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balanced stepped attenuator wiring
balanced stepped attenuator wiring
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Old 25th February 2019, 09:16 PM   #1
nugat is offline nugat  Europe
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Default balanced stepped attenuator wiring

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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Old 25th February 2019, 09:27 PM   #2
Michael Bean is online now Michael Bean  United States
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I presume it uses a four gang rotary switch? If so, are the contacts make-before-break?

Mike
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Old 25th February 2019, 09:32 PM   #3
rayma is online now rayma  United States
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Quote:
Originally Posted by nugat View Post
I am experiencing some noise with a 4-gang balanced stepped attenuator integrated into an amplifier.
Post the schematic. Is this in both channels?
Is there an input coupling capacitor after the attenuator?

Last edited by rayma; 25th February 2019 at 09:36 PM.
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Old 25th February 2019, 09:49 PM   #4
nugat is offline nugat  Europe
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3544427C-EAEC-4812-8CBF-BB64E1BA9C02.gif

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.

Last edited by nugat; 25th February 2019 at 09:52 PM.
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Old 25th February 2019, 10:51 PM   #5
rayma is online now rayma  United States
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Quote:
Originally Posted by nugat View Post
I am trying to integrate the DACT attenuator into a hypex n-core modules based amplifier.
That looks ok. I'd say the problem is in the amplifier input wiring.
Post that schematic also.
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Old 25th February 2019, 11:15 PM   #6
nugat is offline nugat  Europe
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The hypex NC400 input is just one inverting, one non-inverting and ground- when feeding differential signals, page 10 of the manual. What can go wrong here?
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Old 25th February 2019, 11:24 PM   #7
rayma is online now rayma  United States
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Quote:
Originally Posted by nugat View Post
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/..._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.

Last edited by rayma; 25th February 2019 at 11:45 PM.
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Old 26th February 2019, 12:08 AM   #8
jhofland is offline jhofland  United States
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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.
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Old 26th February 2019, 12:10 AM   #9
rayma is online now rayma  United States
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Quote:
Originally Posted by jhofland View Post
You will need to implement a balanced attenuator that does not rely on a connection to ground.
Yeah, I didn't like that circuit much either.
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Old 26th February 2019, 12:40 AM   #10
nugat is offline nugat  Europe
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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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