Hi,
This is my first post to the forum. I studied electronics at college 40 years ago, but ended up in IT so I lack practical experience of analogue circuits. I know the basics of fault finding and have a sig gen, 'scope and schematic of the mixer, but I am stuck with a minor but annoying problem.
The fault is that the left channel output is slightly lower than the right. It isn't a huge amount, and can be balanced by increasing the left channel level, but I'd like to get to the bottom of it.
It doesn't matter which of the 32 channels I use as inputs, and the imbalance appears after the first stage of the master section, which narrows it down considerably, but I can't pinpoint it to a specific stage.
Test setup as follows: 1KHz 1v p-p sine wave as an input on 2 channels (using a splitter cable to make sure the input signals are identical), one channel full pan left, the other full pan right. Using the 'scope to trace the signal path through, the level on left and right is identical as far as the output of the final op-amp in each channel.
The schematic shows the final channel op amp (4580) output going via a 100uF cap and 5k1 resistor in series, then through the main channel routing switch to the -ve input of another 4580 in the master section. This is where it gets really weird! As I said I have a signal on the output of the channel 4580, both sides of the cap, but only one side of the 5k1 resistor. There is no signal on either the + or - inputs of the second 4580, but the signal appears again at the output, with an imbalance between left and right! I definitely have the correct signal path on the board because it checks out with a multimeter. I've changed the op-amp in the master section, but it is still the same.
Hope this makes sense
I understand basic op-amp circuits, but can't work out what is going on here. Can anyone explain it?