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hc167 3rd September 2008 06:14 PM

hum noise from my amp.

I would be greatly appreciated if anyone here can help me. I have a vintage amplifier and it has hum noise from both speaker. I replaced all the electrolytic capacitors (including the power supply cap too). and there are still hum noise. anyone have any idea why that happen? my amp have pre-out/power-in jacks. and I tried the pre-out connected to another power amp. and it was fine. So I guess the pre-amplifier section is fine. So what generally cause the hum noise other than the electrolytic capacitors???

_Attila_ 4th September 2008 07:20 AM

Does the amp hums only when a source is connected or all the time?

hc167 4th September 2008 07:22 AM

all time. volume control does not change the level of hum/buzz

hc167 4th September 2008 08:20 AM

Is it possible that the hum/buzz is coming from the bridge rectifier??

Steerpike 4th September 2008 12:44 PM

Is it a really loud hum/buzz? That could be the rectifier.
If it's a faint, rather unobtrusive hum it's posibly/probably a ground problem - a lose earth connection somewhere perhaps.

Have you added a mains earth where one was not originally fitted?

hc167 4th September 2008 07:14 PM

I think it is ground issue. I check the recifier bridge and it looked ok. and there is a switch at the back of the amp that allow me to use pre-out and main-in for preamp out and power amp input. So I switch to pre-out and pwr-in. the level of hum/buzz change.

So how do I solve the ground issue?

Yes, the hum/buzz is loud enough that if I am a meter away from the speaker, I can hear it.

Steerpike 5th September 2008 04:57 PM

Just to narrow down possible causes:
(1) with NO inputs - i.e., NOTHING except he speakers - plugged into the amp, does it still hum?

(2) does it have an earth, either separate, or as part of the mains wiring (3 pin plug) ?

If it has no earth (2 pin plug) try reversing the plug in the socket & see if that makes any difference.

Also, try earthing it - it may have an earth connection on the back - usually close to the Phono input sockets. If not, the outter/screen connection of any of the input sockets is circuit ground, and will make a suitable earth point.

N.B.: Don't make or unmake connection with the amplifier ON, Always switch it off while you are changing the plug arrangements, or you may destroy a loudspeaker.

hc167 5th September 2008 05:52 PM

I have already tried all the thing that you mentioned. I tried nothing is connected but the speaker only. still hum/buzz. I tried the pre-out to another power amp. it is fine. no hum, So I believe it is somewhere in the power amp circuit that generate the hum/buzz. For this amp, the pre amp and power amp have their own individual power supply circuit. well, anyway, I tried all the possible situation you said already and there are still hum/buzz.

Steerpike 6th September 2008 01:20 PM

It could be a flakey capacitor, even though they are new.
I put new caps in my 1990 amp, it didn't hum before, but after the cap replacement it did hum - better caps solved that.

Another tube amp I have hummed because of a broken solder joint between system ground and the chassis.

east electronics 6th September 2008 06:11 PM

there is a couple of things more
A) did you change may be also any rca input plugs ???? cause if you did there is a chance that the original was isolated from the box and yours are not.

B) when you changed the psu caps is there any chance that you made some mistake with the grounds ???? mistake can be :
1)forget to connect one of ground cables
2)connect some ground cable to the wrong place
3) adding some gound cable that wasnt there in the first place ( like for example from common of both capacitors to the box ground

C) did you maybe changed the power cord that goes to mains from existing 2pin cable to a new one three pin ???? if so ground will require other topology

D) Finally the only thing you need to find out is any of the input ground of your amp comes in contact with the ground of the box ...this will solve the problem

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