Synth2

Analog-Synth

My personal Analog Voltage Controlled Oscilator, wave shapers, and output stage

Design and Decisions

My oscillator and linear to exponential converter due to being notoriously sensitive were based on preexisting circuits shown in Lantertronics series Analog Circuits for Music Synthesis. Some core changes I made to his designs were swapping the 2N4859 JFET with a 2N3906 as the JFET, while likely having slightly better switching performance, is widely no longer sold. I also switched the CA3130, LM301, and TL084 to a TL074 due to its all around quicker slew rate, lower noise along with the added price and density advantage of using the same 4 op-amp package for numerous circuits. Finally I kept the LM311 as it is still readily available and has extremely high slew rates needed for quick switching allowing crisp transitions and creating the overtones needed for the punchy sound of the Saw and Pulse Waves.

Another change I made to his linear to exponential converter was choosing to use the 2N3906 once more (as a NPN model was not stated) along with changing voltage input and resistor for the bias current from 10 volts to 5, along with 1 MΩ to 4 MΩ. This decrease in bias current by 8x allowed my syth to reach much lower frequencies without requiring extremely low input voltages.

Lantertronics Design on the Left, Mine Design on the Right

Note: all LTSpice images bellow usings the LT1011, and OP07 was only used for quick digital prototyping, in my final build I used the LM311 and LT074

For the pulse wave generator i used the LM311 once more. The output of the saw was generatored at 0-5 V. For the pulse generator I compare the saw to a voltage determined by the user via a potentiometer to change the duty cycle.

For the triangle wave I based my design off of Moritz Kleins Sawtooth-to-Triangle Converter which centers the sawtooth around 0 volts, makes a inverted copy via an opamp, and sums them together via a junction using diodes. The main change I made was rather than using a high pass capacitor to center the triangle I used a subtracting op-amp to subtract a voltage determined by a user potentiometer. At higher frequencies the circuit, due to its inability two make quick transitions, may longer be centered from peak to peak around zero when using a capacitor (which can be seen in the simulated image bellow). Instead using a subtracting op amp allows me to perfectly center the sawtooth so no artifact occurs. The triangle is meant to have a smooth almost sine wave sound and so any even small artifacts add many overtones causing a undesired sharpness to the sound reminiscent of a square wave. Due to the waveshapping done, the signal also ends up with a voltage peak to peak of roughly half that of the other waves, to compensate I use a TL074 to double the volume of the triangle wave ensuring all are the same volume.

Triangle using Capacitor instead of user offset

Moritz Klein Original Triangle Design

Modified Triangle Wave Shaper

Finally for all buffering and output stages I use the tl072 to save. I also added a mixer stage allowing the user to combine all wave shapes, along with 2 rotary switches allowing the user to change between 5 different octave settings and the 4 wave settings. Along with the rotary switch, for the user the is a fine tune and course tune nob, a CV input which currently is a nob, a pulse control nob, a volume nob, a trim nob for the exp-converter and a trim nob for the triangle offset.

Mixing and Output Stage

Finally the circuit modeled on KiCAD in a 2 layer design. Originally I had used a 4 layer to ensure a unbroken ground layer which reduces noise, but due to high costs I instead used a 2 layer design which simply uses short, minimal traces on the ground layer. Especially around the VCO region which is especially sensative to noise, I ensured to use short traces to prevent trace impedance from impacting oscillation frequency.

Behavior

Due to the transistor being used in my linear to exponential converters, changes in heat impact the Vt constant leading to the transistor to open up, push higher currents and thus raise the frequency. The frequency of this circuit started at 265.1 and given about 5 minutes it raised to a steady frequency of about 266.18 to 266.57 (error could be due numerous sources including, the oscilloscope, noise from interference and the bread board, a switching power supply and the fact that the transistors are not matched on an IC meaning small unequal changes in heat can cause instability). Regardless, the error once heated up is only about 2.5 cents which is under the range where people start noticing intonation issues which is around 5-10 cents. Even from the start up frequency of 265.1 to the max measured frequency of 266.6 the cent difference is 9.57 meaning that though idling for 5 minutes first is ideal, the temperature drift is still under that 10 cent threshold where it starts mattering.

The oscillator is desired to double frequency every volt added at the input. The measured exponential rate of my circuit at the time of measuring was 1.977 rather than the ideal 2. While only a 1% error, due to the exponential nature of this circuit, at high frequencies the measured frequency is off by a noticeable margin (7975 vs 8372 hz as seen in the table bellow). A large part of this is due to the error in my tuning methods themselves, rather than using a precise digital voltage source or a DAC to input exact control voltages, I used a potentiate meaning that the frequencies I measured were not exact (along with the fact that the potentiometer introduces noise both naturally and if anything in the circuit moves). Hence when recursively tuning the instrument by starting at 1 volt, and comparing it to 2v it was never a truly exact comparison. That small error could have introduced the cascading error seen in my final result, a oscillator that drifts flat with increasing CV.

Wave Shape Photos at decreasing frequency

Audio Demo

https://github.com/user-attachments/assets/6dd11a05-b424-4769-ac00-267ce8dbd173

https://github.com/user-attachments/assets/6da5949c-7cdd-406b-b645-f12ac689b7d8

https://github.com/user-attachments/assets/ecf994ba-5ef1-4e04-878b-3f7e2e97198b

https://github.com/user-attachments/assets/66918d8a-2ba2-4e4a-80ee-363ddde3f1d2