calculating output impedance

[tempus]

Hey all;

This is kind of a follow up question to my previous post "how to make a bass rolloff for condenser mic".

How do I calculate the output impedance of the mic? If I measure at the mic terminals, I get around 1.6K. The bias resistor, however, is 2.2K. If I measure at the XLR pins 2-1, I get around 3.2K.

It has always been my understanding that the output Z of one of these things was basically, the bias resistor, but my readings are making me wonder.

P.S. - I want to impedance balance the output of this electret mic so I'd like to get it as close as possible. I've connected a pot between pins 3-1 to try to hear the greatest noise reduction, but it sounds clean at any setting. I'm assuming I'm not in a high-noise-enough environment to tell any difference.

Thanks

[J M Fahey]

the output Z of one of these things was basically, the bias resistor

Yes, that's basically it. And it's not a "bias" but a *load* resistor.

I want to impedance balance the output of this electret mic so I'd like to get it as close as possible.

Hard to achieve with such a minimalistic circuit.
Not worth the trouble.

I've connected a pot between pins 3-1 to try to hear the greatest noise reduction, but it sounds clean at any setting

Not the way to do it, and you'll "hear" nothing, "clean" is unrelated to line balancing..
If you really want to do it, build a phantom powered mic preamp and drive an output transformer or active balanced out.
PS: post the circuit you are actually using.

[tempus]

Thanks again for your help JM.

Here's my idea:



The 47 ohm resistor is something left over in the case of the bullet.

Also, I should clarify my last post. By "clean" I was referring to common mode noise - hum and the like, not the quality of the sound I was hearing. What I did was plug the mic in and listen for any hum/EMF type noise pickup. This is what I was referring to earlier. As an update, I tried setting the mic near a neon light, and noticed considerable CM noise reduction as I lowered the resistance on the pot. I found this odd. I expected to hear a fair amount of noise on either side (low or high resistance) of the output impedance of the actual mic, but instead I got lower noise with lower resistance.

Thanks

Merry Christmas