[sdiy] SSM2040 clone / discrete 'precision' VCA

Justin Owen juzowen at gmail.com
Wed Dec 14 17:06:12 CET 2011


Excluding the filter cap-to-ground and the output buffer stage on the right hand side of the marked sections of the two SSM2040 clones I linked to here: http://www.sdiy.org/juz/jh2040_02.jpg and here: http://www.sdiy.org/juz/rs2040_02.jpg - I end up with the standard 4 transistor differential amp with a differential pair below and a current mirror above - yes?

Comparing that (which is exactly what I started with before the 2040) with the schematics of the LM13700 and the CA3080 I notice two things...

The first is that on the OTA schems, each side of the differential pair has its own current mirror as opposed to a single current mirror that bridges both differential pair collectors...

The second is that neither the current mirror in the 13700 or the 3080 match any of the current mirror configs referenced in AofE (with the Bases joined).

So, considering I'm after a VCA and not an OTA is there going to be any advantage to using a current mirror with the differential pair or should I just be looking at the differential pair as per the PAiA 2720 and Rene Schmitz VCAs?

And if there is an advantage to adding the current mirrors is there case for using one or two mirrors and is there going to be an advantage to going with the AofE mirrors or the ones on the 13700/3080 schems...

Sorry - that's a lot of questions and I will try just plugging some stuff in as always, be good to count out any no starters though.

Thanks.

Justin






-----Original Message-----
From: Tom Wiltshire [tom at electricdruid.net]
Received: 14.12.2011 12:39:50
To: Justin Owen
Cc: SDIY List
Subject: Re: [sdiy] SSM2040 clone / discrete 'precision' VCA

Yes, the current-mirror thing (the one with four transistors in a row, right?) at the bottom is a exponential current source to drive the four stages and give a exponential response to input CV.
Olivier has a nice (readable, unlike some!) analysis on his site:

http://mutable-instruments.net/static/documentation/smr4_analysis.pdf

This includes the current source, but the rest of it is broadly applicable as well (although he uses integrators rather than cap-to-ground).

Tom


On 14 Dec 2011, at 12:32, Justin Owen wrote:

> Ah, that's one of the things that worried me - I was looking for an integrator but those caps to ground are the poles.
> 
> On one hand that's positive because that 4 transistor OTA config (minus the Darlington pair) is what I was looking at originally - less positive though because I had no luck at all with it when I first tried to breadboard it.
> 
> So does the fact that the response should be linear mean that the odd current mirror looking thing feeding it via the common Base at the bottom of the differential amp at the bottom is some sort of Lin to Exp converter?
> 
> Thanks Tom!
> 
> Justin
> 
> 
> 
> -----Original Message-----
> From: Tom Wiltshire [tom at electricdruid.net]
> Received: 14.12.2011 12:05:29
> To: Justin Owen
> Cc: SDIY List
> Subject: Re: [sdiy] SSM2040 clone / discrete 'precision' VCA
> 
> You've included the filter's cap-to-ground and the output buffer stage too, Justin.
> The 'VCA' part is the part on the left only. Compare with the "voltage-controlled lowpass filter" stage in here:
> 
> http://www.ti.com/lit/ds/symlink/lm13700.pdf
> 
> You can see the extra bits that have been added around the OTA to turn it into a single-pole filter - the cap, the buffer, and the resistors to mix the signals.
> 
> Incidentally, this 'discrete OTA' VCA should have a linear response, unlike the single-transistor VCA you've just been working on.
> 
> HTH,
> Tom
> 
> On 14 Dec 2011, at 11:35, Justin Owen wrote:
> 
>> Hello,
>> 
>> I'd like to start looking at a discrete, precision VCA to pair with the simple little down'n'dirty VCA I've been playing with.
>> 
>> When I say 'precision' - I mean as good as I can get it, better than a single transistor VCA. You can stop laughing now :)
>> 
>> I've been looking at the schematics for Juergen :( and Rene's SSM2040 clone. I know it's a VCF - but am I right in thinking that the section I've marked in red here: http://www.sdiy.org/juz/jh2040_02.jpg and here: http://www.sdiy.org/juz/rs2040_02.jpg is essentially a VCA?
>> 
>> Input is via the 10K resistor into the (inverted?) input of the differential amp pair at the bottom, Output is via the Emitter on the Darlington pair to the right and CV (or current in) is via the common Base at the bottom of the differential amp at the bottom.
>> 
>> Originally I assumed this based on the VCAs used in the traditional SVF schematic and my first quick breadboard of that section had some potential - but now I'm beginning to doubt my initial assumption so I thought I'd ask before I head down a hole.
>> 
>> Any clues? Any suggestions for a better place to start looking? The Blackmer gain cell has also been suggested...
>> 
>> Thanks all.
>> 
>> Justin
>> 
>> 
>> 
>> 
>> 
>> 
>> 
>> _______________________________________________
>> Synth-diy mailing list
>> Synth-diy at synth-diy.org
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> 
> 
> 





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