Hum (ground loop?) issue when using HT pass through on SDS-254 amp - diyAudio
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Old 18th August 2011, 03:43 PM   #1
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Default Hum (ground loop?) issue when using HT pass through on SDS-254 amp

I have a weird hum (ground loop?) issue with an amp I built using an SDS-254 kit from classdaudio.com.

My source right now is a Wyred4Sound DAC-2, which is connected directly to the amp. However, please note that I had the exact same problem when the HT pass through was used when a Parasound 2100 preamp was connected to the amp, so I doubt the problem lies in the DAC-2. More over, I switched the SDS-254 amp with a Denon integrated amp, and I did not encounter the issue, so I think the problem is definitely with the SDS-254 amp.

The problem is that whenever the HT pass through mode is engaged in the DAC-2, I get a significant hum through my speakers. I also get a loud pop when I change to another mode (i.e. USB). Now, before someone suggests the problem might lay in my Denon surround sound receiver (L & R pre-outs used for HT pass through), this problem exists regardless of whether or not anything is connected to the HT pass through inputs on the DAC-2.

If I am using the DAC-2 as the actual source (i.e. digital audio via USB), then I don't have any problems with the amp (no hum or pops).

I have modified the amp into the following configurations. At all times, the unbalanced RCA input jacks have been insulated from the chassis and not connected to anything other than the amp board inputs. The problem has persisted through all of these configurations.

1) Earth ground (AC ground) connected to chassis, amp board connected to chassis.
2) Earth ground NOT connected to chassis, amp board connected to chassis.
3) Earth ground connected to chassis, amp board isolated.

I have not tried disconnected both the earth ground and amp board from the chassis. Also, as far as I know the power supply is not grounded to anything.

Does anyone have any ideas why this strange problem exists? What is causing it, and how do I fix it?

Thanks.
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Old 19th August 2011, 04:06 AM   #2
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No takers?

I just tried unplugging all of my other equipment from the wall. The only things left on were the DAC-2 and the amp. Still, no dice. I also tried grounding the unbalanced RCA inputs to the chassis, so that the input ground and amp circuit ground were tied together. It didn't make any difference.

I also tried replacing the amp with my Denon integrated again, just to make sure the Denon didn't exhibit the problem. As I feared, there was no hum when using the Denon.

Now, this may be a dumb, noobish, question, but should the toroidal transformer be grounded to anything? Unfortunately, the transformer leads are not marked, and the only thing I had to go by was a simple diagram which came with the kit. According to the diagram, the transformer is not connected to ground in any fashion.
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Old 20th August 2011, 02:34 AM   #3
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Update number 3:

I have attempted to join the transformer ground wire (or what I think it should be) to first the chassis and then the AC safety ground. There was no change.
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Old 20th August 2011, 12:01 PM   #4
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Disconnect the transformer center tap and reconnect it in series with a 10 ohm 1 watt resistor.
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Old 21st August 2011, 01:09 AM   #5
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Quote:
Originally Posted by Nico Ras View Post
Disconnect the transformer center tap and reconnect it in series with a 10 ohm 1 watt resistor.
Thanks for the response, but this didn't fix the problem.
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Old 21st August 2011, 01:51 AM   #6
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If you don't have the amp connected to anything but speakers and short the inputs, does it still hum?

Mike
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Old 21st August 2011, 02:36 AM   #7
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Quote:
Originally Posted by Michael Bean View Post
If you don't have the amp connected to anything but speakers and short the inputs, does it still hum?

Mike
No, there is absolutely no hum when the inputs are shorted. Do you think that the cables are picking up some kind of interference?
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Old 21st August 2011, 09:30 AM   #8
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Then it is definitely a ground loop. You have two electrical ground return paths to the inputs of the amplifier. The only resolve is to find it. Try disconnecting one ground from an input terminal inside the amp.
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Old 23rd August 2011, 02:35 AM   #9
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I tried disconnecting one the ground wire from the left input and vice-versa with the right input, but there was no change.
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Old 24th August 2011, 10:26 AM   #10
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The only other thing that I can think of then is that the amplifier is picking up stray magnetic field from the transformer. This can happen as soon as you connect leads to the input. Can you move the transformer away from the amplifier. Maybe move the wiring. What you are experiencing is very annoying and very disappointing.
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