tube amp question

[terriblepaulz]

I am the recent owner of a DIY tube integrated built by a friend. It uses 2 6SN7s for input, and a single 6BX7 (or 6BL7) for output. Pics:

Picasa Web Albums - hagbard - Tube Amp

Here's my issue - there is a very slight hum audible with my Grados (I don't notice it with my higher impedance Beyers). I should note that this is the second iteration of this amp from the same builder. The first one had a significant hum (along with occasional buzzing), which the builder explained was caused by the cheap parts he used in too small a chassis (he told me he threw it together in an afternoon with parts he had laying around). This version is much quieter with only the barest hum mentioned above. Of course I don't notice the hum when music is playing, but I suffer from the audiophile problem of knowing it's there.

I'm not out any money on this amp, so this situation won't kill me, but if there are any simple tweaks or tips I can employ to eliminate the hum, I would be interested. Any suggestions?

[Dusty Chalk]

An RKV Impedanzer or equivalent. What this does is "show" the amp a higher impedance. Someone can probably draw you up a circuit for it, but I think we established it's just a couple of toroids.

Or just listen to your Beyers. (Or find a higher impedance pair of headphones whose sound you enjoy.)

Or get another amp.

[Pars]

Wiring proximity to the transformers can be a cause of this, usually the input wiring, pot, etc. On this amp, it appears that the wiring from the input PRCa comes around the left side of the chassis (behind the board or terminals with caps on left amp wall) to the pot, which is about as far away from the transformers as you can get in this. The output jack is very close to a transformer however.

[cetoole]

Looks like a pretty sweet amp, great gift. There are some things you could try to get rid of the hum, such as increasing the power supply filtering, but just casually looking at it, it would seem your friend used a reasonable amount of capacitance in the supply. No idea how many poles he has on the filter, but maybe he would be the better person to ask. Do you have any sort of schematic that this amp was built to follow? The other issue could be emi from the transformers, especially since it looks like the one toroid is pretty solidly over part of the audio circuit. Is the hum the same in both channels?

[terriblepaulz]

Looks like a pretty sweet amp, great gift. There are some things you could try to get rid of the hum, such as increasing the power supply filtering, but just casually looking at it, it would seem your friend used a reasonable amount of capacitance in the supply. No idea how many poles he has on the filter, but maybe he would be the better person to ask. Do you have any sort of schematic that this amp was built to follow? The other issue could be emi from the transformers, especially since it looks like the one toroid is pretty solidly over part of the audio circuit. Is the hum the same in both channels?

No schematic, but he said it was based on an Eric Barbour design from a back issue of Vacuum Tube Valley Magazine. The hum is equal in both channels. I should emphasize how low this is. Right now I am having difficulty hearing the hum over the ventilation system and the fluorescents.

[AlanY]

It's hard to get rid of all hum beyond -85dB or so in DIY tube amps, especially those where the power supply and amp sections are in the same chassis. If you can't hear it above ambient noise, it may be in the -75 to -80dB range.