PAL

The PAL project started in early February of 2025, and was finished by mid April. The synthesizer is built around a monophonic keyboard voice chip from a RadioShack "Musical Elephant Keyboard" from the early 90's. Only the sound chip and the keyboard keys were used. The rest of the circuit and enclosure were designed and built from scratch.

FEATURES

-Two octaves of micro-keys

-Voice Waveform/Octave modulation joystick

-LFO Rate/Depth modulation joystick

-Single 12db HP/LP resonant VCF

-Single VCA

-Single Looping Attack/Decay envelope generator

-Pitch modulation knob

-PT2399 Delay

-Master Volume knob

-8 Step Sequencer

KEYBOARD

The two octaves of split mini keys can be played manually, as well as held or played by the Sequencer and/or LFO with the SEQ and LFO KEY HOLD switches respectively. The analog voltage from the Sequencer and/or LFO can select any one of the 24 notes. The KEY HOLD note can also be set by external CV source from the 3.5mm KEY CV IN jack. 

WAVEFORM/OCTAVE

The square-wave oscillator coming from the voice chip is sent to a frequency multiplying CD4046 PLL chip. The multiplier can be set to one of eight octaves that are set by the OCTAVE side of the WAVE/OCTAVE joystick. The OCTAVE can also be modulated by the EG, LFO or SEQUENCER with the octave modulation slide switch. The eight frequency multiples of the voice are summed through an R-2R resistor network to create a pseudo-sawtooth waveform. The output waveform can be tapped from different points of the R-2R network to give eight unique waveforms. The waveform is set with the WAVE side of the WAVE/OCTAVE joystick. The waveform can be modulated by the EG, LFO or SEQUENCER with the waveform modulation slide switch. The waveform can also be set by external CV source from the 3.5mm WAVE CV IN jack. 

LFO

The LFO joystick controls the DEPTH and RATE of the LFO. In the center position, the DEPTH is set to zero. The DEPTH of the LFO will invert the wave-shape below zero. The LFO can be set to Triangle or Ramp waves with the triangle switch, or be set to Sample/Hold with the S/H switch. When the LFO is in Sample/Hold mode, the SAMPLE knob controls the frequency of the triangle(or ramp) wave that is being sampled. The RATE side of the joystick controls the gate frequency of the Sample/Hold. The Amount of the LFO modulation can be controlled with the EG when the EG modulation switch is pressed. The LFO has two rate ranges that can be set with the HI/LO switch. The LFO CV can be output externally with the 3.5mm LFO CV OUT jack. 

FILTER

The 12db filter can be set to Highpass or Lowpass with the HP/LP switch. The Cutoff frequency is set by the FILTER knob, and/or by the EG, SEQ and LFO modulation switches. The Resonance of the filter is set with the "Q" knob. 

VCA

The Voltage Controlled Amplifier is set between the VCF and the Delay and Master volume output. The VCA volume is set with the VCA knob, and/or the EG and LFO modulation switches.

ENVELOPE GENERATOR:

The EG consists of an ATTACK stage, DECAY stage, and a DEPTH control. The LOOP switch will cause the EG to restart at the end of the DECAY stage, and loop continuously as long as there is a key-press held.

PITCH

The PITCH is set with the PITCH knob, and can be modulated by the EG, LFO or SEQUENCER with the BEND modulation slide switch.

DELAY

The PT2399 Delay is set between the VCA stage, and the Master volume. There are knobs for Regeneration, Wet/Dry mix, and Delay time with the REGEN, WET and DELAY knobs respectively.

SEQUENCER

The 8-step Sequencer has a control knob for each step, and a corresponding indicator LED. The Sequence CV can be output externally with the 3.5mm SEQ CV OUT jack. 

In the down position, the RUN switch sets the Sequencer step to be driven by the Sequencer Clock. The Rate of the Sequencer Clock is set by the CLOCK knob, or can be set by external clock sources with the 3.5mm CLOCK IN jack. The Sequence Clock can be output externally with the 3.5mm CLOCK OUT jack. In the up position, the RUN knob sets the Sequencer step to be controlled with the STEP knob. The STEP control can be modulated by the LFO with the "LFO" switch next to the STEP knob. The STEP can also be set by external CV source from the 3.5mm STEP CV IN jack. 

