Connecting a PC mic to audio equipment

When I am in Virtual Reality, I sometimes have need to change my voice to match whatever avatar I am inhabiting at the time. Rather than mess about with a software voice changer, I happened to have a Boss Voice Transformer VT-1 lying around. The problem is that a standard PC mic (such as the one in my headset) has electrically different needs to a stage mic, which is what the VT-1 is expecting. A little bit of electronics hacking is required to bridge the gap.

Yes, there is a difference.

A generic stage mic is generally just a passive device, relying on being plugged into a preamplifier to boost its signal to a level that sound equipment can work with.

A standard PC mic has a small amplifier transistor built into the electret microphone. This requires a 5V power source (they can generally handle up to 10V though 5V is the standard). This voltage is applied along the same wire as the audio signal returns on (just to make things awkward! .... actually it is so they can use standard 3.5mm jacks).

Note that some stage mics do have built-in pre-amps and require a battery or 'phantom power' to be supplied by the equipment to function, but this 'phantom power' is very different to the power required by a PC mic (usually 24V or 48V!). Also, the VT-1 doesn't do phantom power anyway.

The above presents two issues: Firstly, we must get a voltage onto the Mic line to power the electret microphone. Secondly, we have to make sure that voltage is isolated from what the mic is plugged into (the VT-1 in this case).

Give me the power!

The following circuit achieves this:

Above, the components achieve the following functions:

2k2 resistor - attenuates the power fed to the Mic. While the input voltage for this purpose is ideally 5V, most PC mics should handle 9V. No guarentees though! I am using 9V primarily because that is the voltage immediately available to me on the back of the equipment nearest the circuit (the VT-1).
10µF Capacitor - Isolates the voltage from the output: fluctuating audio signals can pass through this capacitor fine, but the static DC voltage can't. You really don't want to be feeding DC voltages into audio equipment that isn't expecting it!
10k resistor - any DC voltage that does build up on this side of the capacitor should be able to slowly drain away via this resistor. It is high enough value to not significantly effect the audio signal from the mic.
Mic connections - I wired the Left and Right channels together in the Mic socket as I only need a mono signal (some mics have stereo pick-ups even if this is often pointless, and I want a mono output for the VT-1 anyway).

Putting it together.

The circuit was simple enough to rats-nest*.

* Rats-nest refers to when components are connected by twisting the wires together, no circuit board is used. For simple one-off circuits it is a quite adequate solution.

I am connecting the +9V wire to the nearby 9V power jack of the VT-1. Music equipment is often (contrary to norms) positive-outer plug, so connecting to the outside of the plug with a tight hook made from a paperclip works for now. I may get proper DC male and female line sockets at a later date to neaten it up. For now I am letting the negative rail return via the ground on the VT-1 input. I have a negative wire soldered in for if I later take the neat-path just mentioned.

Finally, I sandwitched the whole thing between some dremeled-out acrylic plates and a big blob of acrylic fixer in the middle to add mechanical strength and stop anything bending about and shorting. I held the acrylic together with a bunch of clothes pegs around the edges, overnight, while the fixer set. I used black acrylic for the back and clear for the top, giving good contrast with the components - everything I do ends up as a student-demo, so displaying the insides of projects is how I tend to do things by default.

Alternative: Of course the other way is using a stage headset (with a non-PC microphone which should plug strait in!) But those can be a bit pricey! And you would have to build a PC-input-compatible pre-amp to use it 'directly' with a PC!

Notes:

All the parts used were from my scrap box, so this project hasn't cost me a cent yet! That is also why the rats-nest's looks a bit more shoddy than it otherwise might!: the components were recycled and had short, twisted, tarnished leads on them. Even if you buy the parts, it will only cost a few dollars (the plugs will be the most expensive components, at a dollar-and-a-bit each).
Passing my mic signal through the VT-1, even with the controls set to keep my voice 'as-is' seems to do a pretty decent job at cleaning up the signal too, resulting in less noise going into the PC's audio-in socket.
The VT-1 is a somewhat pricey piece of equipment. I first tried one of those $40 phone voice-changers, but the results were pretty crappy. Incidentally, it is a lot easier to drop the pitch+formant (you need to do both for the voice to sound real!) than to raise them, so females/children wanting to drop their voice to male or monster levels - or guys who want to sound like a cartoon character - may be able to get away with one of these far cheaper units. There are also software options, of course (the only documented Linux soft-solution I could find looked more complicated than doing this project. YMMV!).
Here is a version I built with the electronics rats-nested to fit inside the 6mm jack's shell: