BPPBP - Bruno Putzey's Purist Balanced Preamp (well a balanced volume control really) - diyAudio
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Old 5th September 2015, 09:40 AM   #1
Bertel is offline Bertel  Germany
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Default BPPBP - Bruno Putzey's Purist Balanced Preamp (well a balanced volume control really)

I'm currently doing some research on preamp concepts which basically resemble a balanced volume controller, if I may say so.

While doing so, I came across a design by Bruno Putzey which he lays out here: http://www.hypex.nl/docs/papers/The%20G%20Word.pdf (originally written for Linear Audio Linear Audio | your tech audio resource , Volume 5 came out in April 2013 with a free PCB for it)

This design includes the following features:
- Buffer as input stage with input biasing network
- Difference amplifier between input stage and variable gain stage, with the output of the difference amplifier being referenced to the cold point of the volume controller
- Linear potentiometer used as feedback element in an inverting amplifier, not as attenuator
- No output stage (almost)
- DC removal circuit (2nd order low-pass filter whose output is subsequently subtracted from the signal)

So basically bal to unbal, followed by unbal active volume control as part of a difference stage, then back to bal with passive balanced impedance output.

Has anyone actually built this preamp? If so, what are your experiences with it? And what do you think of the concept and design as such?



(EDIT: Just saw from AndrewT's post here that the EDN version of the article published here has some very valuable additional comments.)

Last edited by Bertel; 5th September 2015 at 10:07 AM.
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Old 5th September 2015, 11:02 AM   #2
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You need to re-read and get clear the difference between balanced and differential. I will build one one day as it is possibly the lowest distortion DIY design out there.
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Old 5th September 2015, 11:09 AM   #3
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by billshurv View Post
You need to re-read and get clear the difference between balanced and differential. I will build one one day as it is possibly the lowest distortion DIY design out there.
Funny you should state that.
I was trying to get that message across recently and some came back arguing how similar the two philosophies are, rather than seeing where they are different and why they perform differently.
I gave up arguing.
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Old 5th September 2015, 11:11 AM   #4
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It's very easy to get very confused!
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Old 5th September 2015, 11:18 AM   #5
AndrewT is offline AndrewT  Scotland
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The Bruno Putzeys device has balanced impedance input and balanced impedance output, so can be used in a balanced impedance system.

Note it is the CONNECTION system that is balanced impedance and this is what gives the very high interference attenuation, if you get the cabling and impedances just as they NEED to be.

The difference amplifier just about describes every opamp and every power amp.
They all measure the DIFFERENCE in input voltage and apply their processing to arrive at an output voltage. This part has absolutely nothing to do with "Balanced Impedance Connections".

Unfortunately many corrupt our technical language and shorten to "balanced" and that gets misunderstood by thousands of our Members.
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Old 5th September 2015, 11:21 AM   #6
AndrewT is offline AndrewT  Scotland
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Quote:
what do you think of the concept and design as such?
I think it is brilliant.

Jan must have thought so too. He included the project in his "Linear" series of periodicals.
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Old 5th September 2015, 11:25 AM   #7
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But the amplification stages ARE balanced impedance internally. That is the interesting bit of the design. I also like his choice of words for ground as 'the electron dump'.
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Old 5th September 2015, 11:49 AM   #8
Bertel is offline Bertel  Germany
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That's also my understanding gents, I was obviously too sloppy in my language and probably should have called this "a volume control for use in fully balanced impedance connection systems throughout" or so, thanks for pointing that out (Bruno himself calls it "a balanced volume controller" in his article, by the way).

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Originally Posted by AndrewT View Post
I think it is brilliant.
Thanks for your assessment AndrewT, I saw you had recommended this approach in the Wire Thread.

Anyone who actually built this?

Last edited by Bertel; 5th September 2015 at 12:07 PM.
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Old 7th September 2015, 11:00 PM   #9
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Does anybody know how and when to use J4 and J5 ?
I asume that there is also a posibility for unbal ( RCA ) In and unbal ( RCA ) Out.
I could need bal In to unbal Out.
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Old 8th September 2015, 12:41 AM   #10
Pan is offline Pan  Sweden
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I wonder if J4/5 is simply there for letting your PCB layout software accepting a GND connection. Zero ohm resistor could be used as well.

It's mentioned in the text.
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