Friday, August 24, 2007

Quick Voltage Amp

If you need a quick voltage amp for some project requiring a lot of gain and transformer output, a quick and easy way to go is to use the input of a typical low cost solid state condenser microphone, which has a high gain amp in it, behind a super low noise amp, like this one shown here from Miteq. The DC bias at the input must be blocked by a cap, and shielded by the stock headgrille. Use an isolated BNC connector. A regular BNC will bring in a ton of noise.

This mic was about $45, yet the low noise 50 db amp cost at least $400. Together they will produce over 100 db of gain, not without noise, but OK for non critical applications.

I used this one on a "fly's wing" transducer with a piezoelectric base. You can read about that here.


craig said...

$45 for the Mic, at a hamfest?
Will we be able to click on an enlargement pic?

Bob Crowley said...

That's just one of the many cheap LDCs that are out there. This one is an MXL. I bought it new for that price, just to satisfy my curiosity. As you can see, it has been recycled into something else.

That image is only about 230 X 150. I'll see if I can replace it with a bigger one.

Bob Crowley said...

OK I got a couple of emails about this one.

The blocking cap prevents the DC bias that would normally make the diaphram into a variable cap from getting to the amplifier. The cap stores and releases some charge from the low noise amp output. The cap replaces the capsule, in other words. I don't use a high ohm resistor across it but in series with it. That develops the charge even though the output of the LNA is relatively low Z.