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Discussion on 60HZ ground loops for pro audio appl
"Help! 60 Hz Hummmm"
______________________________________________
CompuServe MIDI/Music Forum Discussion
Fm: D.I. Johnson 76636,2277
To: ALL
Yikes!! Someone help me before I do something unspeakable!!!
The situation is this... I have an Alesis RA100 amp powering my studio
monitors.
The *problem* is... whenever I plug the amp inputs into the output of any other
piece of gear I get an annoying, 60 cycle hum. The only way I've found to
eliminate it is to disconnect one of the inputs. I do not get the hum if I
connect the inputs to the outs of two different decks - ex. left channel to
DAT, right to CD - only when they're plugged into the same deck, or mixer. I
can also get the same hum if I simply cross connect the inputs of the amp.
I've tried ground lift adapters, different patch cables, different power cords,
unplugging everything, unplugging some things... I just can't seem to eliminate
the hum. Actually, I haven't tried another amp, but I will do that this
evening.
Does anyone have any suggestions? Is there something I'm overlooking? Is there
some type of in-line filter available that might work. Could it be problem
internal the amplifier? I can't take it anymore! Any help will be appreciated
by myself and my analyst. Thanks!
Fm: Steven Cohn 76377,1500
To: D.I. Johnson 76636,2277
Well, the one thing to know about hum problems like this is that you must
eliminate multiple paths to ground. Usually, a piece of equipment is grounded
either to the wall (on the ground pin), through the chassis, or both. If you
have the amp plugged in to a different outlet than the rest of the gear, then
you have a ground loop through the wall. If you have it rack-mounted with the
other gear, then you could be grounded through the rack rails as well.
Unfortunately, the problem of eliminating grounding hum takes a little
knowledge and a lot of patience.
Best of luck.
Fm: Craig Walsh 70701,1614
To: D.I. Johnson 76636,2277
DIJ: You know, I've had similar problems which I was able to eliminate by
careful routing of my signal paths. Essentially, there are a couple of ways to
get the power station's 60 Hz hum into your audio. One is thru the mains
themselves, and this is usually taken care of by careful grounding and
judicious usage of noise-suppressors. OTOH, when a signal cable is placed
right next to an AC, you are gonna get some transmission of B-field flux
between the two, even if the signal cable is relatively well shielded. I'd
suggest going thru all of your signal cables and making sure that none of them
are even close (3 feet???) to your AC (including the wiring in the walls). Give
it a try and see if it helps. I've been able to get rid of lots of noisy
situations using this approach. Craig
Fm: D.I. Johnson 76636,2277
To: Steven Cohn 76377,1500
Thanks, Steve, for your suggestions.
I have tried lifting the ground of the amp and various other pieces of
equipment but have had no success.
The amp is plugged into the same circuit as the rest of the equipment, but is
not mounted in a common rack, so that eliminates the "loop through the rails"
theory.
I guess I'm still looking. DIJ
Fm: D.I. Johnson 76636,2277
To: Craig Walsh 70701,1614
Craig, thanks for the reply.
>> One is thru the mains themselves, and this is usually taken care of by
careful grounding and judicious usage of noise-suppressors. <<
Can you provide a little more information about the type of suppressors might I
look for, who makes them and what the price ranges are? Does a *line
conditioner* meet these requirements and is this something I should be
considering, versus surge suppression?
One thing that puzzles me is why the hum is there only when the inputs are
routed from the same deck. As I mentioned, I've tried lifting the ground on the
various sources but have had no success with that.
Fm: Dave Tosti-Lane 70334,3165
To: D.I. Johnson 76636,2277
When you "cross connect" the inputs, are you saying you run a jumper
from one input to the other and get hum? What if you simply connect the
ground of one input to the ground of the other? Hot to hot? If you are
getting the hum with all other equipment off and disconnected from the Amp,
the problem is likely to be internal to the amp it seems to me. I also
assume that you are doing this cross connecting at the amp, and not at a
patch panel?
If your test with another amp eliminates the problem, that should
confirm the problem to be in the amp.
