The Gemini – 3rd Mod

I was apprehensive about my third project, my final Stylophone mod for the time being, since it involved major surgery to two individual Stylophones, and I wasn’t really sure at the outset whether it would work.  In the end, in an unpredictable way of its own, it did!

The concept was simple, to generate two notes at the same time, using two Stylophone circuit boards operated by one stylus.  The ability to tune the fundamental note had to be retained, as well as means of selecting any harmony note – and, of course, it had to be possible to play the Stylophone as originally intended.

This photograph shows the main pitch control of the Gemini, the dual concentric potentiometer at the back left, as well as the fine tune, volume and blend controls on the back.  It also shows its decoration with signs of the zodiac stickers – I think the sign of Cancer (not visible) is the wrong way round, but this is not my fault, it was like that when I bought them.

Gemini high angle standard knobs sm

The first extra control to be added would be a 3 way rotary switch, to select ‘Normal’, ‘Harmony’ and ‘Modulated’ outputs.  This is just visible on the right hand side of the picture above.  ‘Normal’ would switch only one of the boards in circuit, with a mono output available to the internal speaker or to the headphone output; ‘Harmony’ would switch both boards in circuit, with one board available to the internal speaker, but both boards available to left and right sides respectively of the headphone output; in ‘Modulated’ mode, the outputs of the two boards were connected by diodes to allow the tones to interact with one another.

A dual 10k lin pot was used as a ‘Blend’ control, ranging from Board One (Oscillator 1) only to the left, and Board Two (Oscillator 2) only to the right, with varying mixes in between of the two tones produced.  A dual 10k log pot controlled the overall volume.  These were located in the circuit in place of the existing volume controls and both original volume pots and associated circuit boards were removed.

Pitch was to be controlled by a dual 500k concentric pot, the inner control for correct tuning of the fundamental pitch, the outer control for the harmony note, the interval being set by ear.  As this pot would cover a wide range, and the fundamental note would be difficult to set precisely, I decided to add a 10k lin pot, normally in centre position, in series with the inner pot, as a ‘Fine Tune’ control.

The first problem was to test if removing one stylus and connecting the ‘keys’ on one board to the corresponding keys on the other board would successfully enable one stylus on one keyboard to create two distinct notes at the same time.  Connecting the keys was easy enough, as each one has a small point – presumably included for testing purposes at the factory – to which wires could be connected.  To my relief, this procedure worked, two notes were produced simultaneously, and the interval set remained steady along the length of the keyboard.

The Stylophone’s original 3-way tone switch, as I had discovered on my first modification project, is a two-pole type, with only one pole used; so it was easy to connect the three wires from the two boards to other side of the same switch, to ensure the same tone was used on both boards.

I hoped the same would be possible with the vibrato, and connected the main boards together at the 3 inputs to the small board containing the power and vibrato switches.  This worked, too, and enabled one power on/off and one vibrato on/off switch to be used for both main boards.

It was the ‘mp3’ input/headphone output that caused difficulty, and I didn’t fully resolve this problem, I just got round it in a not entirely satisfactory way.  I had hoped that the outputs of the two amplifier boards would simply appear separate and equal at the headphone socket, and that a stereo input would appear as a stereo output.  (A stereo input is normally reproduced in mono through the internal speaker and the headphone output, as an unmodified Stylophone has only one mono amplifier in it).

I disconnected one of the mp3 inputs and one of the headphone outputs from Board One and replaced them with an input and an output from Board Two, in order to make this happen, but it was clear that one of the channels was leaking into the other one somewhere and the output was still mono, even though using two amplifier boards.  I had removed the 3.5mm sockets from Board Two, in order to fit this in the original case on top of Board One – maybe this had some effect on the stereo image, I don’t know.  But I eventually created a slightly stereo output by putting a 1k preset into the output of one channel, so Oscillator 1 was slightly to the left in the headphones, and Oscillator 2 slightly to the right.  This was the effect I was after in the first place, it just wasn’t the way I anticipated achieving it.

However, this was close enough to what I set out to create, so I considered the job done.

The following diagrams might help illustrate what I did and how I did it:

Gemini  Board 1b smGemini Board 2 sm

As it turned out, the pitches of the two oscillators couldn’t be set independently – raising the pitch of one would automatically lower the pitch of the other, and vice versa!  Not only that, but as the 500k pots were logarithmic the pitches would vary at different rates.  This apparent disadvantage, I decided, would be turned to an advantage – and would, in fact, be the whole point of ‘The Gemini’: the careful adjustment of the three pots to obtain a suitable pair of notes would become its principal performance feature.

The stylus, of course, was still fully functional, so as soon as a suitable interval was stumbled upon, this was playable up and down the keyboard.

Fitting everything back into the case was not easy, and involved quite a bit of rewiring to reduce the tangle of leads inside.  I had hoped to be able to fit everything in the original Stylophone case, but it soon became apparent that this wouldn’t be possible, so I aimed to deepen the case, as I had with ‘The Hedgehog’ – although not so drastically, as there would be fewer extra controls to add.  In the end doubling the depth proved adequate to contain the extra boards and wiring without unduly squashing them.  The extra piece of bodywork added to the case proved an ideal base for the zodiac motif stickers I added.

In use, the basic pitch seemed somewhat lower than originally intended, but it turns out that ‘The Gemini’ is perfect for bass sounds, and some excellent tones are obtained, especially in ‘Modulation’ mode.

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December 2011

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