Hum in Tone Control Board

[whaleman]

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?

[Tekko]

You may also wanna check the voltage regulators feeding the opamps, if theres any.

[sakis]

look at the circuitry of the switch also ....there might be some loop cretatd from absense of ground in some point .

tekko's point is also corect ...looking at the pcb it says 22+22 volts there should be some regulation for that somewhere

[Tekko]

I do see a zener right above the connector.

[Frank Berry]

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.

[sakis]

very good point i think also.

[whaleman]

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.

[whaleman]

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?

[sakis]

he he ...the usual suspects .... why i thin that this might be caused from the "wired" capacitors?

[whaleman]

Originally Posted by sakis

he he ...the usual suspects .... why i thin that this might be caused from the "wired" capacitors?

Which ones?