Hum in Tone Control Board

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Hi Folks,

I have a DBX CX3 preamp with some slight faint hum when the tone control is engaged (bypass, no hum). I replaced all the power filtering caps and the rectifier bridges, even added some filtering caps since the three 1,000 uF caps seemed not enough.

I am thinking that one of the Op Amps may be to blame. I intend to replace all five with OPA2604. I also replaced all the electrolytics already.

At about $5 a chip, am I wasting money?
 

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The tone control potentiometers are quite a distance from the IC's. It's possible that the traces on the PC board are picking up some stray noise. Is the PC board mounted in a shielded and grounded enclosure?
If so, how is the board connected to chassis ground? If it's grounded at multiple points, it may have a ground loop.
 
Thanks for the hints, I will upload some more pics tomorrow as to the setup. I will also check to see if it is Zener regulated or if those are voltage regulators near the connector.

There is so little documentation on this DBX stuff. I have the amp and it is just as flaky, but they sound remarkably good.
 
Here are some pictures. The board sits bottom left corner as the unit faces you. The only thing under it is the metal cover. The braided color wires are the power supplies.

There are two transistors on the board too, they are audio type 2SA1145Y and 2SC2705O. The regulation seems accomplished by two Zener diodes near the power entry connector. The other semiconductors on the board are the 5 NEC 4570C ICs.

I am supposing that one of the above semiconductors could be the problem, I could not readily see a ground loop potential, but who knows.

What would you all try first?
 

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It won't be the opamps...

You really need to scope it. Can you determine the nature of the hum. Is it a very deep pure line frequency or is there any harshness to it (harmonics( which may give a clue as to where it comes from. Pure line and it's probably pickup. If it's twice line and harsh then it's possible it's PSU or ground related.
 
It won't be the opamps...

You really need to scope it. Can you determine the nature of the hum. Is it a very deep pure line frequency or is there any harshness to it (harmonics( which may give a clue as to where it comes from. Pure line and it's probably pickup. If it's twice line and harsh then it's possible it's PSU or ground related.

I was afraid I might need the scope I passed up buying a couple of weeks back.

The hum is faint, not harsh, just like a transformer hum. It goes up an down with the bass pot. There is no buzz associated with it like an open ground on a guitar amp.

there you go

Yes, I thought that is what you meant. That is just a rig to see if increasing the filtering capacity on the three main power caps would help. It did only slightly.
 
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Is the amp sat on top of anything that has a transformer in it ?
Have you tried unscrewing and rotating and moving the mains tranny to see if it affects it at all.

You mention altering the caps lowered the hum albeit slightly... I would not expect any audible difference tbh with regulated supplies. Just confirm the supplies on the opamp supply pins are OK. As mentioned by others the reg for the board is by two zeners and transistors where the 22v enters. Check the voltage over each zener and make sure that less aroun 0.6v appears across the supply pins of opamps assuming they are configured as simple series pass regs.

I bet that PCB would run ok off two 9 volt batteries in series (centre to ground) if you isolated the incoming 22v. That would prove if supply problem to board in absence of scope.
 
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