This article initiates a fascinating DIY journey through the world of synthesizers. More specifically, a guitar-controlled all-analog synthesizer project that includes a time machine sample & hold section, input signal detectors, noise gate, oscillators, envelope generators, filters, signal converters, and more. This first module is a sample and hold circuit designed to translate the guitar notes to equivalent oscillator frequencies.
I’ve always been fascinated with synthesizers. I grew up in the 1960s when the earliest commercial models were analog, starting with the Moog Modular, a complex expensive beast. Then later the simpler and less expensive MiniMoog and ARP 2600 models were introduced, soon followed by more models from other companies. As a poor teenager I couldn’t afford to buy those, but I had a keen interest in electronics. So I set out to build my own. By the 1970s, I had built several synthesizers starting with a basic version controlled by a Hewlett-Packard (HP) computer. I never had formal schooling for electronics, but working as a technician at various companies gave me access to real engineers who were kind enough to answer my endless beginner questions.
At one company, I became friends with Leo Taylor, who later got a fabulous job as a service tech for HP. Leo’s workbench included all the latest HP test equipment—storage oscilloscopes, signal generators, distortion analyzers, and more. Over a period of several years we met at his company every Monday night to design synthesizers and other audio devices.
Connecting Guitar Pedal Synths
One of the basic principles of analog synthesizers is voltage control. Rather than use only knobs to control the volume, or the pitch of an oscillator, or the cut-off frequency of a filter, in a synthesizer these are also varied by control voltages. So when you press a key on the keyboard, a voltage corresponding to that note is sent to a voltage-controlled oscillator (VCO). Another voltage that ramps up and down at user-defined rates feeds a voltage-controlled amplifier (VCA), and yet another varying voltage might go to a voltage-controlled filter (VCF) to add animation to the sound.
In what was surely one of the earliest attempts to sync a computer with an analog tape recorder, in 1970 Leo and I recorded a short tune, a modern three-part invention written by a friend of mine. We started by recording 60Hz hum onto one track of my Ampex AG-440 4-track professional recorder. That hum was then squared up — overdriven to clip and become a square wave — and played from the recorder into the control port of a small HP computer Leo borrowed from work. In 1970 a “small” computer was the size of a dormitory refrigerator!
Leo wrote a program in binary machine code to read the 60Hz sync tone recorded on Track 1, entering it one instruction at a time on the computer’s 16 front-panel toggle switches. There were no diskettes or portable hard drives back then either. The musical note data for the tune was also entered via the computer’s toggle switches. It took us 11½ hours to program the computer, enter the song data, and record the three tracks one at a time from control voltages output by the computer. You can hear the result in the file phils-hp.mp3 listed in Resources.
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In 1974, I built another synthesizer, shown in Photo 1 and Photo 2. This one was much more ambitious than the first and included a 61-note keyboard, four audio oscillators, two VCFs and two VCAs, a low-frequency oscillator (LFO) for vibrato and tremolo, and portamento (glide from note to note). A 10-step sequencer was also included, along with circuitry to split the keyboard at any point so both synthesizer “halves” could be played simultaneously for a whopping two-note polyphony.
Photo 1: Ethan built this synthesizer in 1970. The power supply heatsinks on top were originally inside the case. But the heat they generated caused the synthesizer oscillators to drift out of tune.
It took Leo and me two years to design this synthesizer, and I spent another nine months building it in the evenings. Leo was the real brains, though by the end of this project I had learned enough to design all the keyboard and control circuits myself. Fourteen separate plug-in circuit cards hold the various modules, and each card was hand-wired manually using crimped wiring posts and 24-gauge buss wire with Teflon sleeving.
You Can Diy! Building A Guitar Controlled Synthesizer: Lfo And Adsr
Fast forward to early 2021. I’m not much of a keyboard player, though I can pound out simple melodies and chords at half speed in a MIDI sequencer program. But my main instrument is the guitar. Where most synthesizers include a keyboard to play the musical notes, I wanted to be able to use my electric guitar. Guitar synths have been available for many years, but most require you to purchase a special guitar having separate pickups for each string. I decided to build one final synthesizer that I could play using my Fender Telecaster instead of a keyboard.
Reading the frequency of notes played by a string instrument reliably is the “holy grail” of guitar synthesizers. Unlike a static wave containing harmonics, such as square and sawtooth waves, the harmonics of a guitar or bass string vary in frequency as the string stretches and relaxes at the fundamental frequency. This creates “rolling harmonics” that ride up and down the fundamental when viewed on an oscilloscope. Figure 1 shows a note played high up on the guitar neck. Between the large fundamental cycles you can see smaller third harmonics whose level and position on the main wave change as the note sustains. So at the start of the note there are two extra zero crossings for each cycle, but 20ms later (at the right edge) there’s now only one zero crossing per cycle.
Figure 1: This high note shows the “rolling harmonics” that occur with all string instruments. Lower notes are even more complex, having many harmonic transitions through zero within each fundamental cycle.
Make: Diy Synthesizers, And A Guitar Hero Diy Controller Challenge
One common way to convert a frequency to an equivalent voltage is to generate a pulse having a fixed duration each time the input signal ascends through zero. These pulses are then accumulated and averaged by a low-pass filter. The higher the frequency, the more pulses that are accumulated, which creates a higher output voltage. But if the input wave passes through zero additional times due to the harmonics, the output voltage jumps one or more octaves higher than it should. So the first thing needed to read a guitar’s frequency is a filter that removes the harmonics. This is surprisingly more difficult to do than you might think, requiring a very steep filter!
My guitar synthesizer first filters out the harmonics, then an equivalent voltage is generated with a Period-to-Voltage converter (P2V) that measures the length (period) of each cycle. This has one big advantage over the more common Frequency-to-Voltage (F2V) converter described earlier. Instead of having to accumulate and average pulses over several cycles, a P2V converter assesses the time span between zero crossings. So it can determine the input frequency on the first cycle!
That voltage is then sent to Voltage-to-Period (V2P) oscillators that generate equivalent frequencies. When the note ends and the input voltage subsides, a noise gate senses the lack of signal and shuts off the guitar input. Then the last voltage that was measured can be held, similar to the Sustain pedal on a piano. Unfortunately, by the time the noise gate closes, the last voltage saved is invalid because the final bits of the note are mostly noise.
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A Sample & Hold (S&H) circuit is very simple comprising a switch, a capacitor, and an op-amp or other “buffer” to prevent draining the voltage currently stored in the capacitor. Figure 2 shows a basic Sample & Hold circuit. The analog input passes through a voltage-controlled switch, which then charges a capacitor. Whenever the switch closes, the current input voltage is “sampled” and applied to the capacitor. Then when the switch opens, the capacitor’s present voltage continues to be sent to the output regardless of the input. That’s the “hold” part. An FET op-amp is often used because its input bias current is too small to drain the charge held in the capacitor.
To avoid sustaining the corrupt voltage that happens at the end of a guitar note requires looking back at least 20mS before the note ended, then quickly switch to sending that to the V2P oscillators. But it’s not enough to simply grab a backup voltage sample every 20mS because the previous sample might have been taken just before the current note ended. To do this reliably with analog electronics, a round-robin set of three S&H capacitors is needed to save the three most recent voltages. While a note plays, a single-pole three-throw (SP3T) electronic switch alternates sending the incoming P2V voltage stream to each S&H in turn. This makes available for output to the oscillators: 1) the current stream; 2) the sample taken and held less than 20mS earlier; or 3) the sample taken and held 20mS before that.
Designing a large complex system, such as an entire synthesizer, is
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