Re: Need experts for vexing hum problem


<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

I have a few observations and questions about the design and the layout. I
don't claim to be an audio expert, and I gave up using tubes about 40 years
ago, so there may be some element of "why" in the following:

1. The AC supply appears to be fed by a two-prong plug. There needs to be
an earth ground somewhere, and it should be at the same point for all
equipment enclosures. The shields for the signals should not be used for
this purpose. A separate ground wire should be used to connect the preamp
to the main amp.

2. The filaments of the tubes are fed by DC, which is a good idea to reduce
hum. But the filament supply ground seems to be connected to the same
ground as the signal reference, so the capacitor current will create some
120 Hz noise in the ground system. It would be better to have the entire
filament circuit isolated (and maybe tied to the chassis with a small RC
network so it doesn't float to a high potential)

3. I don't understand the purpose of V301A in the schematic2 for the
preamp. It seems like it is used as a constant current load resistance for
V300A, which actually provides the amplification. It is only dropping about
840 mW with 140 VDC and 6 mA. Why not a 22 kOhm 2 watt resistor?

4. What sort of output voltage is needed on R312? The +15 dB figure with no
reference indicates a power ratio of 34 times (6 mV/500 ohms), or about 2.5
mW. If it is in dBm, then it is 34 mW. The circuit can provide probably 50
VRMS at 3 mA, which is 150 mW into a 17 k ohm load.

5. What do you mean by ordering resistors to step down the scope probe
signal to the computer? You will be looking for signals in the microvolt
and millivolt range that are causing the hum, and stepping them down won't
help. And you can't just isolate the scope input with resistors. You really
need a differential input, with high impedance probes that can be placed at
nodes in the circuit to measure potentials across components to track down
the source of the noise. Otherwise, the capacitance of the scope ground
will induce noise into the circuit (if it is floating), or else it will
create a ground path. Either way, it won't help you.

6. It seems to me that this whole circuit could be made with an IC audio
amplifier and it could be powered by a 9 volt battery or maybe a wall wart
supply. But, then, I don't understand why anyone would design something
like this using tubes. Maybe there is some rationale behind using them for
high power output stages, where a "warmer" sound (IOW, pleasant distortion)
may be produced, but it seems that it is asking for trouble to use high
voltage components to amplify microvolts to millivolts or a few volts.

Good luck,

Paul



.

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