2. tech-talk
  3. 1191763173

Hum- help needed

It's been very strange and I cannot think of a logical explanation.
It will require all your attention.
1. Two mono blocks plugged in. EVERYTING ELSE IS UNPLUGGED,
ICs disconnected- hum in the R channel (can be heard from
2-3 ft. away)
Hum in the L channel- order of magnitude lower, I would consider it "normal", can only be heard with the ear to the driver.
So I would think, it's not a ground loop (nothing else is plugged in, heard in R ch. only.
2. R Amp. moved to the L speaker (other amp unplugged altogether)- no hum.
L amp. (used to be quiet), connected to the same L speaker, with the same power cord- hum.
3. All tubes switched L to R- no difference.
4. R amp moved back to the R speaker (position, where it used to hum- no hum.
5. L amp (used to be quiet)- hums, no matter what.
To sum it-up:
I think it's not a ground loop (see #1),
It's not the amp.- (see # 2),
It's not a speaker- (see # 2),
It's not the tubes- (see # 3)

So, here is my question- what the hell is it?
maril555
wwshull
157 posts
 
if you can only hear it with your ear to the driver i would not worry about it. i have the same problem and it used to drive me crazy and i have spent days and hours trying to figure it out. i have been told by my stereo person with 20 plus years of experience that it does not hurt the sound and not to worry aobut it. i have finally taken his advice and now just enjoy my systme
zenblaster
504 posts
 
Try an outlet on a different circuit (not just another outlet))
tvad
9,467 posts
 
You don't mention if you moved the L amp to the R speaker, but I suppose you did.

Do you have a halogen lamp nearby? It's possible a lamp transformer is interacting with one of the amps, but not with the other.

Do the L and R amps use two different AC outlets, or do they share one outlet?

It looks like you swapped power cords, yes? Maybe the ground is disconnected on one of them?

Tubes may be the problem, but each tube needs to be swapped one at a time rather than the whole set at once. Just in case you didn't do this. I did notice that in your sequence, the hum shifted from the R amp to the L amp (number 5 in the list) AFTER you switched the tubes from L to R. Did you swap the tubes back to their original amps before observing number 5 on your list? If not, I'd say it could be a tube...and probably a power tube.

My system in the past has had a hum with nothing connected except the amps, but then the hum went away when ICs were connected. Especially in the case of phono ICs not being connected.

Hum is a PITA to track down. Good luck.
maril555 OP
618 posts
 
Shadorne- amps are VAC PA 90C,four chassis mono blocks.
WWShull- the "quiet" one can only be heard with the ear to the driver, but the other one is significantly louder.
Zenblaster- good point:
I forgot to mention, when I took the amps to my technician- they are both dead quiet.
I moved "humming" amp to another floor, effectively different circuit in my own home- quiet.
Tvad- L and R amps were connected with the same power cord to the same outlet.
I simply physically moved "humming" R amp. to the L speaker and hum dissapeared,without changing anything else, but when the same power cord reconnected to the L amp. and the same L speaker, the L amp began to hum.
It hummed BEFORE and AFTER I swapped the tubes, and hummed again after tubes were in the original position, in other words, tubes swapping didn't affect the hum.
tvad
9,467 posts
 
It hummed BEFORE and AFTER I swapped the tubes, and hummed again after tubes were in the original position, in other words, tubes swapping didn't affect the hum.
Maril555 (System | Threads | Answers)

Understood. Good luck.

FWIW...when I have had a hum in one tube amp and not in the other, it was because either a power tube required re-biasing, or a power tube was bad.
More to discover