Paia 9308 Guide

FatMan MIDI Synth 1
94.7.22
FatMan Analog MIDI Synth
Model 9308
Assembly and Using Manual
It's hard to beat analog synths for fat, punchy bass lines. And for
discovering new sounds, nothing comes close to real knobs operating in real
time. The FatMan has all of the features that give analog it's warm, full tone in
a MIDI controlled package. The classic normalization scheme of twin VCO/
VCF/VCA and Dual Transient Generators is brought up to date with the
inclusion of Velocity CV not available on pre-MIDI synths.
FatMan learns from the past by including features that were eccentricities of
classic synths such as a unique "punch" switch that adds a subtle but useful
fifth segment to the standard ADSR response.
(c)2000 PAiA Electronics, Inc.
Fair use copy with this notice only
email [email protected]
Details specific to the installation of the FatMan
circuit board in the 9308C Desk Top enclosure are
covered in the 9308C Supplement. Follow the
assembly instructions in this manual until instructed
to reference the 9308C Supplement.
VCOs
pitch
VCF VCA PUNCH
MIDI / CV GATEPOWER
ADSRA(S)R
AR
ASR
glide offset 1/2 mix velocity pitch frequency resonance velocity
releasesustaindecayattackreleaseattackgatevelocitypitchthrumidi in
output
9308

2 FatMan MIDI Synth 94.7.22
FatMan
Packing List
1 8031 8 Bit MicroController IC1
2 74HC373 8 Bit Latch IC2,IC4
1 2764 8kEPROM IC3
1 DAC08 8 Bit DAC (may be 1408) IC5
1 6N138 Opto Isolator IC6
1 74HC14 Hex Inv. Schmitt Trig. IC7
1 LM339 Quad Comparator IC8
1 4052 Dual1/4CMOSMUX IC9
2 LM324 Quad OpAmp (CA324) IC10,IC13
1 4016 Quad Analog Switch IC11
1 TL084 Quad Bi-fet Amp (CA084)IC12
2 LM13600 DualOTA IC17,IC18
2 555 Timer IC15,IC16
1 7805 +5V Voltage Reg. IC19
1 7808 +8V “ “ IC20
1 7912 -12V “ “ IC14
3 100uF/16V Electrolytic Capacitor C28,C29,C30
2 10uF/16V “ “ C1,C23
5 1uF/16V “ “ C15,C18,C31,
C32,C33
5 2.2uF/16V “ “ C5,C6,C19,C22,
C25
2 470uF/25V “ “ C26,C27
3 .1uF Mylar Capacitor C7,C8,C12
2 .01uF “ “ C14,C17
2 33pF Ceramic Disk Capacitor C2,C3
6 .01uF “ “ “ C4,C9,C10,C11,
C13,C16
1 .001uF “ “ “ C24
3 .05uF “ “ “ C34,C35,C36
2 560pF Polystyrene Capacitor C20,C21
2 1N4001 Power Diodes D10,D11
8 1N4148 SignalDiodes D1,D3,D4,D5,
D6,D7,D8,D9
3 RedLED D2,D12,D13
5 2N4124 NPN Silicon Transistors Q1,Q2,Q7,
Q10,Q11
7 2N4126 PNP Silicon Transistors Q3,Q4,Q5,Q6,
Q8,Q9,Q12
1 1/4" Phone Jack *J6
2 PCMount5 PinDINSockets J1,J2
3 PCMountPhonoJack J3,J4,J5
2 10k ohm PCMountTrimmer R13,R42
3 1k ohm PCMountTrimmer R18,R21,R24
8 10kohm PanelMountPot *R34,*R56,
*R69,*R71,
*R74,*R102,
*R104,*R115
6 1megohm “ “ “ *R32,*R82,
*R84,*R92,
*R94,*R96
1 1k ohm “ “ “ *R90
1 100k ohm " " " *R40
1 500k ohm “ “ “ *R114
1 5k ohm “ “ “ *R113
1/4W 5% resistors
3 10ohm (brown-black-black) R38,R39,R93
10 100 ohm (brown-black-brown) *R73,R16,R20,R26,R44,
R53,R81,R83,R91,R95
15 10k (brown-black-orange) R6,R7,R8,R9,R29,R30,
R46,R55,R67,R76,R77,
R79,R86,R101,R106
1 100k (brown-black-yellow) R57
2 10megohm (brown-black-blue) R41,R100
1 120 ohm (brown-red-brown) R22
5 12k (brown-red-orange) R70,R72,R75,
R78,R108
3 15k (brown-green-orange) R14,R58,R62
1 18k (brown-grey-orange) R89
1 1800 ohm (brown-grey-red) *R33
9 1000 ohm (brown-black-red) R31,R35,R37,
R48,R59,R60,
R63,R64,R80
3 220 ohm (red-red-brown) R2,R4,R5
4 2200 ohm (red-red-red) R49,R85,R116,R119
4 22k (red-red-orange) R36,R66,R68,R107
2 270 ohm (red-violet-brown) R3,R19
1 2700 ohm (red-violet-red) R10
1 330k (orange-orange-yellow) R98
3 33k (orange-orange-orange) R45,R54,R109
2 390 ohm (orange-white-brown) R17,R99
2 39k (orange-white-orange) R103,R105
3 47ohm (yellow-violet-black) R23,R43,R52
2 470 ohm (yellow-violet-brown) R110,R111
10 4700 ohm (yellow-violet-red) R1,R12,R15,R27,R28,
R61,R65,R88,R97,R112
1 470k (yellow-violet-yellow) R99
1 56 ohm (green-blue-black) R25
1 56k (green-blue-orange)` R47
1 6800 ohm (blue-grey-red) R11
2 15 ohm 1W. Power Resistor R117 (see pg 6)
3 SPST Panel Mount Toggle Switches *S1,*S3,*S4
1 8 Position DIP Switch S2
1 12 - 14VAC, PWR1
500mA (or greater) Wall Mount Transf.
