Reducing Hum

From kurt–(at)–r.hp.com Mon Jul 15 18:12:48 CDT 1996
From: kurt–(at)–r.hp.com (Kurt Strain)
Newsgroups: rec.audio.tubes
Subject: Controlling preamp hum
Date: 15 Jul 1996 15:43:36 GMT

This is meant to be hopefully helpful guidelines for reducing hum in
phono or even line level preamps. Hum has two major categories besides
60 Hz and 120 Hz varieties: hum from pickup of nearby sources, and hum
>from pickup of small amounts of nearby sources and amplified through
feedback mechanisms to very high levels. The first kind will be
present in various changing degrees, while the second will transition
rapidly from loud to nonexistent.

A list of things you should look for in reducing the chances of hum:

1. Shield and make short the signal paths – including feedback.
Use coaxial cables, and use a metal enclosure with no AC produced
from inside this enclosure – external DC supplies can help.

2. Place the output of the preamp signal chain as far away in a direct
line as the input of the preamp signal chain.

–> V1 >—-> V2 >—> V3 >–

Don’t let the high output of V3 get fed back through the air to V1
input.

3. If paralleling triodes for reduced noise, etc, then separate at least
the cathodes. You need to let the biases set independently or you
make an unstable condition that could move easily with small amounts of
electromagnetic fields present, most likely hum.

4. Keep the plate and cathode circuit wiring and elements away from the
grid circuit. This is a potential feedback path for hum to grow.
If wires must cross, try to keep them at right angles to avoid
electromagnetic coupling – cases being from plate to grid or output
tube to input tube.

5. I highly recommend using a PC board with a ground plane if possible.
This is very effective at shielding all components and making a
good ground.

6. Ground, as best you can, the power supply negative terminals (including
the grounded end of the heater supply) to the
other circuit ground potentials in the same point. Ideally, this
would make all paths fan out like a star, and it is called star
grounding. In this way, hum currents don’t develop into hum voltages
in other parts of the circuit and amplify up and cause trouble.

7. Make sure one of the heater filament voltage electrodes is tied to
ground. Stay away from a raised heater potential to allow high cathode
potentials – they can be trouble in helping to cause hum.

8. Watch for heater filament currents inducing hum into the rest of
the circuit. I recommend shielding filament wires by using shielded
twin pair or coax with outer conductor grounded. Even DC filaments
can have large 120 Hz current ripple. Keep filament wires short,
not looping around, and away from the rest of the preamp circuits.

8. Lower the input resistance, and lower the resistors in the circuit a
little to help reduce a bit of hum, if necessary.

9. Work back in the circuit when building and testing it – output stage
first. Check for low noise and hum each time. Wiggle wires to find
minimum coupling of hum. When one stage works well, build the other
one as closely as the first.

When all else fails, buy a nice preamp. 🙂

Kurt

From jatwoo–(at)–etcom.com Mon Jul 15 18:13:25 CDT 1996
Newsgroups: rec.audio.tubes
From: jatwoo–(at)–etcom.com (John Atwood)
Subject: Re: Controlling preamp hum
Date: Mon, 15 Jul 1996 17:00:17 GMT
Sender: jatwoo–(at)–etcom14.netcom.com

kurt–(at)–r.hp.com (Kurt Strain) writes:

>This is meant to be hopefully helpful guidelines for reducing hum in
>phono or even line level preamps. …

< A lot of good information on hum reduction.>

However, I have one minor difference of opinion (based on my own experience):

>7. Make sure one of the heater filament voltage electrodes is tied to
> ground. Stay away from a raised heater potential to allow high cathode
> potentials – they can be trouble in helping to cause hum.

This only causes problems if the DC voltage used to raise the filament
supply is not at AC ground. A bypass capacitor (say about 100 uF) to ground
is essential. Also, I found I get the least hum if the center tap of the
filament supply is AC grounded, not one end. If the transformer winding is
not center-tapped, you can use the classic “hum control”, a 20 ohm or so
variable resistor across the filament supply, with the wiper either grounded
or connected to the DC bias supply. By the way, I find that a DC bias voltage
of about +25 to +35 volts works well.

Thank you, Kurt for this posting. Hum is a frequently misunderstood topic,
with many people coming to the conclusion that hum is inevitable in tube amps.
With good design practices, it is possible to get a completely hum-free system,
even with AC on the filaments!

Keep up these good postings,

– John Atwood

John Atwood
Preferred e-mail address: atwoo–(at)–ne-electron.com

From kurt–(at)–r.hp.com Mon Jul 15 18:13:45 CDT 1996
From: kurt–(at)–r.hp.com (Kurt Strain)
Newsgroups: rec.audio.tubes
Subject: Re: Controlling preamp hum
Date: 15 Jul 1996 18:17:10 GMT

John Atwood (jatwoo–(at)–etcom.com) wrote:

: < A lot of good information on hum reduction.>

: However, I have one minor difference of opinion (based on my own experience):

: >7. Make sure one of the heater filament voltage electrodes is tied to
: > ground. Stay away from a raised heater potential to allow high cathode
: > potentials – they can be trouble in helping to cause hum.

: This only causes problems if the DC voltage used to raise the filament
: supply is not at AC ground. A bypass capacitor (say about 100 uF) to ground
: is essential. Also, I found I get the least hum if the center tap of the
: filament supply is AC grounded, not one end. If the transformer winding is
: not center-tapped, you can use the classic “hum control”, a 20 ohm or so
: variable resistor across the filament supply, with the wiper either grounded
: or connected to the DC bias supply. By the way, I find that a DC bias voltage
: of about +25 to +35 volts works well.

For a DC regulated filament supply a center tap is not applicable,
but for an AC filament supply it would definitely help as you state.
In phono preamps, I would not want anything but regulated DC personally.
I don’t know why, but every time I elevated the filament supply voltage
level with AC ground it was not as good as leaving it DC grounded to the
shield of the coax filament supply line I had. Theoretically this
AC grounding should be perfectly fine. I haven’t figured out what has
gone wrong yet.

: Keep up these good postings,

Thanks.

Kurt

 

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