From Playing@Blues.GuitarMon Jun 26 15:06:41 1995
Date: Fri, 23 Jun 1995 18:25:52 -0700
From: Playing@Blues.Guitar
To: analogue@hyperreal.com
Subject: FatMan SubHarmonic Mod Text File

FatMan SubHarmonic Generator                 by Scott R.Gravenhorst
                                             chordman@ix.netcom.com
                                             06-22-95


I ask only one thing of anyone who builds this mod:  Please email me
with your comments.  Good or bad.

NOTE:  The circuit diagram is SUBHARM.GIF and is a separate file.

This FatMan mod requires no trace cutting of the FatMan circuit board.
Connections to different points in the FatMan circuit can be
accomplished by wrapping a wire around an existing resistor (or other
part) lead and soldering.  An alternate, less permanent method I use 
for testing is to tack solder to the bottom side of the board.  The
SubHarmonic Generator produces a square wave signal, pitch locked to
VCO 2 and one octave below it.  A simple RC lowpass filter is provided
to roll off harmonics as desired.  By using both the subharmonic
lowpass filter in combination with the VCF, a near sine waveform is
possible.  A level pot is provided to allow controlling the mix of the
subharmonic and  the output of the 1/2 vco mix control.  

How it works:
A 74L93 TTL binary counter is used to generate a subharmonic (one
octave below) tone from the pulse output of the VCO 2   555 timer chip.
The pulse output from IC16 is available on pin 3 as a -V referenced
square pulse.  This is incompatible with TTL logic so it is level
converted by IC901A, clamped no lower than ground by D901 and then fed
to the B input of the binary divide by 2 counter IC902.  The B output
of IC902 produces a symmetrical square wave that is exactly one octave
below it's input.  This is the raw subharmonic.  The raw subharmonic is
then sent to a simple lowpass filter to remove harmonics as desired,
amplified by IC901B and mixed with the output of the VCO 1/2 MIX 
control
via resistor R901.  The combined output is then sent to the VCF.

As fat as the FatMan is, right out of the box, it is even fatter and
thicker with the subharmonic generator.  This mod has a rather profound
effect on the harmonic structures available.  It's almost like having a
third VCO.  The subharmonic generator also allows the FatMan to produce
tones down to (approximately) 32 Hz!

------------------------------------------------------------------------

I strongly recommend monitoring the amount of heat produced by the
power supply regulator ICs before adding the mod and after.  The +5
volt regulator will heat up a little more, but in open air at 72 
degrees F, it is not hot to the touch.

The resistors I used were all of gold tolerance.

Pinouts of the ICs I used are as follows:

LM1458 (BTW These are just what I happen to have a pile of.  Other
bipolar supply op amps may work as well.  There is nothing special
about either the amplifier or the converter topologies.)

LM1458  Dual Operational Amplifier
Dual inline 8 pins
1... output A
2... inverting input A
3... non inverting input A
4... V-
5... non inverting input B
6... inverting input B
7... output B
8... V+


74L93
Dual inline 14 pins
 1... input B 
 2... RO(1)  (reset 1)
 3... RO(2)  (reset 2)
 4... nc
 5... Vcc  (+5V)
 6... nc
 7... nc
 8... Qc  Output C
 9... Qb  Output B
10... GND
11... Qd  Output D
12... Qa  Output A
13... nc
14... Input A

Standard disclaimer:  Mine works.

-- 
Have you ever wondered why AT&T chose the 'Death Star' as their logo?
To pay more, get poor service, be lied to, to be transfered all over
creation and they *still* _won't_ help...
"AT&T... It just doesn't ring true."