Right, I'm new to electronics, but this is what want to know:

What do I need to take a mic input, and output it to some ear phones (standard jack, can be mono). I want to build a motorcycle intercom, here are some other requirements I have in mind:

  • Volume controllable
  • Turned off by a switch

Also, if I have two inputs that I want to send to one output, is it as simple as joining those two inputs together and the connecting with output? Perhaps with a few diodes... I'm starting to guess here.

Anyway any advice to nudge me in the right direction would be appreciated.

Thanks, Ben :-)


Microphones don't put out much signal. You basically need a amp that has a voltage gain of a few 1000 and a low output impedance, like 8 ohms or less. You probably design the amp with 5000 voltage gain, then the volume control reduces that as necessary. A volume control is basically a variable attenuator.

To make is switchable, just put a SPST switch in series with the power. No power, no output.

Multiple signals can be summed in various ways. The cleanest is probably a "summing amplifier", which is just a inverting amp with one input resistor for each input source. This has the advantage that each source sees a resistive load to ground. One source can'b back drive another, although that may not be a big deal if the sources are buffered for that purpose.

The design of a good audio amp could fill a book. I have to run now and am not going to write a book anyway. Look around for battery operated audio amp, microphone amp, headphone amp, etc. I'm sure there are lots and lots of circuits out there and maybe even whole devices. Mic amps and headphone amps aren't all that unusual. There are also small power amp modules that will be easy to use for the final stage if you want to roll your own.

How about something like a MP3 player to drive the headphones. I'm guessing that those or at least some of those come with a line in or mic input. Look around. Good chunks of what you want should be available off the shelf, especially if you're willing to do a little repurposing.

  • 1
    \$\begingroup\$ Many thanks for your answer Olin, very helpful. On your latter point I already own some basic (read: crappy) motorcycle intercoms, I could frankenstein them together to come up with part of the project, but want to try and bundle it all together if possible. I'll look at the components you suggest and see what I can come up with. Many thanks :-) \$\endgroup\$ – Ben Everard Jan 9 '12 at 23:36

You should get all the Forrest Mims books and pore through them, all your circuits you need are in there, and Forrest has been shedding light on electronics for 40+ years. I have three engineering degrees, EE, MSEE, and a Computer Sci degree, and I still keep Forrest's books on my electronics bench!


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