• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

say it's a hum problem


Disabled Account
2004-09-10 4:57 pm
I don't know where exactly is the problem.If it's my amp's problem or another.
When my amp's input is grounded is dead quiet. My speakers are around 92db and I don’t hear anything.
When it is opened there’s a small hiss or fsss or whatever (I’m not very good describing these sounds) but not audible at listeners place. Not even at very close to speakers.
When I connect the interconnect cable with the preamp there’s an audible (not very loud-you must place your ear next to woofer) hum that looks like it is from a transformer interference. Say it’s sth like a bass mmmmmm. The strange thing is that this sound is the same when the power switch of the preamp is opened or closed.
When I reconnect the interconnect cable only from the preamp’s side it’s like the second purpose above. Like when the input is opened.
I thought that the problem was at the preamp. But I tried another preamp and the results were the same.

I’m a little bit confused.


diyAudio Moderator Emeritus
2003-01-18 7:57 am
Near London. UK
danFrank said:
If this is the case, then lift one of the outlet plugs from ground.

NO! To do so is potentially dangerous. If you remove the mains safety earth from the pre-amplifier's plug it will only receive its earth via the audio cables. The point of earthing an enclosing conductive chassis is that if the live mains conductor should contact the chassis a sufficiently large current will flow to earth to blow the mains fuse QUICKLY. You'd be amazed at how much current is needed to blow a fuse quickly. Ten times the rated current is nice. If you increase the resistance of the earth path by forcing the fault current to flow through the audio cables it might not blow the fuse but instead start a fire. Or, it might leave the chassis at a sufficently high potential to deliver a lethal shock.

So, what to do? If it genuinely turns out to be a hum loop you need to lift the bond between the amplifier's audio ground and the chassis. This bond is usually made at the input sockets or perhaps at the input valve. The best place for the bond is at the most sensitive point, so it should be made at the input of the phono stage, in the pre-amplifier. Thus, the bond that needs to be lifted is at the input of the power amplifier. Once lifted, it can be a good idea to insert a 100 Ohm resistor in parallel with 10nF. It won't cause hum, but it will prevent hum if the input to the power amplifier is removed.


Disabled Account
2004-09-10 4:57 pm
Thanks guys!
Nice answers.
Sounds like a hum loop to me. Unplug the power lead to the pre-amplifier and see if the hum goes away. If it does, it's a hum loop.
:angel: It's the first thing that I'll do.
Check to see if you have more than one piece of equipment in your system that has a 3-prong (grounded) mains plug.
All of my equipment has a 3-prong mains plug.

I'm going to unplug them and see. :smash:


Disabled Account
2004-09-10 4:57 pm
:( :bawling:
Only the amp is ON with all the other devices unplugged.
And nothing changed.The hum is still there.

Now I saw something different that maybe will help to solve the problem (hey,sorry about my bad english).
When I unplug ONLY the left or ONLY the right rca from the preamp the hum is gone at both channels! :)
But :confused: where to look at?
At the preamp or at the amp?
All the mains lead of the other devices are unplugged.
So the problem is somewhere else.


Disabled Account
2004-09-10 4:57 pm
Wait a minute; is the amp now humming with nothing attached to it's inputs?
No,when there's nothing attached to amp's inputs there's no hum.
I'll try to explain it more clearly.
From the power outlet on the wall there's attached a multi-outlet where I plug all the devices (turntable,phono,preamp and amp).
Now I have unplugged all the other devices and on the multi-outlet is plugged ONLY the amp.
The hum is presented when both interconnect cables are attached on the preamp.Of course the preamp is OFF cause the power lead is unplugged from the multi-outlet.
When I unplug ONLY the L or ONLY the R channel (interconnect cable) the hum is gone.Just like when both are unplugged.
The hum is there when both channels are plugged.

Notice that the hum is the same if there's nothing in the inputs (NOT the outputs) of the preamp or is attached the inerconnect cable from the phono.

I don't suppose some of the amps are on another power outlet? Ohmic loss and parasitic pickup inside the walls can cause ground loop. I make sure all my connected amps are plugged into the same power strip.
My listening room has two more power outlets.
I use them with multi outlets for my PC and some other devices like my guitar amp.I'll unplug them to see what will happen.
Do you believe my problem is from there?
Sounds like there is an internal ground loop in your power amplifier. Take a look at how the right and left channel rca input jacks on the amplifier are grounded. They should run to the same point which preferably should be a star ground where all of the amplifier's grounds are connected, if this is not the case lift (unsolder) the ground connection to one of the input jacks and solder a wire between the two input jack grounds. Alternately if you are very intrepid you can rewire the amplifier with a star ground.

It is possible with proper star grounding technique to eliminate the audible consequences of ground loops - almost all of my components use 3 wire cords and are grounded. No hum..

Good luck..