Fm: Craig Walsh 70701,1614
To: D.I. Johnson 76636,2277
D.I.: You should consider noise suppression. Standard output strips usually
have spike and surge suppression which is much different (basically, a little
switch goes off and blocks any large variations in the AC input voltage). For
noise suppression, you need large capacitors across the input voltage to reduce
variations, and a bunch of other stuff. Furmann makes some good line
conditioners which meet this criteria, although I'm not sure this is related
much to the problem to which you speak. Sounds to me like it is definitely
either a grounding problem or a signal routing problem (as mentioned before).
Is your deck balanced (via TRS jacks)? If it is, you need to make sure that
you are using properly shielded cables (i.e. one side should have a TRS and be
grouned to the shield, the other side should have the ground lifted to avoid
these types of ground loop problems). I hope this helps, Craig Walsh
Fm: David G. Hackbarth 71162,1262
To: D.I. Johnson 76636,2277
I'm not familiar that amp, so it may not have the option, but is there a switch
for "bridging" or dual-mono that's in the wrong position. Is there an internal
jumper for this? Pretty unlikely, but thought I'd try a long shot.
You might try lifting the ground (shield) from one side at the amp input. Leave
it connected at the output of the mixer. May not work with unbal input, but
also worth a try.
Fm: James Chandler Jr.-Sysop 71333,2651
To: D.I. Johnson 76636,2277
D.I., here is an odd-ball suggestion, but it turned out to be the problem in an
earlier hum discussion, and is a problem which regularly sends a PA contracter
friend on service calls--
If your system is connected to cable TV, the cable hookup must be transformer
isolated.
Fm: D.I. Johnson 76636,2277
To: James Chandler Jr.-Sysop 71333,2651
Thanks for the suggestion...unfortunately, as far as television goes I am still
living in the age of the Flintstones and haven't sprung for the cable. So this
wouldn't be the problem.
Also, I have been able to swap amps, and still the problem exists, though it
isn't so severe as with the first amp.
And upon further checking, with a line-tracing induction amplifier, I think
I've found the problem to be the wiring of the house. The entire place seems
to be incredably noisy from this side of the main panel. I suppose I should
give the local power company a call and ask them about it. Craig W. suggested
a line conditioner and I may have to give that a try. I've pretty much
eliminated grounding or signal routing as the problem - I can make the speakers
hum with nothing but the amp connected and powered up.
Fm: D.I. Johnson 76636,2277
To: David G. Hackbarth 71162,1262
Thanks for the suggestions. I may have to give the shield-lift that you suggest
a try, or at least consider the option. Also, no, there are no bridging
switches or jumpers on this amp.
Actually though, I'm very suspicious of the wiring in the house. I've
discovered that it's ripe with noise throughout. I think I'll be giving the
poser company a call about it. It's pretty bad. Someone suggested a line
conditioner and that may be the next step. Maybe I can borrow one for a day or
two and see if it helps.
Fm: D.I. Johnson 76636,2277
To: Dave Tosti-Lane 70334,3165
Yes, by simply running a jumper from one input to the other (no patch bay) I
can induce the noise from the amp.
I was able to duplicat this with another, identical amp so I don't think it's
the hardware. I'm suspecting very much the quality of the power within the
house. I was able to give it a listen with a line tracer amp and it's way too
loud. Craig Walsh suggests a line conditioner and that's one option I will be
looking at, as well as calling the power company to find out if they have any
suggestions.
Fm: D.I. Johnson 76636,2277
To: Craig Walsh 70701,1614
I appreciate the response. I've pretty much eliminated the signal routing and
grounding as the problem. I'm suspecting that the problem is severe noise in
the power throughout the house. It hums pretty badly through a tracing amp. I
think your suggestion of a line conditioner may be the way to solve it. I will
also be speaking with the power company to see if they can make any suggestions
as well.
As for the deck, it is an unbalanced Fostex 450, but it isn't the problem...
the speakers hum without the deck being in the signal path. Someone else
suggested that lifting the cable shielding at one end might help, so that is
another option I will explore.
Fm: James Chandler Jr.-Sysop 71333,2651
To: D.I. Johnson 76636,2277
>>Also, I have been able to swap amps, and still the problem exists, though it
isn't so severe as with the first amp.