 The direction of the Sequencer in Clock mode can be set up or down with the U/D switch. An LED indicates the state of the direction setting. The U/D function can also be flipped by the REVERSE clock when the switch is pressed. The frequency of the REVERSE clock is set by the RATE knob. 

BUILD PROCESS

Over the past 10-15 years, I have accumulated small collections of different toy keyboards that I think have potential for super-mod projects. While working as a cook, I had more money than time, so I was actively searching out the RadioShack elephant keyboard from the 90's on Ebay and other online auction sites. Over time, I collected nearly a dozen. Since leaving my day job, my skills in electronics have advanced significantly, and the keyboards seemed less interesting. But what to do with them all? I decided that they were still too interesting to just give away or try to sell, so I tried coming up with a simple standard set of mods that I could add to them all, and sell them as circuit bent toys. Of course, as soon as I started hacking them open, the wheels started turning and I took things a bit further than expected. Rather than trying to fit everything in the cramped elephant keyboard case, I went with a small enclosure design that I had drawn up for fun several years earlier that I wanted to use for something cool like this. I drew up the enclosure in Sketchup around 2016-17 I think, and I called it the "Tank", due to its shape and dimensions. Later on my dad said it looks like a cybertruck.. Haha! Thanks Dad.

The Musical Elephant Keyboard is a monophonic toy organ with basic toy features. There are two envelope modes; "Piano" and "Organ" with the option of vibrato. There are also a bunch of pre-programmed melodies that can be played in "One-Key" and "Demo" mode. The keyboard has a very basic rhythm section, too. There are four patterns to choose from; "March", "Tango", "Waltz" and "Rhumba", that are controlled with the "Start/Stop", and tempo "Up/Down" switches. There are two separate drums sounds to simulate a snare, and kick sound. The rhythm and melody sections of the keyboard have two separate voice chips, each with their own clock resistors and analog envelopes for volume. Rather than try to fit both circuits into the PAL, I decided early on that I would break it up into two separate projects. The PAL would just be the keyboard section of the elephant, the rhythm section would be a separate instrument later on(see TRIP). This way I could fit more modulation into the enclosure, and give the instrument the character it deserves.

The melody-chip has one square-wave output pin, a 0-5 volt VCA CV input pin, and a key-on trigger/gate output pin. The trigger/gate pin is set to trigger, in piano mode, and gate, in organ mode. For my purposes, the chip is permanently set to organ mode with the VCA wide open. The gate output is then used for the envelope generator. 

I thought it would be cool to send the square wave to a discreet 8-bit wave-shaper to generate a sawtooth-wave. The wave-shaper simply divides the incoming frequency into 8 sub divisions that are then reintegrated with an R-2R resistor network. the output is a pseudo-sawtooth wave with 256 steps with some lovely aliasing and micro-distortions. In order for the sawtooth wave to not be 8 octaves below the original voice frequency, I sent the square-wave to a CD4046 PLL first, and used the 8 outputs from the sawtooth divider to the comparator input. This way the sawtooth output frequency can be scaled up or down by 8 octaves. The sawtooth wave was great on its own, but I noticed I could get different variations of waveforms by tapping different points of the R-2R resistor network, all with 256 steps, but different variations and octaves of super-saws, down to almost pseudo-random. The idea then came to be able to shift through the waveforms and octaves with an analog control input, so I set them up with CD4051 multiplexer chips, and programmed a microcontroller as a dual 3-bit ADC's to set the output of the multiplexer chips. This way, I could use a joystick or other modulations sources to shift through the different octaves and waveforms. I highly recommend building discrete digital wave-shapers! The possibilities are endless, and there is something more satisfying about discrete over "programmed" signal processing. I could just be imagining that, though... 

Moving past the wave-shaper, the signal then gets fed to a pretty standard MS-20 style HP/LP filter, then a standard VCA. Both can be modulated by a basic A/D envelope generator. The EG can be set to loop continuously when gated by a key-press or "Key-Hold". After the VCA, the signal goes to a PT2399 Delay circuit that has been specially tuned for the synth that sounds really nice, and adds a lot of charm to the sound. The mixed signal is then sent to the master volume control, then the output jack. If the output jack is not in use, the signal gets sent to an LM386 amplifier that drives a small speaker. 

