Friday, January 23, 2015


Ahhh... internet at last. been having trouble with my connection for the past week or so. haven't had the opportunity to post. the NT03 is the latest of my NT(noystoise) series. i initially started developing the NT03 in early November of 2013. the first of the six was completed at the beginning of December, but a commission came my way, so i put the rest on hold for a month before finally finishing them all. unfortunately the commission fell through. thankfully it was only one month wasted. hoping to quickly recoup my losses, i cranked out the final five NT03's in record time(two weeks), only to find that my 3 month old CenturyLink DSL router was all dried out. one after another, i finally got a replacement that works(for now). now to catch up!

The NT03 consists of two square wave voltage controlled oscillators, a holtek voice modulator circuit, a voltage controlled filter(VCF), and a low frequency oscillator(LFO) that can either/or modulate the filter cutoff, gate the voice modulator transpose sequencer.

The two VCOs are made up of one 74LS124 dual VCO chip. i was able to get a pretty good deal on these chips some years ago, but i haven't really put them to much use. this felt like a good opportunity to use them. i also had a few holtek HT8950 chips that needed to be used. the HT8950 is basically just what you would find in one of those toy megaphones that turns your voice into a robot voice(all circuit benders should know what i'm talking about). the difference in the HT8950 is that in addition to being able to incrementally select the pitch setting of the modulator, there is also a three bit parallel input that can be used to select the chips' eight pitch settings; Robot, x2, x1.6, x1.3, x1, x.9, x.8, x.6. the sample rate(BIT RATE) is resistor set. initially i tried clocking the UP/DOWN inputs on the 8950 with an external oscillator, but found that there is a pretty significant debounce protocol built in. this makes it difficult to achieve faster transitions at lower sample rates, so instead, i built an eight step parallel sequencer from the CD4029 chip. this way the eight pitch modes can be accessed quickly with no debounce(latency/stutter). the sequencer can also advance in either direction.

After the two VCOs are mixed and sent through the voice modulator, they are then sent to a resonant 12db lowpass filter(VCF). the filter's cutoff can be modulated with an envelope derived from the sequencer clock, but the attack and decay of the envelope are preset. the filter cutoff, pitch of VCOs 1&2, and the sequencer clock rate are all controlled by one of four joystick axis. the 3D joysticks are a recurring theme in my NT series. this time i decided to use all white thumbsticks, just because they seem to go with everything. they do tend to get dirty though..


Each of the four joystick parameters can also be controlled externally, although the joystick still controls the maximum threshold of the incoming control voltages(CV). the frequency of the sequencer clock cannot be controlled with a CV, but it can accept incoming clock, or negative going gates. the sequencer clock can be triggered internally and/or externally. to advance the sequence internally, there is an 'ENABLE' switch, and a 'DIRECTION' switch. the DIRECTION switch controls the direction of the sequence with either internal or external clock advance input.

The NT03 also includes BIT-RATE control, resonance control(Q), VCF MOD switch(on/off), POWER switch, VOLUME control, LINE OUT, clock rate LED, and center positive 9VDC supply input jack. the NT03 draws about 60ma max, and can also be powered by internal 9v battery.

The NT03 enclosures are made from laser-cut plywood and laser-cut acrylic faceplates. everything is made inhouse(in my house) from scratch. when i finished the first NT03, i had originally intended on painting them all primary or secondary 'candy' colors. hence the yellow one. after some time away from the project though, i thought it might be nice to come up with some colors of my own. i ended up mixing all of the colors for the final five by eye. some are a clean wash, and some ended up with some inconsistencies. namely the olive green one that has my big fat finger print in it. i suppose i could fix it or change the color, but despite the imperfection, it still looks really clean. if no one buys it, i'll just have to keep this one for myself.


This project began in early December of 2013. it was originally a commission, but the deal ended up falling through, and i never got paid for it. hopefully i can find someone out there to buy it from me to make up for my losses.

The original concept was based around the Synare3 circuit. the Synare3 consisted of two CMOS oscillators, a white noise generator, a four-pole resonant lowpass filter, a voltage controlled amplifier, and two envelope generators that modulated the filter cutoff and VCA amplitude.

