The problem is when i add the 3rd input it goes 15v , and up to 20v with the 4th one. Is it intended to do so??? What can I check, maybe I did something wrong on the inverting Opamp?
Thatās kind of impossible, unless something is really wrong. How are you measuring the voltage? Between which two points? The op amp has, or should have, Ā±12 V power, so if you measure from TL072 pin 7 to ground it has to be less than that, maximum about Ā±10.5 V. Same from pin 1 to ground. Pin 4 to ground should be -12 V and pin 8 to ground should be +12 V.
What are you putting into the inputs?
Check also your multimeter settings, make sure itās DC voltage, in an appropriate range.
Post a picture of your breadboard.
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Thank you for the hint. I made some little mistakes and to limit the output i added a 10k pot to voltage divide the mix out.
Sometime is not a good idea to do electronics after a 11 hours shiftā¦
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Thanks Iām getting 10.8v from pin 8 and -10.8 on pin 4. Now Iām getting no audio from any of the inputs
This is what Iām getting on the chip
No input
Pin: voltage
1: -9
2: -1.4
3: 0
4: -10.8
5: 0
6: 0.7
7: 9
8: 10.8
Input on input 6
Pin: voltage
1: -1.2
2: -2
3: -0.6
4: -10.8
5: -0.6
6: 0.2
7: 1.3
8: 10.8
What is the reference for your measurements?
According to the schematic, pins 3 and 5 are tied directly to ground, they should always measure exactly 0V.
In these inverting amplifier configuration, the - inputs (pins 2 and 6) are virtual grounds and should also measure very close to 0.
Double check the soldering on the feedback resistors R9 and R11.
If you remove U1 and measure between pins 1 and 2 do you see 100K? And between 6 and 7?
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Thanks!!
Iām getting
R9: 9.6v
R11: 0v
U1
Pin 1 - 2: 99.8k
Pin 6 - 7: 99k
So Iām guessing R11 is wrong, Iām going to re flow it
Oops I read the wrong resistor for 9. Both R9 and R11 read as 0v even after replacing them and reflowing. Is there anything else I can check?
Based on your measurements, U1A doesnāt seem to be doing its job properly.
My guess would be that U1 is defective, do you have another TL072 you can swap in?
Another lower probability possibility is that one of your 100K resistors has a much lower resistance or there is a short somewhere. Sorry, I know, thatās not very helpful.
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Tried two other TL072s that looked like they were different brands (different printing on the top), replaced all the 100k resistors and cleaned the flux from between the solder points. Now Iām getting output from channels 1, 4 and 5 as well as a faint signal from channel 6.
Another weird thing is that if I touch the end of the cord from the VCO without plugging it into the mixer I can hear the VCO through the guitar amp I have the mixer plugged into.
Just grasping at straws here.
For the remaining faulty inputs, check your soldering of the switches, and check the continuity across the switches when they are closed.
As for the bleed through when not plugged in, I really donāt know, maybe a weak power supply?
The mute circuit just leaves the inputs floating, it would have been better to short the muted inputs to ground, maybe turning the unused inputsā attenuating pots all the way towards zero will eliminate the bleed through.
@lookmumnocomputer if you do a new batch of these mixers, maybe move the mute switch to between the input pot and the 100K resistor and short the 100K to ground when in the mute position?
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h0ow very odd. however noticing also from the video, when the volume is turned down it still makes the sound? signifying that floating or not floating is not the issue as the pot goes to gnd? unless i am seeing the video wrong.
the fact that all of them are giving out dsifferent signals is confusing as they all just connect up to the opamp in the same way. quite perplexing.
HAve you taken a working jack socekt and tried it on the other inputs?
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Great idea! I tested the wires to the big jack sockets and some of the signal wires are not connected, all the grounds are which is weird, currently in the middle of replacing them. Hopefully this Ship of Theseus will work after that!
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Works 100% now, just goes to show you have to check all connections. Didnāt think the wires would be a problem
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Cool 
Glad you solved the issue
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Just finishing up this module. Which pins from the 5pin jack sockets do I connect to the 2 pins on the pcb?
I will be using this mixer as the final step before connecting to my amplifier and sound system. As all of the modular is mono, but the sound system is stereo, am I correct in thinking I can make the output stereo by just connecting the tip and ring of the output jack socket?
No
for that you would need a module with a stereo output, which is not the case with this module, or you could use 2 mono mixer modules (one Out to the right and the other to the left in your system)
Really? But al I want to do is just send the same mono channel over both stereo channels, no need to adjust channels seperately. And as the socket Iām using for output is actually a stereo socket, I thought this would be a simple Look-Mum-no-Computer-style bodge job. Can you explain to me why this wouldnāt work, or break something?