Valve amp hum

Not strictly a DIY question but I though someone here may be able to help! I am suffering a faint hum in both speakers attached to my integrated valve amplifier.
Some features of the hum:

• I can only hear it when my ear is quite close to the speaker (and there is no music!)
• It sounds like mains frequency.
• It is present in both channels even when there are no other devices attached to the amplifier.
• It is not at all affected by the volume, even at max it is no louder.
• It is present whether or not the ground wire is connected in the plug
• It is present when all the valves are swapped

Any help in eliminating it would be gratefully received!

 

If you haven't done so already, well worth reading this:-
www.sound.westhost.com/earthing.htm

 

Thanks, unless I misunderstood, two feature of my hum makes me think it might not be a grounding loop:

• It is present even when there are no other devices attached to the amplifier.
• It is present whether or not the ground wire is connected in the plug

 

Ah! Couldn't see for looking

When you hear the hum, have you shorted-out the inpunts to ground?

So...
Power amp only; no source conncted, inputs shorted, speakers connected.

 

Only guessing work here, as I don't know anything about what set-up you've got.
A faint hum can be due to the speakers being very sensitive and your valve amp very powerful. Any possible mismatch here? Unless you listen to nearfield monitors, this won't be a real issue when enjoying your music. Maybe a better power cord and/or power conditioner could soften it somehow? Dunno. Try it.

Changing the pre-amp tubes to some military ones providing less gain could be your ticket against hum, if there were a speakers/amp mismatch, I mean.

Any info about your current set-up would be useful for nore specific advice.

Regards,

Vincula

 

Forgot to ask, what amp is it?

 

Quad405 , not sure what you mean by "shorted out the inputs to ground"? I did try disconnecting the ground lead in the plug - potentially dangerous I know but just wanted to try it!

It is an integrated amp, but yes I can hear the hum with no source and just the speakers connected.

Some specs...

My amplifier
Tubes: KT88×4, 6SN7×4, 12AX7×2
Power Output: 50W+50W, Class AB1
Frequency Response: 20Hz - 20KHz
Input Impedance: 250KΩ
Output Impedance: 4Ω or 8Ω

My Floor-standing speakers
Frequency Response: 20kHz
Nominal Impedance: 8 ohms
Sensitivity: 91dB for 1w at 1m
Amplifier Requirements: 25-175 watts

I did try Russian military tubes 6SN7 tubes, no difference.

 

In that case, it sounds like mains hum, possibly induced from your valve heater wiring.
Is it a DIY amp or is it a manufactured product?
If it's a DIY amp, it's most likely due to poor layout, if it's a manufactured product, it might just be poor design! i.e. if it's one of those el-cheapo Chinese amps from ebay.

 

I tend to agree with QUAD405 here, since the speakers look like a sensible match to me, at least on paper... Monitor Audio's perhaps?
Play with power cords and a bit and see whether it gets better, though I may suspect it's not that bad when you're playing music, am I wrong?

Try the different taps too, 4 and 8 ohms and check if you get as much noise. Could be an issue with the transformers as well, the most critical part of a valve amp, and the most expensive...

Regards,

Vincula

 

It is not quite an 'el cheapo' amp! Though it is Chinese/Australian and, I suppose, relatively inexpensive. Its made by Melody HiFi. A H88 ( Mark II I think).

So it is sounding like I'm stuck with the hum. Once music is playing I never notice it but I'd love to have a completely black background!
Would a power conditioner be likely help or just be a waste of money?

 

I'd go with checking the heater circuit first,either convert it to center taped with a couple of resistors,or make a regulated DC supply.

 

It certainly is a very nice amp, and Melody's build quality is usually very high. It sounds like a tour to an specialised technician of your confidence is a must. Contact the dealer/distributor.
If I remember well, your model employs point-to-point handwiring, so it should be reasonable stright to make any mods you wish. Maybe you could even upgrade some parts while at it

Regards,

Vincula

 

Take a look at the signal wires. Do they or the gnd run anywhere near the mains traffo?
I found using thicker sig / gnd wire increased the hum / made routing more critical.
Using thinner wire and routing I eradicated that hum.

From build I always had a slight hum on R.H channel and identified that problem as a cap on the gnd sig rail. Replacing that cured it.
I used a scope to compare L.H and R.H sides and found a discrepancy.

 

I'd suggest that if you can hear it from your seated position ..its not someting to worry about unless it intrudes on the music ..

check the routing of your speaker cables too !

 

I wish I knew more about electronics! A total newbie unfortunately...
I took a pic of the inside in case anyone can spot anything obvious. I had to shrink the pic right down unfortunately so it may be no use at all.
It certainly LOOKS very well built!