My task was to build a modern variation on the circuit that would enable CV/EXP. PEDAL inputs to the oscillators, filter, and VCA. this would mean replacing the CMOS oscillators with voltage controlled oscillators. i was also asked to add the ability to 'transpose' the oscillators frequency range, create multiple waveforms, add a 'crossfade' between one oscillator and the noise generator, add a line-in mix, and a pushbutton switch that would fully trigger the envelope generators.

with all of this on the list, i began designing a circuit that would achieve it all. in the end, the circuit was quite a bit different than the original Synare3 schematics i was able to find. for the VCOs, i used a basic dual op-amp design that produces both square and triangle waveforms, and has a pretty decent frequency range. transposing the VCOs would just be a matter of switching in additional timing capacitors. 

the white noise generator in the original schematic was not giving me very good results, so i ended up using the two transistor type that i have used in the past with the NT02 series. it works well, but the circuit requires at least 10vdc. since the buyer asked that the finished product be powered by a standard 'center-positive' 9vdc supply, i would also have to build a 'charge-pump' circuit to create the necessary voltage for the noise generator circuit. like the NT02 rev.b, i used a mirrored signal from 'oscillator 2' to drive the charge-pump voltage multiplier.

the filter in the Synare3 originally consisted of four separate 'OTA' 6db lowpass filters in series to create the four-pole filter. the circuit originally used the CA3080. in the interest of space and cost, i decided to use the dual package LM13700 instead. i felt that the fourth filter in the chain was not really necessary in this circuit, so i cut the filter down to three poles, and used the fourth OTA for the VCA. i was able to cut down my OTA needs from five single amplifier chips to two dual chips. also, i didn't really see the need for two separate envelope generators, and the buyer only wanted one decay control knob, so i only built one to control both the VCA and VCF. in the end, the only thing that stayed original to the Synare3 was the drum-sensor/envelope-generator circuit. the seller also insisted that the sensor be a 3 inch loudspeaker like the original Synare3. the speaker did have a more natural response(maybe), but when the deal fell through, i ended up switching it out for a piezo instead. the speaker was just not sensitive enough for finger tapping. i also ended up adding a lot of different modulation routing to the LFO, VCO, VCF, VCA, etc. the finished product does well as a drum synth, but excelles as a drone synth. plugging other signals in to the trigger input is especially fun. it can take LINE, MIC, INST, and other sensor inputs safely.

the buyer had also asked that the VCOs, VCF, and VCA have standard 0~5vdc CV inputs that doubled as expression pedal inputs. this was new territory for me. i didn't understand how you could send 5 volts to a jack in a TRS situation, and then just short that 5 volts to ground when you plug in a TS jack, but the buyer knew what he wanted, and it is common for newer moog and roland gear to have this option. after a couple of days trying to come up with some sort of current sensing switch that would disconnect the 5 volt supply when shorted, i ended up just asking to borrow someones moog pedal to see what they do to overcome this. ironically, the instructions on the bottom of the moog sent me to their website, where the manual with the description of the circuit is. all it really ended up consisting of was a dedicated 5 volt supply that was limited to 5ma per output. if one of the four was shorted to ground, the rest would not be able to power an expression pedal. if all four are connected to TS CV inputs, the circuit will draw no more that 20ma. since this thing isn't running off of batteries, it's not a big deal. so i was able to do it the way moog does. i just added a 7805 regulator with four separate 1k resistors coming off of it to the four CV/EXPR inputs.

the LINE-IN MIX jack was pretty basic. the buyer really just wanted to have the option of mixing another LINE-IN without the need of an external mixer. the LINE-IN has a little bit of gain too, so the signal can be summed from the external device's volume control.

the body of the synth is made from laser-cut wood, and the faceplates are laser-etched hand-cut sheet metal. working with sheet metal can be a pretty stressful when you don't have the right tools. my second try turned out much better than the first. i wasted a lot of hours, but it was a good learning experience.


Tuesday, October 28, 2014


This project was supposed to be a quick quick one, but it ended up taking nearly the entire month of October to complete. it was't terribly difficult to build, but this month has been so full of distraction. initially all i had intended on doing was modding up a toy keyboard with some simple mods that could be built quickly, and would yield some interesting sounds. i started with a Rhapsody programmable electronic organ. i forgot to take a picture of mine before i totally dismantled it, but i did find a pretty good picture of it on google images.
The keyboard comes equipped with a built-in sequence recorder and rhythm generator, so i thought it would make a great base for a novelty synthesizer. the rhythm generator was actually just two separate pulse generators that were filtered slightly before being sent to the amplifier. my first thought was to add a step sequencer that could be counted up by one pulse-generator, then counted down by the other. it seemed to work, but for whatever reason, i decided not to include the pulse generators or the internal sequence recorder in the end. i think the reason was that the pulse generators made a popping sound whether they were connected or not. i only wanted to use them as a clock source, and not hear them at all. they could also be heard when the recorded sequence was playing back, so i just got rid of both features and moved on. 

