Making the chipophone

Håkan, organ donor.

• The chipophone
• Making the chipophone
• Spellbound

It all started when a good friend of mine asked me if I wanted an old electronic organ. He had bought it in a thrift store some years ago, but had now realized that it would remain untouched in his cellar forever unless he could find a new home for it. I accepted the gift on the condition that I could do whatever I wanted with it, in terms of modifications and experiments.

Let me clarify at this point that organs like these are not particularly rare. They were mass produced in the seventies, and most thrift stores in Sweden have at least one of them on display. As can be seen in the pictures below, they are based on solid state technology (transistors), but not integrated circuits.

The organ was in good condition, with only a few contact glitches in the pedals and switches. It featured a set of really plain organ patches, tremolo, reverb and volume controls.

Reverse engineering

The first step was to remove the back cover. An overwhelming amount of dust was eager to come out and see the world, having been locked up for all those years.

It's always a pleasure to work with old machines that have been designed with maintenance in mind: As you can see from the pictures, there were several hinges and other details to aid the repairman.

The bottom right of these images shows a spring reverb tank, similar to what you'd find in a guitar amplifier. This makes sense because reverb, as I've discovered earlier, is the key to synthesizing organ-like sounds.

The reverb tank is currently not used in the chipophone, but it might be integrated in the future.

Apart from the reverb, there's a huge loudspeaker, a transformer, capacitors for the transformer, and rows upon rows of switches and wires. And then there are twelve oscillator cards:

Oscillator card

This struck me as odd at the time, because it seemed like a really backwards way of design a synthesizer, but I suppose it made perfect sense in the analogue world.

There are twelve oscillators, one for each semitone. Each tone is divided down into several versions at different octaves. Then, if I'm not mistaken, these oscillating waveforms are attenuated according to a set of volume signals (presumably voltages or currents) arriving from the keys themselves. Every key acts as two or even three physical switches, feeding the volume signals from a few master signals that are controlled using the switches and knobs on the front panel. So, for instance, the volume signal for F#, second octave, might be a weighted sum of the first overtone of the second F# of the upper manual, and the second overtone of the first F# of the lower manual. Think of it as electronics simulating air flowing through ducts and hoses.

Making a MIDI keyboard

After some deliberation, I began cutting the wires. There was no turning back now.

Bank of switches

I had decided to modify the organ into a MIDI keyboard. To do that I just needed a single switch for every key, so a lot of the wires could simply be removed.

The remaining wires were connected to a bunch of 74HC165 chips, 8-bit parallel input latches that can be daisy chained into a single serial line. Two sets of 44 keys, 13 pedals and a couple of switches made for a total of 120 input signals and a daisy chain of 15 chips.

The daisy chain is controlled by an ATmega88 microcontroller, which is responsible for polling all the signals and running a debounce algorithm. The microcontroller also has six analogue inputs, which are directly connected to the five potentiometers on the front panel (the leftmost knob is a switch) and the right foot pedal.

The pedal

The analogue pedal, used as a volume controller in the original design, was not a potentiometer. This came as quite a shock to me, but again, I suppose it made sense in the good old days of no op amps.

The pedal contained a small light bulb that would shine through an opening, the width of which would vary (non-linearly) as the pedal was operated. This would cause a varying amount of light to shine on a photoresistor on the other side of the opening. Rube Goldberg would've been proud. I do not know whether the lamp was designed to shine with a constant light intensity or if it would actually carry an audio signal.

I replaced the light bulb with a high intensity LED.

The synthesizer

Once the MIDI keyboard was up and running (thoroughly tested with a General MIDI softsynth of course), I started experimenting with creating an ATmega88 based synthesizer with typical chiptune sounds. I could re-use code from several earlier projects, of course.

The synthesizer contains eight independent waveform generators capable of generating pulse waves, lo-fi triangle waves and white noise, as well as some experimental features like ring modulation. These eight voices are then allocated dynamically as keys are pressed. Please refer to the chipophone page for further details.

The linear bits

The original electronic organ contained its own amplifier and loudspeaker. I've opted for a traditional line out signal for the time being, so external amplification is necessary. This also enables me to power the chipophone from a single 5V supply.

In the future, I hope to incorporate the loudspeaker and reverb tank back into the chipophone, but then I'm going to need op amps and a dual power supply to drive them. Time will tell if I ever get around to doing this, but it bugs me that I have a perfectly good spring reverb tank just laying around.