"The difference stage
As said, we're out of luck when it comes to wiring a pot differentially so we won't even try. Instead we'll be using the surrounding stages to reference the cold point of the variable gain stage. So between the input buffer and the variable gain stage we insert a difference amplifier. This is the circuit that'll confer CMRR to our little preamp, so resistor matching is of prime importance here. The output of the difference amplifier is referenced to the cold point of the volume controller."
As said, we're out of luck when it comes to wiring a pot differentially so we won't even try. Instead we'll be using the surrounding stages to reference the cold point of the variable gain stage. So between the input buffer and the variable gain stage we insert a difference amplifier. This is the circuit that'll confer CMRR to our little preamp, so resistor matching is of prime importance here. The output of the difference amplifier is referenced to the cold point of the volume controller."
I don't see a need for MELF. you can get std 0805 thin films at 0.05% and 5ppm readily. BUT the more precise the higher the price. To give an idea, Vishay PLT (0.05% 5ppm ) are around £1.50 each +Vat if you buy 500.
So first @Tfboy: can you ask who their preferred distributor is. Then we can get a list of the grades stocked and a rough cost multiplier to argue about.
So first @Tfboy: can you ask who their preferred distributor is. Then we can get a list of the grades stocked and a rough cost multiplier to argue about.
Sorry for starting this confusion! I had assumed that the original BOM for this Group Buy was the correct one.. but I see that it's open (as Linear Audio's own BOM used 1% SMDs) to interpretation.
However! If Bruno used Vishays then by THEIR definition, "precision" = 0.1%, "high precision" = 0.01%.
Bruno specifically differenciated between the two types in his paper's BOM and so I would then presume that Bruno used Vishay's own terminology.
He specifies precision for the following:
and
The G Word - Bruno Putzey
I guess the complication comes, when talking to the circuit maker people, in using more than one type (or maybe it doesn't matter once they've loaded the machines up for the job? I've no experience).
However! If Bruno used Vishays then by THEIR definition, "precision" = 0.1%, "high precision" = 0.01%.
Bruno specifically differenciated between the two types in his paper's BOM and so I would then presume that Bruno used Vishay's own terminology.
He specifies precision for the following:
and
The G Word - Bruno Putzey
I guess the complication comes, when talking to the circuit maker people, in using more than one type (or maybe it doesn't matter once they've loaded the machines up for the job? I've no experience).
Here they are in context of Bruno's basic BOM in tabletised form (from the www.edn.com version of the article ) :
An externally hosted image should be here but it was not working when we last tested it.
(shame that Linear Audio's BOM contradicts this - think that's where the confusion has arisen).
I think it is that BOM which is causing confusion - it is different to Bruno's original parts list as per his paper. I have given his original in the table in post #225 above. Bertel's original BOM was the closest to that.
Also Jan himself used a different BOM list for his own builds as shown by his post #89
Johny Lee Hooker had the same problem building this. He said: "BOM BOM BOM BOM - I'm gonna shoot you right down!"
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Yeah, you are right. We need to keep the components matched. Just need to agree on the BOM being uses
Sent from my ONE A2003 using Tapatalk
Sent from my ONE A2003 using Tapatalk
As mentioned previously in this thread, there was a GB for this preamp in another forum a year ago, and we basically used Bruno's BOM, interpreting the term "precisision thin film" as "0.1% thin film". There was a change however in that we used R2/30/13/40 = 2K2 instead of the 1K in the BOM in order to reduce the total gain of the preamp (suggested by Jan). This change extends the attenuation part of the volume control range by 6.8dB and improves the volume regulation characteristics in that area, at a cost of reducing the CMRR.
Making this change necessitates also changing R8/17/35/44 to 22K in order for the DC compensation to work correctly.
I'm now about to build my second preamp along the above lines.
Regards,
Braca
Making this change necessitates also changing R8/17/35/44 to 22K in order for the DC compensation to work correctly.
I'm now about to build my second preamp along the above lines.
Regards,
Braca
Is post225 from the original article?
I see balanced impedance anomalies in that BoM.
All the components in the balanced impedance parts of the circuit should be precision matched.
i.e. the 100r & 100p input filters add load to the balanced lines.
R6 & R9 add load to the balanced lines.
R4 and R15 add load to the balanced lines.
All of the above should be matched for value to maintain the balance of the balanced lines.
The differential stages also require precision and the article picks out the 1k and 10k as "precision thin film".
But the filters feeding U3 have not been included in the differential matching. They are "805" rather than "thin film"
Are the capacitors (C8, 9, 24, 25, 4, 7, 21 & 23) required to be matched?
I don't know what precision is required in the differential stages to maintain performance through this.
I see balanced impedance anomalies in that BoM.
All the components in the balanced impedance parts of the circuit should be precision matched.
i.e. the 100r & 100p input filters add load to the balanced lines.
R6 & R9 add load to the balanced lines.
R4 and R15 add load to the balanced lines.
All of the above should be matched for value to maintain the balance of the balanced lines.
The differential stages also require precision and the article picks out the 1k and 10k as "precision thin film".
But the filters feeding U3 have not been included in the differential matching. They are "805" rather than "thin film"
Are the capacitors (C8, 9, 24, 25, 4, 7, 21 & 23) required to be matched?
I don't know what precision is required in the differential stages to maintain performance through this.
Re-read the article first before you start to question Bruno he explains all this. If all this worries you take a small hit in distortion and use the whitlock chips.
This BOM is Paul Hendriks version as printed in Linear Audio, not Bruno's origional.
http://linearaudionet.solide-ict.nl/sites/linearaudio.net/files/putzeys demo preamp BoM.PDF
http://linearaudionet.solide-ict.nl/sites/linearaudio.net/files/putzeys demo preamp BoM.PDF
I will read and re-read Bruno's article. I have already read it twice.
But that re-reading does not remove the need to maintain balanced impedance.
Here they are in context of Bruno's basic BOM in tabletised form (from the www.edn.com version of the article ) :
I remember talking to Bruno that R25, 26, 27, 28 should be melf as they should have a higher peak current, and that he forgot to mention that. The others can be melf but not required.
Paul Hendrik added the order numbers for easy ordering.
Jan
Hi Andrew, that particular table is from Bruno's article as published here : http://www.edn.com/design/consumer/4431638/2/The-G-word--Demo-project---A-balanced-volume-controller
It's a web version of the .PDF of his paper on the hypex website. It has been put into table form and turned into a picture of it's own right - i.e. formatted for web publication. But it is exactly the same as the .PDF .
Thanks Braca and Jan! I think combining these two posts with the original BOM should set us on the correct and final path for the SMD populated boards. And of course paying attention to the spacing of the pads on the board and specifing the appropriate SMD package!
And of course, let's remind ourselves of what we are going to get from this - this is the motivation :
And of course, let's remind ourselves of what we are going to get from this - this is the motivation :
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