From ynottnaro@yahoo.com Fri Oct 03 22:51:05 2003
Subject:RE: Next Generation Vibrato/Chorus Clone

I don't know the answer to that, but the Voce has the
WORST chorus/vibrato sound of ANY clone going. (And I
LIKE the v5)

T
--- bilyod@msn.com wrote:
> Does any one know what the VK7 &8, Korg and XK2 use
> in the way of vibrato cirtuit? From what little I've
> heard they sounded OK. They just don't measure up to
> the Voce for true Hammond sound. Bill
>
> ------ Original Message -----
>
> From: Kevin
> Sent: Fri, Oct 3, 2003 9:10pm
> To:
> Subject: [CWSG] Next Generation Vibrato/Chorus Clone
> After reviewing a number of design options over the
> past couple of
> weeks (thanks to all who contributed to the
> discussion), I have
> decided on a design and will post a *preliminary*
> schematic on my web
> site in the next week or so before embarking on this
> project.
>
> As I previously mentioned, Hammond was limited by
> available
> technology based on the era they designed their
> chorus. Functionally
> the Vibrato/Chorus on a Hammond organ has the
> following attributes:
>
> - tapped phase shift line using inductors and
> capacitors (this
> arrangement served no other purpose than to realize
> a multi-stage
> phase shifter with discrete phase shifts (which were
> tapped) rather
> than a continuously variable phase shift from the
> phase shifters we
> know today)
>
> - discrete phase shifts were selected by the vibrato
> scanner which
> behaved as a sort of switch blending the signal from
> any given tap
> with the next one in line as the scanner's rotor was
> swinging
> through - as the scanner rotated the effective phase
> shift increased
> to the max and back to the min as the scanner made
> one complete
> revolution - it did this 412 times per minute (or so
> the research has
> indicated).
>
> - The V1/C1, V2/C2, V3/C3 switch was used to
> determine how far the
> scanner traversed the phase shift line was used to
> achieve the
> desired effect with the "1" setting using the least
> length and "3"
> used the most (see the B3 schematic I posted this
> morning for those
> who are interested).
>
> - Chorus (the sound we all know and love) was
> realized by mixing the
> dry signal with the phase shifted signal.
>
> - the combination of all of the above produces what
> we know as chorus.
>
> My design will incorporate the digitally controlled
> phase shifters of
> Dave Amels' circuit, replacing the digi-pots with a
> binary-weighted 4
> resistor serial line and 4 analog switches (in
> effect, a digitally
> controlled resistor) driven off of an up/down
> counter circuit to
> either increase or decrease the resistor in an
> op-amp phase-lead
> configuration by shorting appropriate resistors.
> This gives 16
> discrete phase shifts that when driven by an up/down
> counter will
> scan back and forth as if scanning a multi-tapped
> delay line. If a
> given phase shift is realized by one op-amp, then 2
> or 3 times that
> phase shift is achieved by cascading the same phase
> shifter with 2 or
> 3 identical circuits (as in Dave's circuit which
> used 4). Each of
> the 3 stages' outputs will feed a V1/C1, V2/C2,
> V3/C3 switch to
> determine the amount of chorus or vibrato depth that
> will be used. A
> pot will be available which can be used to adjust
> the depth of the
> chorus effect. It will probably be a trimpot since
> once the Hammond
> chorus sound is achieved you probably don't want to
> mess with it :)
>
> Individual changes to the digitally controlled
> resistor (during
> scanning) will probably not change instantaneously
> since a resistor
> and capacitor are involved, so I don't suspect
> switching noise to be
> a problem. This will *probably* closely approximate
> the transfer
> function of the scanner as it blends from one tap to
> the next during
> rotation.
>
> The previous discussion on using cross-fading VCAs
> and how much
> overlap is needed to approximate the scanner's
> transfer function,
> while academic, is probably not necessary for this
> effect based on
> the information I have today - Thanks Dave for the
> innovative design
> using discrete scanned phase shifters.
>
> The only issue I might encounter is digital
> bleedthrough into the
> analog signal, but the difference between a digipot
> and my digital
> controlled resistor is that the resistors are not on
> the same
> substrate as the analog switches and may not suffer
> from the same
> behavior. I've used analog switches in similar
> instrumentation and
> industrial control designs before with very little
> if any noise fed
> through.
>
> As soon as I have a schematic I'll post and update
> the group. Thanks
> to all who contributed in any way to the discussion
> and stimulated my
> thinking. Once I've posted the schematic and
> commenced building,
> I'll update with progress and eventually sound
> samples both dry and
> through my Leslie.
>
> Regards,
> Kevin
>
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