5th June 2026 - Maybe it can't all be analog...

My assessed university group project has completely consumed my time over the past couple of weeks. Some highlights include laying out triangular PCBs (not recommended...), measuring miniscule op-amp offsets, and blaming my horrific spaghetti code on sleep deprivation. Hence, I've struggled to find the time to work on the synthesizer. To be completely honest, it's difficult to find the motivation to work on circuits at home having spent 8 hours in the lab diagnosing uninteresting issues.

Regardless, over the past couple of weeks I've learned a few lessons, and weighed up the balance between analog and digital. More details below.

Project deadline update

My deadline to complete the master VCO was the 31st May. I'm pleased to say I've met this deadline despite the unexpected volume of work from my university project. The only element of the first deadline I haven't met is the PCB design - because lead times are long and postage is so expensive, I'd like to wait until I have more designs before ordering.

The next stage is MIDI control integration. I'm less sure I'll meet this deadline, as university project work looks to hotten up even further over the next couple of weeks.

A disappointing accident - digital buffers always need power!

Earlier this week, I connected up one of my TOGs (Top Octave Generators) to the output of my VCO through a CD40106 buffer. For some reason, I decided that the CD40106 didn't need power or ground at all, and connected the floating output, with no reference from the power supply rails, to my TOG.

Around 5 seconds after connecting up the TOG, there was a large spark and audible pop as one of my NPN transistors upstream of the digital buffer blew up :( 

This caused a short circuit between power and ground. The power supply entered overcurrent protection mode but it was too late - the huge current had already fried one of my four delicate TOGs and made it unusable. At almost 2000JPY each, they weren't cheap, and with them being vintage replicas I was a little disappointed at this loss. So how did this happen?

I realised I completely misunderstood what kind of chip the CD40106 actually is. It's not a typical inverter/buffer per se - it's a Schmitt trigger inverter. Schmitt triggers inverters are devices used for converting noisy or erratic analog voltages into cleaner digital ones. They work by latching the output voltage at either +Vcc or GND depending on the input voltage and reversing the polarity of the input. By chaining two inverters together, we can create a digital buffer.

Schmitt triggers require a +Vcc and GND connection so that the output voltage can latch correctly. If there is no power supply rail, the output voltage becomes floating and behaves erratically, potentially drawing huge amounts of current (which I think is what happened in my case).

This damage would have been prevented by a quick look at the datasheet, and it's absurd this happened as I'm usually quite careful with this sort of thing. Perhaps I relied on the schematic a little too much, as it doesn't explicitly show the power supply rails. TLDR: Always read...

22nd May 2026 - Comparing / verifying two master VCOs

I've watched Eurovision every year for the past 10 years or so - this is probably the first time in the contest's history that an analog synth designer has performed on stage. A little disappointed that crazy British synth man didn't do so well this year, but it's been nice to hear murmurs of synths and vintage electronics in the news and I hope it's inspired some people.

This week

This week has been less productive than I thought it'd be, as I've found most of my time being consumed by our university group project. 

I've still been able to work for a couple of hours each evening at home, and have been focusing on breadboarding both the Korg Lambda and Roland RS-505 Paraphonic master clock generators, and building up the MIDI to CV converter which will become a significant point of focus over the next few weeks.

Many pictures this week and less theory :)

So...which VCO is better?

My main concern in last week's update was the compromise between the large rise-time caused by the Lambda's first-order feedback, and the possible increased parasitics of the Paraphonic's second-order feedback.

I had a suspicion that the Paraphonic's master VCO would be better performing, as the Lambda was designed as a budget synthesizer at the time of its release, but also questioned whether first-order feedback could be a better design choice. Over the past week, I've examined both circuits with the oscilloscope to get a real-world comparison, and the results are worth analysing.

This is the output of the Lambda's master VCO. The first-order charging and discharging of the 120pF capacitor is very obvious on the waveform, and the rise-time is close to our estimate, at around 120ns by inspection. Interestingly, I found that when the output resistors R9/R10 were left at 3.3kΩ, the output appeared more like a low-amplitude triangle wave. Therefore, I decreased R9/R10 by a factor of 10 to 330Ω, increasing the quiescent output voltage of Q1, and this seemed to do the job. I still need to play around with different values of R9/R10 that give the best results.

This VCO really impressed me with its fantastic frequency stability. Despite having a much less complicated negative feedback than the Paraphonic, the oscillation never varied by more than 8000Hz from its average frequency of 2.420MHz. That's a 0.3% maximum error, with standard deviation 0.2%! To put this into perspective, if the oscillator was tuned to A = 440Hz, then the oscillator's frequency would never go below 438.5Hz or above 441.5Hz. A perceptible difference, but much better than I was expecting, especially for a breadboard and my questionable wiring.