Keyboard case cutting before and after.

I came up with a rough idea of what i wanted the keyboard to do, and breadboarded the circuit. the keyboard pitch would be sent through a bi-directional 8-step sequencer. the sequencer would also control the pitch of a little drum voice chip i found in the basement. it was an HT82105 from an old drum toy from the 80s. the chip has four drum sounds that would be triggered by the sequencer clock. since the pitch of the drums would be sequence by the same hardware that pitches the keyboard, i thought it would be a good idea to have the two voices be pitched in opposite directions. so basically, pitching the keyboard voice up will effectively pitch the drum voice down. the drum chip is kind of unique in the fact that when the voice is triggered, the drum sound will repeat as long as the gate is high. at higher pitches, the voice is able to repeat more than at lower pitches. to get a little more versatility from this effect, i added a duty-cycle control to the sequencer clock to be able to adjust the gate time to the drum voice. 
The melody voice would be sent to two CD4046 PLL circuits. one would output 1/2 and 1/4 the frequency of the keyboard voice, and the other would output X2, X4, X8, and X16 the frequency of the keyboard voice. each PLL circuit would have individual filter controls, to modulate the tracking speed of the circuit. one of the great things you can do with PLL chips is drastically reduce the tracking speed and efficiency by replacing the filter resistors and capacitors with ones that are not nominal. the result is squirly, phasey, rough and beating tones reminiscent of circuit bent sounds, but much more stable and not overdriven. the voices have a mind of their own, and that is what i want!
Anyway, then the two PLL voices would be mixed together with the original keyboard voice via switches, then sent to a 12db lowpass filter before being sent to the power amplifier. the filter would be slightly resonant, and the cut-off frequency could be controlled either manually, or with an envelope generated by the sequencer clock. there is not really any decent way to shape the modulating envelope to the filter without effecting the sequencer speed and the drum trigger gate time, but because there is a duty cycle control, the envelope can be changed. everything being connected in some way is kind of a recurring them in this synth.

Schematics and PCB layout.

Once all of my ideas were on a breadboard and working, i drew up some circuit boards and faceplates. surprisingly the circuit boards came out with no problems at all. i think i'm getting good at this. the one thing i forgot to include in the faceplate design though, was the Key Hold switches. unfortunately there was no way to hold notes on this keyboard without making a physical connection to the note switches. i wanted to be able to hold any of the 25 notes, so i would have to use multi-position switches. the keyboard chip has 8 data lines, and 4 common busses, but the 4th is only responsible for one note and all of the other unused functions, so i left that one out, so the high F can't be held. also, because the PLL circuits need a gate signal to disable their VCO's while a note is not playing, i used the gate signal coming from the 4th common buss. unfortunately though, the PLLs cannot be gated when the high F is played, only the original keyboard voice will sound. 
Anyway, to hold the 24 notes, i used a combination of a single pole four position switch with a single pole eight position switch. the 8-position switch will select the note address, and the 4-position switch will select the one of three common busses, or off. unfortunately the 8-position switch i used is a "make before break" type, which means that the note cannot be changed with the 8-positions switch alone, the 4-positions switch will need to be turned to activate a changed note. sadly, i didn't realize this until it was already built. good to know for next time though. 

Sequencer board on the right.

Fitting everything in the keyboard enclosure was kind of a challenge. there was not really a lot of room to work with, and it all got filled. the finished keyboard is actually pretty heavy for its size. i had thought about painting the keyboard enclosure a different color just because of how old and sun-stained the white plastic had become, but then i started to like the stains, and started to think that painting the whole thing might just make it look too "arts-and-crafty". it's not like it wouldn't be obvious that i painted it. also, it's been super wet here in Portland lately, and that has caused me more than a few paint-job heartaches in the past. best to let it be itself as much as possible. i am glad i did, because i don't think i could be happier with the way it turned out. 

 Tight fit.