[sdiy] Problems with linearized SSI2164 ?

Tim Ressel timr at circuitabbey.com
Thu Feb 7 07:53:10 CET 2019


I used the same circuit and it works well. I have a 220K for your R82. 
That is the only significant difference I see.

--TimR

On 2/6/2019 3:08 PM, Danjel van Tijn wrote:
> Here is a link to the schematic block: https://ibb.co/njhCcbZ
>
> Really just a standard Mike Irwin configuration that we have used on 
> literally thousands of modules.   Dave Dixon and I have tended to use 
> 30k as the standard input resistance to the VCA cells but on more 
> recent designs we have started using 100k.
>
> Unfortunately we just built a ton of modules with the SSI2164 and our 
> factory did not tell me they had switched to the new chip (from the 
> v2164).   We can re-work these by replacing the three 30k resistors 
> with 100k but I would still like to understand this better.  It also 
> unfortunately means the SSI2164 is not a blind drop in replacement for 
> the V2164 and I am going to have to test out the rest of our circuits 
> very carefully before any more manufacturing.
>
> Hopefully this helps some other folks avoid the same headache!
>
>
>
> On Wed, Feb 6, 2019 at 3:01 PM Neil Johnson <neil.johnson71 at gmail.com 
> <mailto:neil.johnson71 at gmail.com>> wrote:
>
>     Hi Danjel,
>
>     Can you share the circuit you are building?  Do you have some
>     means of pulling the leakage current out of the summing node? 
>     This is usually done with a high-value resistor.  And the issue
>     Matthias raised is that with 30k you are sending more current into
>     the gain cell, so the leakage current will be higher (well, it's
>     not really leakage in the classic sense, but the fact that as this
>     is a log circuit you can never achieve infinite attenuation, so
>     *some* current will leak through.
>
>     As to why there is a difference, I would need to make some
>     measurements on the bench.  One difference I do know is that the
>     SSI2164 has much larger transistors in the gain cell, giving
>     better log conformance, but they may exhibit different "off"
>     behaviours.
>
>     The 0.85V you are seeing - that would equate to about 26dB
>     attenuation.  In the Irwin linearised circuit an input CV of 0V
>     would force the VCA CV to very positive (op-amp output pinned to
>     the +ve rail) as it tries to attenuate the bias current to 0 with
>     infinite attenuation.
>
>     Cheers
>     Neil
>
>
>     On Wed, 6 Feb 2019 at 22:20, Danjel van Tijn
>     <danjel at intellijel.com <mailto:danjel at intellijel.com>> wrote:
>
>         Hi there,
>
>         Has anyone else experienced issues with the SSI2164 used in
>         place of a Coolaudio 2164 in a linearized VCA configuration?
>
>         With no CV applied to the circuit control pin of the VCA
>         should be close to 0 (and it is when I swap back in a
>         coolaudio chip)  but with a SSI2164 the voltage at the control
>         pin is around 0.85V.
>
>         I am using accurate voltage sources for the negative and
>         positive voltage references (-5 and +5). and as aforementioned
>         swapping in a coolaudio chip returns the circuit to working as
>         expected.
>         Opamp is a  TI TL072CPWR.
>
>         The only two differences from the datasheet version are that
>         we are using 30K for Rin and 510R+560p for stability;  both
>         are within range of acceptable values according to datasheet.
>
>         Any thoughts?  possibly bad batch of ICs?
>
>
>         -- 
>         cheers,
>
>         Danjel van Tijn
>         Intellijel Designs Inc.
>         m. +001 604-773-2426 <tel:(604)%20773-2426>
>
>         www.intellijel.com <http://www.intellijel.com>
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>
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>
>     -- 
>     http://www.njohnson.co.uk
>
> -- 
> cheers,
>
> Danjel van Tijn
> Intellijel Designs Inc.
> m. +001 604-773-2426 <tel:(604)%20773-2426>
>
> www.intellijel.com <http://www.intellijel.com>
> https://www.instagram.com/intellijel/
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>
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-- 
--Tim Ressel
Circuit Abbey
timr at circuitabbey.com

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