1 12mHz Crystal X1
18 Set Screw Knobs
1 28PinICSocket
1 40PinICSocket
2 “L” Brackets
3 #4 Nuts
4 4-40 X 1/4" Machine Screws
1 4-40 X 1/2" Machine Screw
1 #4 Flat Washer
1 NylonCableClamp
42" Bare Wire
8" SmallInsulatedSleeving
38' #22 insulated, stranded wire (4 ea. 9.5' lengths)
1 Voltage Regulator Cooling Fin
1 9308 FatMan Printed Circuit Board
parts marked * mount on the front panel
Designations R50, R51and R87 are not used.
(c)2000 PAiA Electronics, Inc.
Fair use copy with this notice only
email [email protected]

FatMan MIDI Synth 3
94.7.22
FatManDesignandTuningAnalysis
As shown in fig 5a, the schematic of the digital circuitry,
FatMan’s brain is an 8031 MicroController (IC1). Firm-
ware for the system is burned into the EPROM (IC3)
which is attached to the uP’s address and data lines with
the Octal Latch IC2. The DIP switch S2 connects to five of
the uP’s input port lines. Four of the switches in this
package are used to select MIDI Channel and the fifth is
an unused input to the processor.
The receive (RxD) line of IC1 receives MIDI Data from the
mandatory optocoupler IC6 which isolates the ground of
the MIDI sending device from FatMan’s ground. The
output of the optocoupler is also buffered by a pair of
Inverter stages (IC7:b & a) which drive the MIDI Thru
output J2. A third Inverter stage, IC7:c, drives the LED D2
to give an indication of MIDI activity on the input J1.
DAC TUNING
FatMan’s VCOs are linear in the way their frequencies
respond to Control Voltage changes. This means that
CVs must change exponentially to produce proper
pitches. For example, to produce a pitch an octave above
the present pitch the CV must double; for an octave lower
the voltage must be halved. Linear Digital to Analog
Converters are generally no good at generating these
kinds of voltage increments because if the DAC is scaled
to produce the largest voltage necessary, a couple of
octaves lower you’re dealing with semi-tone voltage
changes that are much smaller than the resolution of the
Least Significant Bit.
FatMan gets around this problem by having the DAC (IC5)
be responsible for only a single octave’s worth of the CV.
In tech-talk, the voltages for 12 equally tempered pitches
are sparsely mapped along an exponential curve in the
256space of the 8-bit DAC. Octave changes are handled
by the ranging network consisting of a 1/4 Multiplexer
(IC9) that selects one of four taps on the voltage divider
string R17-R26. These component values produce a
voltage at each tap that is 1/2 the voltage of the tap above.
On the digital side of things, the DAC is glued to the uP
data lines with the octal latch IC4. The ranging MUX is
controlled by the processor's T0 and T1 lines. These
signals are level shifted to 8V by discrete transistors Q1
and Q2.
In normal operation, the voltage generated by the DAC
can be thought of as going from C down to C#, with
octave ranging changes happening between C# and the
C immediately below it. So that the maximum output
range of the DAC can be used (for maximum error of less
than one cent), the DAC is ranged to produce a voltage
from a nominal 3V for C (FFh into the DAC) down to a
nominal .177v for C# (0Eh into the DAC). The 3V offset
introduced by the current flow through R12 and R14
causes the voltage from the DAC’s output buffer (pin 7 of
IC10) to go from a nominal 6V down to a nominal 3.177V.
