[sdiy] Experimental VCA RFC

[Tom Wiltshire]

I see. I guess that's the difference with opto-transistors. I know it's the LDR response that slows vactrols down, but I have to say I didn't expect a difference of such magnitude.

T.

On 18 Mar 2011, at 15:13, Jerry Gray-Eskue wrote:

> I have not specifically looked at that but the data sheet shows Turn-on /
> off and Rise and Fall times well under 20 microseconds for the conditions
> listed. 
> 
> My gut feel is that they will be plenty fast for most if not all audio
> application. 
> 
> -----Original Message-----
> From: Tom Wiltshire [mailto:tom at electricdruid.net] 
> Sent: Friday, March 18, 2011 9:25 AM
> To: Jerry Gray-Eskue
> Cc: synth-diy at dropmix.xs4all.nl
> Subject: Re: [sdiy] Experimental VCA RFC
> 
> Very interesting Jerry.
> 
> How fast can it respond? I see you've got about 40ms attack time there. I'd
> expect there to be a significant lower limit with optos, but perhaps it
> depends entirely on the type.
> 
> Thanks,
> Tom
> 
> On 18 Mar 2011, at 13:08, Jerry Gray-Eskue wrote:
> 
>> Request for Comments:
>> 
>> This is a link to a VCA design I have been working on using H11A1 Optos as
>> voltage controlled resistors.
>> This includes a PSpice schematic for the core, and two PSpice plots at 20
>> and 60 degrees C.
>> The biggest design challenge is keeping the circuit from being effected by
>> temperature changes, U3D uses the Thermistor NTC3 for this purpose.
>> U1C is just the signal in buffer, U4A,U5 and U6 are the control /
>> attenuation circuit. R15 sets the current per voltage in for the LEDs of
> the
>> Optos. U4B is a just a spare at the moment.
>> The Schematic shows H11A2s which seem to share the same PSpice model with
>> the H11A1.
>> 
>> In the plots the Green line is the envelope voltage, the red line is
> signal
>> out.
>> The signal in not shown is 1000hz at +-5v
>> 
>> 
>> http://myweb.cableone.net/ankh/VCA.htm
>> 
>> Jerry Gray-Eskue
>> 
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>