Re: Preamp causes hum even when off

On Feb 29, 2:15 pm, PhattyMo <Phatt...@xxxxxxxxxxxx> wrote:
t...@xxxxxxxxxxx wrote:
Can anyone shed some light on possible reasons that a tube-based
preamp could cause the main power amplifier to produce hum, even when
the preamp is not powered?  It also causes the hum when unplugged, as
well as when powered and in use.  Other signal sources do not cause
the main amplifier to hum.  Other than causing the hum, the preamp
sounds good.

Possibly a ground loop..
Does it hum with all of the sources connected to the preamp?
Does it go away if you disconnect the sources one by one?  Perhaps it
only does it when,(say)the CD player or TV/DVD is connected.

Is the internal wiring in the preamp in good shape?
The signal carrying wires are all shielded,etc.?
Might be poor shielding,but probably not likely.

A couple cents. :-)

Thanks, PhattyMo.

Only one source was connected at a time. Only the preamp caused hum.
It still causes hum even when unplugged from power.

The internal ground-scheme was (and still is) pretty ugly. But now,
at least, all circuit and i/o grounds connect to the chassis through
only one point, with a 'safety disconnect' network between that point
and the chassis, consisting of 10R, 0.1uF and two anti-parallel
diodes, all in parallel. Only the earth ground connects directly to
the chassis, which is steel by the way.

Internal (and external) i/o wiring is all shielded.

Note that it is not my preamp. I'm just trying to help out. It was
built from a commercial DIY kit.

Below is a link to thumbnail links to schematics,
layout diagrams and photos, etc:

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

The upper-most thumbnails are the 'originals'. There are some farther
down that may show modifications, or proposed modifications, scope
traces, etc.

[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 was/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, possibly with minimal
modifications.

*****

There is an ongoing discussion thread about this preamp's problem,
where, so
far, some great minds have not found a solution, at:

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

(Please try to ignore my sometimes-embarrassing contributions to that
thread.)

Thanks again, PhattyMo

- Tom
.

Relevant Pages

  • 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, ... so when its off, but with its output connected to the power amp input, ...
    (rec.audio.tubes)
  • 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
    ... the preamp is not powered? ...  It also causes the hum when unplugged, ... It still causes hum even when unplugged from power. ... It would seem the star grounding has worked; the 120 Hz hum is now ...
    (rec.audio.tubes)
  • Re: Need experts for vexing hum problem
    ...  The preamp causes significant hum at the main amplifier's ... output, even when the preamp is NOT powered, and also when unpowered ... trannybox - the orange wire should follow the brown wire from the AC ... Ignore other issues until powered-off hum is defeated. ...
    (sci.electronics.design)