[sdiy] LM13600/13700 VCA circuit recommendations

[Scott Gravenhorst]

Dave Kendall <davekendall at ntlworld.com> wrote:
>Hi Guys.
>
>Thanks for the replies. I'll breadboard a couple of circuits soon....
>
>
>on 28/8/06 20:20, Scott Gravenhorst at music.maker at gte.net wrote:
>>  A real linear current driver will usually be an opamp current
>> source where there's either a Darlington pair or a MOSFET in the feedback
>loop.  The FatMan VCA is done this way (Darlington).
>
>I had a look at the Fatman schemo, but could only see a single 2N4126 going
>to pin 1 of the LM13600. Am I looking at the wrong bit of the schemo?

Heh heh, I forgot about that.  The darlington pair attempts to correct a flaw caused by
the fact that the current coming out of the opamp is fed to the summing node through the
2N4126 base-emitter circuit.  The full current to the summing node is actually the sum
of the load (from the LM13600) as well as the base current of the transistor.  For a
VCA, it isn't as critical to get that "perfect", so PAiA made a cost decision and used a
single transistor instead of a darlington.  If you look at the VCOs, they are fed
current the same way, but use the darlington because it reduces the amount of base
current by a factor of 100 and is therefore a tad more accurate.  So no, you weren't
looking in the wrong place...  A VCA with a more accurate linear current source would
use a darlington pair - what difference that would make I can't imagine.

>BTW, I seem to have lost a few 2N4124s for an unbuilt Midi2CV. I'm finding
>them hard to source in the UK. Is there a more common equivalent tranny -
>someone suggested 2N4402?
>
>cheers,
>
>Dave
>

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-- Scott Gravenhorst | LegoManiac / Lego Trains / RIS 1.5
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