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?
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?
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.
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.
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.
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.
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.
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.
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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.
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.
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