28th February 2012
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#1 | | Gear maniac
Joined: Apr 2007 Location: fr
Posts: 165
Thread Starter  | 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 !
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28th February 2012
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#2 | | Gear maniac
Joined: Apr 2007 Location: fr
Posts: 165
Thread Starter |
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  | |
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28th February 2012
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#3 | | Gear maniac
Joined: Apr 2007 Location: fr
Posts: 165
Thread Starter |
[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 ?
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28th February 2012
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#4 | | Gear addict
Joined: Apr 2009 Location: Seattle, WA
Posts: 453
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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.
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28th February 2012
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#5 | | Lives for gear
Joined: Feb 2010
Posts: 5,476
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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.
__________________
"I believe that entertainment can aspire to be art, and can become art, but if you set out to make art you're an idiot."
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Show business: we're all here because we're not all there.
Resistance is not futile. It is voltage divided by current.
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28th February 2012
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#6 | | Gear maniac
Joined: Apr 2007 Location: fr
Posts: 165
Thread Starter |
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)?
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29th February 2012
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#7 | | Gear addict
Joined: Apr 2009 Location: Seattle, WA
Posts: 453
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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... | |
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29th February 2012
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#8 | | Gear addict
Joined: Apr 2009 Location: Seattle, WA
Posts: 453
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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.
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29th February 2012
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#9 | | Gear maniac
Joined: Apr 2007 Location: fr
Posts: 165
Thread Starter |
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...
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29th February 2012
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#10 | | Gear addict
Joined: Apr 2009 Location: Seattle, WA
Posts: 453
| 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...
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29th February 2012
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#11 | | Gear maniac
Joined: Apr 2007 Location: fr
Posts: 165
Thread Starter |
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 !
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29th February 2012
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#12 | | Gear addict
Joined: Apr 2009 Location: Seattle, WA
Posts: 453
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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.
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29th February 2012
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#13 | | Gear maniac
Joined: Apr 2007 Location: fr
Posts: 165
Thread Starter |
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) !
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29th February 2012
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#14 | | Gear addict
Joined: Apr 2009 Location: Seattle, WA
Posts: 453
| Quote:
Originally Posted by kragg  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.
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29th February 2012
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#15 | | Lives for gear
Joined: Feb 2010
Posts: 5,476
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I would still swap the wires on the internal power supply and get rid of the converter, which is unnecessary.
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1st March 2012
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#16 | | Gear maniac
Joined: Apr 2007 Location: fr
Posts: 165
Thread Starter |
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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