Analog Modular Synthesizer6.101 Final Project

Dominik Martinez

May 20, 2017

Introduction

The modular synthesizer is a musical instrument that produces sound fromelectronic systems rather than mechanical as with traditional musical in-struments. Synthesizers first were developed in the late 18th century andhave since been under the development from the musical and engineeringcommunities for over a century.

This report explores the design and construction of an analog modularsynthesizer. Modular synthesizers are unique because each stage of thesound processing chain is a separate module (i.e. the oscillator, the filter,the amplifier). The sound signal can be routed in different ways throughthe synthesizer, with older modular synths actually using patch cables tomanually route the signal through different modules. This provides musiciansan intuitive way to produce unique sounds. The modular synthesizer is oneof the most pervasive type of synths to this day due to its ease of use.

Figure 1 shows a very general overview of how the analog modular syn-thesizer is designed to work. Generally speaking, there is a power supply, akeyboard for physically playing the notes, an amplifier, and speaker. Figure2 shows the sound processing system in more detail.

The keyboard sends a voltage to the voltage controlled oscillator (VCO)which produces the initial sound wave. In general the wave can be any shape,but for this project the VCO produces a triangle, sine, and ramp wave. Thiswaveform is then propagated through the voltage controlled filter (VCF),which acts as a either a low-pass, high-pass, or band-pass filter dependingon user set parameters, and the voltage controlled amplifier (VCA), whichcontrols the gain of the signal before it is sent to the amplifier. While theVCO is controlled from the keyboard’s linear 1V/octave signal, the VCF andthe VCA are controlled by the Attack-Decay-Sustain-Release (ADSR) module.The ADSR module is an envelope generator that is set to the parameters ofthe user and triggered by the keyboard receiving input (the ADSR module

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Power Supply

Sound ProcessingKeyboard Amplifier

Figure 1: Very general block diagram

will be explored in more detail later in this report).The scope of the project includes the design and contructing of the

amplifier, keyboard, VCO, VCA, and ADSR modules. This report willexplore these modules. The justification for these modules is that they arethe most critical components for a fully functioning synthesizer. The powersupply used is from the 6.101 lab kit, providing 15V rails. The VCF wascompletely left out of the sound chain. While this means that the synthdoes have a bit less flexibility in terms of the different musical timbres itcan produce, the synth is still, by definition, a fully function analog modularsynthesizer.

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VCO1 VCF VCA

VCO2 ADSR ADSR

Keyboard

Figure 2: Detailed sound processing block diagram

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Amplifier

The audio amplifier circuit (seen in Figure 3) is a 3-stage power amplifier. Itsdesign is inspired by the work presented in Douglas Self’s textbook AudioPower Amplifier Design. The 3 stages in this amplifier are the input stage, thevoltage-amplifier stage, and the output stage. The output of the amplifier isthen fed back into the input of the amplifier in the form of negative feedback.Notice that since the amount of feedback is very large, this amplifier only hasa gain of 6 dB. This amount of gain is intentional, since during testing thesound signal voltage from the sound processing module was already found tobe very large (on the scale of 10V peak to peak), so further voltage gain wasnot necessary.

Input Stage

The role of the input stage is to take the input signal and the feedback signaland subtract the latter from the former. It accomplishes this in the form of along-tailed pair differential amplifier. The design used for the synthesizer inthis project uses Q3 to create a tail current-source to provide more stability.

In practice this worked reasonably well, but there was a slight misbalancebetween the two ends which caused the output of the amplifier to sit at 50mV. Potential improvements that would have minimized this issue would havebeen to put a potentiometer across the -15V ends of the amplifier and usethat to fine tune the balance to manually minimize DC offset. The superiormethod for controlling the balance would be to install a current mirror at thebase of this amplifier. A current mirror, seen in Figure 4 would be a nearperfect method of eliminating any DC offset since the circuit mirror forcesthe current at the tails to be equal.

