sontec 1 RU build thread

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Hi helpful people :D!
I am building a sontec clone based on Fabios Baumanns PCBs. Right now with Tl071 and NE5534.
I have one band on the bench and everything works BUT it develops a hum when I turn down the frequency pot ( I have a 24 step one ). This problem is described here a lot but I can not find an obvious solution to it in this thread and the people who had this prob in this thread just disappear without a solution.
Only differences between Fabios and Igors layout are some higher blocking cap values ( changed that already, did nothing to the hum) and some bias options ( also don´t change anything to the prob).
When I disable the band ( Made a switchable Gain Range 6-off-12 ) at the summing resistor I have around 88db snr. Okay for me. When I engage the band a 50/60 hz hum at -72db is clearly visible on the analyser. When I turn the freq up the hum is gone completly. Grounding the knobs doesn´t do anything. Also it seems it is not depending on the freq caps... Should this be gone when the pot is installed at the frontpanel as mentioned here? Could not believe this ;) It should just be working laying on the bench, right?
I am a little afraid of ordering case and fancy knobs just to find out that the unit has this prob in the end. Every hint appreciated ;)
 
Grounding the knobs doesn´t do anything.
Not sure what you mean by 'grounding knobs. Not sure about Fabio's boards but on Igor's board, the pots (pot cases) needed grounding for unit to be noise-free -- just installing the faceplate didn't do it on my build.
 
Not sure what you mean by 'grounding knobs. Not sure about Fabio's boards but on Igor's board, the pots (pot cases) needed grounding for unit to be noise-free -- just installing the faceplate didn't do it on my build.
Yes, grounding the pot cases. Here it is mentioned that when the pots are installed in the frontpanel they would be grounded and then the hum should be gone...

Found out that it´s worse with smaller freq cap values as described several times in this thread.
When I touch ground and cover the pot with my hand without touching it, the hum is significantly lower. Maybe that shows, it could work when it´s cased?....mmhhhh

Edit: it's indeed much better when I put the circuit in a case with normal ground at xlr plus case grounding like usual.
I am always amazed how much difference this makes for some circuits. Some are dead quite on a bread board or even point to point some are an antenna. High impedance magic.
But I think its often more about shielding and resonating than electrical connection there.
 
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Hi ArranDutt do you still have the g-son 5's schematic? I'd like to compare it to Igor's original schematic.
 
PSU and summing pcb’s: schematics or links? The original file links aren’t working. Purchased the G Son case and filter pcb’s but I’m at a loss for the power and summing.

Also OPA1656’s appear to be the posh ic’s now that all the ic’s listed in the BOM are discontinued. Very likely I’ll try with 5532’s also.
 

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