Diagnosing hum???

[jstan]

I just posted this in the chip amp forum, but thought I should also pose the question to the tube crowd. I recently completed a tube buffered chip amp using Joe Rasmussen's design. It sounds great except for the hum. I'm getting a constant hum (probably 60hz, but I don't have a scope) that is way above what could be considered normal or acceptable to even the least demanding listener (I'm getting about 9-10 mVAC with no input). The hum does not vary with changes in volume, connecting or disconnecting sources, shorting the inputs, or touching the chassis. The heater is supplied with DC and the power supply is a separate unit attached via a 3 foot umbilical. Any ideas as to what could be going on here or things to try?
-Jeff

[jstan]

Here's an interesting update...I've been poking around with my meter trying to find AC in the amp. I had my meter in frequency mode to see if I could get a reading between ground V+ for the power amp section. When I ran the meter between V+ and ground, the hum magically disappeared from that channel. I repeated on the opposite channel and got the same result. My first instinct was that I was shorting the power to ground through the meter and basically just de-powering the amp, but when I tried this with music playing, the volume remained. What does this mean?
-Jeff

[aardvarkash10]

Sounds like you have a small component of ripple on your B+ supply. The buffer in your meter is bypassing it.

Try putting a small value snubber cap across each of the diodes in the power supply and a snubber on the main filter cap...

[ChrisA]

Originally Posted by jstan

Here's an interesting update...I've been poking around with my meter trying to find AC in the amp. I had my meter in frequency mode to see if I could get a reading between ground V+ for the power amp section. When I ran the meter between V+ and ground, the hum magically disappeared from that channel. I repeated on the opposite channel and got the same result. My first instinct was that I was shorting the power to ground through the meter and basically just de-powering the amp, but when I tried this with music playing, the volume remained. What does this mean?
-Jeff

I bet the trouble is that 3 foot cord. The last filter cap needs to be physically close to the load. Add another cap, maybe 0.1uF or 1uF From the plate load resistor to ground.

AC may also be getting into the DC that supplies the chip amp. Watch the high voltage secondary transformer leads. Keep them tightly twisted from the time they leave the transformers to when they connect to the rectifier. They can radiate, So can the heater leads before they are rectified

Pull the tube from it's socket. If you still hear hum then it is com
ming from th chip amp part of the amp, likely through the DC power.

[jstan]

Thanks for the suggestions. I'll try these ideas out tonight.

[jstan]

So after trying snubber caps across the diodes and additional filter caps closer to the load with no improvement, I began what I'll call 'desperation experiments': random rearrangements, pixie dust, magic incantations and the like. To my surprise, one of these experiments solved the problem and the amp is absolutely quiet now. I had all of the grounds connected in a star in the ps, running through a single wire in the umbilical, and spreading out again via a star in the amp chassis. When I separated the tube ground from the ss ground, the hum disappeared. The arrangement now has the tube ground (grounds for heater and tube power) running through its own wire in the umbilical, and the ss grounds (grounds for the chip amp power) connected to the chassis and running through the braided shield of the umbilical. I have no idea why this matters, but I won't complain. Still, if anyone can explain why this eliminated the hum, I would appreciate the education. Thanks for all the suggestions.
-Jeff

[Arnulf]

http://en.wikipedia.org/wiki/Ground_loop_(electricity)