"zipzit" wrote ...
Anybody out there work on Audio circuits?

Most likely.

I'm trying to design & build a motorcycle intercom, similar to the
Starcom1. see http://www.starcom1.com/

I found some pretty good reference materials, particularly an old
newsgroup posting for an airplane intercom. This posting is
exceptional, with lots of details on why and how.

Do a google groups search for:
Subject: Intercom Schematic wanted
From: Graham E Laucht )
Newsgroups: rec.aviation.homebuilt
Date: 1997/02/25

The intercom should be able to take multiple inputs (cell phone, cd
player and two way radio) and have the ability to mute music when
radio is used. The unit should be small/ compact and use low power.
We're only driving headphones speakers.

You might find my circuit, description, and construction project of
some interest... http://www.rcrowley.com/ComClone/default.htm

One feature that I found of interest is a automatic volume adjustment
scheme. Things can get pretty loud in a motorcycle helmet at higher
speeds. I'm looking for a cheap man's speed sensitive volume system.
I know the auto industry uses actual vehicle speed input for radio
volume adjustment (I'm an Automotive Engineer. with close ties to
radio design folks.) I'm thinking the Starcom system uses helmet
microphone input to take a measure of increasing ambient sound, then
adjusts audio amplifier gain to match.

I've done lots of searching on the web, and the only postings that I
can find on the subject are for volume actuated switching.

The same ambient sound-level adjustment scheme is used in
many PA/paging systems in public spaces (airports, etc.)

The three basic building-block circuits you need a
1. Microphone preamp
2. Audio signal rectification (and integrating)
3. VCA (voltage-controlled amplifier)

There should be many examples of these circuits out on the
internet. The mic preamp can likely be a single op-amp, and
you could likely use the other half of a dual op-amp for the
active rectification/integration circuit (similar to those used
for audio level metering). And there should be several good
application notes by the vendors of VCA chips.