The PE MINISONIC

After some months of delay I finally found some time to post the first collaboration here.
The following article came from my good friend Jeffrey Jones who built an MINISONIC, the original, el número 1, the mark I. We both found that there is almost nothing of information about this synth on the web, so Jeff therefore wrote about his experience with the MINISONIC.

Minisonic 1

The original minisonic was designed to be as cheap as possible to allow more people to be able to own a synthesizer. All of the circuits were designed to be as cheap and simple as possible. The unit did not even have a keyboard but used a stylus and keys etched on a printed circuit board to operate. It used batteries to avoid a power supply. The whole synthesizer was built around three fairly small printed circuit boards. The kit consisted of the circuit boards and components but nothing else. The constructor therefore had to make a cabinet and etch the keys onto a PCB. On the plus side the synthesizer was very cheap and even allowing for the cabinet other extras the cost was about one tenth of that of the ETI4600 which was available as a kit at about the same time. Additionally the modules were connected by patch cords which made it very flexible.

The PCB's were very easy to construct and the unit worked first time. The obvious drawback was having to make your own cabinet and keyboard. Additionally the patchcord system required a lot of wiring from the PCB's to the front of the cabinet and the provision of sockets.

In use there were a number of problems which would be expected in a unit this cheap. The stylus keyboard was very hit and miss, it was difficult to hit the right note cleanly and there was an obvious gap between lifting from one note to another. Intermitting connection of the stylus also made notes a bit unstable.

The unit that I made covered four octaves but it was not possible to set up the span of the oscillators to cover this range in tune. In practice the keyboard could only be used over two and a half octaves. The sample and hold circuit was quite basic resulting in a drift in tune with fairly long notes. The tuning of the oscillators was not very stable and could drift even during the playing of a single tune. 

One other strange problem was to do with the way the minisonic generated a trigger pulse. It used a high frequency oscillator signal which was sent to the stylus and was detected from the keyboard. This allowed a single stylus contact to generate control voltage and trigger pulse. Holding the stylus and wearing headphones allowed the high frequency signal to be transmitted to the keyboard before actually touching it. This generated a trigger pulse to operate the synthesizer resulting in the previous note being heard.

On the plus side the original unit had two oscillators which could be used to modulate each other. The patchcord system allowed for great flexibility enabling many modules to be connected in different ways. The ring modulator was also a bonus. 

In conclusion the original minisonic synthesizer was a good introduction to sound synthesis and great for producing a wide range of sounds. It was not however very practical for playing tunes. I passed it to a friend who used the basic circuits to build a Minisonic 2. This had none of the problems of the original.

For those of you that don't know what synth we're talking about, here you can find a website with the scanned pages from the Practical Electronics magazine (nov 1974 to mar 1975) including the erratas:
Tim Stinchcombe - PE Minisonic articles ( http://www.timstinchcombe.co.uk/index.php?pge=pemini )

Thanks Jeff! I also encourage others to share info about electronic musical instruments not easily found on the web.