Ground loop hum - what is best way to eliminate?
Curtis Leeds
Stephen Mannino wrote:
>
> I have a ground-loop hum problem caused by using two separate AC
> circuits for my equipment. They are both "dedicated", but one is a 30
> amp (with larger wire), and the other a 15 amp. Although they are
> both grounded at the same place, there is still a difference in the
> ground potential.
>
> Is there any gizmo I can use that would eliminate the hum, while
> still allowing all the equipment to be grounded via the third pins on
> the AC plus? I'd rather not use ground-lift plugs if I don't have to.
> I would also like to use the two separate circuits, rather than
> re-wire my room. Any ideas? Thanks.
If these are truly dedicated lines, and assuming that they are in
satisfactory condition with tight connections, then the only way that is
is likely for a difference in voltage potential to develop is for there
to be yet another (a third) path to ground, e.g. a cable tv feed. (There
is also the possibilty of defective equipment.) The first logical step
would be to insure that it is well grounded near your service panel and
other existing ground.
--
********************************************************
Curtis Leeds cle...@mail.idt.net
"A man hears what he wants to hear, and disregards
the rest."
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Gregory T Freitag
> Stephen Mannino wrote:
> >
> > I have a ground-loop hum problem caused by using two separate AC
> > circuits for my equipment. They are both "dedicated", but one is a 30
> > amp (with larger wire), and the other a 15 amp. Although they are
> > both grounded at the same place, there is still a difference in the
> > ground potential.
> >
> > Is there any gizmo I can use that would eliminate the hum, while
> > still allowing all the equipment to be grounded via the third pins on
> > the AC plus? I'd rather not use ground-lift plugs if I don't have to.
> > I would also like to use the two separate circuits, rather than
> > re-wire my room. Any ideas? Thanks.
Ground loops are formed whenever you have more than one path to ground.
They are inevitable in any system that is single ended such as hi-fi
equipment. I assume that you have your equipment in two different
locations. If you have your amplifier near your speakers and your preamp in
another location each being plugged separate circuits. There is then a
signal cable that connects the pre-amp to the power amp. The power amp now
has two paths to ground, through the AC power ground and through the signal
cable ( shield ) to the preamp to the pre-amp AC power ground. There are
always leakage currents from your equipment that flows in the AC power
grounds and is why you have different potentials at your ground points at
your equipment. Lifting the safety grounds is not a good idea. If you
disconnect the shield at one end the hum will get very loud. I think you
have two options, put all the equipment in one location and plug into the
same circuit (There is still a ground loop but the resistance in the AC
power ground between the two pieces of equipment is reduced and
consequently the hum is also reduced). If you have any electronics skills
you can build a simple differential amp for the input of your amp. The
signal wire and shield are connected to the difference amp so that there is
no ground connection via the signal cable. This can also be done with a
transformer, but they are usually quite expensive for a high quality one.
Greg
Warren Gaiewski
Gregory T Freitag wrote:
>
> > Stephen Mannino wrote:
> > >
> > > I have a ground-loop hum problem caused by using two separate AC
> > > circuits for my equipment. They are both "dedicated", but one is a 30
> > > amp (with larger wire), and the other a 15 amp. Although they are
> > > both grounded at the same place, there is still a difference in the
> > > ground potential.
Not necessarily just a difference in ground potential. What the real
culprit usually is
is induction by transformer action. The loop formed by the two safety
grounds and the input
leads of two or more amps produces a 'secondary' which will get an EMF
(voltage) across
it caused by a nearby current-carrying conductor (the 'primary'). the
bigger teh loop
or the more current, the worse it gets.
> > >
> > > Is there any gizmo I can use that would eliminate the hum, while
> > > still allowing all the equipment to be grounded via the third pins on
> > > the AC plus? I'd rather not use ground-lift plugs if I don't have to.
> > > I would also like to use the two separate circuits, rather than
> > > re-wire my room. Any ideas? Thanks.
>
> Ground loops are formed whenever you have more than one path to ground.
> They are inevitable in any system that is single ended such as hi-fi
> equipment.
Or when using more than one amp.
> Lifting the safety grounds is not a good idea.
Depends how far you "lift" it. If you mean breaking the 3rd prong off,
yeah.
I've gotten zapped enough times due to moisture condensation on
equipment used
outdoors to learn my lesson.
I've seen some pro amps with a 5 ohm or so resistor in series with the
earth ground
wire, which usually eliminates the hum, but may be dangerous in the
event of
a direct short from hot to chassis (the resistor may open before the
breaker blows).
What's a bit safer is to use diodes (that is, heavy duty industrial
rectifiers) to
lift the ground by a couple of volts. Two pairs of inverse parallel
diodes in series
will usually do it. This will _break_ the ground loop for small signals,
but allow a
short or leakage current a low-Z path to power ground. This does require
electronics
skills and is NOT an absolute safety guarantee!!!! I use this on all my
systems, but
then again, I build my own amps.
> If you have any electronics skills
> you can build a simple differential amp for the input of your amp.
Balanced in's is probably the ONLY way to go if you have to conform to
any
rigid safety standards.
Dan Kriwitsky
You may want to contact a company called Sescom. They make a transformer isolator called an IL-19. It's regarded in the broadcast industry as a standard toolkit item. It's a simple isolation transformer in an XLR tube. Works great.
Curtis Leeds <cle...@mail.idt.net> wrote in article <32F37E...@mail.idt.net>...
