• A guitar sub-octave generator
• A guitar to gate & trigger generator
• An audio mixer.
• Two AD/AR generators.
• Two Triangle wave LFOs.
• A 12dB/octave lowpass VCF (Voltage Controlled Filter)
• A exponential response VCA (Voltage Controlled Amplifier)

Buy MFOS SUB-COMMANDER Guitar Synthesizer PC Boards

Tired of etchant eating your hands and the tedious work of drilling hundreds of tiny holes. Then buy a ready-to-go, super high quality PC Board! Purchase Printed Circuit Boards online! Click the Add To Cart button to make a purchase or go to PayPal to sign up for a PayPal account. You can also set the number of boards you would like to buy on the PayPal shopping cart page. Please note that you are buying an un-populated PC board only. The glass epoxy, double sided, plated through-hole PC board is professionally manufactured, pre-drilled and silk-screened with a parts layout legend. You must purchase all of the parts for the project and build it yourself. But since that is the whole idea behind DIY (Do It Yourself) that's a good thing.

MFOS SUB-COMMANDER Guitar Synthesizer (6.3" x 5.3")

Page one of the schematic contains the input preamplifier, 3 pole fixed low pass filter, peak/valley detectors, and pulse generator. Additionally the sub-octave divider and signal to gate generator are shown. U1-A and U1-B along with their associated resistors and capacitors comprise the input signal buffer. The instrument signal is applied to U1-A's non-inverting input via C1 and R1. R7 and R12 cause U1-A to have a gain of 2. R6 holds the high impedance non-inverting input of U1-A at ground potential. U1-A's output is applied via C2 to the non-inverting input of U1-B (also held to ground via R8 1M resistor). R9 and R13 cause U1-B to have a gain of about 4.8. The combination of U1-A nd U1-B provide a gain of about 9.7 to the input signal. Even with a hot guitar signal the level at U1-B pin 7 (point "CG" (Clean Guitar)) is well within the non-distorting level of output voltage for the TL074. U1-C, U1-D and associated components comprise a 3 pole low pass (Sallen Key) filter. This filter's purpose is to reduce the high frequency harmonics contained in the original guitar (or instrument) signal and accentuate the fundemental frequency. This allows the following circuitry to more accurately track the fundamental during pulse generation and subsequent division by two. The output of the filter is capacitively coupled via C6 and dropped on R11 (1M resistor to ground). The signal across R11 is applied to the non-inverting inputs of U2-B, U2-C, and U2-D and the inverting input of U2-A. The diode in the feedback loop of U2-C charges C7 via R17 during the positive half of the guitar signal cycles. D2 prevents the output of U2-C from discharging C7 during the low half of the guitar signal cycles. During the low half of the cycle U2-C acts like a comparator and goes as close to the negative supply level as it can. This is because diode D2 blocks the output signal from reaching the non-inverting input and U2-C pin 8 goes nuts trying to cause the voltage at the inverting input to equal the voltage at the non-inverting input. R15 and R18 bias the inverting input of U2-D up to about 120 mV. R18 tends to discharge C7 to that level with a time constant of about 1/100 of a second. Thus C7 is repeatedly charged to a diode drop below the guitar cycle peaks by U2-C so it tends to maintain that value when the guitar (or instrument) signal is present. When no signal is present C7 goes to about 120 mV (the voltage at the junction of R15 and R18). The reason for holding the non-inverting input slighlty above ground is so the signal from simply holding the guitar or touching the strings doesn't cause the output of U2-D (the positive half cycle comparator) to go high. During positive half cycles the voltage at the non-inverting input of U2-D is higher than the voltage on it's non-inverting input but note that it goes low again faster than the voltage on C7 which has to discharge via R18 so U2-D's output shoots high during the signal peak but then goes low again as soon as the voltage at U2-D pin 12 goes lower than the voltage on U2-D pin 13. The pulse appearing on U2-D's output (pin 14) is fed via forward biased diode D1 and dropped on R19 where it causes U3-A (1/2 of the CD4013 Dual D Flip Flop) to be set. When the set pin is pulsed U3-A's pin 1 goes and stays high until U3-A is reset by a pulse applied to it's reset input. That pulse comes from U2-A pin 1 via diode D3 where it is dropped across R21. Note that U2-B is permitting the low halves of the input signal to discharge C8 to the negative peak voltage of the signal. C8's voltage is presented to the non-inverting input of U2-A (pin 3). When the signal voltage (applied to the inverting input of U2-A pin 2) goes lower than the voltage on U2-A pin 3, U2-A's output goes high until the signal voltage on U2-A pin 3 returns to a level higher than the voltage on U2-A pin 2. At that time the output of U2-A goes low again. The timing diagram below illustrates the pulse generation and the sub-octave generation occurring as the input signal oscillates.

