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Need mircrophone preamp circuit

I have a generic computer microphone that works fine on my desktop computer for voice, which is all I need. It's a standard electret condenser mic. Actually, it's a genuine QuickShot (not some cheap imitation).

It also works fine on my laptop, but only if I'm on battery power. When I run on the A/C adapter, I get really bad hum - so loud that it isn't usable that way.

But if I turn off the mic amplification in Volume Control, and feed in any higher-level line input, the hum drops way down.

In addition, I have an external mic input on my Canon camcorder which has the instruction, "Use commercially available consender microphones with their own power supply", and I've confirmed that there's no DC supplied through that jack.

So, I think it's time for a microphone preamp. It needs to be battery powered, hopefully no more than 3V. I have on hand a TI TLC27M2CP CMOS dual opamp which the data sheet says will run on a minimum of 3V, and I have a variety of bipolar transistors on hand. I would appreciate it if someone could point me to a relatively simple circuit that they like that would operate on 3V. It just needs to be mono voice, but should do that well. If it matters, I would like to be able to use a 15-foot cable run when I use it with the camera.

Thanks for any suggestions.

Reply to
George
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This tells me of a ground loop with the high frequency AC modulated noise generated by the AC adapter. This noise gets rectified by the mic. circuit, so there is bad hum.

First, try to put a big ferrite on the AC power cord. Make ~10 turns of wire around the core and see if it makes any difference.

May be, may be not. First, you need to find out the cause of the problem.

LMV722 is an inexpensive audio opamp specified at 2.7V.

???? If you don't know how to make the basic amplifier, there is no reason to do that. It won't work.

Vladimir Vassilevsky

DSP and Mixed Signal Design Consultant

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Reply to
Vladimir Vassilevsky

On Thu, 19 Jul 2007 08:16:35 -0500, George wrote: ...

When I needed a mic preamp, I just built a plain vanilla, bog-standard, textbook common emitter amp; this basic circuit:

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And did a little DC analysis to figure out resistor values; I ran it on two "C" cells, and used a 10K pot for Rl.

But, I was using a 600 ohm dynamic RS mic - for an electret, you'll have to figure out how to power it.

Have fun! Rich

Reply to
Rich Grise

He can power the mike by adding a 10K from Vcc to the

  • terminal on the mike with the circuit you posted.

Ed

Reply to
ehsjr

Thanks very much. That looks like it should work fine.

Reply to
George

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