impedance balanced out

[tempus]

Hey all;

I'd like to try impedance balancing using the schematic here. Is it necessary to use a metal film resistor to keep the noise contribution down, or does it not matter, since it isn't actually in the signal path?

Thanks

[Steve Conner]

Yes. It is in the signal path.

Metal film resistors are the standard nowadays and only cost a few pennies each, there's no reason to use anything else.

[tempus]

I know, and I would normally not even ask, but I don't actually have the value I need around here, which means I have to order some from Mouser, and the shipping will be 8$ for a few cents worth of resistors...

I hate shipping.

Can you explain how it's in the signal path?


Thanks

[Steve Conner]

Well, if you swapped the two mic signal pins over, then you would still hear signal, right? They're both inputs to the preamp: normal and inverting.

Therefore, you would hear any spurious noises generated by anything you connect to either pin. I'm not a big fan of the concept of "signal path", it's an oversimplification.

Most of the noise from a resistor simply depends on its resistance. This thermal noise is generated irrespective of whether any signal is flowing through it or not. Only really old and crappy resistors have "excess noise" over and above this, any fairly new carbon film ones should be OK.

[tempus]

I see what you mean now. I was thinking of it more being just grounding pin 3 through the resistor, and since no audio signal was actually passing through the resistor it wouldn't matter. But what you're saying makes sense. I also understand about the thermal noise from the resistor, and since I only need 100 ohms to balance it, I think I'll try carbon first. If I find it adds noticeable amounts of noise (it's for a mic preamp output), I'll bite the bullet and go metal.

Thanks again Steve