Re: Need experts for vexing hum problem

On Mar 1, 12:00 am, "MooseFET" <kensm...@xxxxxxxxx> wrote:
<t...@xxxxxxxxxxx> wrote in message

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On Feb 29, 1:32 pm, "MooseFET" <kensm...@xxxxxxxxx> wrote:





<t...@xxxxxxxxxxx> wrote in message

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On Feb 29, 12:51 pm, "MooseFET" <kensm...@xxxxxxxxx> wrote:

<t...@xxxxxxxxxxx> wrote in message

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I am sorry if this is slightly off-topic for this group. But you are
the ones I trust the most.

There is a guy who has built a tube-based preamp, from a commercial
DIY kit. The preamp causes significant hum at the main amplifier's
output, even when the preamp is NOT powered, and also when unpowered
and unplugged. (It also causes hum when powered.) Other signal
sources do not cause hum when connected to the amplifier. Other than
the hum, the preamp sounds good.

Some great minds at diyaudio.com have, so far, been unable to find a
solution. The discussion-thread is here:

http://www.diyaudio.com/forums/showthread.php?s=&threadid=116374

(Please ignore my embarrassing contributions to that thread.)

Here is a link to thumbnails of the preamp's schematics, pcb-layout
diagram, and photos:

http://home.comcast.net/~garyworld/site/?/photos/

[Please note that the filaments' (DC) supply's smoothing capacitance
is much larger than what is indicated on the power supply schematic.]

The grounding scheme is quite ugly, and IMO should at least eventually
be completely converted to a proper star-grounding scheme.

But my current thinking is that if we can solve the hum problem for
when the preamp is NOT powered, then we will probably have mostly
solved the problem for when it IS powered, probably with minimal
modifications.

Thanks, guys.

- Tom Gootee

Check your rectifier, use Inductance+Cap as your filter, and Do not
overload
your final Tube.

The waving at the top of your DC supply is where you need to clean up.
Stick an AC volt-meter on DC line to check your progress. This tip
brought
to you from the Retarded Community

Thanks for the tips, MooseFET.

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

Then your suspected target is not the cause of the hummmmmmmmmm...........

But the amplifier hums when the preamp is its signal source, and
doesn't hum with any other signal source.  And it hums with the preamp
as it source whether or not the preamp is powered, unpowered, or
unplugged.

As I said, it's 'vexing'.

Whoever solves this one will be given genius/hero credit, at
diyaudio.com .

- T

Ah, I've seen this problem, your input MIC/SIGNAL ground maybe opened.  I
used to trace stereo and I've found a broken wire inside a MIC cable.- Hide quoted text -

- Show quoted text -

Thanks for the further suggestion, MooseFET!

I will pass it along. (Actually, I already posted this thread's URL
in the diyaudio.com thread. So your suggestion will be seen whether I
mention it there or not.)

- Tom
.

Relevant Pages

  • Re: Need experts for vexing hum problem
    ... The preamp causes significant hum at the main amplifier's ... But the amplifier hums when the preamp is its signal source, ... doesn't hum with any other signal source. ...
    (sci.electronics.design)
  • Re: Need experts for vexing hum problem
    ... The preamp causes significant hum at the main amplifier's ... But the amplifier hums when the preamp is its signal source, ... doesn't hum with any other signal source. ...
    (sci.electronics.design)
  • Re: Need experts for vexing hum problem
    ... There is a guy who has built a tube-based preamp, ... The preamp causes significant hum at the main amplifier's ... trannybox - the orange wire should follow the brown wire from the AC ... run to the transformer primary.I am surprised that both transformer ...
    (sci.electronics.design)
  • Re: Preamp causes hum even when off
    ... the preamp is not powered? ... It also causes the hum when unplugged, ... so when its off, but with its output connected to the power amp input, ... There should be NO direct connection of the OV buss to the case. ...
    (rec.audio.tubes)
  • Re: Preamp causes hum even when off
    ... preamp could cause the main power amplifier to produce hum, ... the preamp is not powered? ...  It also causes the hum when unplugged, ... It still causes hum even when unplugged from power. ...
    (rec.audio.tubes)