Huh?
What’s this 3.177V business? Well, that is the voltage
corresponding to the octave below 6V (which is 3V) plus
the voltage required to produce the next semi-tone up.
Since in equal temperament each semi-tone has a
frequency 1.059 times the preceding semi-tone, and
since our Voltage/Frequency response is linear, the next
semi-tone above 3V is 3*1.059 =3.177V (if you think it’s
difficult to read, try explaining it some time.)
At the step between C# and the C below it, the DAC
buffer output returns to 6V and the octave switching
network switches to divide this in half so the CV to the
VCOs becomes 3V, which as you now know is the
voltage an octave below 6V.
During calibration the output of the DAC as set by R13 is
adjusted so that it exactly matches the offset voltage
from R12 and R14. When these conditions are met, the
output of the buffer will be some voltage
X in response to
the maximum DAC output (FFh as data) and exactly
X /2
when the DAC is contributing no output at all (00h as
data). We’ve stated the “nominal” value of x as 6V, which
may seem sort of sloppy (the actual voltage may be as
low as 5V.) until you realize that it’s the ratio of 2:1 that
matters, and not the exact value of the voltages.
The DAC must be tuned over the octave from C0 to C1
because C0 is the only C that causes 00h to be sent to
the DAC. In firmware, this lowest C is an exception to the
normal ranging that happens between C# and C.
Once the DAC is tuned, the trimmers that set octave
intervals (R18, R21 and R24) are adjusted so that the
pitch changes by octaves as you go down the keyboard
by octaves. These adjustments do not interact between
themselves or with the tuning of the DAC, so you usually
only have to go through them once for them to be right,
and the circuitry is simple and stable so they tend to stay
right for a long time.
In the final calibration step, the two VCOs are made
identical by adjusting the zero offset of VCO #1 so that
it’s the same as VCO #2. A subtlety of the tuning process
is that it compensates for any zero offset in VCO #2
(which means that exactly zero voltage may not produce
exactly zero frequency, trickier than it sounds). So as
long as VCO #1 is the same, everything is wonderful.
The single output of the DAC and Octave Range
Switcher is split into Pitch and Velocity CVs with the
sample and hold circuits built using OpAmps IC12:a&b,
CMOS switches IC11:a&b and capacitors C7 and C8.
System firmware outputs values to the DAC and Range
Switcher corresponding to the Pitch CV then turns on
IC11:a to sample the voltage by charging capacitor C7.
IC11:a is then turned off to isolate the voltage on C7.
The processor then repeats these actions for the
Velocity CV, turning on the second CMOS switch (IC11:b)
to charge C8. The voltages on the capacitors are read
out by their corresponding OpAmp buffers IC12:a & :b.
Comparators IC8:a&b provide level translation from 5V
to the higher voltage needed for the CMOS switches by
tying their open collector outputs to the 8V rail through
R29 and R30.
Leaving the bits and bytes behind, we turn our attention
(c)2000 PAiA Electronics, Inc.
Fair use copy with this notice only
email [email protected]

4 FatMan MIDI Synth 94.7.22
Fig 5a. An 8031 uProcessor provides the computing horsepower needed to
decode MIDI and keep Control Voltages straight. Equally tempered Control
Voltages are provided by the combination of the DAC and Octave Range
switching.
(c)2000 PAiA Electronics, Inc.
Fair use copy with this notice only
email [email protected]

FatMan MIDI Synth 5
94.7.22
to the analog sound generating and processing part of
FatMan shown in fig 5b.
What would an analog synth be without a GLIDE control to
grab and twist for really expressive portamento? FatMan
uses the common approach of charging a capacitor
(C12) through a variable resistor (R32). IC10:a buffers the
voltage on the capacitor and drives the Master Pitch
control R34 which is used to transpose both oscillators
over slightly more than an octave range.
The two VCOs are identical except for the Offset control
(R40) which allows the pitch of VCO #1 to be raised and
lowered an octave relative to VCO #2. VCO #1 also has a
trimmer that allows it's zero intercept to be adjusted to
match that of VCO #2.
Taking VCO #1 as being otherwise typical, the Pitch CV
drives a voltage to current converter (V/I) consisting of
IC10:c, transistors Q3 and Q4 and the associated
resistors. The current output of this circuit, from the
collectors of the transistors, charges capacitor C14 and
produces a linear voltage ramp which is read out by the
buffer amp IC10:d. IC16 is a 555 type timer that senses
when the voltage ramp at the output of the buffer exceeds
a threshold at which point an internal transistor is turned
on to short out the capacitor and quickly discharge it.
When the capacitor discharges to a lower threshold the
transistor is turned off and the capacitor can once again
charge and repeat the cycle.
