Help with preamp noise/hum

[Brice]

Hi guys,

I am working on an Acro S1001 preamp, and I am facing terrible noise/hum with it.

I located where the noise is coming from: the tone control.



Here are the symptoms:

When the balance it to ground: no noise at all. Dead silent.
When the volume is at ground, noise is present.
If I touch the chassis, which is grounded, the noise is greatly reduced.
When the volume is at max, the noise is not really changed.
When I approach my finger to the tone control resistors/caps the hum gets louder and louder. I don't have to touch them just getting close to it. I checked and the pots are grounded via the chassis.
If I ground the grid pin 7 of the first triode, no noise. It's like the tone control acts as an antenna.

So I looked at the classic Baxandall filter and this one is kinda strange, especially the ground return, going through the balance pot and the enormous value of the pots.

I understand that if I put back the cover, the noise will most likely be reduced but I think there is another underlying problem here.

Do you guys have experience of this kind of problem?

Thank you.
Brice.

 

[BinaryMike]

Looks like a conventional active tone control to me, except for the unusually high impedance, which probably was meant to minimize loading on the volume control wiper.

I'm guessing the problem will be something open-circuited in the tone control section. Does it perform normally but for the hum?

 

[Brice]

Yes for the highs.
The bass at max make the sound bloated and distorted.

 

[gadget73]

Possibly the bass turnover frequency is too high, giving muddy upper bass at an excessive level. Some tone controls are just poorly done. Without a frequency response plot its hard to judge. Also, off-value parts will skew how this acts, pots included. Those high value pots and resistors are somewhat prone to drift too.

The very high impedance will make it prone to hum pickup from less than ideal lead dress, lack of grounding to the pots, or just poor alignment of the moon and Mars. Moving the leads around, maybe adding some shielding, and being very careful about where the grounds are connected may help, but without knowing exactly how the thing was constructed its hard to be more specific.

 

[Brice]

Thanks Gadget for your insight.
Here is the tone control on the right. Since then I replaced all components and I think that makes it worse.
Re-designing the baxandall with lower impedance would help?
I might have too much attenuation then for reaching full amplification after the first triode.

I now understand why on one of them (prototype? ) it has a 7199 there instead of a 7247.

 

[BuzzK]

Sure sounds like an open pot or bad ground to me.

Check the solder joints. Check the pots for continuity and proper function with a VOM.

If that doesn't isolate it, check the initial components in the circuit.

 

[Brice]

Thanks for your input.
I'll review it and report.