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Voltage-Amplifier Stage

This stage takes the current output of the input stage, converts it to avoltage, and amplifies it. This stage is in the form of a common emitteramplifier with a current source as its load. Optimally there would be voltagebiasing between the amplifying transistor and the current transistor to removecrossover distortion. This would generally be in the form of a rubber diode.Practically I found that crossover distortion was not an issue with negativefeedback, so I left the biasing out.

Output Stage

The output stage implemented here is a unity gain voltage follower stage, atopology that Self claims is one of the most commonly used circuits in poweramplifiers. In the implementation here, a complementary feedback pair (CFP)topology is used for current gain. The CFP topology generally shows betterthermal stability than a plain emitter follower topology. Because each CFPis only conducting for half the input signal, this is a Class B power amplifier.The output from here is then fed back into the negative input on the inputstage.

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Figure 3: Schematic for the 3-stage amplifier

Figure 4: Generic current mirror

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Attack-Decay-Sustain-Release

The Attack-Decay-Sustain-Release (ADSR) module is a envelope generatorthat is triggered by the keyboard, has four user controllable parameters, andproduces a voltage envelope which is then used as a control voltage for theVCA and VCF. A graph of the envelope produced by the ADSR and how theparameters control it can be seen in Figure 5. Attack controls the time for thevoltage to reach the maximum voltage, decay controls how long it takes forthe voltage to reach the sustain voltage. Sustain controls the voltage that theenvelope will remain at while the user holds down the key. Release controlshow long it takes for the voltage to return to 0 after the release of the key.Figure 6 shows the implementation of the ADSR module for this project.

The trigger voltage is a pulse of about 10 V, with it normally being heldat around -10 V. In the untriggered state, Q1 is off, which causes Q2 to beon. With Q2 turned on, the 555 reset pin is held to ground, which keeps thecircuit’s output at 0V. When the trigger voltage comes, Q1 turns on, whichturns off Q2 and turns on Q3. This causes the trigger voltage to be groundedwhich causes the output voltage of the 555 to go high. R10 controls howquickly C3 charges which controls the length of the attack. Eventually the555 threshold goes high which causes discharge to be grounded. The topologywith U3, R13, and U2 controls how quickly the circuit sustains and on whatvoltage it sustains on by shunting some of the voltage away from the outputto ground. Finally, when the key trigger is returned to its initial state, Q1is off, Q2 is on, and C3 begins to discharge through R8, which controls therelease.

This design worked very well in practice, requiring no debugging oralterations.

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Figure 5: The ADSR envelope

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Figure 6: Schematic for the ADSR envelope generator

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Voltage Controlled Oscillator(VCO)

The VCO has the role of taking in a linear voltage input, converting thatinput to an exponential signal, and converting that signal into several typesof waveforms. The reason for the conversion between the linear input andexponential conversion is because music is exponential. On a standard pianofor example, the frequency from middle C to the next C an octave up isdoubled. And from that C to the next C it is doubled again. The schematicpresented here drew heavy inspirations from Thomas Henry’s book An AnalogSynthesizer for the 21st Century.

The first stage in the VCO is running the linear input through an adder.R2 is a high value so that the input from the tuning pot has a much smallereffect on the frequency than the actual keyboard input. The output of theadder is then ran to a dual BJT circuit where the exponential relationshipbetween the base voltage and the collector current of a BJT is used to convertthat linear voltage input to an exponential voltage output. The dual transistortopology is use so that the output of the exponential convert is independentof variations in temperature of the transistors. U3 provides Q2 a referencecurrent, which is then modified depending on the voltage at the base of Q1.

The output of the linear to exponential convert is then ran to a transcon-ductance amplifier, so that the circuit can convert the current into a voltage.This dual transconductance amplifier produces a triangle wave, with its basefrequency being controlled by C3. This signal is ran to U6 which amplifiesthe wave. The output of U4 and U5 produces a square wave which is usedlater on in the circuit.

