Studer A807 distortion problem
Old 6th December 2009
  #1
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Studer A807 distortion problem

Something screwy happened to my A807 over the course of doing some upgrades the audio boards, but I'm having trouble figuring out what it is. Here's some background. I upgraded op amps and caps on one board, tested it and it all worked brilliantly with no problems. I upgraded the second board, installed it, and then experienced some level and distortion problems (like the sound of op amp clipping) on both boards. So I tried cleaning contacts and re-seating the boards. At first I got strange pops, level jumps, fuzzy sounds, and then it all went away. Problem fixed I thought.

Fast forward to today. I went to use the deck and noticed the same buzzy clipping distortion on both channels. I again tried re-seating the boards. No luck. Here's what's happening.

1. Machine is in input mode and I'm feeding it a mix from the DAW. I'm monitoring the output of the Studer in the DAW. Both channels are set to cal mode.
2. The monitor level (on the deck) is turned all the way down but there is broken distorted sound coming out of the monitor speaker on the tape deck.
3. I switch off the deck and switch it back on. Initially the sound is clear and distortion free. There is no "broken" sound leaking out of the small monitor speaker.
4. After about 6-8 seconds the sound becomes distorted again and all of a sudden I have distorted sound coming out of the monitor speaker despite the fact that the volume is turned all the way down.

I realize it could be any number of problems, but I'm hoping to eliminate a few variables. Because I'm monitoring the input I know the source of the problem is either in the line input preamp stage, or the output stage. The delayed onset of distortion indicates to me that some capacitor is charging up...or discharging. In the attached schematic of the output circuit, you can see C120 in the upper right, highlighted. I changed that from 470uF to 1000uF. By doing that did I screw up the transistor voltages? I had the board working fine initially with the 1000uF cap in there, but I just need a sanity check.

What else might cause the sound to go from clean and clear to distorted and buzzy after a finite amount of time when the machine is powered up? And what's causing the sound to leak out of the monitor speaker even though it's turned off? The two phenomena are definitely related and it's driving me nuts.

thanks,
Brad
Attached Thumbnails
Studer A807 distortion problem-a807-schematic-mods.jpg  
Old 6th December 2009
  #2
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Studerfreak's Avatar
 

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 ?
Old 6th December 2009
  #3
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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
Old 6th December 2009
  #4
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Studerfreak's Avatar
 

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.
Old 7th December 2009
  #5
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
Old 7th December 2009
  #6
Lives for gear
 

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
Old 7th December 2009
  #7
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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
Old 3rd January 2010
  #8
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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
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