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:
Noise Floor: -116.8dB
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:
Noise Floor: -116.8dB
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.
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.
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.