DIY Synth Page 1: Analog
I have you, Steve to thank (blame) for this latest obsession. OK,
you and the world-wide resurgence of interest in Analog
Synthesizers, combined with my love of analog designs and
music. In the mid 70's an old friend Jeff, a very smart analog
engineer, designed and built an analog synthesizer from scratch. It
was a modular, and had most of the features you would expect. I
played with it a bit, not much. Fast forward 40 years, and the world
has re-discovered these things. Moogs and Arps sell for many times
over their original prices.
Analog Synthesizer in
Blog for this project
The Schematics, PCB files, and
Simulation models are here
Page 2: the Digital
OLEDs and CV-> Midi
I put this off for a while because I had little to offer the synth
community. In fact, most of the circuits were designed 40-50 years
ago and have been refined ever since. One big problem with old
synths and their designs is that the opamps, transistors and other
components used to build the original machines have long gone
obsolete. Originals and replacements are old stock only and sell for
top dollar. Meanwhile, there are few surface mount designs. Surface
mount offers smaller, more modern and thus available parts, and
machine assembly as well as hand assembly. As a pro-fessional
'lectical engineer, I almost never design with thru-hole
technology. Even my test boards and one-offs use SMT. The
small size offers lower parisitics and thus closer to theoretical
performance. Not so much an issue with audio frequencies, but it
doesn't hurt. I find it easier to mount and solder a part on one
side instead of bending the leads, mounting it through the board,
bending the leads again to hold the component in place, soldering it
through the board, flipping the board, soldering the leads on the
rear, then trimming the leads. It's not so bad, but how 70's.
Here is one example: do a Digikey search for 'matched NPN' then
filter by SMT and by thru-hole. There are two old and expensive part
numbers in thru-hole and a couple dozen in SMT including the
precision SSM and MAT series from Analog Devices. Some SMT parts
that are matched to better than 2mV are less than $.50. No more hand
matching and gluing together transistors. Thank you, modern
There are many modular synth systems, starting with the Moog Modular
and others from the '60s. The current rage is Eurorack 3U. An
amazing assortment of Eurorack modules are available in kit, board
or completed form. The Eurorack platform is very simple, a 16 pin
ribbon cable backplane provides power and a couple of bussed control
signals. Mechanically, each module needs only a flat front panel and
a few screws. The card cages use standard hardware, and are strong,
flexible, low-cost and available.
The concept of modular solutions has always appealed to me. When I
was at WPI in the early 70s, I learned to simulate physical systems
on Analog Computers. Yes, I am that old. In the 80's at Datacube I
was one of the inventors of generations of MaxVideo modular image
processing systems. At Analogic and Teradyne, we developed modular
instrumentation for PXI, VXI and several proprietary platforms. At
home I worked on modular solutions for microprocessor development
and for low cost instrumentation solutions, but these never got off
One appeal of Eurorack is its utter simplicity. Each module is sheet
metal, a circuit board or two, a ribbon cable, some knobs and
jacks, and the creativity you bring. I love it.
Here is my first pass synth system on the electronic work bench, in
all its wiry glory. From Left to Right: MIDI2CV,
Sequencer prototype, Dual VCO, three ADSRs, VC-LPF, and a passive
Mixer. It is beginning to look like a synth, but still has a science
I wanted to get started with some existing technology and to gain
some experience with the basic mechanical components, pots, jacks,
switches, etc, as well as with building and packaging. I looked for
VCO, VCA, VCF, and ADSR designs that I could lay out in SMT, and
that would give me full function quality electronics. The VCO is one
challenge. Since it is the heart of any synth, it needs to be
flexible. Accurate voltage control, manual pitch, FM, and voltage
control over parameters such as pulse width are all useful functions
found in most full-featured VCOs. Waveforms should include all the
basics: Sine, Triangle, Ramp and Pulse. It needs to be stable with
temperature and time so that continuous tuning is not required. Most
VCOs use an exponential voltage to current circuit that was
originally published by National Semi in the '60s. The 'standard'
for voltage control is 1 volt per octave (1V/O) and it needs to
operate over the audio range of about 20Hz to 20KHz, or about 10
octaves. Stated as 1000:1 it sounds easier. Most designs use the
exponential current source to charge a capacitor, typically until it
reaches a threshold, then the capacitor is discharged. Charge and
discharge the cap at the same current but opposite polarity, and you
get a triangle wave. Discharge it quickly and you get a ramp, AKA
sawtooth or saw.
