AW: [Re: thermal tips re expo converters:]

[jhaible]

> >I think the MS-20 scheme really has some points to it. 
> 
> This is true if you are building a keyboard synth, but if
> you are doing something more modular, you really need the 
> VCO to track over 8 octaves.

I used to think like this, but I'm not so sure anymore.
You can still modulate the MS-20 VCO way out of the keyboard
octave range, by using the expo input.

If I would start a new modular, I'd at least consider this scheme.
Even with a fixed V/Oct source (midi/CV interface etc.) it might
be worth a thought to have *one* highly stable voltage-in / voltage-
out expo converter, and drive a whole bank of linear VCOs from that.

Why would a VCO have to be able to *precisely track* over 8 octaves
without switching ? It's important to have a VCO that's capable do do
such sweeps with the turn of one knob, sure, but you don't expect this
CV source (the know with 8 octaves, or the deep ENV modulation for sync)
to be precisely temperature compensated. I think there is nothing
wrong with a VCO that has this possibillity (on its expo input), but which
has its keyboard control and footage switchig done from the linear input
at the same time.

These are just rough thoughts at the moment. I don't know where it will
end.
The good thing is that almost every exponential VCO already has a point 
where one can connect a linear CV: the virtual GND summing node at the
servo opamp of the expo converter. This point is rarely used for linear FM
at all, and if it is, it's mostly AC coupled. For good reason, because
offset
voltage is very critical here.

So my proposal would be: Spend a little effort in this offset voltage (with
a cheap opamp like the OP-07) and use the linear input for keyboard
and discrete octave steps. Then the expo input becomes much less critical.
Tempco resistors aren't an absolute must anymore, and even a slight 
misadjustment of the V/oct scale trimmer wouldn't hurt that much anymore.
(You still have to adjust the HFT trimmer, if the circuit requires one,
though.)

Maybe getting rid of tempco stuff and ultra precise V/oct trimming is worth
the little extra effort of using a low offset opamp, and even *one* global
high
precision expo converter, *especially* in a large modular system with
dozens
of VCOs.
(Ok, make that two precision converters in a duophonic system.)

I don't know how critical V/oct trimming is for all of you. I think it's no
problem for the usual 8' range and 5 octaves. But it's real fun to hear
the highest notes in 4' or 2' range on a V/Hz system in comparison.

JH.