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Diagnosed and solved my ground loop issue, but looking for a safe solution

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#21
Sorry, the +10dBu THD sweep should be

TF THD +10dBu.jpg
 
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#23
The very high 3rd and 5th harmonic distortion concerns me.. in terms of percentages what are we looking at? and would you say these would be audible?
Yeah they are high. Sorry I was in a hurry and didn't normalise to the fundamental. As an eg below is the 0dBu sweep with THDs in %. Yes rather too high. >0.1% at frequencies less than 600 Hz. Ideally you would want an order of magnitude less than your amp. That's a SE tube amp right? Specs seem <0.3% so you might not hear it.

TF THD % 0dBu.jpg
 
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#24
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xevman
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Thread Starter #25
Yeah it's a Single ended triode amp so it would have high 2nd and 3 order distortion naturally. I actually just rigged it up to AB test with and without the transformer by splitting the output on the Topping D90 as pictured below and using the input switcher on the Woo to switch between them. So far I'm unable to tell the difference with the transformer in line and not.
 

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#26
I'm struggling with ground loop noise as you are. Mine is reversed from your problem. I have an unbalanced preamp (Schiit Heresy) and a balanced amplifier (Crown XLS 1500) and the noise is awfully distracting. I tried adding extra ground cables from the Heresy to the Crown and that reduced the noise by a few DB but didn't eliminate it. I have tried multiple different RCA cables. Nothing seems to eliminate the noise if I don't drive the amp with a balanced connection.

The problem, as with all ground loop problems, is the difference between the ground potential at the Heresy and the Crown. With an unbalanced connection, any variation in this potential is impressed onto the signal as noise (please excuse any inaccuracies as I'm not a tech). When I added cables to lower the resistance of the ground connection the difference in potential was reduced but the problem didn't go away. It seems that ground noise is not the only problem. I read about "pseudo balanced" connections which appear to be an option. The idea is to use a balanced cable that terminates on an XLR as normal and connect an RCA connecter on the other end, with the shield connected to the RCA body, the + of the twisted pair to the hot pin on the RCA, and the - of the twisted pair to a resistor that is the same size as the output resistor of the Heresy, in this case,100 ohms, to the body of the RCA connecter where the shield is connected. This balances the twisted pair and eliminates noise picked up in the cable (in theory).

I wouldn't be trying this if it wasn't for also owning a Behringer umc404hd which doesn't have any noise driving the Crown and AFAIK is using the "pseudo balanced" topology. I just need to pick up some microphone cable and a few connectors to give it a try.
 
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xevman
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Thread Starter #27
I'm struggling with ground loop noise as you are. Mine is reversed from your problem. I have an unbalanced preamp (Schiit Heresy) and a balanced amplifier (Crown XLS 1500) and the noise is awfully distracting. I tried adding extra ground cables from the Heresy to the Crown and that reduced the noise by a few DB but didn't eliminate it. I have tried multiple different RCA cables. Nothing seems to eliminate the noise if I don't drive the amp with a balanced connection.

The problem, as with all ground loop problems, is the difference between the ground potential at the Heresy and the Crown. With an unbalanced connection, any variation in this potential is impressed onto the signal as noise (please excuse any inaccuracies as I'm not a tech). When I added cables to lower the resistance of the ground connection the difference in potential was reduced but the problem didn't go away. It seems that ground noise is not the only problem. I read about "pseudo balanced" connections which appear to be an option. The idea is to use a balanced cable that terminates on an XLR as normal and connect an RCA connecter on the other end, with the shield connected to the RCA body, the + of the twisted pair to the hot pin on the RCA, and the - of the twisted pair to a resistor that is the same size as the output resistor of the Heresy, in this case,100 ohms, to the body of the RCA connecter where the shield is connected. This balances the twisted pair and eliminates noise picked up in the cable (in theory).

I wouldn't be trying this if it wasn't for also owning a Behringer umc404hd which doesn't have any noise driving the Crown and AFAIK is using the "pseudo balanced" topology. I just need to pick up some microphone cable and a few connectors to give it a try.
Let us know how you go, there are many ways you can easily solve this issue safely. If you dont want to mess with isolation transformers in the signal path you could easily connect all your gear to an AC isolation transformer (a proper one with true isolation not the ones that pass ground through or tie it to neutral) and that will definately kill the mains buzz.
 

AnalogSteph

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#30
Likely a ground loop between the PC and DA converter thats going through the shield of the interconnect.
Or rather, a ground loop between the IEC Class I PC + DAC (connected by cable shield / power ground) and the IEC Class I amplifier.

Said ground loop would occur just the same if you used onboard audio, an internal soundcard or any number of other inexpensive USB DACs with unbalanced output (fancier ones may have USB galvanic isolation).
 
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#31
Hmm, this pseudo-balanced idea seems to work. I didn't have the resistors so I tried a secondary method. I used a male XLR connector on 2 wire + shield microphone cable and then at the RCA end I didn't connect the shield and connected the + wire to the hot on the RCA and the - wire to the ground on the RCA. At the Heresy's low gain setting I no longer have any noise. The high gain setting still is picking up some noise but I'm pretty sure if I connected the shield at the RCA end and added a 100 ohm resistor between the - wire and the RCA ground, I could eliminate the noise.

Interesting experiment. I should add that the noise was there even with the DAC disconnected from the Heresy, so it wasn't just a PC related ground loop. I like this solution as it doesn't require a transformer or anything else that could increase distortion.
 
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