Valve Headphone amp - mains hum

[ReggieB]

I've been doing a lot of commuting recently and as a result have been listening to a lot of music via headphones. I've been finding this very enjoyable and as a result have been upgrading my home headphone system.

My most recent purchase was a Ming Da Valve Headphone amp. I've been blown away by this amp. It is a great upgrade from my Creek OBH-11.

I've connected it to the power amp output of my Audiolab 8000s, so that it acts as a power amp with the Audiolab providing source selection and volume. This allows me to use my speakers (KEF iQ30) when the Audiolab is switched to integrated mode, and headphones (AKG K701) when it is switched to pre-amp mode.

The detail and imaging is great. However, it is the subtly of tone that most impresses me. Nuances in singers voices are apparent that I have not heard before. Intonations in voices have become much clearer bringing extra character to singing. Silly things like being able to hear exactly what the words are that are being sung, are common surprises that bring a smile to my face.

It is easily the best sounding set up I have owned .... with one exception.

There is a low level hum, that to me sounds like mains hum. Once the music starts, I can no longer hear the hum. It is only really obvious between tracks. Once the music starts, the sound is so good that I am happy to live with a hum in the very quiet sections.

This is my first valve amp. Is a low level hum a common artefact of valve amps or is this indicative of something wrong?

[Ali Tait]

Could be a lot of things, including "that's just what it does"

First thing I'd do is try different valves.

[worrasf]

Is it both channels? Is it with all inputs or perhaps just the phono input. I had this issue with my self-build SOHA II valve/hybrid headphone amp - tried everything to cure it. In the end it was a slightly iffy earthing to the volume pot - I soldered a secondary lead and it's as quiet as can be. Not suggesting you do this with yours but it supports Ali Tait's suggestion that "it could be lots of things"

FWIW if its both channels it's unlikely to be a valve whereas if its only one then it could be. Check and re-check all your cables - particularly earth - try disconnecting the input and output cables and see if it still hums. If it does then its an issue with the amp or its earth.

Trial and error I'm afraid.

Steve

[goraman]

My first inclination would be a ground issue, but be aware that tube amps can have far fewer parts than a solid state amp and less capacitors to filter out mains noise.
That's why the quality of the parts in tube audio is very important.
Here in the U.S. we often use a adapter from 3 prongs to 2, eliminate the ground pin.
More often than not it solves the problem.
A test you can do is to take a couple RCA phono plugs solder them shorted so there is continuity from the center pin to the outer return of the jack.
Hook them up to the input going into the headphone amp so there is nothing else but the mains plug,the RCA jacks on the inputs and your headphones.
If the noise is still there it is most likely not a ground problem.
At that point it could be an unsheilded signal wire in the amp to close to the transformer.
You could also try a ferrite power cord,I use PS Audio on all my gear.
They do quiet the power coming in noticeably.
But the cord may cost as much or more than your amp.
Personally I would have chosen the Little Dot over a Ming .

[ReggieB]

Thank you for the feed back. There is a local chap advertising valve amp repairs, so I think I'll drop it round to him.

I have to admin to being swayed toward the Ming da after reading a couple of favourable reviews of their amps in Hi-fi world.

I'm like the amp a lot, so if the hum can be cured, I'll be chuffed to bits.

[ReggieB]

I've got my amp back this weekend, and am currently listening to Britten's War Requiem through it (with the help of my K701s). Splendid. The hum is gone, but all the detail and life are still there. I am very pleased.

I took the amp to AmpLabs which is less that ten miles from my home. Rob who worked on my amp, said he started by replacing a capacitor, but that only had a very small effect. He then worked on shielding the power supply, and that's what had the biggest effect. He fitted three interlocking steel plates around the power supply. He also stated that he was surprised that a toroidal power supply had not been used considering the sensitivity of the system.

The hum is now inaudible to my ears. Rob said the hum is still visible on his oscilloscope and if still a problem there were a couple of further tweaks he could try. However, I'm very happy with the result as is. Splendid job.

[Rare Bird]

Ive had three tube Headphone units, all have a slight amount of low level hum.

[Canetoad]

Yes, mine does.