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Old 20th May 2016, 12:45 AM   #1
surfstu is offline surfstu  United Kingdom
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Default Balanced or Unbalanced

Question for people with far more expertise than me.

I'm using 10m of XLR cable to connect a DJ mixer into a Class D amp that I'm building. The amp NEEDS volume control as I am also integrating a Pi and DAC into the chassis. I can not afford a fancy stepped attenuator and am using an alps blue 10k pot.

My question is... Is it better to change the signal to unbalanced when it arrives at the amp and then attenuate the unbalanced signal or will it be better to implement my volume as shown in the attached image?

I will be using 5K series resistors and a 10k pot as this is what I have

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Old 20th May 2016, 02:35 AM   #2
tomchr is offline tomchr  Canada
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I would either use a differential/balanced volume control or a differential/balanced receiver. An example of a differential receiver would be my THAT Receiver, which uses the THAT1200 to convert from balanced to unbalanced.

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Old 20th May 2016, 12:06 PM   #3
surfstu is offline surfstu  United Kingdom
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Have spent almost 2 days reading up about attenuating balanced signals. I do not have the time or resource so I have developed a workaround and would love for someone to agree with my logic!

I will only attenuate the unbalanced signal from the internal DAC and will allow the XLR input to go full signal to the power amp, volume can be adjusted at source preamp.

I will run XLR input 1 on entry to chassis ground (also safety ground by way of IEC) but then run shielded cable to the switch, connect shield to pin 1 of XLR and not connect the shield to the selector switch

I have attached a diagram but would love some feedback.... and I have purchased 4 highendish 5K resistors +/-0.1% and would like to know if i can place these in series with the hot cold signal of XLR to slightly attenuate the signal - I am concerned that this amp is far too powerful for speakers. This is a gift to a DJ friend and I don't want the Mixer to have to be set to level '1 or 2' before damaging the speakers at level 3!!

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Old 20th May 2016, 03:14 PM   #4
Speedskater is offline Speedskater  United States
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Remember that XLR pin 1 is a shield not audio circuit common (aka ground).
The shield should just shield the signal conductors and not be a separate path.
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Old 20th May 2016, 03:40 PM   #5
AndrewT is offline AndrewT  Scotland
Join Date: Jul 2004
Location: Scottish Borders
Convert your balnced signal to unbalanced. Adjsut the volume of the unbalanced signal. Convert the attenuated unblanced signal back to balanced.

Bruno Putzeys' balanced vol pot does exactly that.

You missed the group buy, but I think Tfboy confirmed he has a very few PCBs and one smd complete board still available.

Read D.Self's article on balanced signal volume attenuation. It tells you why your post1 pic is not good.
For a while after I joined this Forum I was recommending that post1 method to a few Members. I don't now. I know better.
regards Andrew T.

Last edited by AndrewT; 20th May 2016 at 03:43 PM.
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