Tuesday, September 30, 2014


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.

Watch this

Wednesday, August 20, 2014


This is the latest set of NT(noystoise) series novelty synthesizers. following the overwhelming reception of the original NT01 series, it was pretty clear that if i build them, they will sell. the NT02 is simpler in many ways than the original NT01, but the sound and character are a bit more sophisticated than that of the NT01. the NT02 consists of a square wave VCO with chorus/delay, a white noise generator, a ramp LFO, and individual 12db resonant lowpass filters for the VCO and white noise generator. the pitch of the VCO is controlled by one axis of the VCO joystick, while the other axis controls the delay time of the chorus. the chorus circuit is basically just your typical PT2399 delay chip setup, except there is no feedback loop. the VCO is fed to the delay chip, and the delayed signal is recombined with the initial signal before the filter stage. the effect makes the VCO sound much bigger and warmer with the chorusing effect. almost like an old analog poly-synth with detuned VCOs. the white noise generator is your basic two transistor type found in many old synth designs. a common issue with the two transistor noise generators is that they require at least 9 to 10 volts to operate. that's just fine if you are using a plug in power supply, but when using a 9 volt battery, the noise generator usually wont work. in my case, the noise generator wouldn't sound with any less than 10 volts. however, the noise generator doesn't draw all that much current, so i simply used a 555 charge pump voltage multiplier to get the circuit where it needed to be. the problem i ran in to on the breadboard though, was there was some cross-talk from the 555 charge pump oscillator to the VCO, so i only used the 555 charge pump method for the first NT02(revA). for the others i simply buffered the VCO, and made a charge pump out of that. this way, if there was any cross-talk, it would be harmonized and unnoticeable. however, once the NT02 revA was built and working, the cross-talk was gone, so i guess it wasn't necessary. both the VCO and the white noise generator have their own respective filter with resonance control. the cutoff frequency of both filters are controlled by their respective axis on the VCF joystick. the center switch on the joystick toggles either the VCO or noise voice on or off depending on which joystick switch you press. each voice will remain on or off until the switch is pressed again. this feature was not included in the revA NT02. its voices are permanently on. the NT02's LFO is ramp shaped and can be set to modulate one of four parameters, or turned off. the LFO can modulate either the chorus delay time, VCO pitch, VCO filter cutoff, or the noise filter cutoff. on the revA NT02, the VCO pitch setting controls the maximum threshold of the pitch joystick, whereas the revB VCO pitch setting is independent of the pitch joystick. the LFO rate is controlled by the knob above the VCF joystick. the NT02 also has a volume knob, a 1/4 inch switching line out jack that bypasses the built in speaker when plugged in, and a center positive polarity protected DC jack that bypasses the internal battery when plugged in. the internal power supply is regulated, so any DC adapter between 9 and 18 volts will work just fine. unfortunately, unlike the NT01, the NT02 does not have CV inputs or outputs. the NT02 revA(black one) does have two 0-5 volt CV inputs for the VCO and LFO, but the revB models do not because their circuits' operating voltage is 8vdc instead of 5, which is kind of an unusual number... i would have liked to have scaled CV inputs that could turn 0-5 volt CV input to the necessary 1-8 volt CV input that the VCFs, VCO, and LFO require, but space was too limited. the inputs are in there though. i still kind of regret not including the ability to connect CV inputs to the NT02, but it is just safer this way. these were not intended as modular synths. they can be easily modified though, for any advanced users out there. hopefully these little synths do as well as the NT01 did so i can start on an NT03... the NT02s will be available at my for sale page until they are all gone. have a look, tell your friends about this BLOG!!!
Thanks for reading,


REV-B PCB (a ton of parts)





Monday, June 23, 2014


I started this project soon after finishing up the NT01 set. after feeling a bit overwhelmed by the idea of trying to compete with other novelty-synth producers that seem to be popping up all over, i thought it might be a good idea to fall back on my strengths. rather than trying to develop some new and original device that is completely "from scratch", i wanted to get back to modifying obsolete things. this is not to say that there wont be more NT models in the future, but i needed a break from brainstorming. i had a yamaha PSS-170 laying around, so i figured i would just have some fun with that. the initial plan was to keep it simple. an echo circuit, a filter, a sequencer, and so on. i figured this would be a quick little project that i could crank out in a couple of weeks, but it ended up taking about a month to finish.

upon doing some random research for circuit ideas, i decided it might be time to finally find out what PLL's do. when i started circuit bending some 10 years ago, i remember reading about Phase Locked Loops in a textbook. i think i got about two sentences in before my eyes crossed in boredom. since then i have increased my capacity for understanding electronic applications. i couldn't believe how simple it was to understand. basically, in the case of the CD4046, there is a voltage controlled oscillator that synchronize its frequency to the frequency of whatever signal is inputted to the CD4046. i guess i never understood why you would need to do that. why not just use the input frequency? well, apparently if you divide the VCO output frequency before sending it back in to the CD4046's phase-comparator, the frequency of the VCO will be increased until the divided output frequency matches that of the input frequency. so if the phase-comparator input was set to 1/2 the VCO frequency, the VCO frequency would be raised X2 to match the input frequency to the CD4046. BRILLIANT!!! i remember how exited i was when i learned to divide frequencies, and now i can multiply them!! and not just by 2, 4, 8, and so on. with the PLL you can multiply by whatever number you want. there is one drawback though, at lower frequencies the PLL takes longer to stabilize. it kind of sounds cool though because the frequency stutters a bit before it locks in to sync. hearing something fall in to sync is so vindicating.