Undoubtedly a good design; the only thing letting it down is the clock rise time.

...and here is the Paraphonic's master VCO output. I've supplied it at Vcc = +15V and Vee = 0V for simplicity in this picture, different from the negative power supply rail used in the original design. Everything else is unchanged, except for the lack of a varactor diode used for precise frequency trimming which doesn't really affect the behaviour or stability.

The rise time is much less than the Lambda, at around 40ns, which makes it more attractive for feeding into the clock dividers of the top-octave generator to follow. If you look closely, the ringing caused by second-order effects is definitely there, but it's very well controlled and small compared to the oscillation amplitude. I made sure to use a film capacitor in the LC tank instead of a ceramic one to try and help with...

15th May 2026 - Let's Make Stuff! Choosing a VCO architecture

My last few exams went considerably better than the three exams I had before the last update. I might actually pass the year.

As of Tuesday all my exams are finished, which means I have much more time on my hands, and have been able to start building my home electronics lab.

In the post

BB830 solderless breadboard x2	£13.26
47pF film capacitor x4	£1.88
10K rotary pot x3	£3.15
100K trim pot x4	£3.45
180uH inductor x3	£1.38
TL074 quad JFET op-amp 14DIP x10	£5.99
J111 n-channel JFET x5	£1.70
CD4018BE 5-bit divider by N 16DIP x5	£4.70
CD4010BE buffer 16DIP x3	£1.92
8DIP IC sockets x10	£1.50
14DIP IC sockets x10	£1.42
2N3904 NPN transistor x25	£1.63
2N3906 complementary PNP transistor x25	£1.63
2N7000 n-channel MOSFET x10	£1.33
BS250P p-channel MOSFET x5	£5.65
M082AB1 13-note Top Octave Generator 16DIP x4	£35.81

This is a total expenditure of £86.40, bringing the total spend so far up to £129.25. This is a very significant order, and will probably be one of the largest of the whole project.

Choosing a VCO architecture 

There are many different VCO architectures used in synths from the valve era right through to the present day. For musical use, they must all be of excellent accuracy to reproduce pitches well, no matter the temperature, environmental noise or any other initial conditions.

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Phase-shift oscillator

An oscillator, in its simplest form, is an amplifier with gain greater than 1, and a feedback network providing an overall phase shift of 360 degrees (such that the feedback signal superposes with itself). This is a necessary, but not sufficient, condition for oscillation known as the Barkhausen stability criterion.

Above is a schematic of a simple phase-shift oscillator. In the example above, the three RC stages provide a combined 180 degrees of phase shift, and the op-amp adds another 180 degrees of phase shift at the output as the signal is fed into the inverting input.

The circuit produces a sinusoidal output voltage, with frequency of oscillation is given by

There are a couple of problems with this circuit that make it unsuitable for use in our synthesizer. 

• The op-amp output will either grow or shrink exponentially unless the feedback resistor R_fb is set to an exact value. As our op-amp supply rails are limited, significant distortion may occur if the signal grows and clips at the rails, or worse the output voltage collapses as the closed-loop gain becomes less than 1.
• The output is a pure sine wave. Analog synthesizers use the principle of subtractive synthesis. This means that they start with a harmonically rich sound like a sawtooth, square or triangle wave, and shape the sound using VCFs and tone filters. A pure sine wave has no harmonics, so we cannot tailor the sound to our liking.
• Voltage control of the oscillator is challenging, as we need to simultaneously modify the feedback network whilst ensuring the open-loop gain remains 1. This means it's not suitable for producing multiple frequencies.

(N.B a technique called additive synthesis also exists, where sounds are synthesized by summing sinusoidal waveforms together. As many fundamental waveforms require infinite sums of sinusoidal waveforms, additive synthesis uses a lot of hardware very quickly.)

A circuit like this may be useful for low-frequency modulation at a fixed frequency such as tremolo / vibrato, but we need to continue our search to find something better.

Relaxation oscillator

The relaxation oscillator is one of the most commonly used oscillators for producing square and triangle waves. It takes the form of an op-amp configured as a RC discharge network (R3 and C) which creates a periodic oscillation. The relaxation oscillator is the working principle behind the legendary NE555 timer chip.

The output of this circuit will be a square wave as the Schmitt trigger flips between its two output states, with the...