[sdiy] DCO ramp core / capacitor discharge transistors

[Ian Fritz]

At 03:00 PM 12/16/2008, Tom Wiltshire wrote:
>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?

I've had the best luck so far with a low-power MOSFET (VN0104).  I posted 
this core several years ago.  There is some discussion here, where you can 
see the carefully shaped 200 ns discharge pulse:
http://electro-music.com/forum/topic-28713.html
I haven's seen the Roland schematic, so I can't comment on that aspect.

Ian