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,
WIRING HARNESS AND ASSEMBLY LINE
REV-B PCB (a ton of parts)