PreAmp Problem

Hi all, been trying to figure this out without posting but not having any luck so here I am asking.

Project - Have a video surveillance system, bosses now want it to have audio. Current system has inputs for audio so just need a mic and preamp to get to line in strength.

Using - M040 Preamp **broken link removed**

- Electret Mic **broken link removed**
- 10K Resistor
- Unknown value capacitor
- 15VDC Plug pack power supply.


Setup - With out the capacitor it just hums and nothing when I tap the mic, been told I need the capacitor but no idea what value.

So I guess the question is, Is this setup right? Will it do what I need? I don't need crystal clear audio, just good enough to hear any abuse from our clients.

Thanks and regards
Brendan

The 15V source is a little high. Might be allright if you increased the 10K to say 20K. Also, to use the +15V as the source of the bias to the electret, it will have to be very well regulated (no hum). If it isn't, that could be the source of your hum...

The capacitor just has to have an impedance less than the input impedance of the pre-amp at the lowest frequency you want it to pass. Xc=1/(2ΠfC). You will have to look up the input impedance of the pre-amp.

I can go down to 12 or 9V, the 15V was just the first plug pack I found that fitted the range.
Any idea what that Cap should be?

The preamp is designed to drive a power amplifier, not a speaker.