[sdiy] Problems with linearized SSI2164 ?

Mattias Rickardsson mr at analogue.org
Thu Feb 7 08:10:27 CET 2019


Have you tried the suspicious SSI2164 in another application? Just to see
if it behaves in other situations before assuming that the Irwin circuit is
some special case.

Also I'm wondering if there could be some HF oscillation that could lead to
this behavior almost invisibly. Are the SSI2164s from the very early
batches where internal oscillations at loud signals through low Rin would
result in a raised noise floor, or are they the corrected version?

/mr

Den tors 7 feb. 2019 06:58 skrev Danjel van Tijn <danjel at intellijel.com>:

> Definitely not. I actually replaced all of the resistors just to be
> absolutely certain too. A mentioned earlier I desoldered the SSI2164 and
> replaced it with the V2164 from coolaudio and the circuit worked as
> expected.    Dan Parks from SSI  gave me a call earlier this evening and he
> has passed on the details to Phil Gallo who can hopefully shed some more
> light on the situation soon.
> In the meantime we will have to replace resistors on our boards and do
> some extensive testing to make sure there is no other unexpected behavior.
>
>
>
> On Wed, Feb 6, 2019 at 9:50 PM Roman <modular at go2.pl> wrote:
>
>> This is only my wild guess, but since they took the freedom to replace
>> V2164 with SSI2164, then maybe also misplaced 3k instead of 30k as R75?
>> Markings look similar...
>>
>> Roman
>>
>> Dnia 7 lutego 2019 00:13 Danjel van Tijn <danjel at intellijel.com>
>> napisał(a):
>>
>> 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>
>> 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>
>> 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
>>
>> www.intellijel.com
>> https://www.instagram.com/intellijel/
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>> www.youtube.com/intellijel
>> www.twitter.com/intellijel
>>
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>>
>>
>> --
>> http://www.njohnson.co.uk
>>
>> --
>> cheers,
>>
>> Danjel van Tijn
>> Intellijel Designs Inc.
>> m. +001 604-773-2426
>>
>> www.intellijel.com
>> https://www.instagram.com/intellijel/
>> www.facebook.com/intellijel
>> www.youtube.com/intellijel
>> www.twitter.com/intellijel
>>
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>>
>> --
> cheers,
>
> Danjel van Tijn
> Intellijel Designs Inc.
> m. +001 604-773-2426 <(604)%20773-2426>
>
> www.intellijel.com
> https://www.instagram.com/intellijel/
> www.facebook.com/intellijel
> www.youtube.com/intellijel
> www.twitter.com/intellijel
>
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