>>I can make the speakers hum with nothing but the amp connected and powered
up.
That sounds like some serious humm problems. My condolences (G).
Does your rig hum if you listen on the mixer headphone output, or only when a
power amp is connected?
We just got some Crest 601 amps for PA (we're hugely impressed, by the way),
and they have an interesting feature. If all the equipment is powered from the
same AC circuit, and hum still persists, you can lift a jumper on the input
block to separate the ground of the amp from the rest of the system. They warn
not to lift the jumper unless all units are getting good AC ground, and are all
on same circuit, though.
Fm: James Chandler Jr.-Sysop 71333,2651
To: D.I. Johnson 76636,2277
D.I., I noticed in one of your messages you said your amp substitution used an
identical amp. If that means you have tested only two Alesis amps, maybe you
should try another brand. Crest, Crown, or Urei doesn't cost too much more,
and they've had a bit more experience (G).
Fm: Ronald Dobey 73353,1766
To: James Chandler Jr.-Sysop 71333,2651
<< "....the cable hookup must be transformer-isolated." >>
How is this accomplished ???
Fm: D.I. Johnson 76636,2277
To: James Chandler Jr.-Sysop 71333,2651
Now that you mention it and I've had a chance to check, the hum is actually
there at the mixer headphone out also. That is with all inputs muted, faders
down, synths and modules powered down and phones output at 100%.
As for the ground lift on the input block of the Crest 601's, someone else
suggested the same sort of fix on these Alesis amps. I think it may help,
though I haven't tried it yet.
I would like to give another amp a try, but it may be a little while before I
can get hold of one.
Like I mentioned in another message, I'm very suspicious of the house wiring
now. I used a tracing amp to listen in and it's unbelievably noisy. I may be
looking for some sort of power conditioner soon. I've got the luxury of digital
recording, and now I'm hearing this junk. If it's not one thing...
Fm: Craig Walsh 70701,1614
To: D.I. Johnson 76636,2277
Well, it depends on how you are wired. Are you using 1/4" to 1/4" shielded
cable? If you are, then you can certainly eliminate ground loop problems.
Also, is the amp grounded itself? You mentioned that you lifted ground
somewhere (I don't recall) but that didn't help. I'm not sure how well line
conditioners work on 60Hz hum in that respect. Let me get this straight. If
you just have the amp connected to your speakers (no signal in), you get hum?
Or do you have cables coming from your board? What is the config? Good luck
and let us know how it goes. Craig Walsh
Fm: James Chandler Jr.-Sysop 71333,2651
To: D.I. Johnson 76636,2277
>>Like I mentioned in another message, I'm very suspicious of the house wiring
now. I used a tracing amp to listen in and it's unbelievably noisy.
You've done a basic check of the wiring with one of those little Radio Shack AC
line testers? All the right lights come on (G)?
Fm: James Chandler Jr.-Sysop 71333,2651
To: Ronald Dobey 73353,1766
>><< "....the cable hookup must be transformer-isolated." >>
>>How is this accomplished ???
I think Radio Shack sells a video transformer that does the trick. Haven't had
to solve this problem personally.
Fm: James Chandler Jr.-Sysop 71333,2651
To: D.I. Johnson 76636,2277
That dern hummm stuff can be obnoxious. Last weekend, they remodeled the stage
at my gig. After the electricians re-wired the stage, I checked the outlets
for proper grounding and polarity, and everything was cool.
The guitarist plays a Carvin double-neck bass/guitar. He uses a tube guitar
amp and a transistor bass amp. I plug one of my keyboard outputs to the bass
amp too, and we trade out bass duties. Everything on stage is powered off one
outlet, from a big outlet array made from several 4 outlet electrical boxes
bolted together.
Anyway, we got lots of bass amp hum after setting up on the new stage
(remember, guitar amp and bass amp are grounded together thru the double neck
guitar). Also, we were having ground loop between my MPS+ and the bass amp.
With everything plugged into a known good outlet.
