Studer A807 distortion problem

[Studerfreak]

I would suggest checking your power supply voltages and see if DC is really DC (so not a big ripple on the DC).
It may have nothing to do with your modifications, but just a coincidence.
Are you sure the opamps you replaced are pin-to-pin compatible with the ones you removed ?

[Brad McGowan]

All the op mods were checked on on the scope one by one. I installed 0.1uF caps from power supply pins to ground at every socket and added caps to the feedback loops where necessary to deal with oscillations. So I've made them pin for pin compatible. LM6172's replaced 5532's and LME49720's replaced the 4559's except in one spot where it was necessary to use a lower bandwidth part for stability. Coupling caps were reoriented as necessary to properly block DC offsets at chip outputs. Like I mentioned above...after channel one mods were finished it all worked perfectly fine. It wasn't until I removed the first board, did the channel two mods, and re-installed both boards that I began having issues with both boards. I was, however, able to fix the problem by reseating the boards when they were initially both installed. So I thought it was just a bad/dirty connection at the connectors. But apparently it's not.

I'll check out the power supply voltages. Are you thinking that I have a blown filter cap in the power supply?

I should also note that when I applied pressure to the board I could get the leaking monitor speaker sound to change volume, but the sound out of the control room monitors would still be distorted.

Brad

[Studerfreak]

Yes, I think there is something wrong with the power supply.
Check your voltages and also check with an oscilloscope for any large AC ripple.

[Jim Williams]

Replace the opamps with the original ones. If the problem goes away you have stability issues. If not, you have rework issues.

Jim Williams
Audio Upgrades

[ripple_fx1]

If your power supply checks OK, (image is kind of fuzzy, so I can't tell how the monitor speaker is fed) and you can press the board and "fix" it, look for something mechanical (bad solder joint, missing ground) and/or a problem with the feedback resistors for the opamps - sounds like one may be stuck full throttle. Cheers! JR

[Brad McGowan]

Thanks for the tips guys. I'm going to try out some of these suggestions later this week.

Jim--I had the first board working perfectly in the machine with all the op amp swaps so that would indicate I have neither stability or rework issues. I'm wondering if I made a last minute, untested change to something. My memory is a little foggy. I made the mistake of letting some time pass between finishing the first board and getting the machine set up with both boards. I'm going to have to revisit my notes and see if there was anything I did at the very end.

Let me ask you guys something. I think I may have removed the coupling caps between the two op amp stages in front of the transistor pair in the output circuit. I'll need to check my notes to see if I did that or not. If there was a DC going into those transistors, would I experience the symptoms I described above?

Here's another thought...let's say the current draw for two boards' worth of upgraded op amps exceeded the current capability of the power supply. I need to dig out the power supply schematics to see what's going on, but if a circuit tries to draw more current than a voltage regulator can handle, will it starve the op amps of power and cause them to clip?

thanks,
Brad

[Brad McGowan]

I solved the problem today. It was C16 which is a 220uF coupling cap after the first op amp after the line input. I had removed it and put a jumper in its place. It turns out that the small amount of DC offset at the output of that first stage was a little too much for the following stage. When I restored the coupling cap all worked perfectly.

Thanks for everyone's help!

Brad