Analog Modular Synthesizer 6.101 Final Project Dominik Martinez May 20, 2017 Introduction The modular synthesizer is a musical instrument that produces sound from electronic systems rather than mechanical as with traditional musical in- struments. Synthesizers first were developed in the late 18th century and have since been under the development from the musical and engineering communities for over a century. This report explores the design and construction of an analog modular synthesizer. Modular synthesizers are unique because each stage of the sound processing chain is a separate module (i.e. the oscillator, the filter, the amplifier). The sound signal can be routed in different ways through the synthesizer, with older modular synths actually using patch cables to manually route the signal through different modules. This provides musicians an intuitive way to produce unique sounds. The modular synthesizer is one of 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, a keyboard for physically playing the notes, an amplifier, and speaker. Figure 2 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. This waveform is then propagated through the voltage controlled filter (VCF), which acts as a either a low-pass, high-pass, or band-pass filter depending on user set parameters, and the voltage controlled amplifier (VCA), which controls the gain of the signal before it is sent to the amplifier. While the VCO is controlled from the keyboard's linear 1V/octave signal, the VCF and the VCA are controlled by the Attack-Decay-Sustain-Release (ADSR) module. The ADSR module is an envelope generator that is set to the parameters of the user and triggered by the keyboard receiving input (the ADSR module 1 Power Supply Keyboard Sound Processing 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 will explore these modules. The justification for these modules is that they are the most critical components for a fully functioning synthesizer. The power supply used is from the 6.101 lab kit, providing 15V rails. The VCF was completely left out of the sound chain. While this means that the synth does have a bit less flexibility in terms of the different musical timbres it can produce, the synth is still, by definition, a fully function analog modular synthesizer. 2 VCO1 VCF VCA VCO2 ADSR ADSR Keyboard Figure 2: Detailed sound processing block diagram 3 Amplifier The audio amplifier circuit (seen in Figure 3) is a 3-stage power amplifier. Its design is inspired by the work presented in Douglas Self's textbook Audio Power Amplifier Design. The 3 stages in this amplifier are the input stage, the voltage-amplifier stage, and the output stage. The output of the amplifier is then 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 has a gain of 6 dB. This amount of gain is intentional, since during testing the sound signal voltage from the sound processing module was already found to be very large (on the scale of 10V peak to peak), so further voltage gain was not necessary. Input Stage The role of the input stage is to take the input signal and the feedback signal and subtract the latter from the former. It accomplishes this in the form of a long-tailed pair differential amplifier. The design used for the synthesizer in this project uses Q3 to create a tail current-source to provide more stability. In practice this worked reasonably well, but there was a slight misbalance between the two ends which caused the output of the amplifier to sit at 50 mV. Potential improvements that would have minimized this issue would have been to put a potentiometer across the -15V ends of the amplifier and use that to fine tune the balance to manually minimize DC offset. The superior method for controlling the balance would be to install a current mirror at the base of this amplifier. A current mirror, seen in Figure 4 would be a near perfect method of eliminating any DC offset since the circuit mirror forces the current at the tails to be equal. 4 Voltage-Amplifier Stage This stage takes the current output of the input stage, converts it to a voltage, and amplifies it. This stage is in the form of a common emitter amplifier with a current source as its load. Optimally there would be voltage biasing between the amplifying transistor and the current transistor to remove crossover distortion. This would generally be in the form of a rubber diode. Practically I found that crossover distortion was not an issue with negative feedback, so I left the biasing out. Output Stage The output stage implemented here is a unity gain voltage follower stage, a topology that Self claims is one of the most commonly used circuits in power amplifiers. In the implementation here, a complementary feedback pair (CFP) topology is used for current gain. The CFP topology generally shows better thermal stability than a plain emitter follower topology. Because each CFP is 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 input stage. 5 Figure 3: Schematic for the 3-stage amplifier Figure 4: Generic current mirror 6 Attack-Decay-Sustain-Release The Attack-Decay-Sustain-Release (ADSR) module is a envelope generator that is triggered by the keyboard, has four user controllable parameters, and produces a voltage envelope which is then used as a control voltage for the VCA and VCF. A graph of the envelope produced by the ADSR and how the parameters control it can be seen in Figure 5. Attack controls the time for the voltage to reach the maximum voltage, decay controls how long it takes for the voltage to reach the sustain voltage. Sustain controls the voltage that the envelope will remain at while the user holds down the key. Release controls how 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 held at around -10 V. In the untriggered state, Q1 is off, which causes Q2 to be on. With Q2 turned on, the 555 reset pin is held to ground, which keeps the circuit's output at 0V. When the trigger voltage comes, Q1 turns on, which turns off Q2 and turns on Q3. This causes the trigger voltage to be grounded which causes the output voltage of the 555 to go high. R10 controls how quickly C3 charges which controls the length of the attack. Eventually the 555 threshold goes high which causes discharge to be grounded. The topology with U3, R13, and U2 controls how quickly the circuit sustains and on what voltage it sustains on by shunting some of the voltage away from the output to ground. Finally, when the key trigger is returned to its initial state, Q1 is off, Q2 is on, and C3 begins to discharge through R8, which controls the release. This design worked very well in practice, requiring no debugging or alterations. 7 Figure 5: The ADSR envelope 8 Figure 6: Schematic for the ADSR envelope generator 9 Voltage Controlled Oscillator (VCO) The VCO has the role of taking in a linear voltage input, converting that input to an exponential signal, and converting that signal into several types of waveforms. The reason for the conversion between the linear input and exponential conversion is because music is exponential. On a standard piano for example, the frequency from middle C to the next C an octave up is doubled. And from that C to the next C it is doubled again. The schematic presented here drew heavy inspirations from Thomas Henry's book An Analog Synthesizer 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 smaller effect on the frequency than the actual keyboard input. The output of the adder is then ran to a dual BJT circuit where the exponential relationship between the base voltage and the collector current of a BJT is used to convert that linear voltage input to an exponential voltage output. The dual transistor topology is use so that the output of the exponential convert is independent of variations in temperature of the transistors. U3 provides Q2 a reference current, 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 base frequency being controlled by C3. This signal is ran to U6 which amplifies the wave. The output of U4 and U5 produces a square wave which is used later on in the circuit.