Microphone pre-amplifier

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Hi!
I'm building the microphone amplifier found at this page:
**broken link removed**

I'm a bit of a newb to waveform amplifiers, I was just wondering what polarity is the output? If I wanted it to drive a pair of headphones by connecting up the + and - to the right locations (obviously, I'm making the assumption it can drive them) where should I connect the circuit?
 

You should be concerned about the headphone impedance and,limitations of OA's with higher impedance output. For 32 Ohm headphones a low level could be achieved with series mono connections between Out & Gnd

... Note most are tested with only 1k but lower impedance loads will affect max output swing and possibly more distortion.

But with negative feedback ( gain reduced to x22) it significantly lowers the output impedance, but current limited by the internal series R.
 

Why use headphones with a microphone amplifier? Are you trying to make a hearing aid? Since there is no gain adjustment then it will probably produce acoustical feedback howling.

The output capacitor value is much too low to feed low impedance headphones. Only high frequency squeaks will get through.
 

That pre-amp is really designed to feed into a further stage of amplification. The second amplifier would hardly load the output, allowing it to reach it's full level. Most headphones impose quite a heavy load on the amplifier driving them and that design may not be able to cope. As SunnySkyguy points out, the effect of overloading the output will low volume and probably high distortion.

The 'pre-' in it's name is telling you it goes before something, in this case the main power amplifier. Its designed to make the microphone more sensitive but still needs more 'punch' to drive a thirsty load like headphones or a loudspeaker.

Brian.
 

Audioguru said:
The output capacitor value is much too low to feed low impedance headphones. Only high frequency squeaks will get through.
Click to expand...

I'm a little new to this, just checking did you get that information using the equation?
F=1/(2*pi*capacitance*impedance)
 

Yes, also because the output impedance of an amplifier (the signal source to the coupling capacitor) is extremely low.
Both earphones of 32 ohm headphones connected in parallel make 16 ohms. Then the coupling capacitor value for -3dB at 20Hz is 500uF.
The way headphones are connected to their plug, trying to connect the earphones in series makes them out-of-phase that sounds weird.
 

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