Anybody out there work on Audio circuits?
I'm trying to design & build a motorcycle intercom, similar to the Starcom1? see
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: Re: Intercom Schematic wanted From: Graham E Laucht ( snipped-for-privacy@ukavid.demon.co.uk) 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.
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. Do a google group search for: Subject: Re: Sound Activation From: snipped-for-privacy@bellatlantic.net ( snipped-for-privacy@bellatlantic.net) Newsgroups: sci.electronics.components Date: 2002-05-19 21:02:20 PST
The hint from that posting is "to amplify the audio, rectify the output of the amplifier, and use the rectified voltage to charge a capacitor. The voltage on the capacitor then turns on a transistor which operates the relay."
I don't want to operate a relay, I want to affect the gain of dual LM386 audio drivers. Currently the plan is to use a stereo volume control on the input signal. The LM386 also has external gain inputs. The gain modification circuit is normally meant to be fixed with the addition of a RC series circuit. No circuit = 20 gain, 1.2k Ohm +
10uF = 50 gain, 10uF alone = 200 gain.I guess it would be possible to use a comparator, and use the comparator outputs to switch in alternate resistors? and go step function on volume increases. Somehow that just doesn't seem very elegant.
Is there anyway to use microphone sound density input to generate a isolated variable resistance output, to work as an automatic volume control for varying ambient noise levels? Are there other ways to do this?
Thanks in advance for any design hints? LB Detroit, Michigan
Cross posted to: sci.electronics.design rec.audio.tech sci.electronics.basics