[sdiy] LM13700 as VC resistor

Didier Leplae didierleplae at yahoo.com
Sun Feb 9 20:26:43 CET 2020


This is all very helpful. Thank you all!

> On Feb 9, 2020, at 12:10 PM, Jacob Watters <jacobwatters at gmail.com> wrote:
> 
> 
> You could always try switched resistors instead. Then you use pwm to control the duration each resistor is in place. The clock frequency should be well above the audio range.
> 
>> On Sun., Feb. 9, 2020, 12:53 p.m. Tom Wiltshire, <tom at electricdruid.net> wrote:
>> Yes, I agree! So we’ve found two examples for you, Didier! Phasers and OTA filters both use the OTA like a variable resistance.
>> 
>> Tom
>> 
>> ==================
>>        Electric Druid
>> Synth & Stompbox DIY
>> ==================
>> 
>>> On 9 Feb 2020, at 12:16, Richie Burnett <rburnett at richieburnett.co.uk> wrote:
>>> 
>>> The OTA acts as a variable resistor here:
>>> 
>>> https://electricdruid.net/wp-content/uploads/2017/04/CEM3320LowpassFilterStage.png
>>> 
>>> -Richie,
>>> 
>>> 
>>> 
>>> -----Original Message----- From: Tom Wiltshire
>>> Sent: Sunday, February 09, 2020 11:40 AM
>>> To: Didier Leplae
>>> Cc: Synth DIY
>>> Subject: Re: [sdiy] LM13700 as VC resistor
>>> 
>>> OTA-based phaser designs are about the only application I can think of where the OTA functions like a resistor rather than a VCA. Even then it’s perhaps arguable.
>>> 
>>> Have a look at schematics for the EH Small Stone and the many subsequent derivatives.There’s also a good explanation on R.G.Keens page about phasers:
>>> 
>>> http://www.geofex.com/article_folders/phasers/phase.html
>>> 
>>> HTH,
>>> Tom
>>> 
>>> 
>>> ==================
>>>      Electric Druid
>>> Synth & Stompbox DIY
>>> ==================
>>> 
>>> 
>>> On 8 Feb 2020, at 09:58, Didier Leplae via Synth-diy <synth-diy at synth-diy.org> 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.
>>> 
>>> 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?
>>> 
>>> Any insight on this would be appreciated.
>>> I’m interested mostly in the single ended schematic at the moment.
>>> 
>>> Thank you,
>>> 
>>> Didier
>>> 
>>> 
>>> 
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