While plugging/unplugging different signal wires trying to isolate the problem,
I managed to blow out one of the MPS+ outputs, and will have to replace
probably the muting transistor (the headphone output is still good, so probably
the output op amp is still good). Further checking revealed a 78 volt AC
potential between the bass amp ground and all the other equipment, though it is
a very low current potential, because it doesn't cause any shocks if you touch
between it and any other equipment. Argggh!
Fm: D.I. Johnson 76636,2277
To: Craig Walsh 70701,1614
Yes, I tried lifting the ground of the amp and various other pieces of gear,
but nothing seemed to help, so I don't believe the problem is a ground loop.
I'm almost 100% sure the problem is induced by the house wiring.
>> Are you using 1/4" to 1/4" shielded cable? If you are, then you can
certainly eliminate ground loop problems.<<
No. I'm using RCA inputs to the amp.
>> If you just have the amp connected to your speakers (no signal in), you get
hum? <<
Yes, *IF* I run a jumper between the amp inputs I can hear the same hum, *OR*
if I connect the amp inputs from the same deck or mixer. (1 input from each of
2 decks = no hum)
A friend suggested using a humbucking coil on the input cables to see if that
might help...worth a try at least.
Fm: D.I. Johnson 76636,2277
To: James Chandler Jr.-Sysop 71333,2651
The lights come on, but noone's home ;-) Yeah, that Radio Shack tester says
things are okay, but the sniffer says it's awful loud.
Fm: D.I. Johnson 76636,2277
To: James Chandler Jr.-Sysop 71333,2651
Yikes! Sorry to hear about your MPS+.
What was the fix for the problem you encountered?
Fm: James Chandler Jr.-Sysop 71333,2651
To: D.I. Johnson 76636,2277
>>Yikes! Sorry to hear about your MPS+.
What was the fix for the problem you encountered?
Well, I fiddled around with the AC plugs, and the ground-loop hum diminished
somewhat, but it is still there. I zapped the MPS+ output out of vulgar
stupidity, by plugging and un-plugging it when the hum clearly indicated a
problem. Should have tested with the Fluke 77 first thing.
The bass amp is a 15 year old Peavey TNT 100, about 50 watts with a 15"
speaker. We still use it, because it just sounds great for on-stage bass
levels, and we mic it for the house. It is a really good-sounding bass amp,
never breaks up at the levels we use, and is very warm.
I called Peavey today, asking for strategies on fixing it. I've seen lotsa old
guitar amps that had similar AC leakage problems, but never tried to chase down
the problem. The tech said rather obvious things, check to make sure the AC
ground wire is firmly attached to the AC plug and also the chassis, and check
the bypass caps on the power switch. I'm gonna take it home this Sunday and
try to chase the problem down. It couldn't be too serious, or the guitarist
would be dead by now (G).
Fm: K.K. Proffitt -Sysop 76711,555
To: D.I. Johnson 76636,2277
I've covered this topic som many times in the past years, I'm not sure if I've
mentioned it recently, but your idea that the house feed may be the culprit
could be right on target. We had low voltage on one leg of an old studio I used
to have and it wasn't until we transferred the power to the studio to the other
leg that we fixed the problem. Basically, low voltage will cause RFI and that
will sound like hum. It wasn't low enough to cause a brown-out, just low enough
to cause RFI.
Also, I've heard of one case where there was some 1950's type aluminum cable
carrying power from the pole to the installation. Over the years it had turned
to POWDER at points. When the cable was cool, it would conduct, but when the
place started to draw power, the cable would heat up and the powdered areas
wouldn't conduct and the place would get a brown out. They went through two
heat pumps, lost their insurance, etc., before the electric company dug up the
cable and admitted the problem.
Fm: James Chandler Jr.-Sysop 71333,2651
To: D.I. Johnson 76636,2277
D.I., KK's response to you opens up a cheap alternative to try---
Find the breaker your equipment is on. Then turn off the AC main disconnect,
and swap the wiring between that breaker and the adjacent one. In other words,
if you are on an odd breaker, you would now be on an even breaker, and vice
versa. Make sure you don't switch with a breaker that is ganged for 220 Volts
(G). Also make sure the black and white wires still go the right place.