> Stephen Mannino wrote:
> >
> > I have a ground-loop hum problem caused by using two separate AC
> > circuits for my equipment. They are both "dedicated", but one is a 30
> > amp (with larger wire), and the other a 15 amp. Although they are
> > both grounded at the same place, there is still a difference in the
> > ground potential.
> >
Stephen Mannino
First, thanks to all that responded. Second, yes, I am using two
amplifiers (mono), located next to the speakers, and plugged in to a
different circuit than the rest of the equipment. It never occurred
to me that using two mono amps might be part of the problem. There is
no cable TV attached (only a roof mounted aerial, which when
disconnected does not change anything). The amplifiers are equipped
with "ground lift" switches, which I believe are resistors. This
reduces but does not eliminate the hum. I may be able to configure
the system to "balanced" connections (differential amp) without too
much difficulty. Is this preferable to connecting all the equipment
to the same circuit? Thanks again for the help.
Stephen Mannino
Curtis Leeds
Warren Gaiewski wrote:
> Not necessarily just a difference in ground potential. What the real
> culprit usually is
> is induction by transformer action. The loop formed by the two safety
> grounds....
No, it's usually just a simple difference in voltage potential between
the grounds. It's measurable, and it's not terribly complicated, really.
> Balanced in's is probably the ONLY way to go if you have to conform to
> any
> rigid safety standards.
Sorry, but this is incorrect. Providing true dedicated AC lines - of
proper construction - and assuring clean grounds, is usually all that is
necessary to eliminate ground-loop hum in the home.
BobCleary
The first order of business is to find the offending equipment. First
disconnect all the equipment from each other and plug each one into a
ground adaptor. using a multimeter, measure from the power ground to the
chassis of the equipment. You should not see more than a couple of volts
of AC. If any of the equipment shows more than a couple of volts leakage,
have it fixed.
One scheme that sometimes works is to lift everything, but the preamp off
ground and then try grounding each piece of equipment to ground with a #12
copper wire. This usually will find the offending equipment. Of course you
may have to add the ground back to each component to find a multipathed
loop.
The key is that if all the equipment uses the chassis for both audio and
safety ground you are usually screwed! My systm consists of a 60 wpc tubed
stereo amp, 2ea 200w mono blocks, a preamp, cd player, tuner, cassette
deck, open reel recorder, and a turntable. The preamp and the tube amp
came with grounding ac cordsets. I lifted the ground on the tube amp and
all is quiet.
Some people think this is not good practice. Having worked in elecctronics
for 35 years I ALWAYS lift the ground on scopes, AC VTVM's, signal
generators, and spectrum analyzers. I usually will test the equipment I am
working on (switchnode power supplies up to 7kv output and 4kw) with an
isolation transformer to be sure i an\m not at line potential.This is
usually the only way to get clean measurements on very low signals. Every
manufacturer warns you not to do this (no doubt prompted by his legal
dept), but if you call and ask how to do low level work the engineer on
the other end of the line will usually tell you to buy a $900 differential
probe 9and hope for the best) or lift the safety ground.
Consumer audio gear is put through very stringent safety tests by UL and
CSA. These people are absolutely looney about any possibility of AC mains
voltage leaking over to the people end of the equipment. If the equipment
is in good working order, I would not hesitate to lift the ground on a
offending piece of equipment. I assume this equipment is not subject to
rain or moisture.
Bob
Most men lead lives of quiet desperation
Bob
Curtis Leeds
BobCleary wrote:
> The key is that if all the equipment uses the chassis for both audio
>and
> safety ground you are usually screwed! My systm consists of a 60 wpc
>tubed
> stereo amp, 2ea 200w mono blocks, a preamp, cd player, tuner, cassette
> deck, open reel recorder, and a turntable. The preamp and the tube amp
> came with grounding ac cordsets. I lifted the ground on the tube amp and
> all is quiet.
>
> Some people think this is not good practice.
It's a lousy practice, regardless of the degree of safety hazard, and
it's more of a kludge than a real fix. Nor is it necessary. Generally -
even in the most complex systems - dedicated AC lines are the surest,
safest, best solution to ground loops. Of course, it's hardly practical
for most apartment dwellers. But lifting the grounds on amplifiers is
just not a solution IMO.
> Every
> manufacturer warns you not to do this (no doubt prompted by his legal
> dept)
Even before we entered these horribly litigious times, manufacturers
cautioned against this practice. That a legal department MAY have an
interest in this matter is only because such a practice does indeed
create a potential hazard.
>
> Consumer audio gear is put through very stringent safety tests by UL and
> CSA. These people are absolutely looney about any possibility of AC mains
> voltage leaking over to the people end of the equipment. If the equipment
> is in good working order, I would not hesitate to lift the ground on a
> offending piece of equipment. I assume this equipment is not subject to
> rain or moisture.
Of course, something which is working perfectly well today may enter the
failure mode tomorrow. Hence the reason for such things as safety
grounds, air bags, hardhats, back-up tapes, etc. You are also quite
mistaken to believe that all consumer electronics are rated by UL. That's
an option left to the manufacturer. In the high-end, much gear is not UL
listed.
This conversation reminds me of when I purchased a motorcycle
helmet many years ago. The salesman explained the differences between the
various models and price points by saying, "Well, if you've got a $50
head, get a $50 helmet. Otherwise..."
--
********************************************************
Curtis Leeds cle...@idt.net