[sdiy] electronotes vcf question..

[Tim Stinchcombe]

Hi Al,
	Since you've not been too specific about *which* EN VCF, I picked
one more or less at random, EN#71 p16, 'ENS-76 VCF option 1', which in fact
*does* have a 2N3904 in the frequency CV subcircuit. If your set-up is
similar, then no, it won't work: T1 relies on the log relationship in
interaction with T2 & T3 to generate the 'volts/octave' law, and literally
replacing T1 with an op amp follower will mess this up. However it can be
done, but the op amp needs to have one of the transistors in its feedback
loop - and in fact this is exactly what is done in 'option 2' on page 20
,IC-3 with T1 & T2!

Not sure if this helps, but if you can be more specific about the exact
reference, I can probably check that too!

Tim
__________________________________________________________
Tim Stinchcombe 

Cheltenham, Glos, UK
email: tim102 at tstinchcombe.freeserve.co.uk


> -----Original Message-----
> From: synth-diy-bounces at dropmix.xs4all.nl 
> [mailto:synth-diy-bounces at dropmix.xs4all.nl] On Behalf Of 
> alfred.pear at gmail.com
> Sent: 27 June 2008 18:42
> To: synth-diy at dropmix.xs4all.nl
> Subject: [sdiy] electronotes vcf question..
> 
> 
> hi -
> 
> working on the electronotes  VCF Option 1, with some slight 
> modifications..
> 
> i'm currently using an op-amp as a buffer instead of the 
> 2n3904 configuration, but the filter range seems severely impacted..
> 
> any thoughts?
> 
> -al
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