new Formant book arrived

[jh.]

Took a second look at the new Formant "Moog" filter.
It's worse than I thought.
They use a single-opamp instrumentation amplifier
for buffering the ladder. Which means that the non-inverting
input of this buffer amp has an impedance of 1.01MegOhm,
and the inverting input has an impedance of 10kOhm.
So the ladder is loaded in a quite unsymmetrical way.
I'm sure it works, somehow, nevertheless.
But I'm also sure much of the recommended transistor
matching "to 2.5mV at room temperature" is more or
less in vain.

JH.



-----Ursprüngliche Nachricht-----
Von: jh. <jhaible at t-online.de>
An: diy <synth-diy at node12b53.a2000.nl>
Gesendet: Samstag, 4. November 2000 18:59
Betreff: new Formant book arrived


> Got the new Formant book today.
>
> First impression brings up a couple of questions:
>
> TL081's as noninverting buffers after 3080's in VCF.
> Should lock up due to phase reversal when heavily overdriven,
> shouldn't it ?
>
> Heated transistor array for the VCOs. Adjustment by measuring
> the voltage drop over a be junction and emitter resistor. They say
> the voltage depends on the manufacturer of the chip - I'd
> expect it to be different for each chip, not just for each manufactuer.
>
> Moog ladder with with single opamp buffer stage ... debatable.
> But what's really bad about the Moog VCF article is the theory:
> They show an unbuffered symmetrical RC filter and say the
> transistors take the place of the resistors - way over-simplified.
>
> A lot of low current BiFet opamps (TL061) are used, mainly for the
> output stages. Why use the low current version here, but TL081's
> in the VCF's internal stages ?
>
> The nice old OTA-based 4pole VCF is gone. The tiny jacks remain.
>
> Still a lot of words about the quality a VCA should have - and still
> 3080's used.
>
> Nice little sequencers added.
>
> (Just a first impression.)
>
> JH.
>
>
>
>