Greetings RAHE-
Here goes. I have a solid state preamp (Audio Research LS-3) that's
been in use for about 10 years. I noticed recently that it seems to be
generating an electrical signal when vibrated (heavy footfalls;
closing turntable dustcover too hard, etc.) which manifests as a low,
staticky rumble audible thru the speakers. I was fairly sure that this
sound was originating in the preamp because it does not occur at all
when the muting is engaged. I first checked all of the cable
connections and cleaned all the RCA jacks & pins. When this failed to
correct the condition, I spoke to a local stereo repair person who
immediately said "it sounds like a bad solder joint". I also
mentioned that the volume control had no effect on the loudness of the
noise, and he said that probably meant that the problem was most
likely in the output stage.

Anyway, he looked the thing over and said it was "solid as a rock",
adding that he'd been unable to duplicate the phenomenon when he'd set
it up with other gear in his shop. Of course, when I got the thing
back home, there was that noise again. He said maybe my power amp
(McCormack DNA-0.5, also 10 years old) was the culprit, but it seems
to me that, if this was the case, the noise would occur regardless of
whether the preamp was muted or not, which it doesn't. Also, the amp
is at least 6 feet away from the preamp on a separate stand between
the speakers, and is not subjected to the same vibrations that seem to
cause the sound.

So I'm kind of at a loss. Does anyone care to speculate about what
might be going on here? This phenomenon doesn't necessarily interfere
with my music listening, but it "ain't right". Could it actually be my
amp?

appreciate any insight,
Santos

a few other factoids (possibly relevant):

- system is left "on" pretty much constantly, so these amps have been
up & running for the better part of 10 years, give or take a few moves
and a few electrical storms

- condition occurs with preamp on & unmuted; volume control has no
effect; condition not present with preamp muted.

- condition present with CD & phono inputs disconnected

- condition present with either pair of "main out" jacks of preamp
used

- it's been very humid this summer, and this is not an air-conditioned
home

Santos L Halper wrote:

Greetings RAHE-
Here goes. I have a solid state preamp (Audio Research LS-3) that's
been in use for about 10 years. I noticed recently that it seems to be
generating an electrical signal when vibrated (heavy footfalls;
closing turntable dustcover too hard, etc.) which manifests as a low,
staticky rumble audible thru the speakers. I was fairly sure that this
sound was originating in the preamp because it does not occur at all
when the muting is engaged. I first checked all of the cable
connections and cleaned all the RCA jacks & pins. When this failed to
correct the condition, I spoke to a local stereo repair person who
immediately said "it sounds like a bad solder joint". I also
mentioned that the volume control had no effect on the loudness of the
noise, and he said that probably meant that the problem was most
likely in the output stage.

Anyway, he looked the thing over and said it was "solid as a rock",
adding that he'd been unable to duplicate the phenomenon when he'd set
it up with other gear in his shop. Of course, when I got the thing
back home, there was that noise again. He said maybe my power amp
(McCormack DNA-0.5, also 10 years old) was the culprit, but it seems
to me that, if this was the case, the noise would occur regardless of
whether the preamp was muted or not, which it doesn't. Also, the amp
is at least 6 feet away from the preamp on a separate stand between
the speakers, and is not subjected to the same vibrations that seem to
cause the sound.

So I'm kind of at a loss. Does anyone care to speculate about what
might be going on here? This phenomenon doesn't necessarily interfere
with my music listening, but it "ain't right". Could it actually be my
amp?

appreciate any insight,
Santos

a few other factoids (possibly relevant):

- system is left "on" pretty much constantly, so these amps have been
up & running for the better part of 10 years, give or take a few moves
and a few electrical storms

- condition occurs with preamp on & unmuted; volume control has no
effect; condition not present with preamp muted.

- condition present with CD & phono inputs disconnected

- condition present with either pair of "main out" jacks of preamp
used

- it's been very humid this summer, and this is not an air-conditioned
home


If that condition is not present when the preamp is muted, the problem
would have to be prior to the muting switch, and after the volume
control. Several other things to check:

1. Does that rumble occur on both channels? If so, it may indicate the
problem is from a circuit that is common to both channels. The power
supply, for instance.
2. Can you tap various parts of your preamp and reproduce the rumble?
That can localize the problem area for you.
3. It can be an intermittent ground loop problem between your preamp
and power amp. Some grounding connection may be coming loose, and any
vibration would open it, causing hum to come through. Can you connect a
ground wire between the preamp and the power amp? Can you try a
different power amp?
4. Does it matter which preamp input is used? From your description,
it should not matter.
5. What if you just connect one channel (left or right). Do you still
have the same rumble?

Chung wrote:
I also
mentioned that the volume control had no effect on the loudness of
the noise, and he said that probably meant that the problem was most
likely in the output stage.

