[sdiy] using LM13700 in differential mode to reduce trimmers

[Phillip Gallo]

I have built  VCAs using the LM3700 in a differential configuration.  They
all worked well.
Back when; I had been told the matching of the amplifiers was overstated in
the spec sheet but found it to be accurate for the chips i had available.

I didn't use the Darlingtons instead using I/V op-amps into Diff amp
config.  I also used trimmers and this paid-off nicely.  If you don't use
linearization you can exploit the "3080-ish" overdrive wave shaping with
higher S/N.  With linearization, the VCA's were quite clean, ready to
compete with alternate solutions.

p






On Tue, Oct 27, 2020 at 3:37 PM Eric Schlappi <eric.schlappi at gmail.com>
wrote:

> @aaron That's the paper I was talking about, thanks for linking it into
> the discussion!
>
> @Ian Looking at the wrong things I guess! I only remember seeing it
> (well... the 3080) used with differential input as the VCA in the
> radioshack mg-1, and that's presumably because the filter output is
> differential.
>
> Thanks again, I guess I'll keep on using balanced inputs for OTAs but
> won't get rid of the trimmers.
>
> On Tue, Oct 27, 2020 at 2:01 PM Ian Fritz <ijfritz at comcast.net> wrote:
>
>>
>> > On Oct 27, 2020, at 2:18 PM, Eric Schlappi <eric.schlappi at gmail.com>
>> wrote:
>> >
>> > 
>> > You guys are right.
>>
>> Why, thank you! Please bear in mind that a number of us guys have worked
>> with these chips since they first came out in 1979.
>>
>> > I guess I was wondering why I don't see the lm13700 used with
>> differential input more often, or am I still missing something?
>>
>> I’ve always used balanced inputs for mine in everything I have ever
>> built, except when they are used as switches.
>>
>> Dunno what you have been looking at; this is pretty standard as far as I
>> know.
>>
>> Ian
>> >
>>
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