Behringer mixer problem

[PeteW1959]

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?

[frozenfrogz]

Hello Pete,

it will be considerably easier to help you if you could provide as much information as possible. If you share the schematics with us and point to the part you are working on we would not have to do brain acrobatics while trying to figure things out

Kind regards,
Frederik

[Andy Watson]

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.

It is mixing-down into a virtual earth. You need to look at the output of this op-amp directly on the op-amp pin.

[PeteW1959]

Thanks for the replies. I have attached part of the schematic and tried to make it as clear as I can, but it is from two different pages. The left hand is the final stage of a stereo channel (but I get the same results with two mono channels configured appropriately) and the right hand the start of the master section; I have drawn the lines in for the mix-L and mix-R busses.

With the pan control set to centre (it is indented, so it is always correct), I get 18v p-p signals at pins 1 & 7 of IC58, and this appears both sides of C303 & C304 and then at LH side of R712 and R728. With SW81 down (signal routed to main mix) I get nothing on the RH of R712 & R728 and subsequently nothing on pins 2 & 6 of IC63, but I get 19v p-p on pin 1 & 21v on pin 7.

As I said it doesn't matter which input channels I use, I get the same result, and if I reduce the channel input gain I still get an imbalance, just lower signal levels.

What I don't understand is how I can get an output from IC63 with apparently no input! I am guessing this something to do with the theory of how mixers work, but it seems to defy logic!

[frozenfrogz]

I am not an op-amp expert, but I would start by checking R698 / R699 and R635 / R635. They should be matched to have similar resistance. A variance in those parts would result in different bias of the two op-amps inside the 4580 package.