120hz preamp hum

[mannytheseacow]

I built this schematic as a preamp with a 100k volume pot:
https://www.decware.com/newsite/diy/decware/ZKIT4.html
It’s pretty simple, and has a 120hz hum that increases with volume.
I’ve rewired the ground a couple times but to no avail. Any ideas?

 

[Osvaldo de Banfield]

Schematic needed porfi (please).

 

[mannytheseacow]

The schematic is linked in my original post. The power supply is the tube regulated option.

 

[Osvaldo de Banfield]

Did you grounded almost one pin of the heater wiring?

 

[rayma]

Is the volume control at the input of the preamp?

I would suggest that you first replace the 0C2 tube with the 33k 2W resistor instead.
The OC2 is not regulating, but rather is simply dropping some DC voltage.
But the OC2 directly couples the 120Hz rectifier hum to the B+, which is not usually a good thing.

If that does not resolve the hum, and there is a good chance that it will help a lot,
then there is likely to be a ground loop problem.

 

[mannytheseacow]

I did put the volume on the input.

 

[mannytheseacow]

I think it is ground loop since it is 120hz hum and not 60hz hum. I cannot locate a better grounding scheme though.

 

[rayma]

Post #5 describes how 120Hz hum could happen without a ground loop.
Try that first. Just pull the OC2 and tack in a 33k 2W on the socket pins.

It appears that "regulated power supply" schematic is goofed up really badly,
and does the opposite of what it is supposed to do.

 

[mannytheseacow]

I’ll try that. Thanks.

 

[rayma]

Here's what the regulator circuit should look like. The resistor value may have to be reduced.
However, these tubes are usually kind of noisy (wideband, not just hum).

 

[mannytheseacow]

Nice. Thank you. I don’t have a 33k on hand but will in a few days.

 

[rayma]

I would think the resistor will have to be more like 4.7k at 2W for the OC2 to function properly.
The given 33k value will probably not let it fire up, so it would not regulate at all with the 33k.

 

[mannytheseacow]

There is a note at the bottom of the schematic to load pin 2 with a 10k resistor, which I did. I had hum before and after adding the 10k resistor.
I initially used a 75c1 without the resistor. It worked but hummed. Then I tried a 0C2- wouldn’t stay lit without the 10k resistor. Same hum either way.

 

[rayma]

Ok, adding that 10k will let the OC2 fire, but it still cannot regulate, because it is connected completely wrong.
In that circuit, it actually works WORSE than a simple series resistor

 

[pinholer]

Where is your audio ground point? It should be taken from the negative terminal of the last power supply filter capacitor. The are high ripple currents at the ground point where the first filter capacitor and rectifiers are grounded. Move the ground point for the audio circuit away from this point.

 

[Osvaldo de Banfield]

I insist on grounding one side of the heater. The schematic doesn't indicate it, and both cathodes aren't decoupled, so any leackage from floating heaters AC powered will infiltrate in the audio path. I suggest again to ground one side of heater wiring, and in extreme case, elevate the heater path about 20/30 DCV over ground.

 

[mannytheseacow]

Pinholer solved it for me. I moved the audio section ground to after the power supply section and the hum is fixed. Now I’ve got another amp with a similar problem and I think I know how to fix it!
Thanks.

 

[mannytheseacow]

Osvaldo, the schematic doesn’t show it but I did have pin 9 grounded. I believe that is the center tap and grounds the heater.