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Preamp Hum Issue

A

Albrerta

Member
Joined 2007
The preamp is aikido buffer with 6922, and the potentiometer is 100k Motorized alps, have some hum on both channel, get louder with volume up BUT disappear when reach near the max of potentiometer.

Any one has an idea what kind of hum of this? I have DC heater and main PS with SSHV.
 
Probably a hum current into the wiper of the potmeter, or into something that is connected or AC coupled to the wiper. Is there anything that can capacitively couple hum into the potmeter or the grid of the first valve after the potmeter? What does SSHV mean?
 
I've had this many times. In my case, it was RF oscillation.
Running RG174 for the signal wiring sorted it. Adding a 10k grid stopper can also help.
Changing the pot to 10k can also sort it, but you have the pot already 🙂
 
That reminds me of a really weird hum issue I once had in a solid-state preamplifier. It turned out to be due to a parasitic oscillation in a relay control circuit that worked off an unregulated supply with a substantial ripple. The relay control circuit unintendedly worked as a transmitter of amplitude-modulated RF signals, modulated with hum, that got picked up and unintendedly demodulated somewhere in the signal path.

That is, if it has something to do with a parasitic oscillation, it doesn't necessarily have to be an oscillation in the signal path.
 
Albrerta said:
The preamp is aikido buffer with 6922, and the potentiometer is 100k Motorized alps, have some hum on both channel, get louder with volume up BUT disappear when reach near the max of potentiometer.

Any one has an idea what kind of hum of this? I have DC heater and main PS with SSHV.
Click to expand...
I had a similar problem with an E88cc preamp a long time ago. It was solved by splitting the power supply;
  • each tube needed it's own crc decoupling, furthermore
  • as mentioned before, grid stoppers at each tube
  • one central ground
 
Thanks for the helps, now found the problem. I get the pot and control board off ebay, there is a relay on board for mute, the control board is powered with a tiny smps, 110v AC to 12v DC, I thought it's irrelevant, but clearly it does. Tested with 12v battery, it silent as the dead.

Just need to found out if a LM317 is adequate, or need really low noise PS.
 
That's good news.
If you have 6.3V AC somewhere, I have a little board that will turn it into 12VDC for you.
Lemme know if you're interested... 5$ for a complete plus post.
1665538012013.png
 
If I went through the hassle of building anything worthwhile out of tubes, I would never let a nasty SMPS anywhere near it. Those things cause so many issues with leakage and noise. Its almost like having your audio stuff on a dimmer circuit. Nasty nasty garbage.
 
kodabmx said:
That's good news.
If you have 6.3V AC somewhere, I have a little board that will turn it into 12VDC for you.
Lemme know if you're interested... 5$ for a complete plus post.
View attachment 1098937
Click to expand...

I always have 6.3VAC somewhere... I could use a couple of these little boards. Looks nice.
$5 + post for... complete? Not sure what that means. Would that be for the bare PCB?
 
Its a clever, elegant solution, as it puts very little if any current induced noise into the ground plane if wired correctly (input ground running directly to center tap.)
 
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