Weird distorting problem in mixer. DC voltage on op amp output - Page 1
Forum section: electronics / repair

author: AndreyS
author_rank: Regular Contributor
timestamp: April 03, 2020, 11:20:06 am
content: The mixer I've got (Behringer x1204usb) has issues on 5/6 and 7/8 channels, to be no precise in the section closest to jack sockets.The problem occurs only when gain switch is on -10 and TS jack inserted. In any other condition it seems working fine.When I connected to that line input socket an audio source (it was guitar multi-effect pedal), the sound was distorted, but mixer didn't clipped. Switch was on -10 position, as +4 was quite.What I did:1) traced components and drew a diagram2) desoldered in+ in- out lead from pcb, the voltage was 2.1 2.9 14.2 V respectively.3) desoldered those leads of another op amp in mic section. The results were the same. Therefore, that's probably normal operation.What is the culprit?PS: Could you please recommend any articles/books on op amps, where their operation are described in detail?

author: AndreyS
author_rank: Regular Contributor
timestamp: April 03, 2020, 12:02:52 pm
content: I did a simulation and it appears that everything seems ok. But why then I'm getting clippings? The sound isn't loud, but it is distorted

author: orbanp
author_rank: Regular Contributor
timestamp: April 03, 2020, 01:51:12 pm
content: Hi Andrey,I did a simulation too, I did get different results.The DC voltages are 0V everywhere in my case, even if I leave the inputs floating.It boils down to what is at the input?I assumed 0V DC at both inputs.Why is at the tip 5.82V?Usually at the input there are DC blocking capacitors preventing any DC voltage present there.Find out why is there 5.82V!It is possible that the IC is damaged.Your diagram also looks confusing.The amp stage looks like a differential amplifier.The "tip" and "ring" are the left and right channels of a stereo signal.Why would you amplify the difference of it? Some fancy audio effect?Still, you need to track down why are there dc voltages.Regards, Peter

author: AndreyS
author_rank: Regular Contributor
timestamp: April 03, 2020, 02:32:41 pm
content: Thank you for your input, orbanp,that is indeed a differential amplifier. It supports unbalanced trs jacks as well.I'm starting figuring out that at switch closed it is just way too sensitive. When i connected my guitar, it seemed worked without distortions.But still why there are 5.82 V at tip input? No, there are no any input capacitors, only those that do the filtering

author: AndreyS
author_rank: Regular Contributor
timestamp: April 03, 2020, 02:33:44 pm
content: I am wondering how did you get 0 V at input? Have you changed any components in schematic of your simulation? Is it exact as mine?As for line input, I just insert tip-sleeve jack (which shorts ring and sleeve) and leave it floating and touch only multimeter leads with it

author: orbanp
author_rank: Regular Contributor
timestamp: April 03, 2020, 10:55:08 pm
content: Hi Andrey,An op-amp circuit usually gets is bias from the input at its non-inverting (+) input.If there is nothing attached to the inputs the circuit gets its DC bias through resistor R2, and that is attached to the ground.I used in the simulation DC-blocking caps, but also did the simulation with the voltage sources from the ground.In all cases the output DC voltage was 0V.If you have +14.16V at the output no wonder it starts clipping!Do remove everything from the inputs in the circuit, from the "tip" and "ring", the output should be at 0V. If not chances are that the IC is damaged, do replace it.Regards, Peter