ASM-1 VCO

[jorgen.bergfors at idg.se]

Yes, it is probably the FET. The 2N3819 might not be ideal. You should use the 2N4391, if you can find it. Most FETs don't discharge the capacitor fully in the time given. You can increase the time by increasing the 18p cap. But that makes the saw ramp less steep, which is a problem for saw to triangle converters.

/Jorgen





MIME:s.grainger at staffs.ac.uk on 99-11-01 12:59:08
To:  synth-diy at mailhost.bpa.nl @ SMTP cc:  (bcc: Jorgen Bergfors/IDGSE)
Subject: ASM-1 VCO 

Dear All,

 In amoment of madness had a go at the ASM-1 VCO at the weekend.

 It works fine except for the amplitude of the sawtooth which instead
 of being 0 to +5V is up at +2.5V to +5V (peak to peak) ie. it has
 a large DC offset.

 I suspect this might be caused by the ON resistance of the JFET
 (Rds (ON)) for which I am using a 2N3819, causing the integrator
 capacitor to not discharge completely.

 Any comments on this problem - given that this is the UK and  JFETs seem as rare as hen's teeth !

 Regards

 Steve Grainger