Electret mics and Pro Audio

Powering a mono electret mic and converting to a Pro XLR connection

This project came about for a variety of reasons.

One is to use a headset mic on my cell or PC for video conferencing.
I dislike using headsets for calls. The earpieces bother my ears, and the sudden volume changes are painful and reduce intelligibility.
I wind up riding the volume control and plucking the earpiece from my head a lot. However, the headset mic position (near the mouth) is optimal for voice clarity, and these little electrets are light and inconspicuous, perfect for this application.

Two is to have a simple standard platform to test and evaluate these mics for intermittents and audio quality. Since they are often used for performances with a wireless beltpack transmitter, they take quite a beating. Which means a stable test platform for evaluation and repair is very handy.

Three is to have a backup for my audio board talkback mic, a trusty old Shure SM10A headset.

tl;dr:

It turned out to to be simplest to build this into an old DI box; it already had the high to low impedance transformer and unbalanced to balanced signal change. Also, metal for shielding and a bit of extra room for the bias supply and the various jacks all works nicely.

The first issue: power

These mics need a bias voltage to work, basically a lower voltage variant of phantom power. Getting that figured out was the obvious and first issue.
These mics generally work on voltage from 1.5 to 10 volts, with the higher voltages improving the headroom. Since the current drain is very low, a 9V battery works nicely. Using 48V phantom power may seem obvious, but a lot of ground loop and regulator noise issues can pop up. Meh.

Here’s the basic electret mic hookup:

The second issue: lots of different but similar jacks

A rather unobvious but crucial issue is there are three main 3.5mm connector variants used for mono electret mics (2 conductor TS, 3 conductor TRS, and 4 conductor TRRS). This necessitates three different jacks to host them all.
(Note there was another TRRS variant, but it’s mostly obsolete now; see below for details.)

Let’s begin decoding this mess. (2 and 5 can be ignored here.)

The variants…

Diagram #3 is the simplest, the two wire mic on a TS plug. It needs the bias voltage thru a current limit resistor on the tip, and then a blocking capacitor for the audio out from the tip. This is a common wireless mic pack connection.
(Note that old mono portable radios and tape recorders used this jack for both mics and earpieces, thereby establishing an enduring legacy of confusion about 3.5 mm jacks in general.)

Diagram #4 is a TRS mic input; the ring is used as a separate separate power connection here. This is what most of PC’s use as a standalone mic input, and also it’s a standard wireless bodypack mic connection, eg Sennheiser.

Diagram #2 is a TRS connection for stereo headphones. This is not applicable for a mic application, but observe the complicated compatibility requirement for single jack headset (TRRS) connection #1.

Diagram #1 is the now-standard single jack headset TRRS connection.
It supports both headphones and stereo headsets (ie headphones with mics). To do so, the sleeve is changed from the standard of ground to the mic signal; ring 2 now becomes the common ground. The support circuits’ current limit resistor means when a standard headphone TRS plug is plugged in, it can safely ground out the headset power and mic signal.
In our case, we want to support the mic, so we now we need a 4 pin jack and then we support it like the two wire mic version, except the signal and power is on the sleeve and ring 1 is now the ground. Note that you’ll need a TRRS jack that has the sleeve insulated from the mounting hardware, otherwise your mic signal will be shorted to ground.

Note there was briefly a TRRS variant that placed the mic signal on the tip. As this messed up the afore-mentioned stereo headphone compatibility, it was short lived in these 3.5 mm implementations. I think a couple of cell phones (Nokia?) used this briefly.

The third issue: convert to a clean XLR mic input

This is both high to low impedance and unbalanced to balanced signal conversion.
I cheated here, I just used an old DI box with plenty of room inside. Ha!

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