new Formant book arrived

[Bjorn Julin]

Design fault, they have to
AC coupling the diff buffer!!

BJ


>From: jhaible at t-online.de (jh.)
>To: "diy" <synth-diy at node12b53.a2000.nl>
>Subject: Re: new Formant book arrived
>Date: Sat, 4 Nov 2000 20:46:47 +0100
>
>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.
> >
> >
> >
> >
>
>

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