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 headset headphones 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.
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. Or, Rode makes the “VXLR Plus” that works well with 3 connection TRS mics, an off the shelf solution.
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 mics dynamic 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 with simple circuits. Meh. )
Here’s the basic electret mic hookup:
The second issue: lots of similar looking jacks/plugs, all differently wired
A unobvious but rather crucial issue is there are three(!) popular 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. Illustration 5 stereo mic can be ignored here.)
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.
(Note that old 60s/70s era mono portable radios and tape recorders used this type of connection for both dynamic mic inputs and earphone outputs, thereby establishing an enduring legacy of confusion about 3.5 mm connections that persists to this day.)
Diagram #4 is the standard TRS mic input; the ring is used as a separate separate power line here. This is what most PC’s use for 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 the 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/mic signal.
The mic connection isn’t so fortunate, neither the two conductor TS nor the three conductor TRS mic connections work without a wiring adapter in this case.
In our case, we want to support the mic portion of the TRRS headset directly, so we now we need a 4 pin jack. We then wire 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 can get shorted to ground.
Historical footnote: There were some TRRS variants that placed the mic signal on the tip or ring 2. Although this kept the ground in the normal place, it messed up the aforementioned stereo headphone compatibility. It was short lived in these 3.5 mm connections, I think only a couple of cell phones (Nokia?) used this variant.
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 empty space inside. Ha!
Postscript: Rode makes a handy adapter gadget, the “Rode VXLR Plus” , that works well with 3 connection TRS mics, like Sennheiser’s ME-3 headset mic and Rode’s own lineup.
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