BPPBP - Bruno Putzey's Purist Balanced Preamp (well a balanced volume control really)

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#1,022
DNi said:
I'm not sure that the schematic of #1015 would work at all - I've never seen such an instrumentation amplifier.
As regards gain, the volume control stage in the original preamp schematic can provide gain at will.
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I guess you refer to the wrong polarity of the upper opamp also?


Regarding gain: i using a stepped relay attenuator there's no gain in the circuit - if you need some, as previously stated, I'd put it on the input stages.
 
#1,025
To make sure there are no misunderstandings, especially for future readers, here's the correct schematic of the instrumentation fontend with gain:
 

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#1,026
I had a quick look at your github-files: I think VolCtrlDR.csv is out of date - compared to your schematic (i.e. Ds, OPAs...)

I wouldn't do the RCA-Output. Better to use only balanced outputs and if RCA outputs are necessary, better use one "very good" way (CMRR!), written here at Jensen.
 
#1,027
TJF said:
I had a quick look at your github-files: I think VolCtrlDR.csv is out of date - compared to your schematic (i.e. Ds, OPAs...)
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Yes, that might well be the case. Only one PCB was ever built, then we decided to include the volume control in our DACs directly. And well, that was scrapped as well as we will go full digital volume control only.


TJF said:
I wouldn't do the RCA-Output. Better to use only balanced outputs and if RCA outputs are necessary, better use one "very good" way (CMRR!), written here at Jensen.
Click to expand...


I'm all with you. Just tried to make the PCB rather universal at that time. At times I wonder why even "High end home equipment" still uses RCA. There's no such thing as a ground or "unbalanced" - But I tend to repeat myself 🙂
 
#1,029
Has anyone ever integrated a digital potentiometer, such as AD5171 (AD5171BRJ1) simply in the circuit instead of Bruno Putzeys Poti and controlled via I2C? So i.e. by means of microprocessor or Arduino ... Is there a way that is minimal in effort and at the same time unmatched in precision?
 
#1,030
That's in theory possible but it would not give you linear dB steps because the pot is used in the negative feedback path of the opamp. All the digital pots I know of have to be wired differently to how Bruno uses the pot in his circuit.

Edit: you could always cut the circuit after the diff amp of the instrumentation frontend, insert the digital pot there as a potential divider and feed the output buffer from the pots output. That would be very similar to how I implemented volume vitro with the relays, they also form a resistive divider, albeit a switchable one.
 
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#1,032
TJF said:
Has anyone ever integrated a digital potentiometer, such as AD5171 (AD5171BRJ1) simply in the circuit instead of Bruno Putzeys Poti and controlled via I2C?
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It is possible in principle, but you'd need a digital pot (i) with linear (not log) code/resistance law and (ii) that can handle +/-12V analog signals. AD5171 is linear law, but it runs from Vdd=+5V and can only handle positive voltage between 0 and Vdd, so no luck with that particular digital pot.

I took a different path and integrated three different digitally controlled volume controls into the circuit.
 
#1,035
Tfive said:
That's in theory possible but it would not give you linear dB steps because the pot is used in the negative feedback path of the opamp. All the digital pots I know of have to be wired differently to how Bruno uses the pot in his circuit.
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I did not understand this part. Bruno uses a linear 10k, 2-channel potentiometer (Bourns "PTD902", 2x3 poles). Why shouldn't that be 1:1 interchangeable with this, for example? Voltage supply should be no problem.
 
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#1,036
See its datasheet, section "Absolute Maximum Ratings". It says, Supply voltage: Vdd to GND, -0.3V to 7V; All other pins to GND: -0.3V to Vdd. If you want to connect one of "all other pins" to an opamp powered by +/-12V rails, the digipot will see + or -12V at some point, which will exceed its ratings. Those digipots are made for a different application (replacing trimpots, hence nonvolatile memory), and their distortion performance is probably not great.

A much better idea IMO would be to fit in a specifically designed audio volume control. The closest match would be the MUSES 72320/72323 or the now-obsolete WM8816, and in the thread linked above I show how to do it.
 

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#1,040
alexcp said:
A much better idea IMO would be to fit in a specifically designed audio volume control. The closest match would be the MUSES 72320/72323 or the now-obsolete WM8816, and in the thread linked above I show how to do it.
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MAS6116 is a direct equivalent to the WM8816, it's still current and in UK is available from Profusion. It works great when incorporated into Bruno's balanced amplifier circuit.

I've currently got this running as my pre-amp controlled by an Arduino Nano with rotary encoder and IR remote.
 
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