Build a Battery-Powered Amp for an old Carbon Mic

A friend of mine acquired a 1970's aviation-style headset for use as a live gaming prop.  This Instructable documents the development and construction of a stand-alone amplifier and general-purpose adapter for it.  I was much aided in my task by other Instructables, as well as helpful hints in various audio forums.

Step 1 Breadboarding

The first problem to solve is that carbon-button microphones, like condenser mics, need an external source of power.  A battery and a resistor in series with the mic gets you a current loop (the resistor limits the current) and then you can pick off the audio at any two points with a voltage drop between them.  A single AA battery and a 1K resistor is enough to get a signal you can pick up with a mixer or an audio interface.

To get the rest of it, I bought up several fresh Jim-Packs of capacitors and resistors in useful-sounding values, opened up the National Semiconductor datasheet on the LM386, and started playing.  When I got in trouble (which was most of the time) back-posts in random audio forums gave me new ideas to try. One of these led me to a zobel filter, and  Wikipedia (of all things!) told me how to make a passive low-pass filter.

Okay, I still think you can do better with the same resources.  There are lots of good circuit diagrams floating about the net.  But here's my schematic anyhow (see below).

Comments

[Quercus austrina]

JimPak??!!! Where did you get them? I haven't seen them for years since the local independent  Radio Shack went out of business.

Nice build. I especially like the way you adapted the drain cover and handle for use. The Sidetone control is a great function, something most people would not think of.

Qa

[nomuse]

Heh. My local electronics store; Al Lasher's electronics. Seriously, though, especially with most franchise owners of Radio Shacks getting out of the components business, you are better just ordering from Digikey.

[Quercus austrina]

Ah, to have a local electronics store. (Sigh) Actually, the independent RS went out of business before the internet was around and mail order was a looonnnggg process.

Thanks for the info,
Qa

[nomuse]

Don't get me started! When I was growing up, not only did Radio Shack have parts, but in our area was the giant surplus electronics warehouse, Mike Quinn's. http://www.imsai.net/history/quinn/quinn-1.htm I find the turn-around at Digikey is under a week, but it just isn't the same as actually sorting through the parts in those great dusty bins.

[xxInSAniACxx]

HEY!!!!! I've been looking every where for instructions on how to build an AMP. does this only work with carbon, or with modern equipment too?

[nomuse]

Try the National Semiconductor datasheet -- http://www.national.com/mpf/LM/LM386.html#Documents It describes several basic amplifier circuits around the LM386 (which is available almost everywhere...it's been manufactured since 1986) Component values and everything. The big difference between a carbon mic and other mics is that the carbon mic puts out a LOT of signal. You can drive an earphone with it! For building an amp for one of the cheap electret condensers you see around these days you'd need to have a pre-amp, too. Here's a typical circuit using the oft-cursed LM741 Op-Amp: http://www.freeinfosociety.com/electronics/schemview.php?id=377 Really, though, I'd look for a kit, where someone else has already figured out the component values, bought the components for you, and did most of the soldering already (aka a printed circuit board).

[randofo]

Very cool! I have a headset exactly like this that I have been too lazy to figure out how to wire up. This will be very helpful. I like your casing too. It has a very nice aesthetic.

[nomuse]

Thanks -- it was your circuit that got me started! ( http://www.instructables.com/id/Telephone-Handset-Microphone/step6/I-lied-about-the-circuitry/ )