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| Tau Hydrae in the wild |
Over the past few years I had collected the schematic designs of Thomas Henry, well known in the synth DIY realm. In particular his "An Analog Synthesizer for the 21st Century". What attracted me was his unconventional use of non-synth parts to create synthesizer modules. From these initial schematics came the design for the Tau Hydrae. The layout is a stock standard subtractive synthesizer signal flow but with some special features to make the exploration of sound creation more flexible.
Thomas's designs are aimed at modular synthesis so I needed to think about routing within this new build. My initial design of the Tau Hydrae had a 1/8" jack patch panel for all the modulation input selections but I harkened back to my old Roland SH-5 and I realized I wanted something that was a bit easier to setup than a patch bay with the result being lots of switches. The patch bay would have made this unit quite a bit larger and it's already quite big and heavy at 900mm wide.
I mentioned that this is a monophonic synthesizer but this isn't strictly true. Due to a hardware choice I have two 12 bit DACs on the MIDI to CV converter which might as well be used. The Tau Hydrae has all the hardware in place for duophonic to work where LM-VCO is one voice and the XR-VCO is the other but I have not implemented this in the software.
One of the aims for this project was less wiring. Wiring is my most hated part of any project so I started out with the idea of having a bus which contained almost all modulation and audio data on a ribbon cable. In all but a few places, the ribbon cable and power is all that connects between the modules which I consider a great result.
Before you read on, you can always watch the video I've made describing the modules with small demonstrations of the features - Tau Hydrae
Module Design
With the reduction in wiring in mind, the modules are made up on two printed circuit boards. One board contains mostly the front panel controls and the other board is the main part of the modules functionality. Some of the PCB designs I used are designed by other synth builders. This made the front panel boards a little more difficult to design as connections were not located in the idea places but the time it would have taken to redesign the main boards would have been longer. So I managed to get around these issues.
Where I had to design a totally new board, I had the flexibility to design the main board and the front panel board at the same time to position components in more appropriate locations to avoid clashes with hardware controls like switches and potentiometers.
I have to admit that I could have placed the bus connections in more appropriate locations should I have thought of the boards locations within the front panel. In all cases, the bus connector is located on the front panel PCBs which means they are slightly obscured by the front panel itself.
The choice of knobs was really down to what I could find that had a larger and smaller variety. Having the two sizes helps break up the sea of knobs and switches. Most of my parts come from Tayda Electronics and I had purchased these knobs a while back with the idea of using them on the Expanse project. I wanted to have a larger knob which took control on the mostly used control within a module though in many cases I sadly suspect ascetics overrode practicality.
The Case
I set out for the case to be similar in style to the Yamaha CS range of the late 70's. In particular the CS-15D which was my first synthesizer I got when I was a teenager. This particular CS15D was never the same again after I opened it up and adjust too many trim pots. I also wanted the design to match the John Bowen Solaris to some degree, as this is the other synth I use for jamming, which is why I went for a white case with the wooden end pieces. Always wanting to try different approaches to case design, this unit has a piano hinge on the rear for access to the inside of the case. This is a pretty common approach with manufactured synthesizers. The original planning of this worried me as the case is mostly made from MDF and as many will know MDF isn't the strongest when it comes to screwing into, but it seems to have worked.
The front panel is also made from MDF which I have cut holes for the modules to sit in. This was done on my CNC router. The wood piece is too large for a single pass on the machine but I could simply move the piece along the cutting area and start the second cut as the accuracy could be out by a millimeter or two without issue. The mounting of the modules was originally designed to be screw inserts. This would allow the modules to be removed for whatever reason quite easily. Alas, MDF doesn't really like being countersunk and when I tested the screw inserts, the wood starts to peel apart and go fluffy. Adding to this was trying to paint said fluffy wood led me to use nuts and bolts to hold them to the panels.
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| Front panel has been CNC routed - note mistake on the left hand side - the wonders of Spak Filla |
Having followed woodworker and painter Peter Millard on YouTube for some time, I learnt that I could probably get a decent roller finish on MDF. It worked quite well though I could have done a better job at filling gaps before painting. Sorry Peter.
The side pieces of wood are varnished Tasmanian Oak. This is a nice hard wood that is sustainable in Australia. The symbols I have engraved on the sides are the hydra symbol from Greek mythology. Tau Hydrae actually being a star of the Hydra or water snake constellation.
Module Panels
I tried a new technique for these panels which I am not 100% happy with but it's an improvement on my previous method.
For many of my projects I use my CNC router to cut all the markings into 3mm acrylic. I then clean all the excess acrylic swarf off and fill the engravings with white acrylic paint. After this has dried, I use a mild abrasive cleaning paste to remove the excess acrylic paint and there we have a nicely filled front panel.
The issue I've had with acrylic is that glossy acrylic shows up grease from hands quite quickly. For this project I took a slightly different approach. Previously I would roughen the surface to take the shine off the acrylic as it helps mask grease and other marks. I continued to use this approach but using a heavier scourer to give quite a matt finish but being careful not to scratch the acrylic too much as the acrylic paint used to fill the engraving would start to fill those scratches.
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| My first complete panel - very happy man |
Once I had done the acrylic clean up, I sprayed the panel with three coats of satin clear spray. What this has done is provide a slightly more durable and polished finish compared to a burnished only panel. I still have to be careful when tightening up screws on the pots and switches though. I think it might be my penultimate approach for future projects. I can't see how I could improve the technique without going to a metal panel which would have to be manufactured by someone else. There goes DIY out the window.
