Re: Need experts for vexing hum problem

On Sat, 1 Mar 2008 19:24:45 -0500, "Paul E. Schoen" <pstech@xxxxxxxxx>
wrote:


<tomg@xxxxxxxxxxx> wrote in message
news:e9faffa5-c53a-4bee-965a-c6fcff75eb10@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx
On Mar 1, 12:26 pm, legg <l...@xxxxxxxxxxxxxxx> wrote:
On Fri, 29 Feb 2008 23:28:49 GMT, Rich Grise <r...@xxxxxxxxxxx> wrote:
On Fri, 29 Feb 2008 10:10:49 -0800, tomg wrote:

But none of that would explain why it also causes hum when it is not
even powered on.

Your preamp has a loose ground on its output, and is acting like an
antenna.

Yes, it really sounds like a lousy patch cord with an open ground on
one end. - the extreme case of 'ground loop'.

RL

Hi RL,

Some good news!

I posted your message in the diyaudio.com thread. But, by the time I
got there to post it, I found the message below, from the preamp's
owner/builder

- Tom


------- QUOTE:

Great News...

It would seem the star grounding has worked; the 120 Hz hum is now
absent from the preamp out and amp out when preamp is off. There is
still 100mV 60Hz hum when they are on but I see this as an important
victory and feel very happy. It was previously theorized that this hum
was the sum of several issues possibly compounding each other.

Here is a pic of the current state. Once again, thanks for the
constant support and advice:

http://home.comcast.net/~garyworld/preampmarch1.jpg

Here is a close up of the starground:

http://home.comcast.net/~garyworld/starground.jpg

Please let me know if this can be improved to make it final.

Here is where you guys are going to get annoyed with me...I made the
grounding changes prescribed most recently by megajocke. I tested the
system without a successful outcome and proceeded to install the 4
(replacement) output caps and scrub all the pcb connections with
alcohol and toothbrush. Then I realized that the rca output cable was
still plugged into the old, now floating, output jack. So I can't
conclude which one of the three changes caused the improvement. I'm
assuming it's the ground scheme. Mark, I'm sorry about this. I will
gladly replace the old caps if you are interested in a conclusive
answer. Let me know.

Divide and conquer. Tom, I'm going to order resistors to step down the
scope probe signal to my computer. What is the next step?

You guys are the best. Talk to you on Monday.

gary

ps, I cleaned up the picture site:
http://home.comcast.net/~garyworld/site/?/photos/

I will open up the tranny box and take pictures, I get the feeling
there are some improvements I can make with the fuse and switch
wiring.

------ END QUOTE

That is good news.

120Hz characteristic ripple is more likely in the powered-on circuit
that the powered-down version. The fact that the frequency didn't
change to predominantly 60Hz in the PC Scope waveforms in the
powered-down scope plots suggests that it originates in another piece
of operating equipment.- like the PC Scope itself.

RL
.

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