The View Up Here

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

  • Flickr Gallery

  • Advertisements

Posts Tagged ‘Field Recording’

Powering the EM172 Capsule – Part 3: Capitulation

Posted by Tom Benedict on 23/02/2016

I made up my mind about powering my EM172 microphones. Ultimately this decision had less to do with how I was powering the microphones than how I was plugging the mics into the recorder. One of the things I discovered when I wrote my last post was that the Tascam DR-70D uses completely different amplifiers for the XLR inputs and the 1/8″ inputs. Different form factor, obviously; different impedance; different gain. It’s that last part that really drove this decision.

The gain ranges on the 1/8″ plug are +3dB, +11dB, +26dB, and +38dB. The XLR gain ranges are +21dB, +36dB, +51dB, and +63dB. While I was performing side-by-side tests I kept having to crank back the gain on the XLR input to match the levels on the 1/8″ input. As I tested with quieter and quieter subjects it finally hit me: +38dB of gain just wasn’t enough to bring up the levels of some of the subjects I want to record. The XLR input gave me more gain to play with. The last test I ran was what finally convinced me. Even with the gain cranked all the way up on the 1/8″ input mics, I couldn’t get the sound levels over -25dBFS. The recording was just too quiet to use. I cranked up the gain on the XLR input, and was able to get -12dBFS with the same subject.

Good news is the mics really do perform better with the 9.6v bias voltage David McGriffy’s circuit provides. So this is a win-win.

The lavalier mics were no problem to convert. I bought a stash of Neutrik XLR connectors when I started this whole investigation, so it was just a matter of lopping off the 1/8″ connectors and soldering up the XLRs with the resistor and capacitor from McGriffy’s circuit.

XLR-Converted Lavalier

My SASS was another story. I really hate having things with cords that can’t be unplugged, so I wanted to connectorize everything and use extension cables. Only problem: I’m a beginner! So I had no idea how all the connectors worked.

After some Googling and image searching I learned that:

  • XLR extension cables are gender-inspecific. One end is male, the other is female.
  • Female XLR connectors are the ones with the latch. This is true of both panel and cable connectors. So female panel connectors have a latch, but male panel connectors don’t. (This confused me.)
  • Neutrik makes a crapload of XLR connectors you can choose from. It’s worth looking them up in multiple catalogs to find out which series were developed to fix the bugs in previous series. Though it’s really hard to go wrong, so long as you get all the genders right. These things are built like tanks.

I picked up a pair of pre-built 10′ extension cables for a little over the price of the connectors themselves along with some male panel jacks to install in the SASS. Installation meant cutting into the back of my SASS, but it went quite smoothly and the results look (and sound!) nice. (Yeah, this is an infrared photo. Ironwood trees look like Dr. Seuss trees in the IR, so I just had to play.)

SASS Back in the Field

Meanwhile I figured it was finally time to solve the issue of wind protection. A few months back I learned I’m really REALLY bad at sewing fake fur. I did some reading since then, so I think I know what I did wrong. But rather than getting stalled on my own lack of sewing skill I ordered a pair of lavalier windscreens from Cat Ears. They fit over my oversized mic bodies, but they’re too small to go over a foam windscreen. I probably needed the larger ones. They do a decent job by themselves, but in wind over 15-20kts the mics still suffer from wind noise. Good enough to use the lavs as tree ears, but not enough to use them at the beach in solid wind.

Cat Ears Windscreens

Now I just need to solve the issue of wind protection for my SASS. Back to learning to sew fur…

In any case my gear and I are off the soldering bench and back out in the field. Finally. YAAAAAAAY!

Tom

Advertisements

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

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.

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