[sdiy] DCO ramp core / capacitor discharge transistors

[mark at arpeggi8.com]

I used a FET (ZVN3303) for the reset - thought I was being quite novel in my redesign of the TB-303 VCO 7 years ago...

Apparently it's very similar to what Roland did!

http://www.arpeggi8.com/synth/tb303/tb303sch.PDF

mark




Hi All,

I'm having one of my periodic "Should I build a DCO?" moments...

Consequently, I've been looking at the Electronotes/ASM ramp core,  
and various schematics for DCOs, including the JX8P.

I'd have thought that since the ramp core of both of these circuits  
is identical, you'd see similar components, but you don't. The ASM  
VCO uses a FET chosen for low on resistance (2N4856 seems favourite)  
whereas the JX8P uses a normal NPN, the 2SC945. The JX8P also uses  
half the capacitor value (1n versus 2n2) so probably doesn't need  
such a low on resistance.

Can anyone tell me why we need to use a FET for the VCO, if the DCO  
can get by with a NPN?

Thanks,
Tom


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