[sdiy] LM13700 as VC resistor

[Ian Fritz]

Yeah, lots of interesting timbres. Mostly I just use the morphers, though. I have them built into a couple of Saw-core VCOs, and they typically drive multiple pulse waveshapers. So the up- and down-ramps produce different pulse patterns. There is a video on my youtube channel showing a drone patch using one of these modules.

Ian

> On Feb 8, 2020, at 8:49 PM, Didier Leplae <didierleplae at yahoo.com> wrote:
> 
> BTW, the SNICSter looks super interesting!
> 
>> On Feb 8, 2020, at 9:19 PM, Ian Fritz <ijfritz at comcast.net> wrote:
>> 
>> 
>> It looks like the reply I sent earlier didn’t make it through. I used an OTA as a VC resistor a while back. Didn’t have any trouble with it, although IIRC the range was a bit limited. Certainly did not need AC coupling. Does anyone else have actual experience in incorporating one in a design?
>> http://ijfritz.byethost4.com/sy_cir10.htm
>> 
>> Ian
>> 
>> 
>>> On Feb 8, 2020, at 6:25 PM, Didier Leplae via Synth-diy <synth-diy at synth-diy.org> wrote:
>>> 
>>> René,
>>> Sorry for my ignorance, but when you say “ Use Iabc to get gm (19.2*Iabc = gm), and the rest should be easy.”
>>> 
>>> What is “labc”?
>>> 
>>>> On Feb 8, 2020, at 11:51 AM, René Schmitz <synth at schmitzbits.de> wrote:
>>>> 
>>>> Hi Didier and all,
>>>> 
>>>>> On 08.02.2020 10:58, Didier Leplae via Synth-diy wrote:
>>>>> I am curious to know anyone’s opinions on using the LM13700 as a voltage controlled resistor.
>>>>> The LM13700 datasheet has a couple schematics which I have tried building, with mixed results. I’m having a hard time knowing what sort of resistance I am getting as the multimeter gives strange and inconsistent readings.
>>>>> The information on the datasheet is somewhat incomplete in my opinion and it is not easy to find much info online either.
>>>> 
>>>> Yes, it doesn't tell you the fact that the resistor is AC coupled (single ended circuit). See the Cap at the output...
>>>> So poking around with a multimeter is not going to give meaningful results.
>>>> But the formula for Rx is given. Use Iabc to get gm (19.2*Iabc = gm), and the rest should be easy.
>>>> 
>>>> 
>>>>> It doesn’t seem like it is common practice to use these as VC resistors. Is that because it doesn’t work very well? Or use too many parts?
>>>> 
>>>> Wouldn't be my first choice due to complexity and parts count.
>>>> 
>>>> Best,
>>>> René
>>>> 
>>>> --
>>>> synth at schmitzbits.de
>>>> http://schmitzbits.de
>>> 
>>> 
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