A ramp can be converted to a triangle using full-wave-rectification,
also known as the absolute value function. A triangle can be
converted to a sine with a non-linear circuit, and a triangle or
ramp can be converted to to a square wave or varying pulse width via
Turns out a triangle can be converted to a ramp using an
inverting-non-inverting circuit. So the question is, which waveform
to begin with, a ramp or a triangle? There are numerous examples of
both. Converting a ramp to a triangle is harder than it looks. The
fast rise-time of the ramp causes a fast glitch on one peak of the
triangle, which causes unwanted harmonics. This glitch will show up
on sine waves derived from the triangle. Just what you don't want on
triangles and sines: harmonics. I have seen circuits with two
trimpots to try to null out the glitch but that seems like a hack.
So I decided to start with a nice mellow triangle, and then use a
fast switch to add the fast transients that a ramp needs. I
came up with the idea of using a current mirror to invert the
current, and CMOS switches or transistors to route the currents.
It also turns out that the symmetry of a triangle works better than
a ramp when operating at high frequencies. A ramp's capacitor
discharge takes a finite amount of time, and that time is added to
the desired ramp period, causing an error in frequency that gets
worse at higher frequencies. To fix this, most VCOs have a high
frequency adjustment which is not a simple adjustment to make. With
a triangle, the time to switch direction is due to a comparator and
a flip-flop, generally faster than a transistor discharging a cap. I
haven't implemented a high frequency trim yet, and the VCOs work
well without it. Also if my 10KHz is a bit off, who will know? Most
dogs don't have perfect pitch. That was the theory, and the Spice
simulation agreed. When I built an dtested four of these VCOs, I
found that the high frequency linearity was excellent up to 16KHz.
If anything, the oscillator was a bit sharp (too high frequency) at
16KHz, or about C6, without any compensation trim. Good enough. I
suspect this is an advantage to the triangle VCO design vs. a ramp
For the control logic, I pictured something akin to a 555 timer. A
555 compares a voltage (usually on a capacitor) to 1/3 and 2/3 of
the power supply voltage, and sets or resets a flip-flop. It is
basically a triangle and square wave generator. I did a google
search for triangle VCO and came up with Ian Fritz page. His
schematic for the triangle VCO looks a lot like my pencil block
He uses a "Wilson current mirror" and CMOS switches to get the
current inversion accuracy and symmetry, and used a couple of
comparators and CMOS switches to replace the flip-flop. His design
is nice but uses kind-of a lot of parts. Bergfoton also has a
transistor based Triangle VCO here.
Again, kind of a lot of parts and trims.
Then I thought of using an OTA, the LM13700 as the switchable
polarity current switch. These operate over 5 or more decades of
current, and I only need 3-4. I googled "triangle VCO with LM13700"
and found Thomas Henry's excellent "555-VCO" on Muffwiggler. This is
just what I had in mind. The core oscillator is half of an LM13700,
a buffer amp for the triangle, and a 555 timer. No symmetry
trimpots, and minimum component count. Then he cleverly uses
the Reset pin of the 555 as a VCO 'SYNC' input, and the TRIG output
to switch the gain of the ramp generator. Very clever design using
all the features of the little 555. To convert the triangle to a
ramp, he uses the 555 trigger output to switch the polarity of an
op-amp. I thought this was pretty genius, however a few weeks later
I was perusing some old 1960's EML synthesizer schematics to see how
their VCOs worked, and there was the same triangle to ramp circuit
using the same inverting/non-inverting circuit. Many good
ideas have been around for a long time.
Thomas published his design and offered PC board builds including a
Eurorack front panel for short money. Unfortunately I missed his
buying window. Besides I'd like to lay out and build the boards
myself. I looked in my basement and found a couple of ancient
LM13600's, TL084's and CMOS 555's. In a few hours I had the basic
VCO working on a solderless breadboard. I tried a couple of triangle
to sine circuits and they worked OK. Henry uses the other half of
the LM13700 to make a sine wave converter. One problem with sine
wave converters from triangles is that it is hard to remove the
pointy top and bottom. If you run the amp at higher gain, you get a
flat-top sine which adds odd harmonics. What he does is to subtract
some triangle from the sine to null out the peaks. This works pretty
I changed his design a bit:
Converted it to surface mount. Mine uses one
board with 2 channels instead of two larger boards (one front panel
board and one electronics board) for just one channel.
