BPPBP - Bruno Putzey's Purist Balanced Preamp (well a balanced volume control really)

I have read in Hans' short article in linearaudio the following:

And what, you say, about audible clicks in the sound when changing the gain? I can hear none at all, not even when going from 0111 to 1000 where all 4 relays are changing position. Mandatory is that no DC is entering P1 and P4, but OpAmp U3 on Bruno’s PCB should take care of that.
So, I have I placed my multimeter probes between gnd1 and P1 and gnd2 and P4 and, surprise, it measures exactly 1 VDC. Not sure if I did something wrong or not...

I've experimented with setting the attenuation to 0x00 for 1 to 5 mSec and still horrible clicks.

I'll give it a rest for today.
 
Update: alright so I went back and read the entire thread and the one for the group buy. Seems my problem was DC offset. I switched the TL072 for an OPA1612 (that’s what I had around here). No more clicks on one channel. I still hear the clicking noise on the other channel, there is also more noise like ground noise and I can hear noise from the arduino too.

Only on one channel. Same one that still has a DC of 0.8mV at the output and lowest attenuation. At 0db it goes to around 2mV.

The rest of the opamps are OPA1656.
 
Good that you have one channel working Ok now. From past experience working with SMD assembly, the TL072 was probably fine except that one or more pin wasn't connecting to the circuit board pad. Before you remove the TL072 from the other channel, try reflowing the solder joints with plenty of flux and inspect the joints with a good magnifier to make sure each pin is connecting to its relevant pad on the board.

Keep it up, you're doing great.
 
Funny stuff :)
Today I have changed the wiring to the arduino and installed new power supplies and installed the preamp board on a temporary plexiglass “case”.

When I powered it, the clicking noise came back. DC offset on one channel was 1 volt when measured at the opamp and gnd.
Upon someone’s suggestion I have shorted R10/37, the 2M2 resistors that go to GND and the dc voltage dropped to 60 milivolts. Huge difference. There’s still clicking noise but lower and now there’s a buzz of course.

This sounds like a bad pcb or something. I can’t explain how yesterday it was ok after reflowing most of the parts and today it no longer is.

Anyway, I’ll switch to using the board from @geoffw1 :)
 
Two likely possibilities
  • damaged resistor (they can look fine but film has cracked
  • device pin still not soldered onto PCB pad.
It's worth checking dc voltages on each pin of the devices and comparing with what's expected. Mark the values on a diagram and upload here so we may be able to help you.
Also inspect solder joints with decent magnifier and good light. Do you have something like a jewellers loupe?
 
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I’ll try to find some time after work to measure on each pin and write it down.
I don’t have a jewelers loupe but I do have a microscope from AliExpress which is pretty good for inspecting solder joints.

I’m surprised that 2 boards I’ve soldered and reflowed with flux and still have issues. I’m more surprised that 2 days ago it was fine and the next day without touching any solder joints or bending the board, it started failing. Weird.
 
The Bruno Putzeys board is pretty tightly packed so can be more difficult to work on or inspect.

When looking at soldered joints under the microscope, it's worth looking a joints from an angle. It may look good when looking down vertically but when seen at an angle you may find the connection doesn't touch the PCB pad. You may also find you missed actually soldering a pin!

Intermittent faults can be difficult to find. I've had frustrating times with boards in the past but usually been able to find the problem in the end.
 
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These are my build, inspection and test aids. I can't remember if they are final rev or if I've posted them here before. I like to print these out highlight and check things off, this process is a must if your dealing with thousands of parts. I remember having problems related to grounding when pre-chassis. If something seems off let me know and I'll check my build.
Caps.png
Non RC.png
Res.png
 
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