can fill in the gaps on an object that naturally damps sound, like a chalk board.

Or you can use them to pick up four parts of an object simultaneously, like a coffee maker or a vacuum pump or a helium compressor.

Or maybe I just had four inputs and wanted to stuff them all full of contact mics. Who knows? The cost was minimal, and now I have the option to record four channels of contact mic.

One last thing: In addition to being good mics to have in your arsenal, contact mics are useful for other projects as well. June last year, I wrote an article about a shock mount for my SASS I’d made from a re-purposed multi-rotor anti-vibration mount. I used contact mics to characterize the shock mount and arrive at the -21dB of attenuation figure for that mount.

More recently I’ve been trying to solve a handling noise issue on the Alice mics I’ve been building. Having four channels to work with may finally let me tackle that in a more systematic way, or at least characterize the work I’ve already done.

But I may need some smaller discs to work with. The big Murata discs I got for this project are beasts.

Tom

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LDC Alice – Take Two

Posted by Tom Benedict on 16/09/2017

I decided to build another set of Alice microphones along the lines of Jules Ryckebusch’s Instructable, using the circuit originated by Scott Helmke, modified by Jules, and put in PCB form by Homero Leal. For these, though, I made a couple of changes:

When I built my first Alice microphone, I’d intended to use it for field recording. Unfortunately I found that the very qualities that give a vocal mic a lot of clarity and presence wind up making it extremely sensitive to the sound of wind in the treetops. It did a beautiful job, but was overly sensitive to wind hiss.

Homero came to my rescue and told me about a document written by another member of the Yahoo! micbuilder forum, Ricardo Lee, describing a modification I could make to the Alice circuit to tame the microphone’s presence peak. I rolled this change on my first Alice and on the Mid-Side Alice I built shortly after. The modification worked, and tamed the microphone’s over-sensitivity to wind hiss. I eventually put the modification on a switch so I could choose how the mic was voiced.

For this set of Alice mics I put those components on the PCB with a dedicated set of pads to wire in the switch, visible just to the left of the filter cap on the back of the board.

New PCBs

The next change I made was to use the Transound TSB-25AX capsule rather than the TSB-2555B capsule I’d used in my first Alice. From the datasheets on the Transound web page the two should behave almost identically, with the S/N performance of the 25AX just barely edging out that of the 2555B. To my ear the two sound identical.

Probably the biggest change was how I mounted the capsules. When I built my first Alice I didn’t own a 3D printer and still thought entirely in terms of subtractive machining (taking a chunk of stuff and cutting away whatever doesn’t look like what you want). This time I started over and designed a completely new saddle and post with 3D printing in mind. I wanted at least rudimentary shock mounting and a cleaner run for the wires.

Shock-Isolated Capsule Mount

I tried a number of approaches on shock mounting, but most of them strayed from something else I wanted out of this design: I wanted it to be something anyone could print and use. So I stuck with easily accessible materials for the shock mount itself, in the form of anti-vibration servo grommets available at practically any hobby shop.

Shock-Isolated Capsule Mount with Grommets

Shock-Isolated Capsule Mount Grommets Installed

As with all of the capsule saddles I’ve designed thus far, the capsule is attached using E6000 adhesive. This stuff is at least somewhat compliant, extremely sticky, and can be removed after the fact by passing a fine wire between the capsule and saddle. So far I haven’t needed to remove one, though.

I printed the final parts using Proto-Pasta Carbon Fiber PLA filament. It’s not the cheapest filament out there so I try to save it for lightweight structural parts, but it just looks so danged good, I couldn’t resist.

TSB-25AX Installed Front Quarter View

One of the things I like about 3D printing is that you can add features that are difficult to get any other way, except possibly by casting. In the case of this saddle and post, a 3mm tunnel runs from the back facet of the post down the center to take the capsule wires to the PCB inside the mic body. The tunnel has a 6mm radius curve inside the post, which makes wire installation easier.

TSB-25AX Installed Rear Quarter View

The final change I made was to switch from the admittedly gaudy BM-800 mic bodies I’d used on the previous two microphones to a slightly less gaudy un-branded BM-700 body I picked up off of Ebay for under $20USD. This had the added benefit of switching to a different headbasket geometry that doesn’t contribute as much ringing to the handling noise.

Here are all the bits and pieces going together. (I haven’t installed the voicing switch on this mic yet.)

Microphone Innards Front

Microphone Innards Rear

And a closer look at the wire routing, which is a lot more satisfying than the routing on my previous mics:

Headbasket Innards Rear

Here’s what it looks like closed up and ready to roll:

Microphone Front

For anyone else building their own condenser microphones, I’ve made this and a saddle for the TSB-2555B capsule available on Shapeways and Thingiverse. Be sure to print the right one for the capsule you’re using:

TSB-25AXZ3 Shapeways

TSB-25AXZ3 Thingiverse

TSB-2555BXZ3 Shapeways

TSB-2555BZ3 Thingiverse

 

Happy recording!

Tom

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