Anyway, he looked the thing over and said it was "solid as a rock",
adding that he'd been unable to duplicate the phenomenon when he'd
set it up with other gear in his shop. Of course, when I got the
thing back home, there was that noise again. He said maybe my power
amp (McCormack DNA-0.5, also 10 years old) was the culprit, but it
seems to me that, if this was the case, the noise would occur
regardless of whether the preamp was muted or not, which it doesn't.
Also, the amp is at least 6 feet away from the preamp on a separate
stand between the speakers, and is not subjected to the same
vibrations that seem to cause the sound.

So I'm kind of at a loss. Does anyone care to speculate about what
might be going on here? This phenomenon doesn't necessarily interfere
with my music listening, but it "ain't right". Could it actually be
my amp?

appreciate any insight,
Santos

a few other factoids (possibly relevant):

- system is left "on" pretty much constantly, so these amps have been
up & running for the better part of 10 years, give or take a few
moves and a few electrical storms

- condition occurs with preamp on & unmuted; volume control has no
effect; condition not present with preamp muted.

- condition present with CD & phono inputs disconnected

- condition present with either pair of "main out" jacks of preamp
used

- it's been very humid this summer, and this is not an
air-conditioned home


If that condition is not present when the preamp is muted, the problem
would have to be prior to the muting switch, and after the volume
control. Several other things to check:

1. Does that rumble occur on both channels? If so, it may indicate
the problem is from a circuit that is common to both channels. The
power supply, for instance.
2. Can you tap various parts of your preamp and reproduce the
rumble? That can localize the problem area for you.
3. It can be an intermittent ground loop problem between your preamp
and power amp. Some grounding connection may be coming loose, and any
vibration would open it, causing hum to come through. Can you connect
a ground wire between the preamp and the power amp? Can you try a
different power amp?
4. Does it matter which preamp input is used? From your description,
it should not matter.
5. What if you just connect one channel (left or right). Do you
still have the same rumble?

I am with that repair guy, sounds like a bad solder joint. It also seems to
oscillate. These faults are difficult to find. Your approach seems to be
right, to localize the problem open the preamp, so you can see the circuit
board. Now hit the PCB with a small pen on different places and see if the
problem aggravates or goes away. then resolder all solder joints in that
area.
There might be a partially damaged IC as well, so when the problem is
unchanged by the tapping procedure, you have to exchange the output
opamp(s). If you are not good at soldering, let this be done by the repair
tech.
It can also be an electrolytic capacitor, which dry out with time and loose
their capacity. This would indicate some hum superimposed on the signal. You
will need an oscilloscope to quantify this suspicion. Another case for the
tech I suppose.
--
ciao Ban
Bordighera, Italy

Chung wrote:


If that condition is not present when the preamp is muted, the problem
would have to be prior to the muting switch, and after the volume
control.

Not necessarily. It depends on how the preamp's muting circuit operates
(I don't know how the Audio Research LS-3 muting circuit is designed).
If it simply shorts the signal line to ground, it will also effectively
ground the base of the first transistor in the input of the power
amplifier and would probably thus mute whatever noise may be generated
there, if that transistor is defective.

Gene Poon wrote:
Chung wrote:


If that condition is not present when the preamp is muted, the problem
would have to be prior to the muting switch, and after the volume
control.

Not necessarily. It depends on how the preamp's muting circuit operates
(I don't know how the Audio Research LS-3 muting circuit is designed).
If it simply shorts the signal line to ground, it will also effectively
ground the base of the first transistor in the input of the power
amplifier and would probably thus mute whatever noise may be generated
there, if that transistor is defective.

I am not sure what you are saying there. If the mute switch in the
preamp shorts the signal lines to ground, whatever noise generated by
the first transistor in the power amp is still going to make its way to
the output of the power amp.

Ban wrote:

Chung wrote:
I also
mentioned that the volume control had no effect on the loudness of
the noise, and he said that probably meant that the problem was most
likely in the output stage.

Anyway, he looked the thing over and said it was "solid as a rock",
adding that he'd been unable to duplicate the phenomenon when he'd
set it up with other gear in his shop. Of course, when I got the
thing back home, there was that noise again. He said maybe my power
amp (McCormack DNA-0.5, also 10 years old) was the culprit, but it
seems to me that, if this was the case, the noise would occur
regardless of whether the preamp was muted or not, which it doesn't.
Also, the amp is at least 6 feet away from the preamp on a separate
stand between the speakers, and is not subjected to the same
vibrations that seem to cause the sound.

So I'm kind of at a loss. Does anyone care to speculate about what
might be going on here? This phenomenon doesn't necessarily interfere
with my music listening, but it "ain't right". Could it actually be
my amp?

appreciate any insight,
Santos

a few other factoids (possibly relevant):

- system is left "on" pretty much constantly, so these amps have been
up & running for the better part of 10 years, give or take a few
moves and a few electrical storms

- condition occurs with preamp on & unmuted; volume control has no
effect; condition not present with preamp muted.

