This project started shortly after completing the NT02 series. i wanted to get back to experimenting with the CD4046 PLL chip, and i thought it would be kind of cool to use a toy enclosure as the body for the new synth. i shuffled around in my basement for a while until i found the perfect toy. it was an old radioshack animal sound keyboard. originally i had planned to use the internal circuitry of the toy, but soon realized that the sound quality would not be suitable. instead, i grabbed a little toy keyboard i had laying around, and began prototyping around that circuit. the keyboard was nice because the keys were all different colors, and they were small enough to mount in the toy organ body. i thought it was a perfect match since the keys kind of resembled a rainbow and the organ body kind of resembled a cloud.
Once i had a rough idea of how i wanted the keyboard to look with the toy organ body, i began prototyping the circuit and coming up with a general idea of what the final product would do. i knew that i wanted to use a PLL chip to multiply the voice frequency, and then send that frequency to a pseudo-noise generator made from a 74ls273 shift register and CD4030 XOR chip. i was pleasantly surprised with some positive results, but unfortunately there were problems with the toy keyboard chip i was using. the toy keyboard was a very cheap generic toy keyboard. it put out a square wave voice for a clock-determined gate interval, so changing the pitch of the keyboard would effectively change the gate time of the voice. then an envelope capacitor would shape the output of the square-wave. the problem came with the gate time, since not only did the gate time dictate the length of the note, it also put the chip in standby mode each time the note was allowed to end. then when another key was pressed, the chip would have to power back on, and the voice would spike. the sound was especially noticeable since the signal was being sent to the PLL frequency multiplier, so i decided to scrap that keyboard chip, and go with one that was more compatible. i had an old purple DSI keyboard that had just the chip for the job. the chip had a square wave output that would stay on as long as a key was pressed in"organ" mode, or hold the last note continuously in "piano" mode, granted the envelope circuitry is removed from the circuit. the keyboard also puts out a gate signal for the length of the held note in both modes. the chip also had a record and playback mode, but i decided to leave those out since i knew there would be space limitations on the control panel. the only drawback to the chip was that there is a battery alarm sound that chimes after a minute or so of inactivity. unfortunately the battery alarm timer will only reset when a key is re-triggered, so re-triggering redundant functions like "play" mode and "piano" mode with an LFO was not an option.
moving on, i came up with a general idea for the architecture of the signal path. the main voice coming from the keyboard would be split in two directions. one voice would be put through a frequency divider. the four sub-frequencies would then be mixed together with a joystick before being sent to a VCA. the other direction of the signal would be sent to the PLL frequency multiplier, then sent to the clock input of the pseudo-noise generator circuit. that signal would then be sent to another VCA that would be mixed to the other one. the two signals would then be sent to a PT2399 echo circuit. after the echo circuit, the signal would be sent to a resonant 12db LP filter, and finally the power amplifier stage. i then added two individual ramp LFOs to be able to modulate the VCAs and VCF. LFO "A" has "RISE" and "FALL" inputs to control the shape and speed of the LFO. LFO "B" has a rate knob and a three way toggle switch to set the shape to saw, ramp, or reverse-saw. the filter's cutoff can be modulated by LFO "A", but the LFO input is inverted from that going to the VCAs, so their peaks will be out of phase 180 degrees. this makes for a more versatile effect, i think. once everything was built and working on the breadboard, i designed, cut, and populated the circuit board for all of the new circuitry. i also started playing with ideas for the faceplate, cutting prototypes out of old cereal box cardboard.
It was about this point in the project when i had received an inquiry into buying and modding my last NT02 and my long-for sale NOISE synth. see THIS UPDATE. that took my attention away from the keyboard for a week or so, and then i got really sick... i haven't been sick for a long long time, and this time i was laid out for a good ten days. my cold finally passed though, and by the time it had, i realized that all i really had left to do was wire up the hardware, cut a faceplate, debug, and assemble the keyboard once and for all. debugging took a few days, and the faceplate gave me a few problems, but eventually this little creature came to life. this little guy really turned out great!
Re-wiring the key pads was tricky, but i've done this all before.
Some guts-shots after all of the hardware was wired up.
Blue LEDs under red lenses.
Speaker in the back. remarkably loud, considering...
Portamento knob on the side by the line out.
My schematic. WARNING: there may be errors.