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DIY balanced low noise and distortion connection of electret condenser microphone to audio interface

Mr. Haelscheir

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This is a follow-up to https://www.audiosciencereview.com/...hones-with-motu-m2-and-rew.49384/post-1783062 (post #6) where I identified limitations with the off-the-shelf RØDE VXLR Pro and VXLR+ adapters for converting 48V Phantom Power to plug-in-power: the VXLR Pro incurs low noise, but suffers from increasing third-order harmonic distortion in the bass from the transformer or whatever it is using to convert from unbalanced to balanced, while the VXLR+ while having excellent distortion performance incurs some kind of common noise hump, whether or not this is being picked up internally by the MOTU M2 when in unbalanced operation or is coming from the use of Zener diodes to do the conversion. I have also been picking up significant mains noise and its respective harmonics which is mitigated with shielding, though said shielding was implemented on different capsules for which I had been encountering too much second-order distortion perhaps due to my not supplying the optimum bias voltage and load impedance.

I lately came across the "Using a separate bias supply with a split load resistance" section under https://diymics.com/how-to-use-elec...balanced-input-that-provides-phantom-power-pp which showed a fairly simple way to get a balanced signal out of an electret condenser microphone (ECM) without a transformer while providing the correct bias voltage and load impedance. Given this, I am looking for feedback on the following wiring schematic for my upcoming in-ear microphones:

20241213_012103.jpg


  1. I intend on connecting the 0.5*R_L (in this case, 1.1 kΩ) resistors as close as possible to the capsule, maybe even as precision SMD resistors soldered directly to the back of the 8 mm or 9.7 mm capsule so it fits nearly within the custom in-ear mount.
  2. Regarding shielding, I've seen https://www.audiosciencereview.com/...o-wire-an-electret-microphone-properly.26912/, whereby this would necessarily be following Configuration #3 since I cannot connect the ECM's ground terminal directly to ground in this split load resistance design. I might be able to short the ECM capsule's case to the shielding.
  3. For ergonomics, I would have a thinner (in order to minimize compromising the seal of on-head headphone measurements) balanced and shielded wire running from the in-ear mic to a support necklace or strap to which I would mount the bias voltage rechargeable batteries (in this case, 3 V). Here, I would like feedback on whether shielding is necessary for the DC connection from the rechargeable battery around my neck back up to the in-ear mic; if anything, I might as well use a shield to transmit the ground connection for the battery. I am also supposing that it wouldn't be a good idea to have the DC connection bundled within the same shielding as the balanced pair.
  4. From the support necklace/strap, I would then splice the thinner balanced and shielded connections with a regular XLR run, say, about 6' in length to reach my audio interface and accommodate rotating on a swivel chair when taking HRTF measurements.
  5. Within the male XLR connector's housing, I would have inline electrolytic capacitors to isolate the DC bias from the audio interface for which the 48V Phantom Power will be kept OFF.
 
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Re: 2, please check whether the capsule case isn't tied to the source (-) terminal already. It certainly doesn't make much sense to tie it to cable shield.

Re: 1 and 3, I would consider going with a 9 V block while upping the resistors accordingly (3k3 then). Should help linearity and the wire gauge required for the battery cable if nothing else, not to mention that I'd trust contact on the annoying terminals more than the average AA/AAA holder. Battery voltage should be blocked with some capacitance, I'd give it 10-100 nF at the ends of both resistors and a few dozen µF electrolytic at the battery. Especially if the battery cable is unshielded, you'll probably want to give the mic side another few hundred pF in parallel.

Are you aware of this gentleman's approach to linearizing a capsule like that by cascoding?

Though I don't think the circuit is easily adapted to the balanced approach, and you may not want any more complications than there already are.

BTW, have you considered a more "Schoeps-y" circuit, is is that what one of the Rode adapters already does?
 
Re: 2, please check whether the capsule case isn't tied to the source (-) terminal already. It certainly doesn't make much sense to tie it to cable shield.

Re: 1 and 3, I would consider going with a 9 V block while upping the resistors accordingly (3k3 then). Should help linearity and the wire gauge required for the battery cable if nothing else, not to mention that I'd trust contact on the annoying terminals more than the average AA/AAA holder. Battery voltage should be blocked with some capacitance, I'd give it 10-100 nF at the ends of both resistors and a few dozen µF electrolytic at the battery. Especially if the battery cable is unshielded, you'll probably want to give the mic side another few hundred pF in parallel.

Are you aware of this gentleman's approach to linearizing a capsule like that by cascoding?

Though I don't think the circuit is easily adapted to the balanced approach, and you may not want any more complications than there already are.

BTW, have you considered a more "Schoeps-y" circuit, is is that what one of the Rode adapters already does?
Thank you for the feedback.

The kind of capsule I am looking into is https://docs.google.com/gview?url=https://api.puiaudio.com/filename/AOM-5024L-HD-R.pdf&embedded=true.
  1. I wouldn't know whether the case is already tied to the source terminal. The capsule already has capacitances between the terminals (I'm not sure if these are symbolic representations of the device's capacitance or actual internal capacitor devices). By "the ends of the resistors", do you mean the left, right, or both ends in my diagram? Are those suggested capacitors bridging those nodes to ground? And for the battery, I guess the capacitor would effectively be bridging its terminals.
  2. Said capsule has a nominal Vs of 3 V and a maximum Vs of 10 V. Are you saying that operating at an upper Vs of 9 V while upping the resistors to maintain the same current draw would improve linearity and allow getting away with a lower gauge?
  3. Regarding https://www.johncon.com/john/wm61a/, I guess it wouldn't be practical to fit the additional circuity all within the in-ear mic, and I would imagine it would not be as useful if the external JFET had to be situated all the way amid my support necklace/strap insofar as I want the balanced driving to start as close to the microphone as possible. Regarding linearity, I had already gotten exceptional distortion results with https://www.farnell.com/datasheets/1653468.pdf when driven by the VXLR+ adapter, the same adapter yielding high second-order distortion out of https://docs.google.com/gview?url=h...ame/TOM-1537L-HD-LW100-B-R.pdf&embedded=true; I want to maintain or exceed the performance of the former while at least getting rid of the noise hump seen in https://www.audiosciencereview.com/...hones-with-motu-m2-and-rew.49384/post-1783062 (post #6).
  4. I suppose my schematic based on the following schematic from https://diymics.com/how-to-use-elec...balanced-input-that-provides-phantom-power-pp is similar to the lower schematic within https://audioimprov.com/AudioImprov/Mics/Entries/2015/4/23_Basic_FET_Microphone_Circuits.html where the FET is already within the capsule, except that I am solely using a battery for the red power supply parts, and I presume the green output stage is already implemented by my MOTU M2's mic preamp, else I would vie that an upstream preamp closer to the capsule shouldn't be necessary for this length of run. Are you saying the VXLR Pro is similar in implementation to that Schoeps-y circuit? It is advertised as being transformer-balanced which is what I believe to be responsible for the third-order bass distortion in Figure 1 of https://www.audiosciencereview.com/...hones-with-motu-m2-and-rew.49384/post-1783062 (post #6).
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2024-12-13 - Electret condenser microphone schematic question.png
 
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