- condition present with CD & phono inputs disconnected

- condition present with either pair of "main out" jacks of preamp
used

- it's been very humid this summer, and this is not an
air-conditioned home


If that condition is not present when the preamp is muted, the problem
would have to be prior to the muting switch, and after the volume
control. Several other things to check:

1. Does that rumble occur on both channels? If so, it may indicate
the problem is from a circuit that is common to both channels. The
power supply, for instance.
2. Can you tap various parts of your preamp and reproduce the
rumble? That can localize the problem area for you.
3. It can be an intermittent ground loop problem between your preamp
and power amp. Some grounding connection may be coming loose, and any
vibration would open it, causing hum to come through. Can you connect
a ground wire between the preamp and the power amp? Can you try a
different power amp?
4. Does it matter which preamp input is used? From your description,
it should not matter.
5. What if you just connect one channel (left or right). Do you
still have the same rumble?

I am with that repair guy, sounds like a bad solder joint. It also seems to
oscillate. These faults are difficult to find. Your approach seems to be
right, to localize the problem open the preamp, so you can see the circuit
board. Now hit the PCB with a small pen on different places and see if the
problem aggravates or goes away. then resolder all solder joints in that
area.
There might be a partially damaged IC as well, so when the problem is
unchanged by the tapping procedure, you have to exchange the output
opamp(s). If you are not good at soldering, let this be done by the repair
tech.
It can also be an electrolytic capacitor, which dry out with time and loose
their capacity. This would indicate some hum superimposed on the signal. You
will need an oscilloscope to quantify this suspicion. Another case for the
tech I suppose.

Well, this is the kind of problem that can be solved (or at least
troubleshot) fairly easily if one has access to some basic equipment
like a scope. You can monitor the power supply lines and see if tapping
creates any large disturbance on the supplies.

If the noise occurs in both channels, it is unlikely that a coupling
capacitor is the culprit, because both channels will have the same bad
coupling cap. It could be a filtering cap on the supply lines, but I
would expect the problem to not be intermittent.

My guess is that there is some failing solder joint. I wonder if the
repair guy saw any problem while tapping on various parts of the preamp?
Sometimes using a heat gun (or a hair dryer) can help locate the problem
quickly.

Chung wrote:


If that condition is not present when the preamp is muted, the problem
would have to be prior to the muting switch, and after the volume
control.

Not necessarily. It depends on how the preamp's muting circuit operates
(I don't know how the Audio Research LS-3 muting circuit is designed).
If it simply shorts the signal line to ground, it will also effectively
ground the base of the first transistor in the input of the power
amplifier and would probably thus mute whatever noise may be generated
there, if that transistor is defective.


If this is the case, would that mean that there IS a possibility that the
fault is
in the power amp? What's the overall consensus - should I have Local Repair
Guy take a look at my power amp before thinking about sending the LS-3 to
Audio Research for evaluation?

Santos

Chung wrote:

Gene Poon wrote:

Chung wrote:


If that condition is not present when the preamp is muted, the
problem would have to be prior to the muting switch, and after the
volume control.


Not necessarily. It depends on how the preamp's muting circuit
operates (I don't know how the Audio Research LS-3 muting circuit is
designed). If it simply shorts the signal line to ground, it will also
effectively ground the base of the first transistor in the input of
the power amplifier and would probably thus mute whatever noise may be
generated there, if that transistor is defective.


I am not sure what you are saying there. If the mute switch in the
preamp shorts the signal lines to ground, whatever noise generated by
the first transistor in the power amp is still going to make its way to
the output of the power amp.


He's saying that if you hold the base of the transistor at ground, you
are turning the transistor OFF and it won't generate anything.

LouieHow wrote:
Chung wrote:

Gene Poon wrote:

Chung wrote:


If that condition is not present when the preamp is muted, the
problem would have to be prior to the muting switch, and after the
volume control.


Not necessarily. It depends on how the preamp's muting circuit
operates (I don't know how the Audio Research LS-3 muting circuit is
designed). If it simply shorts the signal line to ground, it will also
effectively ground the base of the first transistor in the input of
the power amplifier and would probably thus mute whatever noise may be
generated there, if that transistor is defective.


I am not sure what you are saying there. If the mute switch in the
preamp shorts the signal lines to ground, whatever noise generated by
the first transistor in the power amp is still going to make its way to
the output of the power amp.


He's saying that if you hold the base of the transistor at ground, you
are turning the transistor OFF and it won't generate anything.