At the positive peak of the input signal C7 is charged to slightly less than the original signal voltage being presented to the non-inverting input of U2-D, pin 12. Since C7's voltage is presented to the inverting input of U2-D (being used as a comparator) U2-D's output goes from about -10.5 volts to about +10.5 volts and sets U3-A (1/2 of Dual D-Flip Flop CD4013) via diode D1. D1 blocks the negative voltage at U2-D's output from reaching U3-A pin 6 since negative voltage applied to the single supply CMOS chip will damage it. U3-A's pin 6 sees ground via R19 (when U2-D's output is at -10.5 volts) or +10.5 volts (when U2-D's output is high). When the input signal starts to go low it eventually goes below the voltage that is still seen on C7 since C7 discharges relatively slowly via R18 (100K). At that point the output of U2-D returns low (about -10.5 volts). At the negative peak of the input signal C8 is dis-charged to slightly more than the original signal voltage being presented to the inverting input of U2-A, pin 2. Since C8's voltage is presented to the non-inverting input of U2-A (being used as a comparator) U2-A's output goes from about -10.5 volts to about +10.5 volts and resets U3-A (1/2 of Dual D-Flip Flop CD4013) via diode D3. D3 blocks the negative voltage at U2-A's output from reaching U3-A pin 4. U3-A's pin 4 sees ground via R21 (when U2-A's output is at -10.5 volts) or +10.5 volts (when U2-A's output is high). When the input signal starts to go high it eventually goes above the voltage that is still seen on C8 since C8 recharges relatively slowly via R25 (100K). At that point the output of U2-A returns low (about -10.5 volts). Thus U3-A is set and reset on each input signal cycle resulting in a rectangular wave that has the same frequency as the input signal at pin 1 of U3-A. That signal is used to clock the D-Flip Flop U3-B which is set up to divide the input clock by 2. Every time a clock edge is applied to U3-B pin 11 the logic level applied to the data input (in this case the logic level seen at the /Q (not-Q) output is propogated to the Q output (and then the /Q output assumes the opposite logic level). Thus the D-Flip Flop clocks in a 1 then clocks in a 0 then a 1 then a 0, etc. This results in two clock cycles being required for every one cycle of low to high (or high to low) at the Q output of U3-A. Thus, Hence and so… the Q output of U3-A becomes the sub-octave signal.

The signal applied to the non-inverting input of U4-A (pin 3) is rectified by the diode in the feedback loop of U4-A. U4-A also provides a bit of gain to the input signal that it is rectifying (via R33 and R32). The positive excursions of the input signal are presented to the inverting input of U4-B (pin 6). U4-B is used as a comparator and voltage divider R27 and R28 apply 1/3 of the positive supply voltage to it's the non-inverting input (pin 5). When the output of U4-A (pin 1) goes above the threshold set by R27 and R28 the output of U4-B (pin 7) goes to about -10.5 volts. When the output of U4-A (pin 1) goes below the threshold set by R27 and R28 the output of U4-B (pin 7) goes to about 10.5 volts. The positive excursions of U4-B's output (pin 7) cause pin 2 of U5 and the base of PNP transistor Q1 to be held at about +10 volts. When U4-B's output goes low D6 blocks the negative excursion from being seen on U5 pin 2 and and the base of PNP transistor Q1 but allows both of them to see ground through R35 20K resistor. The high to low transition does two things it triggers the LMC555 timer to begin timing out (U5 pin 3 goes high) and it turns on Q1 so that Q1 discharges U5's timing cap C17. Having the PNP Q1 in the circuit is called using the 555 in retriggered mode. This causes U5 to stay high as long as an input signal with amplitude sufficient to keep toggling U4-B's output is present. Once the input signal subsides (U4-B is no longer toggled) the 555 times out the last cycle and returns low.