This triangle wave is then ran into a long tailed pair amplifier and amplifiedinto its nonlinear region. This is a easy trick to force a triangle wave to looklike a sine wave, obviously it’s not a true sine wave, but practically speaking

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for music it is close enough. The first few terms of the sine wave’s Taylorseries is x − x3

3!+ x5

5!and for the wave produced by this circuit the Taylor

series is x− x3

3+ 2x5

15. Both waves are linear to the first order so the circuit’s

sine wave is nearly indistinguishable from a true sine wave. For the rampwave, the triangle wave is first ran through U9, which allows us to controlthe gain of the wave, then it is ran to U11, which in conjunction with J1acts as either a voltage follower or an inverter. Since the square wave fromthe transconductance amplifier is at the same frequency as the triangle wave,this allows U11 to flip about half of the triangle wave to produce a rampwave. This can be fined tuned with pot U10. The output impedance of everywaveform used for sound is 1k. This value was chosen experimentally, andproduced the best results when amplified.

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Figure 7: Schematic for the VCO

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Figure 8: Schematic for the VCO cont.

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Voltage Controlled Amplifier(VCA)

The VCA is a long tail pair with the tail current being controlled by thecontrol voltage (the output of the ADSR envelope generator). The schematiccan be seen in Figure 9. U3, U4, and U6 are used to fine tune the effects of theCV on the output. Q1 controls the current of the tail which in turn controlsthe amplification of this circuit. This module is particularly important sincethe VCO is always producing a waveform, the only way to control when anote is sounding or not is through this module. In practice this circuit workedextremely well and required practically no debugging.

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Figure 9: Schematic for the VCA

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Keyboard

The physical keyboard is 62 resistors in series with 5V applied across themas seen in Figure 10; this is 1V/Octave. When a key is played, a connectorconnects at the junction of one of the resistors which creates a large voltagedivider. As different junctions are selected, a linear voltage change can beseen. This linear change is then fed to the circuit seen in Figure 11. This isa simple comparator circuit that simply checks if a nonnegative voltage isbeing applied (a negative voltage is the default state when no key is pressed).If there is an input, the comparator outputs a +V signal for use in theADSR module. If there is no input signal (i.e. no key is pressed), then thecomparator holds the output at ground.

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Figure 10: The voltage divider that creates the keyboard

Figure 11: Schematic for the keyboard

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Results

With all of the modules connected together, the synthesizer proved to bequite successful. The musical notes were very clear and undistorted even atvery loud volumes, and after properly adjusting the pots on the VCO for the1 V/octave control, the synthesizer was very well tuned and stayed in tune forthe entire 5 octave range of the keyboard. The final checkoffs and test weredone running on the 6.101 lab kit’s current limited power supply. I found thatfor a computer speaker 500 mA was plenty to power both the amplifier andthe sound circuitry. Although not tested, it is safe to assume that to power alarger speaker a higher current power supply would have been required.

There is a severe issue with this design that was not addressed duringthe construction of the synth. With the keyboard schematic, when a key isno longer pressed, the input being fed to the VCO is floating. The VCOstill generates waveforms even though there is a voltage input of around 0V.Remembering back to the ADSR module as seen in Figure 5, when the key islifted, the sound from that note should not disappear. This issue is apparentwhen the ADSR’s release is set to a long time, then when the key is lifted,instead of hearing the same note, one could hear the note switch to a differentnote and then fade out when the key was lifted.

The solution to this would be to design a keyboard circuit that couldessentially “save” the voltage of the last key that had been pressed even afterthe key was lifted. While this is certainly possible with analog circuity, itwas a significantly more challenging design project than the author had timeto deal with, so it was ignored. Assuming the release on the ADSR moduleis small, this issue was not noticeable. Practically speaking, this is not anactual solution to the problem.

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Conclusion

While I set out to design and construct every module that were presentedin Figures 1 and 2, due to time constraints this was not reasonably possible.Because of this, I had to shrink the scope of the project to what you seereported here. Overall, I was very happy with the result even if I didn’taccomplish all I set out to build. In the future I plan on finishing the VCFand power supply and printing a PCB so that I can use the synthesizer forcasual music playing.

I am very thankful for the help of the instructors Gim, Yanni, and theCI-M instructor Dave.

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