The result of this relatively slow charging and quick
discharging is a ramp (sawtooth) waveform and in the
interest of simplicity this is the only oscillator waveform
available. A ramp is the most harmonically rich of the
common waveforms, having both the even harmonics of a
triangle and the odd harmonics of a pulse. The filter can
be used to track the pitch of the oscillators and reject all
harmonics in the ramp leaving only the fundamental sine
wave.
Potentiometer R56, the Osc1/Osc2 Mix control, allows the
VCF to be driven by either VCO1 or VCO2 or a mix of the
two. The VCF design is a State Variable Filter which has
been configured to give a low-pass response with
resonance, adjustable with R114, at the corner frequency.
The filter is built around IC17, an LM13600 type Dual
Operational Transconductance Amplifiers (OTA) with C20
and C21 as the tuning capacitors. Two control currents for
setting the gain of the two OTAs in IC17 are produced by
the V/I consisting of IC13:d, Q8, Q9 and associated
resistors. Four separate voltages are summed to set the
corner frequency of the filter; a static voltage that sets the
initial frequency is adjustable with R74, Velocity CV
adjustable by R69, Pitch CV adjustable by R71, and finally
the output of the filter’s dedicated transient generator
adjustable with R115.
The filter’s AR transient generator works by charging C22
through R83 and R84 for the Attack portion of the cycle
and discharging it through R81 and R82 for the Release
section. Charging and discharging currents are steered
by D3 and D4 as Q7 is switched on and off by the TxD line
of the uP. Voltage on the capacitor is buffered by IC12:c
and the comparator IC8:c monitors the buffer’s voltage
and switches the processor’s INT1 input when the peak
voltage is reached. The firmware’s response to this is to
switch from Attack to Release. Closing the Sustain switch
S3 prevents this “peak reached” signal from getting back
to the uP so that the Release portion of the cycle won’t
happen until the key that initiated the response is
released. The result is to switch the transient from a non-
sustaining AR to an Attack / Sustain / Release (ASR)
response.
FatMan’s Voltage Controlled Amplifier uses one OTA from
IC18. The main components of the V/I that control this
element are IC13:c and Q12. This voltage to current
converter is unlike the others in that it must be stable for
zero control voltage (so the VCA can turn off completely).
Adding D9 to the circuit clamps the output of IC13:c and
keeps it from going negative and C24 provides frequency
compensation for the high loop-gain state that exists at
near-zero control voltages
The Attack/Decay/Sustain/Release (ADSR) transient
generator dedicated to the VCA is similar to the filter’s
A(S)R. Under control of a pair of the uP’s output lines
(P12 & P13), capacitor C19 charges and discharges
though steering diodes D6-D8 at rates set by R92, R94
and R96. The Sustain control R90 sets the voltage level
to which the Decay portion of the cycle falls. IC12:d buffers
the voltage on the capacitor and comparator IC8:d signals
the processor when the peak of the Attack is reached.
When the Punch switch S1 is closed the combination of
C34 and R98 add a slight delay (about 20 ms.) between
the time that the ADSR reaches its Attack peak and the
time that this information reaches the uP. The result is a
short Sustain interval that adds punch to sounds with fast
Attack and Decay dynamics. When S1 is open, the ADSR
behaves in the normal, technically correct way.
FIRMWARE
The FatMan firmware is responsible for recognizing MIDI
Note On and Off messages and breaking them down into
Note number and Velocity values. Note number is
checked for being in the range of 36-84 and then con-
verted into octave ranges by division and the data
required to drive the DAC by look-up table.
The Velocity data from Note On and Off messages are
handled in much the same way, except that the 0 to 127
step range of this data is first scaled to range from
36-84.
Pitch Wheel messages are also supported. In the
FatMan, Wheel data modulates the Pitch data before it
gets to the DAC. This is possible because only 12 of the
256 possible values of the DAC are used for pitch and the
space between these values is available for modulation.
Musical range of FatMan’s Pitch Wheel is +/- a semi-tone.
Since there are no pitches available above the highest C
or below the lowest, wheel data is ignored on these
bends.
The firmware is also responsible for turning on and off
the proper sample and hold at the proper time to produce
Pitch and Velocity CVs. It manages the A(S)R and ADSR
transient generators, turning on their Attack cycle when a
note is played and managing Decay, Sustain and
Release as appropriate for the status of the transient and
any Note Off messages which may be received.

6 FatMan MIDI Synth 94.7.22
Fig 5b. The FatMan analog circuitry comporises two VCOs, low-pass VCF
with AR Transient Generator, and VCA with ADSR Transient Generator
(c)2000 PAiA Electronics, Inc.
Fair use copy with this notice only
email [email protected]