This is the mixer section of the SUB-COMMANDER. The PLS (pulse) output from page 1 and SUB (sub-octave) output from page 1 are summed into the inverting input of U6-A via 300K resistors R48 and R49 respectively. The PLS and SUB signals both go through 100K resistors (R22 and R16 on page 1) and are the dropped on R41 and R42 respectively. R41 adjusts the voltage of the pulses appearing at point MX1 between about 6V P-P and 0V as it is adjusted. R42 adjusts the voltage of the sub octave square waves appearing at point MX2 between about 6V P-P and 0V as it is adjusted. U6-A actually attenuates the level of PLS and SUB (gain of 1/3) but passes CG through with a gain of about 1.33. R37 adjusts the voltage of the clean guitar signal appearing at point MX3 between about 4V P-P and 0V as it is adjusted. The clean guitar (CG) signal is applied to the non-inverting input of U6-A and is added to the pulse and sub-octave signals. Summer U6-B provides a gain of 6.6 for the clean guitar signal and 1.6 for the PLS and SUB signals. The output of this mix appears at the output of U6-A (pin 1) where it connects to circuit point "vcfin" (the input of the VCF). That mix makes it's way through the VCF and the VCA and then is fed into summer U6-B at the circuit point "vcao" (VCA out). The clean guitar signal is also fed to summer U6-B. In this way the clean signal can be mixed with the other mix (PLS, SUB, CG) for additional effects and sounds. The output of U6-B goes through C24 and R40 and is dropped on Master Output Level control R125 where it is fed from the wiper terminal to the hot terminal of the output jack.

Q2 is used to invert the output signal of the 555. When the 555's output is low Q2 is turned on an drops 12 volts on R36. This voltage is applied through 1K (current limiter) to point (reset). When the 555's output is high Q2 is turned off and point (reset) sees ground through R36 100K. The gate and reset signals are used to control the two AR generators in the SUB-COMMANDER. One of the AR generators is used to modulate the VCF cutoff frequency and one is used to modulate the VCA amplitude. Since the two AR generator are identical I will only describe the operation of the AR generator whose output terminates in circuit point (AD1 & ADA1). With no input signal, gate low (and reset high), Gate/Trig switch to Gate (closed), and AR/AD switch to AR (closed) the Set input of U7-A (pin 6) is low and the Reset input of U7-A (pin 4) is held high. In this condition the Q output is low and the envelope capacitor C35 is discharged to ground via the Decay control (R56) and D9 and R52 (470 ohm resistor). If a signal is applied causing gate to go high and reset to simultaneously go low then D Flip-Flop U7-A's Q output goes high and C35 begins to charge via R52, D7 and R53 at a rate determined by R53 (Attack control). The voltage on C35 is applied to the non-inverting input of voltage follower U8-A whose output feeds the AR generator output level control and the non-inverting input of U8-B used as a comparator. When the voltage rises above the threshold of U8-B set by resistors R58 and R59 then U8-B's output goes high and a high level is applied to pin 4 of U7-A D Flip-Flop via D16. D16 protects U7-A's reset input from seeing the negative excursion of U8-B's output. Since switch S1 is in Gate mode (closed) the voltage on C35 continues to climb as high as 12 volts until the set input of U7-A (pin 6) is brought low by a cessation of the input signal. Once U7-A pin 6 is brought low the high level on U7-A pin 4 causes the D Flip-Flop to reset bringing U7-A output Q low and discharging C35 via R56, D9 and R52 at a rate determined by R56 (Decay pot). Note that since the AR/AD switch was in AR mode (closed) that point (reset) brings U7-A pin 4 high also. If S1 is in Trigger mode (open) then only the front edge of the positive going gate signal is applied to the set input of U7-A (pin 6). In this mode when comparator U8-B goes high (voltage on C35 exceeds the threshold set by R58 and R59) D Flip-Flop U7-A is immediately reset bringing U7-A output Q low and discharging C35 via R56, D9 and R52 at a rate determined by R56 (Decay pot). This is because in Trigger mode U7-A's pin 6 is not held high by the gate signal (thus forcing Q to stay high). In AR mode (AR/AD switch S2 closed) cessation of input signal causes an immediate entry of the release state whether or not the voltage on C35 has attained the threshold set by R58 and R59 because the inverted gate signal is applied to the reset input of U7-A. In AD mode the envelope generator attains the threshold voltage after it has been triggered whether or not there is a cessation of input signal. R57 and R65 AR/AD Level controls adjust the amount of modulation provided to the VCA and VCF modules respectively.

