[sdiy] LM13700 as VC resistor

Jacob Watters jacobwatters at gmail.com
Sun Feb 9 19:10:04 CET 2020


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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