By the way, the colour scheme was inspired by the East German synth, the Subharchord which was orange and blue and I was looking for something a little less scientific looking that most of my designs follow. The wonderful Hainback does a great demonstration here -
Here's a breakdown of all the modules
Main MCU
1. Arduino Mega 2560 MCU - I chose the larger Arduino so I could have two hardware serial ports in the form of MIDI interfaces
2. Reads in the Arp panels switches
3. Runs the Arpeggiator
4. MIDI to CV converter for the two VCOs
5. Analogue Portamento
6. Output interface - sends for both CVs, Trigger, Gate and both LFO outputs
7. Deals with various Arp Clock inputs
8. Transpose function for Arp
9. Arp idea based on Sh-101 - my second synthesizer
LFO A and B - Thomas Henry design
1. Wonderful module with combination of Delay, Lag, and VCA which gives you a delayed vibrato for example
2. Slightly complex input triggering setup
3. Custom square wave output with pulse width control
4. Voltage controlled speeds from various sources - LFO-B, ENV-A, S&H and Bender
5. LFO-B can be controlled from LFO-A, ENV-B, S&H and Keyboard CV
LM-VCO - Thomas Henry design
1. Uses a LM13700 as an oscillator which is usually used with VCAs and VCFs
2. Fixed or Keyboard voltage controlled
3. Coarse and Fine Pitch Control - large knob controls the Coarse Frequency
4. Lin Modulation input - AC / DC filtering - LFO-A, ENV-B, S&H and Bender
5. Waveforms - Sawtooth, Triangle and Sine but also Square via the Wave Wiper or a mix of those first three and square - maybe show a oscilloscope screen grab of a mix
6. PWM via a knob and LFO-B or ENV-A as source
7. Exponential Modulation - LFO-B, ENV-A, S&H and XR-VCO
8. Wave Wiper Modulation - LFO-B, ENV-A, S&H, and XR-VCO
9. Octave switch
XR-VCO - Thomas Henry design - Mostly the same as LM-VCO
1. Uses a XR-2206 function generator chip as the oscillator
2. Instead of PWM we have SKEW which bends the shape of the selected waveform
3. Sine and Triangle waveform options but they become sawtooth pretty quickly
Noise - Thomas Henry design
1. Digital noise generator - gives interesting sound on the lower end - quite digital
2. Keyboard Tracking control source
3. Modulation - LFO-A, LFO-B, ENV-A, ENV-B, S&H and Bender
4. Linear or Exponential Modulation input selector
Filter - Elka Synthex copy by Xnotox
1. As used in my FloriVoxTron
2. AS3320 filter chip
3. Two separate Frequency modulation inputs - one has LFO-A, ENV-A with AC/DC filtering with the other having LFO-B, Bender, ENV-B and S&H
4. Resonance with modulation control - LFO-A, LFO-B, S&H, ENV-B
5. Small caveat is a doubling of signal level when using the LP24 filter type
6. Voltage controlled Resonance
ADSR A and B - Thomas Henry design
1. Standard ADSR setup with Short or Long timer switch
2. Inverted signal output
3. Extra - looping ADSR function - retriggers when the envelope returns to 0v
4. Built in LFO for triggering the ADSR
5. A little complicated as you have to select whether it's in Trigger or Gate mode
6. Can be triggered from the Keyboard Trigger, Gate, LFO-A or LFO-B
Sample and Hold - Thomas Henry design
1. Addition of a comparator on the Trigger input to allow for any signal to trigger the S&H
2. Uses a MCU to create the internal clock sampling signal which also doubles as a divider or multiplier when in external triggering mode
3. External triggering options - LFO-A and LFO-B
4. Sample source input options - LFO-A, Noise, LFO-B and ENV-A
5. Also has a Lag control to smooth out the sampling
Sub Oscillator - Shed Synth Design
1. Has some great artefacts should it get some unusual waveform inputs
2. Source for tracking is LM-VCO or XR-VCO
3. Output level controls for -1 octave and -2 octave
4. Filter control to smooth out the square wave output
Mixer - my own design plus Moritz Klein additions
1. Basic virtual earth mixer for mixing - LM-VCO, XR-VCO, Sub Osc and Noise
2. 440hz tone generator for tuning
3. Moritz Klein overdrive section
4. Blend control to mix the overdrive with the original plus an invert for the original input - interesting results
5. Overall output level control
Amp - Mark Irwin Design
1. AS2164 VCA
2. Two independent modulation input options - one with LFO-A, LFO-B, ENV-A and LM-VCO
3. One input has the addition of a AC/DC filter
4. Second Modulation input has S&H and Bender as an input option
5. The VCA can be run in Linear or Exponential mode
Keyboard
1. Original planned to be a salvaged key bed but decided on a new controller purchase where I used the controllers MCU to interface to this project. This was a Roland A-49 controller keyboard.
Control Panel
1. Volume control - where the output can be muted while the rear accessed headphones is still active - independent level control as well
2. Bender - this outputs a CV on the bender
3. Modulation - both LFOs are fed through here via another LM13700 VCA where two switches control whether the outputs of the LFOs are affected by the modulation control
4. Arp Speed control which doubles as a MIDI CLOCK divider/multiplier when the MIDI clock source is used for the arpeggiator
5. Portamento - this is applied to both control voltages at the same time.
External Control
1. MIDI input - accepts MIDI note data on the input
2. MIDI output - sends out polyphonic MIDI note data along with velocity information allowing more control over an external keyboard
3. The internal keyboard can be isolated from the synth allowing only external control of the synth but also allowing the keyboard to control and external synth
4. CV-A and CV-B control voltage outputs on the rear along with Trigger and Gate signals
5. LFO-A and LFO-B are also output which is planned as a control source for the Azure Dragon effects rack
6. Sustain pedal connection - which is not implemented as yet
7. Two audio outputs - not stereo but two independent mono balanced outputs
8. Headphone output and level control - headphone amp is a Rod Elliot Electronics Project
9. 240vac IEC connector, power switch and fuse