Possibly the other leg is healthier!
Fm: Craig Walsh 70701,1614
To: D.I. Johnson 76636,2277
I'm stunned! Perhaps this is gonna require some better expertise. Good luck
and let us all know what you come up with. Sounds to me as though you are
gonna need an electrical engineer around here. Craig
Fm: D.I. Johnson 76636,2277
To: K.K. Proffitt -Sysop 76711,555
It's interesting that the low voltage would cause RFI. I wouldn't have expected
that. Could that be analogous to decreasing the signal:noise ratio in an audio
chain?
As far as the older, aluminum wiring goes, I have also seen some of the
problems this can cause, including fires in the worst cases. Fortunately, my
home is only a few years old and was wired with copper. Still, I could check
the voltage levels as well. Actually I would be surprised to find it low. The
power company here is actually pretty reliable, but since they should be out
sometime soon anyway, I'll add this to their list of things to do. Thanks for
the suggestions.
Fm: D.I. Johnson 76636,2277
To: James Chandler Jr.-Sysop 71333,2651
>>Find the breaker your equipment is on. Then turn off the AC main disconnect,
and swap the wiring between that breaker and the adjacent one.
This sounds like it might be something worth trying in some instances, but I
seriously doubt it would be of much help in this case. The house is only a few
years old with copper wiring, and the strength of the AC hum seems pretty
constant throughout. I've mostly eliminated that as a possibility, anyway, as I
have powered the amp from other circuits in the house.
I was just wondering about something...would it have helped if the wiring had
been run through that metal Romex (isn't that what it's called?) conduit when
the place had been constructed? Wouldn't this act as a shield? Is this
something that is used in the construction of studios, or do the electricians
simply run the cable, unshielded, from the main panel?
Fm: D.I. Johnson 76636,2277
To: Craig Walsh 70701,1614
Yeah, it's sounding more like that every day. I've gotten a lot of great
suggestions and hints, and as soon as I'm able to come up with a solution I
will certainly let everyone know what it is. Thanks for all of your
suggestions. I do appreciate them.
I'm still wishing to win the lottery so I can start over from scratch and just
build my dream studio. Wouldn't *that* be something :-) I should probably
start *playing* the lottery first....
Fm: D.I. Johnson 76636,2277
To: James Chandler Jr.-Sysop 71333,2651
>>It couldn't be too serious, or the guitarist would be dead by now (G).<<
And *that* would be a real shame! (ROFL) DIJ
Fm: K.K. Proffitt -Sysop 76711,555
To: D.I. Johnson 76636,2277
I meant the wire from the pole to the house, not the internal wiring. In the
example I gave, the house internal wiring was all copper, but the underground
wire from the pole to the breaker box was aluminum. I know that laws for
internal wiring have been in place for some time--I'm not sure about external
wiring codes.
With low power, some circuits start acting like an antenna and pick up RFI. It
may sound like hum, strangely enough, but it is RFI based.
You should also check that you actually have a strong third prong earth. This
can get knocked loose or corroded pretty easily and it has to be maintained
every few years.
Fm: Mike Mortilla 71333,2704
To: D.I. Johnson 76636,2277
Dear DIJ (sorry I don't know your name),
I missed the top of this thread, but recently I went *CRAZY* finding the source
of a "hum" in my TEAC cassette deck (top of their line).
Assuming a bad head, I bought an Aiwa (which I needed, of course...<g>) and
it's one of their best 3 head machines. I plugged it in and the hum was still
there!!!!!
So I took the deck to various locations and the hum was gone, but when I put it
back on the amp (!) the hum came back!
Bottom line: It was too close to the power source in my amp! Maybe that's a
potential problem?
Fm: James Chandler Jr.-Sysop 71333,2651
To: D.I. Johnson 76636,2277
>>I was just wondering about something...would it have helped if the wiring had
been run through that metal Romex (isn't that what it's called?) conduit when
the place had been constructed? Wouldn't this act as a shield?
