It Works! ... kinda - Green Pre

[AudioJunkie]

Alright, so i finally got around to finishing my green pre's. Just tesint one channel i come up with some strange waveforms.

http://pg.photos.yahoo.com/ph/klari.../pg.photos.yahoo.com/ph/klarinet_18/my_photos

These are all at 100 Hz. It seems only the top of the waveform is amplified for some reason. This was using the two 100 ohm resistors soldered in a V shape going to the + and - inputs. The 1k trimmer all the way clockwise. CCW gives an even funkier waveform. Also this is at output, where if i take measurements from the outputs of the opamps, it's the same just with a little less amplification.

Testing on a real mic, seems to sound okay except the last two gain steps distort really easily.

Any ideas?
Daniel

 

[tmbg]

through those 100ohm resistors, your test signal is still ending up in phase at both inputs, which means the pre is trying to treat it like common mode noise and reject it!

You'd need to feed it the signal with polarity reversed on the cold input.

 

[alk509]

I just googled this up for you. It may help you understand what's going on (and why it works OK when you plug a mic in)

http://www.mediacollege.com/audio/balanced/cable-balanced.html

Hope it helps!

Peace,
Al.

 

[AudioJunkie]

D'oh! Like i've said before. Things that make the most sense sometimes escape me, and more complicated things seem plain as day. I wasn't thinking about the fact that both legs of the resistors were the same polarity.

But in that case, wouldn't the signals be added to each other and then double in size? instead of give a weird half amplified wave?

Daniel

 

[tmbg]

no, the trick with common mode amplification is that one leg is going to the inverting input of an amp, the other the non-inverting. So, in essence, one signal is inverted before they're summed. That way, the two opposite polarity mic signals become two same polarity, and sum, whereas the same-polarity noise becomes opposite polarity, and cancels.

 

[peterc]

Hi Daniel

In the "setup" doc I sent you (I'm pretty sure everyone got them, if not email me), this is the setup for nulling out the CMRR signal. Use 50 or 60Hz & trim for least signal.

>>>Testing on a real mic, seems to sound okay except the last two gain steps distort really easily.

You should be able to get close to 24v pk-pk before distortion (at the top of the Output trim pot) using a sine wave input.

Enjoy!

Peter