multiple electret mics connected to pro audio gear

[viperlabs]

Here's the overview:
- 30-40 electret mics; each connected to a 2 conductor wire about 6-8 feet in length
- the wires go into the ever-popular "magical black box"
- one balanced or unbalanced, line or mic level signal comes out of the box to be plugged into an audio mixer

The problem I've had is answering the question, "what's in the black box?"  I need to create a circuit which will mix all the signals and send them out as one.  I've also wondered if I can get away with using strips of ribbon cable for the 2 conductor wire and not receive much in the way of interference, or is that not possible?

My goal is to have a circuit with as few components as possible, and I just can't seem to put together the information I found on op amps and other audio preamp-related ICs to feel confident.

Can someone out there help me?
j.

[audioguru]

Electret mics pickup sounds from all around them. Directional ones are directional only at high frequencies unless they are very long. Your 30 to 40 mics will all pickup the same sounds so why have so many of them so close together?

The Jfet in an electret mic has a pretty high output impedance. I had 12 connected in parallel along a big boardroom table and powered from a 820 ohm resistor from 9VDC. One picked up a voice and the others picked up echoes and background noises. It worked poorly. Later I added a mixer with automatic switching on of its inputs to fix it.

The black box will be a simple microphone mixer.
A single opamp makes a good mixer. Each input is a capacitor in series with a resistor. One feedback resistor sets the amount of gain. Each electret mic will need to have a resistor to power it.

The high impedance along with the very low output level from an electret mic makes using shielded audio cable very important to stop pickup of interference and mains hum.

[viperlabs]

The mics will be mounted deep down inside a set of tone chimes (see pic), which sound kind of like hand bells but are more rugged...and cheaper  ;)



Hopefully I'm headed in the right direction.  Would you please check out the attached image and see if this is something you think would work?  Is there a specific opamp you might recommend for this application?  Also, do you know of a thin, flexible shielded audio cable that may be somewhat durable?

Thanks for your help  ;D

[audioguru]

Hi Viper,
Nice try.
The signals from your mics were shorted by the power supply and the opamp was amplifying noise from the power supply.
Try this mixer circuit:

[viperlabs]

Thank you soooo much for your help!  People like you are few and far between these days  ;D

j.

[audioguru]

Thank you soooo much for your help!  People like you are few and far between these days

thanks. I am on most of the electronics chat forums on the web. Some guys post the same question on all of them.