Power amp hum.

[henrymop]

Hey, it's not a stompbox, but I hope you don't mind. If so, sorry.

I have an amp I have made, but when I turn it on, it has quite a bit of hum. So, I grounded the circuit(not boxed up yet) on my big metal desk, and then I plugged in my guitar. The hum went down a huge amount, but it's still there.

My question is, how much hum should I expect from the preamp(which has a clean and drive channel) and the power amp?

Thank you.

[Electron Tornado]

Not knowing anything else about your amp, or how much hum you are getting, here's a couple of things off the top of my head:

1. Are you using a 3 prong wall plug?
2. Is the power supply well filtered?
3. Do you have flourescent lights near your work bench?

[henrymop]

1. No, I'm using a two prong, going into a transformer. (Check schematic.)
2. I have no idea. There is little pcb that came with the transformer(got it off a radio). I'll have to make a schematic and upload it here.
3. Yeah. I turn them off, nothing happens. Never does.

Here's the schematic:
http://imageshack.us/a/img809/4503/fulld.png

[Electron Tornado]

Using a 3 prong plug will help. I have a vintage tube amp that hummed. I replaced the 2 prong cord with a 3 prong cord and it's very quiet.

[PRR]

C13 is surely drawn wrong? (reverse polarity)

That's _not_ much filtering for a loudspeaker load. Unless maybe a 3-inch radio (no guts no audible hum). I'd never use less than 1,000uFd, more often 2,000-5,000uFd.

Closed metal box?

3-pin groundy power-cord really can help.

The "virtual ground" voltage-splitter has real problems when driving a low-impedance load. Especially with a distorted signal. It could be interesting to monitor the rail-to-ground voltage, idle and at full roar.

[teemuk]

Good calls on very low filtering capacitance and using rail splitters on circuits with high current draw. I would actually redesign the entire power supply section: first of all, higher filtering capacitance, secondly - since the power transformer doesn't have a center tap - I would simply half wave rectify the rails to create a dual rail supply with a solid, true ground reference.

I would also like to contribute a very important aspect: Improper noding of ground return currents can and will introduce serious hum issues.

[henrymop]

Hey, thanks for the help, guys.

As far as the third prong goes, it won't make a difference because my mains outlet is wired as a 2 prong, meaning one of the first two prongs is wired to the third, "ground" input inside the actual wall outlet. It's an old house. I would like to, though.

As far as C13 and the power amp section, I actually used a schematic from general guitar gadgets.
C13 in my schematic is C6 in this one.
Here it is:
http://www.generalguitargadgets.com/projects/13-amps/59-tda20x0-amp

PRR, I'll definatly try out a 2200uF, 16V cap, but do you mean at C13 or else were?
And, of course, I'll make a closed metal box to enclose the amp after it's done. I have a metal desk with metal drawers, so I'll hook the ground to the desk and just put the amp in one of the drawers and see what happens.

I'll go monitor the rail-to-ground voltage and see what I get. "Rail" meaning the positive voltage and "ground" meaning virtual ground, right?

Teemuk, I'll have to think what you said over because I'm a newb and I'm kindof confused at what you are saying. I'll add the "AC to DC" schematic in jsut a bit.

Thanks for all the help.

[henrymop]

Here's the Ac to DC circuit.

http://imageshack.us/a/img5/9466/actodc.png

[PRR]

> one of the first two prongs is wired to the third, "ground" input inside the actual wall outlet.

I hope you are wrong about that.

A proper 2-prong outlet, the 'neutral' connects to Ground *only* back at the fuse-box (or at the meter, depending on country and custom).

Connecting white to green/bare (US colors) *at* the outlet is not only wrong, and illegal, it is UN-safe.

[oldschoolanalog]

I implore you to not play around with anything that gets plugged into the wall unless you are 1000% sure of what you are doing.
Have somebody that does that for a living or on a regular basis help you with this. It is not only dangerous but it can kill you. As in dead.
Please re-think what you are doing and proceed with caution.
Life is too short as it is...

[reveal]

Add a fuse.