My house was built in the '30s, but we had a new run from the pole, new main
disconnect, and entirely new breaker box installed when we moved in about 11
years ago. The internal house wiring is steel jacket copper, dating from the
'30s. The internal wiring is still good, so thankfully we didn't have to cut
any plaster walls to replace it (yet). The wire is stapled to studs in the
walls, so we could never fish new romex to all the locations. My external
block building (where the audio equipment resides) is all new romex in conduit.
Anyway, when we were discussing this with the electrician, he said that
ordinary plastic romex is recommended for housing, and the armored stuff is
recommended for industrial applications, especially if they need to run wiring
in places which would otherwise require conduit. He went so far as to suggest
that if we ever sell, if we get a picky housing inspector, he might make us
replace the metal jacket with ordinary romex.
The electricians could have been selling us a load of bull trying to get more
work, but we got about the same story from several contractors.
Fm: Craig Walsh 70701,1614
To: D.I. Johnson 76636,2277
DIJ: Lottery is a good choice. You're certain to make some cash there and you
don't rack up the big credit card bills like you do in the studio. Craig
Fm: D.I. Johnson 76636,2277
To: K.K. Proffitt -Sysop 76711,555
Thanks for the clarification. I don't expect that would be the problem, though.
The housing development is only a few years old and I don't believe APS (power
co.) is currently using sluminum in any of their installations. Still, I can
ask them while I have them here.
I will also make sure that the ground is solid. I appreciate the suggestions.
Fm: D.I. Johnson 76636,2277
To: Mike Mortilla 71333,2704
That idea also crossed my mind. I did separate the amp from the speakers and
the rest of the equipment as far as I could (which was really only about 10
feet at the time) but found no change in the problem. Perhaps I should try a
more distant connection. Thanks.
Fm: D.I. Johnson 76636,2277
To: James Chandler Jr.-Sysop 71333,2651
That's interesting to know about the romex. I would have expected it to be a
plus when it came time to sell. Oh, well.
Thanks for the info. DIJ
Fm: Mike Mortilla 71333,2704
To: D.I. Johnson 76636,2277
DIG (Sorry, I *STILL* don't know your name):
Can you restate the original problem? IS it just plain old 60 cycle hum? When
and on what pieces of gear does it happen?
Sorry I missed the top of the thread.
Fm: James Chandler Jr.-Sysop 71333,2651
To: D.I. Johnson 76636,2277
>>That's interesting to know about the romex. I would have expected it to be a
plus when it came time to sell. Oh, well.
Well, every locality has different building codes, so it may not apply in your
area.
Fm: rick a. chinn 72163,3406
To: James Chandler Jr.-Sysop 71333,2651
if you replace any of the AC line connected capacitors, use at least a 600V
cap, preferably one that is UL recognized. These caps take the brunt of the AC
line, and for safety's sake, should be UL rated. These caps are usually why
guitar amps arc & spark!
Fm: rick a. chinn 72163,3406
To: D.I. Johnson 76636,2277
OK, i read thru this thread and nobody mentioned this one:
a friend has the same amp, same problem, exactly!
the amp has a ground loop or something internally. i modified his amp to
isolate the input jack grounding from the dirty ground in the amp, but it
didin't help much. I think it may have helped the general case (stereo, driven
from the same source), but i don't think it helped the patch the two inputs
together case.
this is something alesis should attend to as it is a design flaw. I would have
dug deeper into my friend's unit, but it is "under warranty" and I didn't want
to do anything that couldn't be undone later if we sent it off to them.
good luck, you're not crazy, it's the bloody amp!
Fm: rick a. chinn 72163,3406
To: D.I. Johnson 76636,2277
oh yes, you might try floating the shield on one channel only (that is, use the
tip connection only on the phone plug). always make sure that this plug is the
last one to be plugged in (especially if the amp is ON)!
Fm: K.K. Proffitt -Sysop 76711,555
To: rick a. chinn 72163,3406
Wow, Rick. That's really interesting. I'm glad you piped up with the
observation -- it's certainly useful information for future discussions. I
wonder if others have bought the amp and have the same problem?
Fm: Dave Nutzell 71422,2004
To: D.I. Johnson 76636,2277
You can transformer isolate your inputs with a direct box or similar device.
