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Old 14th May 2002, 04:37 PM   #1
swede is offline swede  Switzerland
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Default Leach Amp: Ground loop problem

Hi all,

I started to discuss my ground loop problems in another thread, not dealing with that problem, so I decided to start another thread.

I have a slight problem with my Leach Amp, that is I have a small hum on my outputs. When I look at it on my scope, I can see that it is about 10mV "spikes" with a frequency of 50Hz there. It goes away when I disconnect one input from my CD-player.

Prof. Leach has one solution on:
http://users.ece.gatech.edu/~mleach/lowtim/part2.html

As is says there there is a more or less speedy solution for it:


1) Turn the amplifier off and wait for the power supply to discharge. Do not perform this procedure with the amplifier on.
2) Cut the wire to the central ground on the input side of one circuit board.
3) Solder a short circuit jumper wire between the ground lugs on the two input jacks.
4) The circuit board with the cut ground wire is now grounded back through its input ground lead to the ground of the other circuit board. Use an ohmmeter to verify the new ground connection before turning the amp back on.


I'm not too kean to draw my signal ground around my amp like that, since I've built it modular, so I can disconnect any of the boards whenever I like. In that case, I have to take special precautions if I want to run only one channel.

Well, what if I just cut both grounds on the input connectors instead, and draw a new ground lead from the inputs to the central ground. Would that be really wrong? I'll just go ahead and try it, but if anyone of you have a better idea, please let me know.

What can cause this kind of ground loops in a Leach Amp?

//magus
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Old 14th May 2002, 05:00 PM   #2
BrianGT is offline BrianGT  United States
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I would think that since you have isolated the problem to one channel, and it is a modular design, you should be able to farther isolate the problem.

I would check check everything on the board and see if component is improperly grounded. It could be as simple as one of the output devices is touching the heatsink.

I would start with checking your output devices, and checking. One thing that you can do, which Prof. Leach recommends doing is running the circuit without the output devices. You can easily do this by:

1. Connect two 100 ohm resistors from the loudspeaker output to one side of R36 tothe loudspeaker output to the other side of R36.

2. place a short circuit jumper in parallel with C12.

This will allow you to test the circuit without the output devices, which will help determine the source of your problem.

As for doing this, you need to do no soldiering. When I was testing my boards in Dr. Leach's lab, he has a setup for the resistors, which just consists of 3 alligator clips and some resistors that does #1, and then 2 interconnected alligator clips for #2. You can connect to other components on the board to reference those points. If you look at the circuit diagram and the component placement, you should be able to find a place on the board to clip onto. I do not have my boards with me, so I can't give you exact places. I do remember that for #2, the two alligator clips connected to Q14 and Q15.

If you determine that your board runs fine without the output devices, I would check everything referenced to ground, to see if anything is tied wrong.

This should narrow the problem down much more. I would do this before cutting wires and doing the task that you mentioned.

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Brian
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Old 14th May 2002, 07:19 PM   #3
Apogee is offline Apogee  United States
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You might also check your power supply.

I don't know if you've built monoblocks or exactly how the channels are configured but check the polarity on your transformer leads. If you have two transformers and the polarity on one is reversed, I've seen it create ground loops...

Simply try reversing the plug one one unit (if monoblocks) where it enters the wall... If common chassis with two transformers them try reversing the polarity on one of them...

Just an idea,

Steve
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Old 14th May 2002, 07:30 PM   #4
BrianGT is offline BrianGT  United States
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Good point. Another thing that you could do is bypass the power supply and use a bench power supply to test it. You could use a lower voltage power supply even, and it would not matter. That would narrow the problem down even more.

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Old 14th May 2002, 08:08 PM   #5
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Probaly is hard to believe,that i give you advices,but anyway
How loud is hum?In my amp hum is there but you cannot hear it from lets say 20cm.When you put ear close to speakers you can hear slight hum.
Maybe you have noise in the mains or something is out of phase like fellow members say.
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Old 14th May 2002, 11:03 PM   #6
swede is offline swede  Switzerland
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Hi all,

I will do some more testing like BrianGT suggest. That is also the way Prof. Leach recommends on the web-page to do the testing. I had not thought on the solution with the aligator clips before, though. Really nice. I've been delaying that kind of testing far to long now, since I thought I had to solder a whole bunch of stuff to my boards.

I have to admit that I have about -30mV offset on my right board and about +10mV offset on my left board. It can have some thing to do with my lack of matching on both the zener diodes and my input transistors on one board. <---------- gag warning...

In my setup I have one toroid transformer in one box for two channels. I've connected it according to the "pic 2" schematic in

the Hey Helix thread . I will check on the polarity from my transformer to the rectifiers. Could that have any effect on the ripple on my output? Well, yes (when I think about it). I'll try that tomorrow (too late now).

Have a nice day,
//magnus
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Old 15th May 2002, 06:10 PM   #7
swede is offline swede  Switzerland
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Ok, I tried to draw the ground leads on the input terminals directly to my reference ground point, but it just got worse that way. Lesson learned.

I tried Prof. Leach's recommendation, and there... Some 25+ years of experience won again. The hum disappeared. Good for me. New project please... ;=)

Well, and I found out that I didn't have to sacrifice my "modularity", since I have used dual pole connectors for my grounds from my boards, so I just disconnected the signal ground from one of the boards after I joined the ground leads on the input terminals. I just need to connect it again if I want to remove one of the boards.

Theory is one thing, practice another.

Thanks for your input!
//magnus
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Old 15th May 2002, 06:19 PM   #8
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Good for you!
Pictures of my amp soon...very soon
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