[sdiy] using LM13700 in differential mode to reduce trimmers

[Ian Fritz]

Agree with all that!  But there have been some pretty wild variation between manufacturers and batches as far as matching and variation with Iabc.  I went so far as to make up a test measurement jig, characterizing dozens of chips and labeling them for future use. What was tough was finding devices where both sides were good enough.

Ian

> On Oct 28, 2020, at 2:39 PM, Phillip Gallo <philgallo at gmail.com> wrote:
> 
> 
> 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
>>> > 
>>> 
>> _______________________________________________
>> Synth-diy mailing list
>> Synth-diy at synth-diy.org
>> http://synth-diy.org/mailman/listinfo/synth-diy
> _______________________________________________
> Synth-diy mailing list
> Synth-diy at synth-diy.org
> http://synth-diy.org/mailman/listinfo/synth-diy
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://synth-diy.org/pipermail/synth-diy/attachments/20201028/15c6af7c/attachment.htm>