Weird ground loop from the amp
Old 28th February 2012
  #1
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Weird ground loop from the amp

Hi

I have spent quite some time in the studio hunting ground loops & hum noise, and all is nearly fine, unless one -quite annoying- thing : i have a nasty and quite unusal hum noise coming from the amp when it is powered ON.

(My amp is a Crown D75a, and, as i bought it in the USA but live in Europe, it is powered by a 110>220v adapter.

The weird stuff is that the hum noise is louder in the 2th and 3d harmonics area (150Hz), instead of coming from the usual 50-60Hz...

My amp of course doesn't have a "ground lift" switch, and i am already using symetric cables whenever possible. I don't want to disconnect the ground from the pug, as it would become hazardous, but i don't know any other solution to get rid of 150Hz humming : could anyone help ?


PS : here is a screenshot of the audio signal coming inside my computer when the amp is powered OFF :


... and powered ON :


Thanks in advance !
Old 28th February 2012
  #2
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Hi, Frank, thank you for your reply.
Yes, i guess it must come from the PSA, but i really don't know what to do to fix it
Old 28th February 2012
  #3
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[edited : the issue comes from the adapter, not from the amp, as the hum can be seen on the spectrum analyzer as soon as the adapter is powered ON]

Ok, just to check, i tried to ground lift the PSA (using a 3-to-2-pole plug adapter), and the humming is gone.
Now, as it is not a safe solution, i would be curious to know how to fix it safely and for good ?
Old 28th February 2012
  #4
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You could try an isolating transformer on the lines feeding the Crown. Maybe the ebtech line level converter to bring your signal from +4 bal down to -10unbal to drive the amp. I believe this would isolate the ground as well.
Old 28th February 2012
  #5
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are you sure that the amp cannot be powered via 220? A lot of gear is made for worldwide sales. Changing from one voltage to another is a matter of swapping a wire on the power supply. I don't know about such an old Crown.

Short of that, get a better AC converter. I lived in Europe years ago and those budget models are really junky. But they are made for razors and stuff like that.
Old 28th February 2012
  #6
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No, there is no switch on the back, sadly.

I know the PSA is not the best thing in the world, but i took the one which was supposed to be the best (i didn't had much choice ).
I know you're not in Europe, so maybe my question is dumb, but would you know any PSA model or brand with a good reputation which might work better ?

Quote:
You could try an isolating transformer on the lines feeding the Crown.
Yes,i have a Behringer HD-400 which works fine on some other gear that i will try inserting there. I guess it does quite the same than the EBTEch, isn't it (at least, it is supposed to, afaik)?
Old 29th February 2012
  #7
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My D150 can be switched 220 via the internal transformer wiring.

I'll scan the part of the manual that describes how to do it. You'd want to be sure that the D75 has the same wire colors etc..

uno momento...
Old 29th February 2012
  #8
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Here's the Power Supply info for the D150, I'm not guaranteeing that it's the same as a D75. You can open it up and see if it's the same.
Attached Thumbnails
Weird ground loop from the amp-d150-ps003.jpg   Weird ground loop from the amp-d150-ps003-wiring.jpg  
Old 29th February 2012
  #9
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Thank you very much, A Zombie, it is very kind from you
But i have contacted the Crown tech support, and told me that my amp didn't feature such a switch, and the only way to change the voltage without an external PSA was to change the PSU inside...
Old 29th February 2012
  #10
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Quote:
. . .was to change the PSU inside
What I posted explains how to do just that. Basically you're just reconfiguring the way the transformer is hooked up to change the voltage ratio.

It involves desoldering and resoldering some wires.

If this is not something you feel comfortable doing, you might want to get a tech to do it for you. It is a simple job...
Old 29th February 2012
  #11
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Ok, i thought it required to change the whole internal psu (not just rewiring/soldering).
You're right, i am not a tech guy at all, so i will go and see a proper tech to give the unit a skilled eye.
Thank you again for your help !
Old 29th February 2012
  #12
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Check out page 26 of the crown D60 manual

The D60 is pretty similar I believe (more similar than the D150) so it will be more useful than what I posted earlier.
Old 29th February 2012
  #13
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Ok, so, a little report : a friend came today, and told me that those 110>220v PSA were not connecting the earth/ground plug from the amp, so he made a custom plug to connect back the earth/ground from the amp to the main plug, and now indeed the humming is gone...
I guess it is a bit the ghetto solution but it works (and it is safe) !
Old 29th February 2012
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Quote:
Originally Posted by kragg View Post
Ok, so, a little report : a friend came today, and told me that those 110>220v PSA were not connecting the earth/ground plug from the amp, so he made a custom plug to connect back the earth/ground from the amp to the main plug, and now indeed the humming is gone...
I guess it is a bit the ghetto solution but it works (and it is safe) !
Nice, that makes perfect sense.
Old 29th February 2012
  #15
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I would still swap the wires on the internal power supply and get rid of the converter, which is unnecessary.
Old 1st March 2012
  #16
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Thanks everyone for all the tips.
So far, i think i will go making music, and let the things as they are for a while :D
If i want a cleaner/smarter solution, i will try to rewire and resolder the internal amp, but for now my 2$ self-made plug shoudl do the trick (as far as it is safe and the audio is clean) !
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