Passive Balanced Attenuator (between Pre & Power) - diyAudio
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Old 9th December 2014, 03:38 AM   #1
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Default Passive Balanced Attenuator (between Pre & Power)

Hi,

I need to attenuate the overall gain of my pre + power amp.
The simplest solution is of course to purchase a in-line attenuator like the Rothwell Audio "balanced in-line attenuators".
balanced in-line attenuators
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But am think might as well DIY-ing it & combine them into the XLR interconnect itself.

Just to check from the experts here which circuits is correct (using the U-pad design).

Many thanks in advance.

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Old 9th December 2014, 11:07 AM   #2
AndrewT is offline AndrewT  Scotland
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I used to recommend circuit A

D.Self explains why circuit B is the better.

I now don't recommend circuit A.

BUT !!!!!!!
Pin1 goes to Chassis. Pin1 NEVER goes to Audio Ground.
Change the labels to audio ground.
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Old 9th December 2014, 02:53 PM   #3
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Am I correct to say that both circuit will work ?
(ie it will not blow up my pre or power amp ?)

For circuit B, do I need to half the R2 value ?
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Old 9th December 2014, 03:18 PM   #4
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Use circuit A as it's easier to implement. If you are not totally clued up on balanced lines using circuit B could result in ground loop problems.
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Old 9th December 2014, 03:24 PM   #5
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by richie00boy View Post
Use circuit A as it's easier to implement. If you are not totally clued up on balanced lines using circuit B could result in ground loop problems.
circuit B is the correct way to do it. Provided Pin1 is NOT connected to Signal Return/Ground.

Self tells us why Circuit A is not correct.
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Old 9th December 2014, 03:27 PM   #6
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Thanks.

My power amp is DIY NC400. Pin 1 is connected to the Chassis which is also connected to the XLR socket casing (I think).

Audio ground is connected to XLR socket casing.

I think that will mean that Pin 1 is also connected indirectly to audio ground of the NC400 board, right ??
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Old 11th December 2014, 01:53 AM   #7
CZ101 is offline CZ101  United States
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Quote:
Originally Posted by AndrewT View Post
circuit B is the correct way to do it. Provided Pin1 is NOT connected to Signal Return/Ground.

Self tells us why Circuit A is not correct.
Can you elaborate on why Circuit A is worse than Circuit B?

Circuit B requires tight matching of R2 for CMR.
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Old 11th December 2014, 02:27 AM   #8
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Where in Doug Self's many books and papers does he enplane that?
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Old 11th December 2014, 09:58 AM   #9
AndrewT is offline AndrewT  Scotland
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The papers that discuss balanced impedance connections.

It came up again in a Thread very recently. I think the posts were relating to a Linear Audio article and a reprint in one of the electrical webzines.
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Old 11th December 2014, 10:27 AM   #10
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From all the posts here, seem like both can work (ie will not blow up my pre, power or speakers).

But circuit B is idea provided ........... audio ground no good, but only chassis ...................... this I'm confuse.
Also, if I can get this correct, do I need to half my R2 value (like in the case of R1) for circuit B ??

The resistors are cheap, if needed to, can always do 2 to compare any different. Most important is not to blow up anything.


My pre-amp is ARC DS-pre. (which I think their output impedance is 500 Ohms)
My power amp is DIY mono Hypex Ncore NC400 (which I think their input impedance is 104 K-Ohms).

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Last edited by tane0019; 11th December 2014 at 10:32 AM.
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