balanced stepped attenuator wiring

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I am experiencing some noise with a 4-gang balanced stepped attenuator integrated into an amplifier. The noise starts from ca 2/3 of the scale and grows, but the top position=no attenuation is noise free. I see solutions with resistors added to ground lines out. Am I on the right track? What is it done for? TIA
PS. no noise in the single-ended configuration.
 
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I am trying to integrate the DACT attenuator into a hypex n-core modules based amplifier.
The noise (electronic buzz) appears on both channels. There are only cables between the DACT and
the amp modules inputs.
 
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The hypex NC400 input is just one inverting, one non-inverting and ground- when feeding differential signals

Is it wired per figure 2 here? https://www.lautsprechershop.de/pdf/hypex/NC400_datasheet.pdf
Does the buzz go away if you only connect one cable? Is this a monoblock or a stereo chassis?
Are you running balanced or single-ended source? Have you tried a different source?

The noise appears when it does, because that point is at the maximum source resistance
presented by the attenuator to the amplifier (-6dB), so it's probably a ground loop.
 
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my 2 cents. A balanced input is supposed to work on the difference between the two input signals and should not be connected to ground. Whatever common mode interference exists on the ground connection is now part of your intended signals. See Bruno Putzey's article regarding the "G" word, which is his attempt at trying to denote that ground is not an absolute. You will need to implement a balanced attenuator that does not rely on a connection to ground.
 
My set up uses mains safety earth. Inputs are only balanced. Pin 1 to chassis.
Now the news for me is that the balanced attenuator should use no ground. DACT is a fairly
known brand, at least in Europe. Is this really the original sin and the cause of my troubles?
All my grounds go to chassis/ safety earth. How can it be done differently with the balanced attenuator? Is there any way I can still use the DACT noise-free?
What really puzzles me is that in the max volume ie. fully open/no attenuation position the signal is clean.
 
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What really puzzles me is that in the max volume ie. fully open/no attenuation position the signal is clean.

The noise appears when it does, because that point is at the maximum source resistance
presented by the attenuator to the amplifier (at the -6dB setting). It's probably a ground loop.

You may have to add an impedance from the chassis to the audio ground, say 10R for each channel.
 
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Balanced connection are really needed for decoupling from common mode noise and ground loops.

It’s a really bad idea to trying regulate volume there.

You need true another thing - first put a balanced-to-unbalanced converter with good CMRR, then regulate volume at unbalanced signal and then inside chassis wire your Hypex balanced input as to unbalanced source needs. Or at last step provide good unbalanced-to-balanced converter and wire it between different chassis straightly to the Hypex balanced input.
 
On one hand Putzeys writes that balanced volume control is tricky (except when done with his non-standard solution), on the other, n-core modules invite a differential signal input.
4-deck passive balanced attenuators are manufactured, sold and used. Resistor based ones and TVC or autoformer ones. So surely there is some experience and practical solutions for
dealing with ground loops etc. ?
 

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...noise starts from ca 2/3 of the scale and grows, but the top position=no attenuation is noise free......

If the noise goes-away at bottom and at top, it is not in the ground.

It is most likely due to wiper-wires going past bad places. See attached. Full-down or full-up, the ground or source shorts-out the interference. But at mid-way the wiper impedance is 1/4 the value of the pot, which may be many K. It takes just a little coupling to inject crap into such medium-high impedances.

(Perfect balance of pot and cable would cancel this; but no pot is perfect balanced.)

So where are your wiper wires going??
 

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Balanced connection are really needed for decoupling from common mode noise and ground loops.

It’s a really bad idea to trying regulate volume there.

You need true another thing - first put a balanced-to-unbalanced converter with good CMRR, then regulate volume at unbalanced signal and then inside chassis wire your Hypex balanced input as to unbalanced source needs. Or at last step provide good unbalanced-to-balanced converter and wire it between different chassis straightly to the Hypex balanced input.

I have a couple of Jensen Isomaxes so I could easily do it. My single-ended amps
have no problems without them. However, I set myself a goal of building a truly balanced differential amp with a passive volume control built-in and that's the fun I am after. Another amp module on the table is fully bridged, so the differential signal will go down all the way to subs' drivers.
 
If the noise goes-away at bottom and at top, it is not in the ground.

It is most likely due to wiper-wires going past bad places. See attached. Full-down or full-up, the ground or source shorts-out the interference. But at mid-way the wiper impedance is 1/4 the value of the pot, which may be many K. It takes just a little coupling to inject crap into such medium-high impedances.

(Perfect balance of pot and cable would cancel this; but no pot is perfect balanced.)

So where are your wiper wires going??

Now, this is some possible explanation or a hypothesis to falsify. I am thinking
about an experiment. Perhaps star-quad canare cabling would help? BTW, the switching amp modules have their dedicated switching power supplies. All in the same cabinet.
PS. What is this violet plasma potential EMI source ? Don't have that one at home;)
 
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