after coming up with a decent PLL circuit, i devised a simple bidirectional sequencer that would drive an LTC1799 chip to pitch the YM2413 voice chip of the PSS170. i wanted to add an envelope generator to the keyboard that would be triggered by note-on messages, but unfortunately there was no way to obtain a decent note-on gate from the PSS170 other than to use the audio signal as a gate. and since there are so many different voices in the PSS170, there would be no way to get the envelope generator to respond to all of the voice presets. instead i just synced the envelope generator to the sequencer clock. i also threw in a divider so that the envelope generator could be triggered by 1/1, 1/2, 1/4, or 1/8 the clock frequency.

the envelope generator would be set up to modulate the cut-off frequency of the melody filter before being sent to an echo circuit. while i was experimenting with creating a note-on gate from the audio signal, i realized that the voices coming from the YM2413 sounded really crunchy when you filter them just right and sent them through a frequency divider, so i ran those sub-frequencies through a joystick mixer and mixed them back with the original voice before they get sent to the filter stage. this is when i realized something else... although there are separate outputs for the melody and rhythm voices coming from the YM2413 voice chip, there is some bleed through from the rhythm output to the melody output, even when the rhythm output is fully attenuated. i suddenly remembered this same problem i had with other yamaha PSS models, namely the PSS-30. i think it is due to the fact that the melody and rhythm sections share some similar functions when it comes to the auto-accompaniment. the accompaniment melody comes through the melody output, but the accompaniment volume switch affects both the accompaniment melody volume, and the rhythm volume regardless of the analog volume outputs of the two sections from the YM2413. and of course, the accompaniment volume has no effect on the melody output volume. with all of this in mind, i safely concluded that there was no solution to the bleed through. the only real problem it gave me was that when the rhythm volume was set all the way down, and the melody divider was engaged, the bleed through would get picked up by the melody divider and get amplified quite a bit. the divider seemed to be picking up some aliasing noise too, so to solve the problem, i used a potentiometer with a switch at one end that would disconnect the divider circuit when its volume was set to its minimum. in addition to the independent volume controls for the melody and rhythm sections, i thought it would be kind of cool to have independent echo circuits for the two too.

after about a week of reverse engineering the keyboard and coming up with an overall circuit, i came up with a design plan. the PSS170 has a lot of room inside, but i would need a lot more if i wanted to fit all of the circuits i had built. the first thing i did was replace the huge 6xC battery compartment with a much smaller 6xAA battery compartment. then i cut as much of the front panel off as i could without compromising the structural integrity of the keyboard case. the next step was to clean the hell out of what was left. this keyboard was absolutely filthy. once it was all clean, i took some measurements and started drawing up the new face-plates. after playing around with the orientation of the interface for a few days, i settled on a design, and got to work drawing up a circuit board for the whole thing. rather than using multiple small circuit boards, i decided to be lazy and just put it all on one big circuit board. the circuit board took most of a day just to cut and drill. CNC engraving a circuit board of that size can be slow going. i had to keep going back over it a little deeper each time around the corners to get all of the traces it missed the first time around.
populating the circuit board was pretty fast. the only thing that bothered me was seeing the huge number of parts that this circuit consumed. it's not that i hate ordering more parts, it's when i need parts that i don't have anymore. oh well, i can always pull parts from scrap circuit boards if i get desperate.

once the circuit board was built and ready to go i cut out some prototype face-plates on my laser cutter using cardboard from a cereal box. i have found this to be the best way to connect the hardware to the main-board without having to worry about over abusing the face-plates. then once everything is wired up and working properly, i just swap the disposable face-plate for the real thing. i had originally planned to paint the three face-plates all different colors, kind of going for a crazy candy color scheme, but i kind of liked the way the cereal box cardboard looked with the bright orange buttons. i had a can of krylon that was basically the same color, but i've had a lot of trouble in the past using solvent based paints with acrylic face-plates. i read somewhere that if you reheat the acrylic and let it cool, it will reduce the risk of cracking the acrylic when the paint dries. once the panels were engraved, cut, and i had colored the graphics, i hit the the panels with a MAPP torch for a few seconds on each side until the acrylic started to bow out. i kept heating them up until they wouldn't warp any more, and then i let them cool. once they cool, they return to their original size, and all of the stress in the plastic is relieved. then they are ready to paint. there was a little blistering from the torch, but it is barely noticeable.

i would have to say the most rewarding part of a project that takes so much planning, is when it's time to assemble everything, and it all comes together smoothly. however, being the most satisfying phase of the project can in turn create a perfectly devastating situation. that's usually what happens, and this time was no exception. once all of the new keyboard was assembled and working, and i was putting in the very last screw that would finish the piece using a very powerful screw-gun, i went too deep and cracked a face-plate... i cut out a replacement, painted it, and was able to swap it in by the end of the day. lesson learned.

once the keyboard was closed up i notice a couple of things that i wasn't entirely satisfied with. for one, the envelope generator was biased too low, so it would make this irritating popping noise every time it swept down. not only that but the envelope generator didn't sweep deep enough because the envelope was buffered by an op-amp, so it couldn't reach more than 4 volts. i opened it back up and swapped out the envelope buffer with a couple of transistors to fix both issues. i seem to remember having the same problem with IVAN. why didn't i think of it when i was designing the circuit? oh well, fixed now. while i had the keyboard open, i threw in a little input jack to the CD4046. now the sequencer can accept incoming clock signals and/or the rhythm tempo. i thought about adding a tempo sync output too, but i'll leave it out for now.