Smaller front panel so I could get more in a
rack. His is 16HP, mine is 9HP per VCO.
Used a low-cost SMT matched-pair PNP. No matching
Added a ground plane (easy with surface mount)
Used low-cost 16mm pots
Used a 3 position switch for FM input: Linear,
Exponential, or none
Eliminated trimpots wherever possible. Only the
one Volts/Octave trim is really needed.
I laid out the board with ExpressPCB on a Mini board. This is 3.8" x
2.5", a reasonable size for Eurorack systems, and only $75 for 3
boards. I plan to build a full synth this way on about 3 PC boards.
Not one to waste PC board area, I put as much electronics as
possible on one PC board, and then hand wired the front panels. But
when I built up a couple of dual VCOs with 10 pots (only 5 are PC
mounted) 20 jacks and 2 switches, I found that the front panel
wiring was a *lot* more work than soldering PC boards. The boards
were straight SMT assembly, but the front panel required many hand
built connectors and cables, and lots of hand wiring to the numerous
pots and jacks. I decided that this was not a lot of fun. Wiring the
first VCO front panel was pretty exciting. After that it was
drudgery repeating the work 3x.
I had begun laying out an ADSR and a dual VCA, and decided that
instead of one board and lots of hand wiring, to use two parallel
boards for each module. One for the front panel controls and one for
the circuit. This was how the Midi2CV was built, and it was fun and
easy to build. I changed from 17mm pots to 9mm, and from hand-wired
jacks to the Earthervar PC mounted types. These parts work pretty
well and are low cost, but their PCB mounting heights are not the
same. So I soldering the tallest parts first, the jacks, and then
mount the pots to the panel so their pins aren't fully inserted in
the PCB. (see photo).
While debugging it I came up with a few issues. I decided to use two
1V/Octave inputs to allow the sequencer and the keyboard to drive it
at the same time. To do this, the two channels should match really
well. So I matched the two 100K input resistors to about .1%. Next
time I'll just buy .1% resistors since they are cheap enough. The
matched PNP transistor pair I selected, PN5201, comes in a very tiny
package, smaller than a sot23-6. I was able to just barely fit the
pins to the pads. Then I found a nice low cost matched pair of
2N3906 PNPs, in a SOT23 package. I'll use these or their
2N3904 NPN brothers for future designs that need matching.
When I looked at the sawtooth waves, there was a good-sized step in
the middle of the ramp. Worse, it varied from board to board.
My prototype had a tiny step, so what had changed? I had used
Intersil ICL7555 CMOS timers on the prototype and TI's TLC555 on the
PC board. The 1/3 - 2/3 voltages on the TLC555 parts are not
as accurate as the 7555s, and so caused DC errors in the triangle
wave, that then caused glitches in the ramp as well as offset errors
on all the other waves. A quick order to Digikey for ICL7555s, and
the problem was fixed. There are still visible glitches, but much
smaller. To really reduce the glitches, the original design
uses a trimpot which I wanted to remove. Below, I discuss other
'7555 problems found when I built the first sequencer.
I wanted an LFO and realized that it should probably have the same
basic waveforms as a VCO. More even. Ramps of both polarity sound
the same, but opposite polarity ramp VCOs definitely sound
different. I found that the basic VCO works down below 1Hz, low
enough for now. So the VCO works as an LFO without increasing the
core capacitor. Cool! To operate it as a really low frequency
VCO I probably should add a switch or change the cap. With the
second VCO, I can experiment with using one as an FM source for the
second from low frequency modulation (tremolo) to high frequency (FM
synthesis), also mixing a fundamental and one harmonic. Fun with
Dual VCO front view
Dual VCO rear view, with lots of hand wired components.
Dual VCO PC board
One channel of Dual VCO Schematic
MIDI to CV
Like most synths, I needed a keyboard interface. After investigating
Arduino and other DIY Midi projects, and due to my lack of
Midi experience, I decided to buy, not build, at least as a
start. I bought the HexInverter Midi2CV converter kit ($120)
and in one evening of soldering, had it working. This is a nice unit
with accurate and easy to calibrate DACs. With a couple of Vector
rack rails, I built up a first Eurorack system. Bought a Synthrotek
Eurorack power board on Ebay and connected it to a dual +/- 12V lab
supply. Finally I can play a tune, woohoo!
