[sdiy] Discrete OTA

[Andrew Simper]

Hi David,

Can you please elaborate as to why the 33u is needed after each sallen
key? As you can see from the schematic:
http://machines.hyperreal.org/manufacturers/Korg/MS-synths/schematics/KLM-307.GIF
, the design actually doesn't use dc blocking caps after each stage of
the sallen key, they only have them after the last stage. I would have
thought there was enough DC blocking from the negative feedback to not
need them at all, the final 2SC945 x 2 transistors already distort a
lot anyway, so a tiny amount of DC input here isn't going to matter
much, and the amp adds dc anyway which needs blocking before the final
output.

So the only thing I can think of is they are protecting the 4558 from
DC, since there is a 10u / 33u DC blocker just prior to each one of
those. So is this a case of protecting 4558s rather than a need for an
OTA + dc adding buffer design needing AC coupling? The IR3109 roland
designs are basically an OTA + dc adding buffer (in this case a p-mos
buffer) and if you check the schematic of the Juno-6 or similar there
is a single 10u DC blocking cap at the input, and output, but nowhere
else in the filter core or resonance feedback path, and the filter
will work just fine if they were removed and operate on DC signals.

--Andy

On 7 April 2014 06:36, David G Dixon <dixon at mail.ubc.ca> wrote:
>> > Also, OTA + Cap + Buffer filters often need internal AC coupling
>>
>> Any chance you could point out some examples of AC-coupled
>> OTA/Cap/Buffer (e.g. SSM2040?) designs?
>
> The later MS-20 filter.  Coupled with 33u caps after each stage.
>
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