Hum from Power Supply?

[bluemonk]

I'm still in trouble with hum in DIY preamp (previous thread is here: http://music-electronics-forum.com/t17650/).
I found that hum goes totally away when feeding the preamp with an external PS. So I believe it depends on inside PS.
Components bypassed by the external PS are: two back-to-back salvaged transformers and a voltage doubler (with 33uf caps).
Can be a transformer be bad and noisy? Should I try different filter caps first?

Thanks in advance.

[loudthud]

You mentioned that there were no center taps on your transformers. You need some kind of ground for the heater supply. Pick the minus side of one of the caps and ground it if there isn't a ground somewhere. You can't let the heater supply float, it will pickup common mode noise from the transformers unless they have internal shields.

Did you build the power supply like the one in the article? Did you leave out any filtering like R1, C7, R2, C9? It's really a voltage tripler, it needs the extra filtering.

You mentioned that you get hum even when the preamp is turned off unless it is un-plugged from the line. This suggests to me that your power amp has the HV CT tied to a transformer bolt. It's going to hum until you fix that. A bunch of spade lugs on a transformer bolt is not a good star ground.

[txstrat]

You mentioned that you get hum even when the preamp is turned off unless it is un-plugged from the line. This suggests to me that your power amp has the HV CT tied to a transformer bolt. It's going to hum until you fix that. A bunch of spade lugs on a transformer bolt is not a good star ground.

That's quite interesting. I usually use a PT bolt for my star grounds too. What would you suggest is a better way?

[bluemonk]

Pick the minus side of one of the caps and ground it if there isn't a ground somewhere.

First of all many thanks for you attention.
Heaters supply runs on DC. Do you mean the minus side of the cap used to filter heaters suppy?

Did you build the power supply like the one in the article? Did you leave out any filtering like R1, C7, R2, C9? It's really a voltage tripler, it needs the extra filtering.

No, I didn't. I built the PS as shown in PSU Designer II, as a voltage doubler.

[loudthud]

Any time you try to pass a single ended audio signal between two pieces of equipment that each have a power line safety ground, there is a very high likelihood that you will pickup some hum. If there is any small voltage difference between the safety ground and the signal ground, it appears in series with the signal passing between the two pieces of equipment when they are connected. In the case where one piece of equipment doesn't have to even be turned on, it would seem that the other piece of equipment has a small voltage difference (a millivolt or two) between the safety ground and the signal ground and connecting the two grounds injects the hum into the signal path.

The most common way to remedy the situation is to disconnect one of the safety grounds. This violates most safety regulations that I am aware of and I do not endorse or suggest it. Another solution mentioned by Steve is to insert back to back diodes in series with one of the safety grounds. This seems like a good solution but I haven't read anything definitive on how it is viewed by safety agencies.

Star grounds usually fall short of the ideal. There cannot be any other connections to the chassis with the possible exception of the safety ground and any shared ground wires can cause problems. If you are using a stack of spade connectors, the order in which they are stacked will make a difference. Try stacking them with the input jacks closest to the chassis followed by a progression of the grounds of the various gain stages with any decoupling caps, then the rectifier and/or transformer main filter grounds.

Heaters supply runs on DC. Do you mean the minus side of the cap used to filter heaters suppy?

Yes, one of the caps C10-C14 or ground it where shown on the schematic. You could use an artificial center tap, a resistor from each side of the AC to ground. 100 ohm 1/2W resistors if it's 6.3Vac, 150 ohm 1/2W if it's 12.6Vac.

[bluemonk]

You need some kind of ground for the heater supply. Pick the minus side of one of the caps and ground it if there isn't a ground somewhere. You can't let the heater supply float.

Quiet true! Hum has gone! Thanks a lot.

Another issue: I have too much gain.
Perhaps is it because I replaced 12ax7 with 6N2P (heaters rewired)? It shouldn't, I think.
Can I move the volume control to input in place of output?
Other suggestions?

[loudthud]

IIRC the 6N2P wants 6.3V between pins 4 and 5. Pin 9 is used as an internal shield. Is that how you have it connected, I don't have a spec sheet on that tube.

The preamp's gain is determined by resistors in the feedback network, probably not wise to change anything there. Try moving the volume control to the input.

[bluemonk]

I believe it is not simply too much gain.
Sound is somewhat distorted even with very low input volume and it becomes more distorted when volume goes up.

I'm confused because it WAS NOT distorted BEFORE.
I checked for oscillation moving leads. Replaced tubes. Used an external PS. No results.
Might some component be bad?
Any suggestion?

[loudthud]

It could be a bad component or the power supply. A weakness of the circuit is that it doesn't adjust it's bias in relation to the power supply, it's more or less fixed by the neon bulbs. What is the voltage on pins 6 and 8 of V2B (use the schematic as a guide of you have used different pins) ?

[bluemonk]

I read these values:
V2b - pin 6 = 365V
V2b - pin 8 = 300V

[loudthud]

You mentioned using an external power supply. Were the voltages any higher when it was used? The schematic says pin 6 should be 430V. Any way you can get the B+ higher to see if the distortion goes away?

[bluemonk]

I've tried the external supply I used in past. It gives heaters' supply and HV to FIRST filter cap. I've read 410V at pin 6. But it doesn't fix distortion.
Is there some clue?

[Steve Conner]

Maybe try a grid stopper resistor on the first stage? Nachbaur's schematic doesn't show one, but maybe the 6N2P is more prone to oscillation than the 12AX7.

[bluemonk]

Tried a 33k grid stopper and it didn't make a difference.
BTW I came back to 12ax7. Also tried 5751+12au7: got less gain but not less dirt tone.