Hum in DIY preamp

[bluemonk]

I built this preamp: A Studio Preamplifier
It does hum.... even when powered OFF! When I unplug it from wall AC then hum stops.
Power supply is from two salvaged PTs wired back-to-back, and a voltage doubler. PTs are not center-tapped.
There are two ground lugs to chassis: one is the circuit star ground; the other carries the earth from wall AC supply.
Filaments run DC from the first PT.

Please help!

[bluemonk]

Any suggestion?
Please help!

[guitardude57]

How about some pix.............
Volt dub circuits can be noisy, pretty easy. Lets see what you have there.

[bluemonk]

Here's a pic: http://home.tele2.it/gtweb/IMG_0880.jpg

There is only one ground spot to the chassis.
I tried to feed the preamp by an external HV source and I got hum anyway
There's hum also when input is shorted to ground.
When I power off the preamp hum is still there and it stops only when the device is unplugged from wall AC.

Can you help please?
TIA

[bluemonk]

Here's the pic.

Excuse the double posting. The link above could be locked.

[Steve Conner]

It's probably not the unit itself that's humming, but a ground loop between the unit and the other equipment you have it plugged into.

Try disconnecting the AC ground from the chassis, but be warned this is dangerous and you should only do it briefly as a test.

Edit: Having seen your layout, it looks a bit scary... Do you have signal wiring running close to the mains wiring? Or maybe try a different colour of duct tape on your transformers

[bluemonk]

Hello Steve, I'm glad to meet you again here. You already were very helpful with my valve troubles

I disconnected the ground from ac plug and the big hum has gone away (it remains hum but as "normal" debugging).
I know the preamp can't run un-earthed.
Is there a solution?

(Yes my lead dressing is not neat as I'd like. But it seems to me that no signal wire runs close to mains wires or power supply section).

[Steve Conner]

OK, lifting the ground fixes it, so we know your lead dress is fine and the problem was just a ground loop.

Is the output jack grounded directly to the chassis? As in, it's a metal jack and screwed straight to the chassis with no insulating washer? If so, then try connecting your mains earth wire to the output jack's ring terminal. This will still ground the chassis through the jack body, but it should defeat any ground loop in the preamp itself. If it still hums, then your other equipment is to blame!

Some people "lift" the ground connection with two large back-to-back diodes. 1N5408 or similar. This allows it to float for small signals, but hopefully they still conduct and blow the fuse in the event of a fault.

Also, it sometimes helps to plug the preamp and power amp into the same wall outlet.

[Amp Kat]

Star grounding preamps is almost a must. I take the first stage and make sure I can get it hum free to the output then go down the line adding stages until you can get it totally hum free. Bypass caps and gain in general can lead to hum so make sure they are star grounded all to one point. Basically as Steve said it creates a ground loop if you don't do that..

[guitardude57]

Wherever you put your grounds.........clean the hell out of the metal with the edge of a flat screw driver or small chisel. I have one specifically for scraping ground spots or solder blobs. Then drill a new hole.....use a star type ring lug to attach ground with a screw.

Carefully remove any metal shavings, since you are dealing with a populated assembly.

Any rust in the way is bad. Including under the PT.

[bluemonk]

ALL your suggestions have been helpful. Once again I've learnt how much important are details. The ugly hum has gone. Thanks a lot.
Now I get a more usual hum and I'm going to debug it beginning from grounding path. I'll let you know hoping in further help.

[bluemonk]

I rewired the grounding path. Hum is still there even when I pull off both the valves (6N2P and 12AT7). PS if based upon a voltage doubler. Filter caps issue?
Any suggestion?
Thanks in advance.

[errnst]

i have similar troubles, i have been modifying an old conrad johnson pv3, new russian teflon caps, stepped attenuator, star ground, grid chokes, and different power supply. Everything was going great until the power supply mod.

Has anyone ever experience creeping hum, ie the thing hums, i attach some grounding, it stops for a while, comes back, i manage to solve it again, but it keeps coming back and coming back until now, it is a full volume hum with the volume turned down, with nothing on the inputs.

Any ideas will be greatly appreciated.

I started this modification process in order to learn about capacitor sound signatures, the improvement in the sound was amazing, so i got attached to it, now i cannot use it. sad

[guitardude57]

I have built doubler PS amps that have had hum issues too.

The last one was a Trainwreck Express circuit on my old integrated amp chassis (my test bed chassis....steel) to see if I liked it before building a real one.

I liked the circuit well enough, and sounds good.....except for the higher than desirable hum. I still use it once in a while since it is a fun amp.

The issue is, since it is a high gain amp, is in V1. Yank the tube, is quiet. I don't worry about it anyway........will become another circuit again after I finish a proper Express I'm in process of.

Never tried snubbers over the diodes on this type of PS..........wonder if that might help.
.oo2 1KV
What do you think?