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Analog Line Level Preamplifiers , Passive Pre-amps, Crossovers, etc.

BPPBP - Bruno Putzey's Purist Balanced Preamp (well a balanced volume control really)
BPPBP - Bruno Putzey's Purist Balanced Preamp (well a balanced volume control really)
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Old 18th February 2016, 08:10 PM   #301
r_merola is offline r_merola  Brazil
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moumousganou

I suppose the best replacement to the pot is the Stepped Attenuator by Hans Polak.
By the way can you post the link of tje project of the Hans Polak project please?
Regards
Ronaldo
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Old 18th February 2016, 09:23 PM   #302
misterdog is offline misterdog  United Kingdom
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It's on Jan's Linear audio site.

I have already contacted Hans and asked but he has no PCB's remaining.

I may contact him again to ask if he would be prepared to produce some more, or allow us to do so.

Lets Concentrate on getting the main PCB into production first.

http://linearaudio.net/sites/lineara...tmeter%203.pdf
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Old 18th February 2016, 09:37 PM   #303
DNi is online now DNi  Switzerland
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When discussing an alternative solution to Bruno's original design, please note that using a digital potentiometer, or a relay-switched resistor ladder, or a PGA, changes the preamp topology in that the original design has a variable-gain last stage, whereas the new one would be based on a constant-gain one with an attenuator in front of it.
The above is a bit complicated way of saying that a digital pot, or a PGA can not be used to control the volume in the way proposed by Bruno; and I'm not aware of a relay-switched resistor ladder that can be used with good results within the negative feedback loop either.

Regards,
Braca
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Old 18th February 2016, 10:39 PM   #304
randytsuch is offline randytsuch
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Quote:
Originally Posted by DNi View Post
When discussing an alternative solution to Bruno's original design, please note that using a digital potentiometer, or a relay-switched resistor ladder, or a PGA, changes the preamp topology in that the original design has a variable-gain last stage, whereas the new one would be based on a constant-gain one with an attenuator in front of it.
The above is a bit complicated way of saying that a digital pot, or a PGA can not be used to control the volume in the way proposed by Bruno; and I'm not aware of a relay-switched resistor ladder that can be used with good results within the negative feedback loop either.

Regards,
Braca
Seems like you can use a digital pot to replace the normal pot in Bruno's circuit. Use an Arduino or something similar to control the pot, and you have remote controllable volume with this project.

If it was me, I would add isolation on the SPI bus (or whatever interface you use to control the digital pot), to keep the Arduino noise out of your preamp.

Am I missing something?

Randy
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Old 19th February 2016, 08:54 AM   #305
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by DNi View Post
When discussing an alternative solution to Bruno's original design, please note that using a digital potentiometer, or a relay-switched resistor ladder, or a PGA, changes the preamp topology in that the original design has a variable-gain last stage, whereas the new one would be based on a constant-gain one with an attenuator in front of it.
The above is a bit complicated way of saying that a digital pot, or a PGA can not be used to control the volume in the way proposed by Bruno; and I'm not aware of a relay-switched resistor ladder that can be used with good results within the negative feedback loop either.

Regards,
Braca
The vol pot part of the Blanced voulme control is a pair of resistances in the feedback loop.

You can put in switched or sliding resistors here. Both work equally.

BUT !!!!!
the wiper must never be allowed to go open circuit.
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Old 19th February 2016, 09:20 AM   #306
tfboy is offline tfboy  United Kingdom
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I'm actually toying with using Vincent's LDR-based volume controller in conjunction with Bruno's pre-amp. I'm hoping to finish it today.

Assuming I can get it working reliably (I'm tweaking the code a bit to adjust for linear response, modify impedences, etc), it's a great way to add Arduino volume control, remote control, source selection, etc.
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Old 19th February 2016, 09:34 AM   #307
AndrewT is offline AndrewT  Scotland
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Originally Posted by tfboy View Post
I'm actually toying with using Vincent's LDR-based volume controller in conjunction with Bruno's pre-amp. I'm hoping to finish it today.

Assuming I can get it working reliably (I'm tweaking the code a bit to adjust for linear response, modify impedences, etc), it's a great way to add Arduino volume control, remote control, source selection, etc.
Using LED/LDR may make matching of channel to channel almost impossible.

The main advantage of using an inverting opamp with volume controlled by feedback is that a linear potentiometer gives a log law control over much of the rotation range of the conventional vol pot.
If you adopt some other resistance topology for the linear potentiometer, you are throwing away this major advantage.
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Old 19th February 2016, 10:40 AM   #308
DNi is online now DNi  Switzerland
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Quote:
Originally Posted by randytsuch View Post
Seems like you can use a digital pot to replace the normal pot in Bruno's circuit. Use an Arduino or something similar to control the pot, and you have remote controllable volume with this project.

If it was me, I would add isolation on the SPI bus (or whatever interface you use to control the digital pot), to keep the Arduino noise out of your preamp.

Am I missing something?

Randy
I stand corrected on this one. Studying the MUSES 72320 data sheet I see that it is possible to insert a digital pot into the feddback path, albeit in a rather complicated way.
My original thinking was that an electronic component in the feedback path of a linear opamp circuit would only be used for obtaining special transfer functions.
This notwithstanding, I would still not recommend this sort of volume control in this particular case. Assuming the MUSES chip as the volume controller and the manufacturer-recommended application circuit for bipolar opamps, the last stage of the preamp would have to be modified in a nontrivial way in order to accomodate this solution; and adding three electrolytic capacitors to the original circuit which previously had only a single linear pot entails IMHO a risk for impairing the sound quality.
This is a purist preamp designed with a view to achieving maximum linearity; and unless someone comes up with an economical solution for a relay-switched resistor ladder in the feedback path, the next best alternative to it still appears to be a relay-switched attenuator between the diff. amp output and the last stage.

Regards,
Braca
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Old 19th February 2016, 11:02 AM   #309
tfboy is offline tfboy  United Kingdom
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Quote:
Originally Posted by AndrewT View Post
Using LED/LDR may make matching of channel to channel almost impossible.

The main advantage of using an inverting opamp with volume controlled by feedback is that a linear potentiometer gives a log law control over much of the rotation range of the conventional vol pot.
If you adopt some other resistance topology for the linear potentiometer, you are throwing away this major advantage.
Hi Andrew, I get that. Vincent's original design emulates a log response, but that can easily be changed in the code to replicate the behaviour of a linear pot.

Matching LDRs isn't an issue as there's a calibration process to ensure they track each other within a percent or so.

But I digress. I'll see how I get on with it and test hopefully this weekend.
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Old 19th February 2016, 01:07 PM   #310
r_merola is offline r_merola  Brazil
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Default Stepped Attenuator by Hans Polak Information

Thank you misterdog very much

Regards
Ronaldo
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