The VCF is a voltage controlled two pole state variable filter with adjustable Q (or resonance). Only the low pass output is used in the SUB-COMMANDER. The two halves of the LM13700 act as voltage controlled resistors that integrate the input signal onto the .001uF capacitors C45 and C46. The darlington buffer amplifiers in the LM13700 buffer the integrated signal and the feedback arrangement results in the state variable response. The control voltages are summed by U10-A whose output feeds the linear voltage to exponential current generator made up by U10-B and associated components R70, Q3, and R75. Current mirror Q4 controls complementary current generator Q5 so that instead of sinking current the generator sources current through the collector of Q5. The resulting current is fed into the bias inputs of U11-A and U11-B via R77 and R78 respectively and controls the cut off frequency of the filter. The output of the first stage is fed back to the input via 10K Resonance contrl R90 to provide the Q adjusment. When the resonance control is set towards the higher end of adjustment the filter adds harmonics to the signal resulting in the classic low pass filter wah type sounds. When the resonance control is set towards the lower end of adjustment the filter adds almost no harmonics to the signal resulting in more mellow horn type sounds. The VCF's cut off frequency is controllable by it's dedicated LFO, it's dedicated AD/AR generator and by the Initial Cut Off Frequency potentiometer R73.

The VCA is realized using 1/2 of an LM13700. It's response is logarithmic in relation to control voltage. By controlling the current through the transconductance op-amp we correspondingly control the level of signal dropped on R104. The exponential control current generator is identical to the one used for the VCF. The VCA's amplitude is controllable by it's dedicated LFO, it's dedicated AD/AR generator and by the Initial Amplitude potentiometer R73. To adjust the CV Reject trimmer start with the trimmer in the center adjustment position. Remove the input signal from the circuit. Turn the VCA's LFO frequency to maximum. Observe the signal at "vcao" and adjust R103 for minimum modulation (which is actually control voltage feed-thru). You will not be able to get it to zero so just find the minimum and leave it there.

There are two simple low frequency oscillators (LFOs) in the SUB-COMMANDER. One is used to modulate the VCF cutoff frequency and one is used to modulate the VCA amplitude. The LFO used for the VCF frequency has a much lower frequency range than the one used to modulate the VCA. Other than that they are identical so only the operation of the LFO whose output terminates in circuit point LFO1 will be described. This is an integrator and comparator style oscillator. Essentially U14-A's output is always driving integrator U14-B in the direction opposite it's output state. That is, when U14-A is high U14-B is ramping low and when U14-A is low U14-B is ramping high. We set up U14-A as a comparator whose threshold levels are at the limits we want U14-B to oscillatre between and voila and oscillator is born. By changing the amount of current applied to the integrator via R13 (the Rate control) we vary how quickly U14-B ramps up and down. R108 and C58 are there to blunt the edge of the output of U14-A so that it doesn't radiate into out wiring and cause "ticking" to occur. The level of the output of U14-B applied to the VCA is controlled by R109 (Depth control). The component that determines the frequency range difference between LFO1 and LFO2 is R111 (33K resistor) in LFO1 and R118 (680K resistor) in LFO2. Since LFO1 has a 33K resistor in front of the integrator it oscillates 20 times faster than LFO2 (who has a 680K resistor in front of the integrator). R109 and R119 Depth controls adjust the amount of modulation provided to the VCA and VCF modules respectively.

Approx. Current Consumption
+12V	32mA
-12V	30mA
+15V	35mA
-15V	32mA

I have shown (in green) some convenient ground points if you desire to run twisted pair or coax for the MX1, MX2, MX3, MX4 and or INPUT connections. The connection near the INPUT connection is actually the leads of R6 and R12.

This image needs to be scaled so that DIP pad centers are spaced 1/10" apart (and 3/10" apart for pins across from one another). Consult your graphics aware friend for how to do that if you need to.

I recommend using an aluminum panel so that it can be attached to ground and thus ground all of the pot bodies.

SUB-COMMANDER Guitar Synthesizer Project Parts List

LM13700 16 pin DIP Equivalents:

Mouser	513-NJM#13600D NJM13600D DIP-16 Dual Operational Transconductance
CoolAudio	V13700D DIL-16

