Anyone in the L.A. area with scope willing to troubleshoot?

[phrarod]

I'm chasing a 60 cycle hum with a DMM which is obviously not going to work. Anyone in the greater Los Angeles area who might not charge too much just to tell me where the culprits may be in the circuit.

This is a super simple preamp with hardly any components so its not intense like a complex circuit or PCB.

PSU is in one box. Amp in another. Its my design not a commercial piece.

[tomchr]

Would you share a schematic and a diagram of the connections between the supply and amp? You may find the help you need right here in this forum if you ask the right questions...

My first question: Are you sure it's 60 Hz hum? Could it be 120 Hz? Does your DMM measure frequency? Is it able to measure the frequency of the hum?

60 Hz is nearly always caused by a ground loop. Other causes include inductive and/or capacitive pick-up.

Is the hum present at all levels of the volume control? Does it vary when you adjust the volume control? If so, the hum comes in before the volume control.

Is the hum present in both channels or just one? What happens if you pull the tubes associated with one channel and disconnect its signal input and output cables?

~Tom

[phrarod]

Hi Tom. I had several threads about this and the conclusion was need a scope.

Hum is the same at all levels. Identical in both channels.

I have the top Extech EX570meter It does have Hz setting.

Two box preamp amp. PSU in one. Here's grounding and PSU schematic.

[boywonder]

I'm in OC (south county) and have a scope....no problem if you want to stop by......where are you located?

[bob91343]

And I am in the San Fernando Valley.

[phrarod]

I'm Santa Monica. Thanks Boywonder for the offer! SFV would be much closer, though. Bob would you be up for it? You can PM if you want or email me.

[tomchr]

What are you using for the heater on the triode? AC? DC? If you're using AC, I'd expect some residual hum. DC should be quiet.

What's the bias current in the triode? (Measure voltage across the 120 ohm resistor).

What's the frequency of the hum on the output? You say 60 Hz. Have you measured 60 Hz or are you relying on your hearing to determine the frequency?

Also, measure the AC voltage (yes AC voltage) on the B+ with the DMM. That will give you an idea of what the ripple voltage is. I don't know your setup, but in my 300B power amp, I can hear hum in the speaker when the ripple voltage gets above a few 100 mV. My speakers are 87 dB/W*m efficient.

I honestly don't think you'll get much out of poking around in the amp with a scope. I've looked at a few of your threads, and I think participated in some of them as well. But generally, I've found them lacking in detail as far as a problem description and measured data go. That said, I've probably missed the one thread that has everything in it...

How bad is the hum? Can you tell from the listening position? What power amp are you using?

How is the rest of the system connected? Speaking of.... Did you try my suggestion in Post 2 and disconnected one channel? What happened? The reason I'm asking is that I cured a ground loop by disconnecting the input cable for one channel. This exposed the fact that there was a ground loop and I could then focus on finding said ground loop.

~Tom

[phrarod]

DC. Heater ripple is .004vac so no problem there. Bias is 12mA Plate has 200VDC.

Relying on hearing. I know what 120Hz sounds like its not that. Right, B+ has too much ripple. .3vac On the wood board with my hybrid choke/regulated supply I had it down to .045vac. Different circuit though and the regulated supply handled the ripple.

I have 96dB speakers. You can hear it from the listening position. Not when the music is on but in the quiet sections it bleeds through. Its not overpoweringly loud but noticeable enough to ruin the experience. Unless the music doesn't have any quiet sections

I think you did help in some of the threads. The first long one should've had all the measurements.

I have tried that in the past and no difference. - your comment in post 2 -- remember this is pure dual mono with separate ground planes for each channel.

[phrarod]

Appears I'm still looking for someone in the Los Angeles area. It would be great to meet another DIY Audiophile plus this pre-amp is really sounding special and would be nice to hear.

I'm sure I could return the favor in other ways.

[cerrem]

It may be helpfull to use a current proble to find this noise...
Also a 1x voltage probe would also work provided you have an LNA on the front end of the scope would be needed to see anything small...
You can also find the source of noise by putting small resistance in series with the local AC current loops in the circuit....when the noise increases...you found the source loop....
Chris