From obxwindsurf@yahoo.com Mon Sep 29 14:26:04 2003
Subject:Re: Solid State scanner project

If it helps, the design I've started down is recreating the delay
line using a series of passive LC networks (2nd order LPF) after
something similar I found elsewhere on the 'net but which the design
didn't appear to be finished. This design will use an identical
configuration to a B3 using PC-board mount inductors with the taps in
the same positions as those I've found on a B3 schematic.

Banks of analog switches select which 9 taps of the delay line are
run to the scanner for the V1/C1, V2/C2, and V3/C3 settings.

The "scanner" in my design is implemented using twin circuits each of
which consists of a 1 of 8 analog multiplexer addressed by a 4 bit
binary counter (using the 3 LSB to select which analog input routes
to the analog output). The analog output feeds the input of 1/2 of a
dual OTA (operational transconductance amplifier) configured as a VCA.

Of the 9 taps selected with the analog switch banks, even taps are
run to one multiplexer, in the sequence 0, 2, 4, 6, 8, 6, 4, 2 odd
taps are run to the other multiplexer in the sequence, 1, 3, 5, 7, 7,
5, 3, 1.

A triangle LFO with a corresponding square wave output is constructed
of an op-amp integrator/comparator circuit which is a classic dual op-
amp LFO configuration (integrator integrates output of comparator
which is monitoring output of integrator). The square wave output of
the LFO clocks the even multiplexer on its rising edge and the odd
multiplexer on its falling edge.

The triangle is half-wave rectified (HWR) which provides one VCA's
control voltage, and inverted and half-wave rectified which provides
the other VCA's control voltage. The result is that the pair of VCAs
crossfade between the even mux tap signal and the odd mux tap signal.

The even mux makes a new selection while the odd mux VCA is fully on
and the odd mux makes a new selection while the even mux VCA is fully
on.

LFO frequency is set to give an equivalent "rotational scan" rate of
412 RPM which I've found from research on the net to be the rate in
use on a B3. The VCAs feed a summing amplifier along with the "dry"
signal (for chorus).

I'm still working through the details of the design and in particular
modifying the waveshape of the HWR signal feeding the VCAs. My
design uses a 2 half triangles 180 deg. out of phase with only one
VCA partially or fully on at any given point in time. The Hammond
scanner was a different story however with its "rotational capacitor"
design and instead blended between two sets of inputs with overlap
before one input was fully selected. From a switching standpoint it
was "inefficient" but this may fall into the category of those so-
called "design-flaws" which greatly contributed to the character of
the sound. I think I have a technique to more accurately mimic this
blended-switching of the rotational scanner. I just need to finish
working it out on paper.

Best regards,
Kevin

--- In CloneWheel@yahoogroups.com, wrote:
> Dan we both have the same idea. I've been wanting to make a solid