Re: Need experts for vexing hum problem

On Fri, 29 Feb 2008 09:31:29 -0800 (PST), tomg@xxxxxxxxxxx wrote:

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.


Sounds (reads) like you've tackled a lot of issues, so far. There's
always a problem with iterations, that makes it possible to
accidentally pass over or defeat solutions.

First thong to fix is hum when off. Ignore all other issues.

With hum present, disconnect all other equipment. Leave disconnected
until powered-off hum is solved. This includes your 'computer scope'.

These connections will just complicate things. Use your ears to
measure the hum. When you can't hear objectionable levels, the problem
is on the road to being solved.

trannybox - the orange wire should follow the brown wire from the AC
line socket to the switch. Twisting uses the least hardware. You are
switching the neutral wire in this photo. This could explain a 60Hz
hum when the preamp is off.

You said that you reversed wires going to the switch. This wording is
subject to misinterpretation. The live terminal of the IEC inlet
(brown wire in your trannybox photo) should go through the fuse, to
the switch. From the switched terminal the returning connection should
run to the transformer primary.I am surprised that both transformer
primary wires are colour-coded brown. The orange wire on the polarized
neutral of the IEC inlet should follow the brown wire to the switch
and return unbroken to the transformer primary. Twisting it around the
live wire uses less hardware to accomplish this routing.

This is actually how the wiring is laid out in the 'preamp1' photo,
using printed wiring, but without the polarized inlet. The 2-pin 120V
hardware can also be polarized - the wider blade is supposed to be
neutral. Old Dynaco hardware may predate this convention.

There is no ground wire connection. Assuming plastic hardware, this
would still have connected to the transformer case, followed by a lead
to the chassis of the target load, to allow for leakage current return
to safety earth. If not present, connect the transformer case to the
chassis, anyways, even from a remote plastic box.

Check the line cord phasing of the power amplifier, it should match
that of the preamp, even if line and neutral are not properly wired
into your wall socket.

The power supply schematic does not use the same transformer you are
using. The original has a colour-coded primary, a grounded
core/screen. If you have an isolation transformer, use this
temporarily for the preamp, power amp and combined set to see if
there's a difference one way or another.

Ignore other issues until powered-off hum is defeated.

RL
.

Relevant Pages

  • 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)
  • 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)
  • Re: Telephone Lines and Ground Loops
    ... you can measure about a quarter volt of AC between them. ... They do ground the phone line boxes every couple hundred feet or so, ... The answer to the unsolvable hum problem is "The Balanced Pair". ... wire grounded only at the far end. ...
    (rec.audio.misc)