[sdiy] Problems with linearized SSI2164 ?

Danjel van Tijn danjel at intellijel.com
Thu Feb 7 00:19:11 CET 2019


Yes I sent some e-mails to Dan and Dave Rossum today.  Dave is on vacation
so I may not hear back for a while.



On Wed, Feb 6, 2019 at 3:15 PM Mattias Rickardsson <mr at analogue.org> wrote:

> Have you asked Dan et al. about this matter?
>
> /mr
>
> Den tors 7 feb. 2019 00:08 skrev Danjel van Tijn <danjel at intellijel.com>:
>
>> 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>
>> 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>
>>> 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 <(604)%20773-2426>
>>>>
>>>> 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 <(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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> --
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
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