You cannot hold the base at ground and turn the transistors of the power
amp for the following reasons:

1. There is most likely an ac coupling capacitor at the input of the
power before the base of the first transistor, and

2. The power amp has bipolar power supplies. In fact the nominal
voltage at the input to the power should be zero volt DC. The input
stage does NOT get turned off when you short the inputs to ground.

Santos L Halper wrote:
Chung wrote:



If that condition is not present when the preamp is muted, the problem
would have to be prior to the muting switch, and after the volume
control.

Not necessarily. It depends on how the preamp's muting circuit operates
(I don't know how the Audio Research LS-3 muting circuit is designed).
If it simply shorts the signal line to ground, it will also effectively
ground the base of the first transistor in the input of the power
amplifier and would probably thus mute whatever noise may be generated
there, if that transistor is defective.



If this is the case, would that mean that there IS a possibility that the
fault is
in the power amp? What's the overall consensus - should I have Local Repair
Guy take a look at my power amp before thinking about sending the LS-3 to
Audio Research for evaluation?

Santos

Take ur CD player - plug it into your amp (with the power off, of
course). Turn the CD on, then the amp. No noise? Not in the amp.
Or use a tuner, Cassette Player, R-R tape deck, etc... any line level
piece of gear. No noise, not in the amp.

IF this noise happens ONLY in PHONO mode - then you are picking up LF
through the cartridge, eh? Common, use a good form of isolation for the
base of ur TT in that case - ask here if you need ways to do that.

IF it is happening in ONLY one channel, then that isolates it to a fault
in that channel. IF it is happening in TWO channels, it may be a fault
in the voltage regulation circuit in the preamp.

Much depends on what the noise sounds like - if it happens on its own,
then it well may be a fault in the circuit. If something mechanical or
physical needs to happen externally for it to happen, AND it happens in
OTHER THAN the phono position then we have to try it without the TT
plugged in, and determine that it only happens in the phono position...

Etc...

Open the top of the AR preamp.

With a PLASTIC object - non conductive and relatively thin, like a
plastic pen with the point not showing, or similar, poke and bang around
on the board and on anything you see, *with the power on* and connected
to the amp - IF it is mechanical you will produce the noise.

You should do the above tests, and determine *exactly* what the
conditions are when the noise happens and does not, and what that noise
sounds like. Let's leave it up to you to describe the conditions and
what the noise sounds like. Then you can get a better answer about what
to do or not to do.

_-_-bear

Chung wrote:

You cannot hold the base at ground and turn the transistors of the power
amp for the following reasons:

1. There is most likely an ac coupling capacitor at the input of the
power before the base of the first transistor, and
==============================

True, but that only isolates the base of a transistor from an input
short for DC. The noise is AC and at audible frequencies, you will be
grounding it. In most cases on a noisy transistor, you can see the
noise on the base of the transistor, if you look for it. If you haven't
TRIED it, you don't know.

====================================

2. The power amp has bipolar power supplies. In fact the nominal
voltage at the input to the power should be zero volt DC. The input
stage does NOT get turned off when you short the inputs to ground.
================================

Again, you haven't tried it and are confusing DC circuit behavior with AC.

LouieHow wrote:
Chung wrote:

You cannot hold the base at ground and turn the transistors of the
power amp for the following reasons:

1. There is most likely an ac coupling capacitor at the input of the
power before the base of the first transistor, and

==============================

True, but that only isolates the base of a transistor from an input
short for DC. The noise is AC and at audible frequencies, you will be
grounding it. In most cases on a noisy transistor, you can see the
noise on the base of the transistor, if you look for it. If you haven't
TRIED it, you don't know.

====================================

I suggest that you read more carefully the posts by the OP and by the
respondents. The OP said that the static went away when the preamp was
muted. Regardless of whether the muting circuit grounds the output of
the preamp, that is an indiaction that the static is not noise from the
power amp's input transistor. Note that it was not white noise, it was
an interference like signal that was heard when vibration picked up by
the preamp.

You cannot see the noise on the base of the transistor if the transistor
is noisy. You do not understand the noise mechanisms in transistors. You
can ground the base, and still see noise at the output (collector) of
the transistor. In fact, that's how we measure the equivalent input
noise voltage of transistors and amplifiers. We short the input to
ground, and then measure output noise. The amount of output noise
divided by gain is then the equivalent input noise.

Specifically, you said that if you short the base of the power amp's
input to ground, you turn off that transistor. You were clearly wrong.
And that was what I objected to.



2. The power amp has bipolar power supplies. In fact the nominal
voltage at the input to the power should be zero volt DC. The input
stage does NOT get turned off when you short the inputs to ground.

================================

Again, you haven't tried it and are confusing DC circuit behavior with AC.

Again, you seem to have no idea about how the input transistors work.
You are the one who said that shorting the input of the power amp will
turn off the input transistor. This is a major misunderstanding on your
part.