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Posts Tagged ‘BT-EM172’

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

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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

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Finishing the DIY Microphones (v.1.1)

Posted by Tom Benedict on 14/07/2015

The more I thought about the hot glue closure on the back of the microphones, the less I liked it. Don’t get me wrong. It works well. But it’s… permanent. I know the BT-EM172 capsules are only $10, and I know the rest of the microphone is largely scrap-boxed, but I hate to make a thing that can’t be serviced when it needs it.

So I re-designed the enclosure to include an end-cap. It’s drilled out 1/4″ to take a cable grommet, and has three #2-56 screws placed every 120 degrees around the periphery to hold it in place.

BT-EM172 Microphone Enclosure - Exploded View

The end caps took about fifteen minutes apiece to make, and were a comfortable fit in the back of the microphone bodies I made previously. Unfortunately, drilling and counter-sinking the screw holes for the end cap meant I needed to re-coat the microphone bodies along with the end caps. Since I had to re-coat them, I added grooves to each mic body to accommodate a Shure RK183T1 lavalier clip. I’m pretty sure a generic clip for a 9/16″ diameter mic would’ve worked fine, but these turn out to be tough to find. There are several listed on Ebay, but if you look at the metric equivalents, the specs say they fit something around 7-9mm in diameter. 9/16″ is closer to 14mm, so I think something was lost in translation. The clips from Shure will fit. (For shure! Har!)

Countersinking Screw Holes

I wasn’t happy with my previous coating job, so I came up with another way to apply the coating. I shoved each part onto a wooden dowel of the appropriate diameter (3/8″ for the end caps, 1/2″ for the mic body), and chucked it in a drill. I applied the Cerakote with the drill spinning. This gave each part a very uniform coating, and let me hit every outside surface without running into my fixture. I loaded the parts in the oven, dowels and all. On a whim I coated the screw heads, too, so I wouldn’t have shiny stainless screws in a black microphone body. Unfortunately the spray gun malfunctioned, so two of the mic bodies didn’t turn out as nice as I’d like. I slated those for the pseudo-SASS array, where they won’t be seen, and saved the two “good” ones for lavalier mics. Note to self: test the spray gun before loading product into it!

Parts Ready to Cerakote

Once the Cerakote cured it should’ve been a simple matter of assembling each of the mics. But I love to fiddle. I assembled the two for the pseudo-SASS array since I already had that cable made. But I needed more cable for the lavalier mics. Even though I’m already using Mogami W3031 cable for the other mics, I ordered 100′ of Mogami W2697 from Redco Audio to use for the generic lavs (only 20′ of which I plan to use). W2697 is almost identical to W3031, except for the way the shield is constructed. W3031 uses a braided shield. W2697’s shield is served (wrapped). Electrically they’re identical. But a served shield is easier to work with when making cables. I’ll have to wait for the cable and clips to come in before finishing the generic lavs.

Completed Mic Bodies

Rather than waiting like I did with the mono mic I built out, I grabbed my pseudo-SASS array and my recorder, and hiked out to the rocks south of Hapuna Beach. The last time I was there the waves were big, and made big, dramatic crash-bam-booms on the rocks. Of course that was in the winter. The summer wave pattern is a lot more bathtub-like, so the sound was a lot more subtle. Still, I ran several side-by-side comparisons of the pseudo-SASS against the built-in mics on the Tascam DR-05. I put together a set of 30-second clips comparing the two. The recording has eight tracks, alternating between the DR-05 built-in mics and the BT-EM172 array, done at four locations. When listening, keep in mind that the gains are different on the two mics, as are the frequency responses. I did no processing on the tracks aside from cutting and fading, so some tracks are louder than others. That’s a function of my technique in the field (or lack thereof), not the microphones themselves. This test was only so I could tell how well the pseudo-SASS array was separating the two channels.

The pseudo-SASS performed well enough I want to build a real one out of some 1/4″ baltic birch plywood I have in the shop. I still haven’t tested my prototype from the air, but it’s easy enough to include 1/4″-20 sockets top and bottom so I can mount it either way. More photos and sound samples to come!

– Tom

P.S. I’m not keen on the way clips from Soundcloud show up on my web site. I’ve seen other people include Soundcloud clips on their sites that are nice, small, and easily worked with. This thing is ungainly! If you know how to fix this, please let me know.

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