However, since you can hear the hum at the headphones, I would suspect the AC
line. See if your local dealer will let you demo a Furman AR-117 line
conditioner. It may work wonders!
Good Luck!
Fm: D.I. Johnson 76636,2277
To: Mike Mortilla 71333,2704
My original question was about reducing/eliminating the 60 cycle hum from the
output of an Alesis amplifier to my studio monitors. I *think* what is
happening is that the hum is being induced into the input side of the amp.
This happens whenever the amp is plugged into the outputs from any of the gear
in my studio (mixer, cd, dat, cass). The hum is not there if I connect from two
different units - ex. left out from the cd and right out from the dat. It also
happens if I place a jumper across the amp inputs with nothing else connected
but the outs to the monitors.
Several people have made some very good suggestions from lifting ground to
humbucking coils to emi/rfi filters to repositioning the amp. I haven't had
much time the past few day to pursue it, but I'm hoping one of these
suggestions will work.
I've pretty much eliminated ground loop as the problem and all of the cables
check out, so I'm still working on a solution. Thanks for the interest.
Fm: D.I. Johnson 76636,2277
To: rick a. chinn 72163,3406
I did try floating the shield on one channel only. Unfortunately this didn't
seem to help at all. I am planning to swap with a different model amp to see if
that has any effect.
It may well be that the Alesis does have some sort of internal grounding
problem, so I may have to open it up and see what I can see. Thanks.
Fm: D.I. Johnson 76636,2277
To: Dave Nutzell 71422,2004
Can you tell me anything about the Furman line conditioner that you mentioned?
Why, where, how and how much? Thanks.
Fm: J.TUNO[THUNDER] 72110,2326
To: Mike Mortilla 71333,2704
I had the sme problem! Having gear to close to outlets and very "hot" stuff
can cause a hum. I my case, as best as possible anything that was hot from not
and still had a hum as well as a little buzz or two. I moved all of my gear
away and used a differnt outlet disovering everything that the culprit was my
cable line. Can you believe that? although it solved my problem yours will
take a lot of trial and error. Wish I could offer more. Joe Tuno 72110.2326
Fm: rick a. chinn 72163,3406
To: D.I. Johnson 76636,2277
what i found is that the input diffamp shares the same ground as the dirty
ground from the output stage.
in most amps, the input grounds (ground ref for diffamp and feedback divider
ground) are separated from the other (dirty) grounds, and returned back to the
power supply magic-point via their own (SEPARATE) wire. Then the two grounds
are tied together via a low-value resistor to ensure that they're close
together for RF, and (probably) to facilitate factory testing.
look at a crown amp, that's what they've been doing since the DC300.
btw, it might help to shield the cabling from the input jacks to the pots, and
then to the amp boards, and it might help to play with the orientation of this
wiring vs the power transformer.
Fm: James Chandler Jr.-Sysop 71333,2651
To: rick a. chinn 72163,3406
>>if you replace any of the AC line connected capacitors, use at least a 600V
cap, preferably one that is UL recognized.
Thanks for the tip. I think I'll go get a new grounded AC cable and two big
orange Mylar caps, do everything in one shot.
Fm: D.I. Johnson 76636,2277
To: rick a. chinn 72163,3406
What would you suggest as an acceptable range of values for the resistor tie-in
to the ground? (I can do the practical work, but my knowledge of component
electronics is weak.)
>>btw, it might help to shield the cabling from the input jacks to the pots,
and then to the amp boards, and it might help to play with the orientation of
this wiring vs the power transformer.
I'll look at this as well if I open the critter up. Thanks.
Fm: Jeff Gilbert 71333,2424
To: D.I. Johnson 76636,2277
Sounds to me like you want to telscope the shield on one side of a stereo cable
pair feeding the amp. At the amp end of one cable, disconnect the shield wire
from the plug's shell. You'll still have a ground reference via the other
cable, but you will have eliminated a ground redundancy loop.
Jeff Gilbert / 71333,2424 / Mackie Designs
Fm: D.I. Johnson 76636,2277
To: Jeff Gilbert 71333,2424
Thanks for the suggestion. I have tried this approach, but there seems to be
little or no change in the problem.
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