I bought the 3-in to 1 out mixer and have built it up but I am getting distortion and a gate like cutting out from all channels... Has anyone else experienced this? I am wondering if I can get the schematic since you have not posted the build documents on this pcb yet... Yes it is a simple circuit but I am not sure where I have gone wrong... I get sound from all inputs it is just distorted and cuts out as it fades on all channels... I thought maybe I had a bad chip and since I socketed them I swapped the output chip and no change... at any rate if you can provide me the schematic for this I can troubleshoot further... If anyone else has any odeas I am open as well... could a bad solder joint somewhere cause this? thanks! Bryan
SOLVED 3 in 1 out mixer issues...
- Thread starter gwyador
- Start date
Feral Feline
Well-known member
The schematics for both the Splitter and Mixer have been uploaded elsewhere on the forum, (in another thread I posted a link to them just yesterday) you'll find them if you SEARCH.
SOP for a trouble-shooting thread is to post well-lit pics of your build including front/back/all wiring.
*BTW, I've tried 3 times now to edit this post to add links to the schematics, and it's not working, but the link to the search engine posted just fine.
SOP for a trouble-shooting thread is to post well-lit pics of your build including front/back/all wiring.
*BTW, I've tried 3 times now to edit this post to add links to the schematics, and it's not working, but the link to the search engine posted just fine.
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Robert
Reverse Engineer
I really do not know what pictures will help with but here goes... As I said, rails look good but I swapped the tl072's out for tl972's in case the signal level was somehow reaching close to the rails and I found if I turn the volumes down about halfway with the 972's installed I can get continuous sound but if I turn it up past about halfway I lose sound... I would have thought it was an op-amp issue except for that symptom. Was this mixer designed for line signals? If so, are the caps there to gate the signal below a threshold? I'm tempted to remove them and see. does anyone have thoughts on this?
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Feral Feline
Well-known member
Pretty sure (99.8%) this mixer is meant for instrument-level.
Danged if I know what the problem is. I was going to suggest re-seating IC #3 (3rd from left), but if you've already swapped out the ICs...
A short/partial-short under one of the IC-sockets? Perhaps the one handling the actual mixing, as you said the problem's happening on all channels.
The fading indicates it might be a problem with a cap, so peut-etre a bad cap? That could also explain the distortion, a bad cap.
I'm not a good trouble-shooter, I know, but people tell me I'm trying...
Danged if I know what the problem is. I was going to suggest re-seating IC #3 (3rd from left), but if you've already swapped out the ICs...
A short/partial-short under one of the IC-sockets? Perhaps the one handling the actual mixing, as you said the problem's happening on all channels.
The fading indicates it might be a problem with a cap, so peut-etre a bad cap? That could also explain the distortion, a bad cap.
I'm not a good trouble-shooter, I know, but people tell me I'm trying...
SpringReverbFan1234
Active member
Normally I'd hear this symptom and think it was a volume pot with legs mixed up. But your wiring looks good from the picture, and based on the schematic all four pots are gain knobs with legs 2 and 3 connected together anyway.
You used 100 pf caps and not 100 nf for C2, C4, C6, and C7, correct? If that's not it I'm out of ideas. Maybe the next thing I'd consider would be a defective volume pot going open circuit.
the DC Block cap in this is a 22nf.... in the splitter it is a 1uf Not sure if that would help but I am running out of ideas... any suggestions??
Yes, using a standard 9v pedalboard power supply so .6 volt drop for the diode in the circuit puts me at rails of 8.8 and ground with the vref at 4.4v but other then that is acts to me like the signals are getting too close to the rails and when I put in the tl972's I DID get less distortion... I raised the resistors between the buffer and the gain stages to 100k instead of 10k effectively making my max gain in the inuts at unity which is fine for my purpose but that got rid of 100% of the distortion now the only issue I have is in the main output... if I turn the master volume up past about halfway I get a complete cutout of volume and some weird digital noise... I'm tempted to make my summing resistors 100k as well but that means my max all around would be unity... I'd like a LITTLE but more gain then that... but this is weird to me... I've built many mixers and not seen this but most of them were on higher supply voltages... I am really not sure about this one!!
Yes 100pf... I don't think it is a bad pot... I swapped it for a smaller pot... 50k... admittedly it was a B-taper in the swap but I could only get it to about 5k before I had the same issue... PS since the 100pf cap is in parallel wouldn't an open circuit pot just take me to max volume and high cut on the output?... I guess that might make the amp hit it's rails and shutdown... hmm...
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Assuming all components are rated for it, have you tried it at 18v for S&Gs?
Your not getting any DC offset out of the input buffers are you(I.e. DC on output =vref, not >vref)? That would push the signal into the rails easier in the mixer stage since those stages aren't decoupled.
Your not getting any DC offset out of the input buffers are you(I.e. DC on output =vref, not >vref)? That would push the signal into the rails easier in the mixer stage since those stages aren't decoupled.
well, I bit the bullet and changed out r3 r6 and r9 with 100k resistors and R10, R13 and R14 for 51K resistors and r101 and r102 for 100k resistors and now things are beautiful... full range of reductions on inputs and still about 2x boost on the master output. I am happy with this but I am including the changes I made in case anyone else runs into this... I did NOT bother to remove the tl972 op-amps... so that is the other difference... I may try putting the tl072's back to see if that helps but either way I now have a working mixer... Yay!! ps for those unhappy with the noise of this mixer I found that with this gain structure I get very little noise until my master is just over 2x boost!! cheers everyone!!