[sdiy] Dual power supply solution? - Kits vs 2600

[Harry Bissell Jr]

--- Paul D Brooks <pdb at sbcglobal.net> wrote: <snipp'd>

> Sure, but Harry, all three of the Arp 2600's i've
> had do this... (well, with the VCF freq slider, not
the Q), and i'm not sure we can call the designers of
that machine unenlightend. 

No but if you noticed the 'problem'... they were too
cheap to do it better. That is a trade-off that
manufacturer and kit designer alike must make...

Yeah, they could have done
> better with respect to this (and i wish they
> woulda!)... but what are you suggesting in
> particular?

In particular...

1) The reference for the expo converter sould come
from a regulated (reference) source... NOT from the
positive or negative rails. Reference might be LM336
or LM4040 etc...

2) The reference for the sawtooth reset level should
come from a reference source... NOT from a resistive
divider on the rails.  If it 'is' from the rails it
could be heavily bypassed, but I usually see NO
capacitor at that point. Even bypassing will not
protect from very low frequencies...

3) Front panel pots that control coarse and fine
frequency should come from reference sources, not the
rails.  These might be able to survive with cheaper
zener diodes if you are willing to re-tune every so
often.  If they are from the rails they need to have
resistors and capacitors to filter the voltage...and I
might add it is DAMN hard to filter .01Hz (from an
LFO) with an RC filter. Shunt regulators would be
better.

Most designers do not want to deal with the added cost
and complexity, so they make a circuit that works fine
when it is the ONLY cirucit on the power supply. Put
these in a modular and you will quickly have troubles.

Most complete instruments can forego some of these
components IF the designer is fully aware of the
issues involved.  If you can minimize the changes in
supply current, a lot of interactions will 'go away'

SOME folk might like the interactions, it might give
character, warmth, or some special quality to the
sound... This is usually unintentional and is like
discovering that the " sow's ear " really does make a
pretty good purse.

> This is a hard problem to tackle for all, and it's
> kind of harsh to come down on the kit manufacturers
> in
> particular.  

I'm not 'coming down' on them. They have made
(hopefully) informed and enlightened engineering
decisions.  In particular I'll mention EFM.  These are
often missing some of the subtle points I covered
above... but OTOH they are targeted at a price point
that would not allow including the extra components.
The whole design point of EFM is to keep everything
dead simple.  Also note that a lot of people might be
confused with all the extra components and the
additional 'room for errors'.   In some cases I've
detailed mods that I did to 'improve' the EFM units to
meet my expectations... which may differ from other
peoples. Still... for like $10 for a PCB Tom is almost
running a charity and deserves credit for what he's
doing !!!   :^P

H^) harry