*** Dual Inline Package (plastic or ceramic)

Qty.	Description	Value	Designators
2	LM13700 Dual gm Op Amp	LM13700 ***	U11, U13
1	LMC555 CMOS Timer	LMC555 ***	U5
8	TL072 Dual Op Amp	TL072 ***	U4, U6, U8, U9, U10, U12, U14, U15
2	TL074 Quad Op Amp	TL074 ***	U1, U2
2	CD4013 Dual D Flip Flop	CD4013 ***	U3, U7
16	1N914 Sw. Diode	VALUE	D4, D2, D3, D5, D6, D1, D7, D9, D11, D8, D10, D12, D14, D16, D13, D15
4	2N3904	2N3904	Q4, Q3, Q7, Q6
4	2N3906	2N3906	Q1, Q2, Q5, Q8
4	Audio Taper Potentiometer	1M	R53, R56, R61, R64
12	Potentiometer	100K	R43, R42, R45, R41, R57, R65, R73, R97, R109, R113, R119, R122
1	Potentiometer	10K	R90
1	Trim Pot	2K	R103
28	Resistor 1/4 Watt 5%	100K	R12, R19, R33, R25, R36, R7, R22, R16, R18, R21, R34, R50, R51, R124, R37, R38, R55, R54, R63, R62, R84, R86, R71, R72, R95, R96, R110, R120
12	Resistor 1/4 Watt 5%	10K	R3, R4, R20, R26, R32, R14, R2, R44, R70, R94, R108, R117
2	Resistor 1/4 Watt 5%	10M	R15, R24
1	Resistor 1/4 Watt 5%	120K	R46
2	Resistor 1/4 Watt 5%	15K	R5, R10
4	Resistor 1/4 Watt 5%	1K	R31, R82, R83, R81
6	Resistor 1/4 Watt 5%	1M	R6, R8, R11, R27, R75, R99
2	Resistor 1/4 Watt 5%	200K	R47, R102
7	Resistor 1/4 Watt 5%	20K	R35, R29, R80, R89, R77, R78, R101
1	Resistor 1/4 Watt 5%	22K	R88
1	Resistor 1/4 Watt 5%	270K	R30
3	Resistor 1/4 Watt 5%	2K	R40, R74, R98
1	Resistor 1/4 Watt 5%	2M	R28
2	Resistor 1/4 Watt 5%	300K	R48, R49
10	Resistor 1/4 Watt 5%	30K	R1, R39, R58, R66, R69, R76, R93, R100, R106, R115
2	Resistor 1/4 Watt 5%	33K	R9, R111
2	Resistor 1/4 Watt 5%	39K	R112, R121
2	Resistor 1/4 Watt 5%	3K	R23, R17
3	Resistor 1/4 Watt 5%	4.7K	R85, R87, R105
1	Resistor 1/4 Watt 5%	43K	R104
4	Resistor 1/4 Watt 5%	470 ohms	R52, R60, R114, R123
3	Resistor 1/4 Watt 5%	47K	R91, R68, R92
1	Resistor 1/4 Watt 5%	6.8K	R13
1	Resistor 1/4 Watt 5%	680K	R118
1	Resistor 1/4 Watt 5%	68K	R107
2	Resistor 1/4 Watt 5%	82K	R79, R116
2	Resistor 1/4 Watt 5%	91K	R59, R67
2	Capacitor (film or ceramic)	.001uF	C46, C45
3	Capacitor (film or ceramic)	.01uF	C19, C34, C36
3	Capacitor (film or ceramic)	.022uF	C5, C3, C4
2	Capacitor (film or ceramic)	.047uF	C17, C58
41	Capacitor (film or ceramic)	.1uF	C8, C1, C2, C7, C10, C12, C15, C13, C11, C9, C14, C16, C20, C21, C22, C25, C23, C27, C30, C38, C41, C39, C42, C40, C48, C50, C49, C51, C54, C56, C55, C57, C61, C63, C62, C64, C53, C60, C59, C67, C66
3	Capacitor (film or ceramic)	.22uF	C6, C44, C65
1	Capacitor (film or ceramic)	220pF	C18
3	Capacitor (film or ceramic)	22pF	C26, C33, C47
2	Capacitor (film or ceramic)	100pF	C43, C52
1	Electrolytic Capacitor	22uF	C24
2	Tantalum Capacitor	10uF	C29, C32
2	Tantalum Capacitor	3.3uF	C37, C35
4	SPST Switch	SPST	S1, S2, S3, S4
2	1/4" Guitar Jacks		In and Out Jacks
18	Control Knobs		Knobs for all pots

Miscellaneous

• (1) 1/16" thick Aluminum plate for mounting the pots and switches.
• Assorted hardware 1" 6-32 nuts and bolts, 1/2" #8 wood screws, etc
• Knobs for potentiometers, wire, solder and typical assorted electronics hand tools.
• Volt Meter and an Amplifier (or oscilloscope) for testing and enjoying.