vco8 (Axxe VCO)

[tomg]

:Hello Joachim,

> 1. Are you sure you got the schematic right in regard to the
> retrace FET's gate circuit?

No...I drew it wrong!...How can I be looking right at something
and draw it wrong?....Anyway, it's right now, thanks...

> 2. ARP used a 3086 array for the comparator NPN pair, which
> helps keep the trip point stable. You're design apparently
> uses two discrete devices. This might theoretically introduce
> stability problems, especially if the two devices aren't
> thermally coupled. Have you compared the performance of your
> circuit to one that uses an array

No I didn't try an array for the schmitt trigger pair. I was thinking
about using a 3096 (2 pnp + 3 npn). That would put everything
but the fets on the same chip, I thnk it's pretty stable using
discrete trannys.

If I wanted to do temp-comp I think I would use the npn expo-tranny
instead of the schmitt trigger pair. Am I wrong?

Now a question for you...Do you have an example of this vco that
has a sync and/or fm option?

Tom




> > I may have stumbled over something you want.....It's the vco from
> > the Arp Axxe.
> 
> > http://www.xavax.com/efm/synth/cbook/pics/vco8a.gif
> 
> Hi,
> 
> This is the VCO design that ARP used for practically all their
> synths from 1973 onwards, or so. It also appeared as the 4027-1
> submodule in the model 2600; schematic at:
> 
> http://www.netcontrol.fi/~jocke/arptech/pdf/4027.pdf
> 
> A few questions about tomg's version:
> 
> 1. Are you sure you got the schematic right in regard to the
> retrace FET's gate circuit?
> 
> 2. ARP used a 3086 array for the comparator NPN pair, which
> helps keep the trip point stable. You're design apparently
> uses two discrete devices. This might theoretically introduce
> stability problems, especially if the two devices aren't
> thermally coupled. Have you compared the performance of your
> circuit to one that uses an array?
> 
> Best,
> 
> -joachim
> 
>