[sdiy] Using both OTAs in 13700 as one OTA

[Quincas]

Hi guys

This thread got me thinking a lot about LM13700 VCAs, so I went to the datasheet and noticed they suggest using the linearizing diodes to reduce harmonic distortion. So I breadboarded it, changed some values to fit synth levels and added a voltage to current converter as seen in Thomas Henry's VCA1. I don't have adequate tools to measure THD or SNR, but it sounds dead clean to me! No noticeable noise and no audible distortion using the nice sine wave from my Norton filter in self oscillation. Is there a reason none of the designs discussed above use the linearizing diodes? Here's my shabby hand drawn schem....  
http://s1070.photobucket.com/user/Quincas_Moreira/media/image_zpsu5cm6fz8.jpeg.html



Sent from my iPad

> On 10/02/2016, at 6:57 a.m., Simon Brouwer <simon.o at brousant.nl> wrote:
> 
> Hi Mattias,
>  
> I mean the circuit like this:
> http://www.bernacomp.com/elec/lockbox/lockbox_vca2.gif
>  
>  
> So, no.
>  
> Best regards
> Simon
>  
> 
> > Op 10 februari 2016 om 12:31 schreef Mattias Rickardsson <mr at analogue.org>:
> > 
> > 
> > Simon,
> > 
> > what about the temperature dependencies in the transistor case?
> > Is that control circuit more sensitive for temperature variations?
> > 
> > /mr
> > 
> > 
> > On 10 February 2016 at 09:08, Simon Brouwer <simon.o at brousant.nl> wrote:
> > > Yes it was.
> > >
> > > However, this circuit still uses one opamp, a resistor and at least two
> > > diodes.
> > > The more straightforward approach adds two resistors, and replaces one diode
> > > by a
> > > transistor.
> > > So this doesn't add much in complexity and cost, and you can control both
> > > halves of an LM13700 independently.
> > >
> > > Best regards
> > > Simon
> > >
> > >
> > >> Op 9 februari 2016 om 20:20 schreef Mattias Rickardsson <mr at analogue.org>:
> > >
> > >>
> > >>
> > >> Aha!
> > >> It *was* a beautiful trick. :-)
> > >>
> > >> http://electronotes.netfirms.com/AN399.pdf
> > >>
> > >> /mr
> > >>
> > >>
> > >> On 8 February 2016 at 19:09, Ian Fritz <ijfritz at comcast.net> wrote:
> > >> > The reference to the original Jung article is JAES, 23, 1975. This
> > >> > method
> > >> > was widely used in EN projects. I looked at using this method with the
> > >> > LM13700 to aid people who want to build off of these projects, but who
> > >> > can't
> > >> > obtain the CA3080. It's also an easy way to improve S/N by 3dB, as
> > >> > pointed
> > >> > out by Serge T. in his EN article from when the chip was new.
> > >> >
> > >> > I couldn't wade through Aaron's article either. It sounded like negating
> > >> > then subtracting? Isn't that the same as adding?
> > >> >
> > >> > Ian
> > >> >
> > >> >
> > >> >
> > >> > On 2/8/2016 2:41 AM, Mattias Rickardsson wrote:
> > >> >>
> > >> >> On 4 February 2016 at 05:52, <ijfritz at comcast.net> wrote:
> > >> >>>
> > >> >>> Good grief! Just connect them directly in parallel. An example,
> > >> >>> complete
> > >> >>> with the W. Jung trick:
> > >> >>>
> > >> >>>
> > >> >>> http://electro-music.com/forum/viewtopic.php?highlight=jung+ota&t=50884
> > >> >>
> > >> >>
> > >> >> What does it do? What is the "trick"?
> > >> >> I feel like one of the answers in that thread:
> > >> >> "I hadn't heard of this trick before, is there an article somewhere
> > >> >> that explains the benefit of drawing bias current from V-? I feel like
> > >> >> there's something obvious that I'm missing."
> > >> >>
> > >> >> I spontaneously found Aaron's idea potentially better than nothing,
> > >> >> but I haven't plunged into it.
> > >> >>
> > >> >> /mr
> > >> >>
> > >> >
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> > >
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