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.

Tuesday, May 13, 2014

NT 01

This project started at the beginning of April. the main motivation for building these noise boxes was to create a product that could be sold easily to try and recoup some of my losses from going a month over my deadline with the Ivan synth. it seems like everywhere i go, people are selling Atari-Punk-Consoles, and other simple square-wave drone synths. the one thing that most of them are lacking is any kind of design aesthetic, so i figured i might be at an advantage there. i got to work designing a wood console for them using my google sketchup demo.

I had recently built a hot-wire acrylic bender. this was the perfect opportunity to use it. i designed the face-plate to have two folds that follow the shape of the console. 
Coming up with a circuit was pretty straight forward. all it really had to do was out-perform the APC. i wanted to keep the cost as low as possible so that i could sell these boxes for less, but still spend some time personalizing each one. the circuit is comprised of one square wave 'voice' oscillator. the oscillator's timing capacitor is hooked up to 10 capacitor-bank switch that is controlled by an LM3914 chip. the LM3914 chip is designed to be used as an analog level meter. there is one analog input, and 10 outputs for driving LEDs. i had originally bought these chips with the intention of using them as sequencers, but soon ran in to the issue of the 10 outputs overlapping with each other. there are lots of other creative ways to use these chips too though. my design was inspired by a schematic i had seen of an LED level meter being used as a Comb-filter. a small capacitor would be connected to each output. the other ends of the 10 capacitors would then be connected to the timing capacitor terminal in the oscillator circuit. the higher the input voltage to the LM3914, the more capacitors are sourced. this effect creates a very crude transposing/staccato sound up and down 11 octaves in total. additionally the 'voice' oscillator can pitched lineally. controlling the pitch and the transpose parameters can be done using either the X axis of the second joystick(CV-1), or the slide pot(CV-2). there is a switch that will swap the two parameters between CV-1 and CV-2. 
The square-wave voice is sent through a frequency divider. four sub-octaves are then mixed back together with the 'wave-shape' joystick. the output is sent through a vactrol-based VCA before the output stage. the VCA i set by the 'VCA threshold' slide pot. the circuit also includes an LFO. the LFO is a triangle wave, but can be set as a sawtooth, or reverse-sawtooth wave at double the LFO speed using the two 'LFO Shape' buttons. when both buttons are pressed, the LFO is a narrow pulse-wave set to the LFO's maximum speed. the LFO can be set to modulate the pitch, transpose, or VCA of the 'voice' oscillator. if the LFO is engaged, the hardware of the parameter it is set to will act as a threshold control for the LFO. the LFO's rate is set by the Y axis of the second joystick. 
 Additionally, there are four CV inputs for VCA, CV-1, CV-2, and LFO rate. when the CV inputs are in use, the respective parameter's hardware is used to set the CV input threshold to the parameter. CV-1, CV-2, and LFO rate are all vactrol-coupled. because each vactrol's response varies, some level of filtering should be expected. inputting a square-wave LFO to a vactrol-coupled CV input will likely produce rounded edges. this kind of gives it character though. after all, these are not intended to be used as precision test equipment. there is also a buffered LFO CV output jack. with 'modular-synth' being as popular as it is today, i thought that this stuff might appeal to people starting out. 

The noise box also has a line out jack, a 9 volt DC input jack, a 9 volt battery compartment, a power switch, and a pretty loud built in speaker. the DC input jack is regulated and reverse-voltage protected, so it can handle anything between 9 and 18 volts. the circuit has a lot of LEDs in it, so you would think the current consumption would be pretty high, but actually, at full volume the entire circuit only draws about 70ma from the battery. 
 The prototype took about three or four weeks to design and build. once it was finished, i redesigned my circuit board, and built three more. each one has its own color scheme. depending on whether or not these first four sell, i may build more in the future. for now though, i am going to experiment with some other circuit designs. my plan is to build small batches of different circuits, all built with the same enclosure with modular inputs and outputs. until recently, i had been working on an idea to use the ISD1820 voice sampler chip for the next series, but unfortunately those ISD chips are just not good enough... oh well, i'm sure i'll think of something else. any suggestions?