[sdiy] Cry Baby, was: Unique sounding modules that can't have voltage control?

[Ingo Debus]

> Am 09.02.2024 um 21:42 schrieb Mike Bryant <mbryant at futurehorizons.com <mailto:mbryant at futurehorizons.com>>:
> 
> R5 C4 VR1 also form a variable frequency element as well as controlling the feedback level.  Hence the waa effect.
> From: Synth-diy <synth-diy-bounces at synth-diy.org <mailto:synth-diy-bounces at synth-diy.org>> on behalf of Ingo Debus via Synth-diy <synth-diy at synth-diy.org <mailto:synth-diy at synth-diy.org>>
> Sent: 09 February 2024 20:28
> To: SDIY List <synth-diy at synth-diy.org <mailto:synth-diy at synth-diy.org>>
> Subject: Re: [sdiy] Unique sounding modules that can't have voltage control?
>  
[…]
> Looking at this description of the Cry Baby:
> https://www.electrosmash.com/crybaby-gcb-95 <https://www.electrosmash.com/crybaby-gcb-95>
> 
> It seems to me that the pot just sets the amount of feedback. This could be easily replaced by a VCA. Am I missing something?
> 

So I downloaded the LTSpice model of the Cry Baby here:
https://cushychicken.github.io/ltspice-dunlop-crybaby/ <https://cushychicken.github.io/ltspice-dunlop-crybaby/>

Here’s the frequency plot for various positions of the potentiometer:


Then I replaced the potentiometer R14/R12 with a „voltage controlled voltage source“, emulating an ideal VCA. The gain was set to the same values the (unloaded) potentiometer would yield. Here’s the result:


Almost the same as the real thing. The peaks at the low frequencies are slightly higher.
So my conclusion is, the pot in the Cry Baby can be replaced by a VCA without altering the performance very much.

Ingo
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