When designing the enclosure, the first step was to come up with a shape, and a kind of dimensional flow. The mini keys would be split and layered, and to the side would be two mini joysticks. What the joysticks would control, and everything else to be added was kind of up in the air. I had decided one joystick would control the voice's Octave and Wave-shape. I thought it would be a good idea to have the other joystick controlling the speed and depth of an LFO. I designed a little LFO that could do Triangle or Ramp waves that stay in the same frequency range as each other, and added a separate Pulse LFO to use as a Sample/Hold option. In the Triangle/Ramp mode, the Rate axis of the joystick controls the rate of the Triangle/Ramp LFO. In S/H mode, the Triangle/Ramp LFO rate is then controlled by the "Sample" knob, and the Pulse LFO Rate for the S/H shape is then controlled by the Rate axis of the joystick. I added a switch to be able to set the LFO frequency range to high or low, since the LFO range is pretty narrow with only 0-5 volts. Oh yeah, did I mention everything in this synth is 5 volts? Somehow it has taken me this long to discover the magical-beauty of the MCP6002 rail-to-rail op-amps. Goodbye forever LM358! Haha, maybe not. Sometimes you need more than 5 volts, I guess. 

Back to the LFO... Rather than have the Depth control axis of the LFO joystick simply attenuate the LFO, I thought it would be better to add an attenuverter. This way the LFO wave output is set to zero in the center position, is positive in one direction, and inverted in the other. I still think it was a good idea, but in hindsight, having so much control on one tiny joystick only, kind of makes it difficult to set the LFO to a desired range. If I had to do it over, I would probably have dedicated knobs for the LFO, and just use the joystick as a modulations source, kind of like the NT04 I design years ago.

While I was at it with the LFO design, I borrowed an idea from the PT100 build I did last year, and added a VCA to the output of the LFO that can be modulated by the EG, or set to fully open. This way, the amount of the final LFO wave-shape being sent out can be shaped by the Envelope Generator before its modulation destination. 

The sequencer is also taken from the PT100 build. It is a bi-directional 8-step sequencer, but with an additional clock input for flipping the UP/DOWN mode if so desired. This adds a little bit of randomness to the step progression of the sequencer. The sequencer is based around the CD4051 analog multiplexer chip. The steps can be advanced by the sequencer clock in RUN mode, or can be individually set with the STEP knob when the RUN switch is off. The STEP knob controls another 3-bit ADC that sets the multiplexer output in "Preset" mode through the "Jam" pins. The ADC can also be modulated by the LFO if so desired. 

The sequencer can be used to modulate the Pitch VCO of the synth voice, but it can also be used to actually play the notes of the keyboard. Much like the PT100, another ADC going to a network of digital multiplexers is used to convert 0-5 volts to 1-of-24 key presses. The "Key-Hold" function can be turned on and off, and used to play 8-note arrangements using the step sequencer in Run mode, or just be used as a note-hold  when the sequencer is not running. It can also be modulated by the LFO for a more chaotic arpeggio type of effect. It is very useful to be able to control the notes with the sequencer because it frees up the pitch-bend to the LFO or EG. The keyboard keys take priority over the Key-Hold, so it is possible to play the keys in addition to sequencing them.

Of all of the possible 0-5 cv inputs and outputs on this synth, I was only able to include 7 for external use, due to limited space. I also am not really into modular, so having a 3.5mm jack for every little thing is not very important to me. In the end I decided to include jacks for Clock-Out, Sequencer-CV-Out, LFO-CV-Out, Clock-In, Waveform-CV-In, Key-Hold-CV-In, and Step-CV-In. 

Building the enclosure was a bit of trial and error, but in the end it came out pretty good I think. Getting the keys to set perfectly was a bit of a hassle since I didn't really plan a decent way to mount them. I've since come up with a way to mount them easily without all the extra measuring, grinding, and glue that was needed for the first one. Laminating the side cheeks is always kind of a learning experience, too. No matter how many times I have made them, I always seem to make a catastrophic mistake at some critical step. Eventually they come out looking good, but fine woodworkers should look away!..

Coming up with the name for the PAL didn't have too much thought put into it. I love the idea of naming things, but I'm also very indecisive, so I usually just stick with the first thing that comes to mind. The word PAL works for this synth because it is small and portable, so I think of it as a little PAL to me. The word also seems like it could mean something interesting or technical if one was so inclined. It doesn't have any actual meaning to me, though. Just Pals!