I like that the Midi2CV has 4 note poly-synth mode, but 2 note or 3
note poly-synth would be useful too. I can foresee new modes in the
future. HexInverter provides source code that runs on the Microchip
Pic processor, so maybe modify it. This is an interesting area to
play with. I like the idea of using an Arduino processor... So many
The voltage outputs of the MIDI2CV are quite accurate and operate
from 0 V to 8.00V, 9 octave range. However, there is an output
protection resistor of 220 ohms on each CV output. This causes a
slight error of -.2% in voltage when a 100K VCO load is applied. The
bad news is that when you drive multiple loads, each load causes an
additional -.2% error. At high frequencies, this error is a larger
pitch error. Say 1% of 8V is -80mV. at 1V per octave, an error of a
full semitone. Since I use my MIDI2CV regularly to calibrate and
test VCOs, and to drive multiple VCOs and filters, and possibly
other stuff as well, I plan to address this error. An external
precision buffered multiple would address this, but even it will
have some input load. There are a couple of ways to address this.
The resistor is there to limit the current output in the case of a
short circuit. Just plugging in a cable to a jack causes a momentary
short to ground as the tip of the plug slides past the ground ring
of the jack. If you were to mistakenly partially plug one in, a
short circuit could be present for a long time. The CA3140 op-amps
specify a maximum short circuit output current as about 50mA so the
power dissipated in the op-amp during a direct short to ground is
about 12V x .05A = .6W, quite hot for a plastic 8 pin DIP package.
The 220 ohm resistor reduces this power by about 1/2, a safer value.
Like many op-amps, the CA3140 is specified to drive a short to
ground or to either supply voltage continuously without damage. So I
may just replace the 220 ohm resistor with a short circuit. If I do
this I'll have a few spare CA3140s around in case they get damaged.
A more elegant solution is to take the feedback of the circuit from
the output of the 220 ohm resistor instead of the output of the
op-amp. This way, voltage drop across the resistor is compensated
for. This requires cutting the traces to the wipers of the gain
trimmers and adding 4 wires. Then because the 220 ohm R and
any cable capacitance forms an additional high frequency pole to the
feedback, an additional compensating capacitor may be needed from
the op-amp output to the - input. Cable has about 10-20pF per foot,
and maybe 10 feet of cable to multiple loads, so about 200pF
of capacitance worst case. I should be able to apply, say a 470pF
cap to the output and test the step response. Then add
compensation if there is excess overshoot, ringing, or oscillation.
As a shortcut you could apply this fix only on the main
output, CVA and then make sure to use this one for heavily
loaded or precision patches.
MIDI2CV, built from a kit from HexInverter
My keyboard skills are limited to say the least, and I wanted a way
to play simple sequences automatically, without having to hit keys.
So I investigated sequencer circuits, initially just as a test
generator. I like the old SimpleSeq from Hexinverter. Their clever
circuit uses a 3 way toggle switch per note. The 3-way switch
positions are ON, OFF and Loop, allowing the sequence to be set from
1 to 8 notes and to skip any notes. The clever part is that diodes
connected to the pots and switches control the analog output, gate,
and reset. I cut a front panel and wired it up. Again, lots of hand
wiring, I built the first one using eight old 10 turn pots I had
lying around. This was a mistake since the 10 turn pots don't have
an indication of the note settings. I rebuilt it with smaller 16mm
single- turn pots with indicator knobs, which work better, and are
smaller and cheaper. Again, lots of hand wiring. This really needs a
PC board. I built a second panel with smaller one-turn pots.
It is a work in process.
For its gate output, it occurs to me that having a variable width
output would be useful. Most sequencers use a variable clock
frequency and a fixed 50% duty cycle. It's a bit tricky to
build an oscillator with independent frequency and duty cycle
outputs. I picture a triangle oscillator and a variable threshold to
set the duty cycle. Someday...
Here is the current sequencer schematic, a work in progress. One
interesting bug I found. At first I used an ICM7555 for the clock.
The circuit regularly skipped notes in the sequence. It turns out
that the 7555 output occasionally had glitches on one edge
transition, maybe when operating at slower clock speeds. I tried
lots of grounding and bypassing tricks, but couldn't stop it. I
substituted it with a TLC555 from TI, and no glitches. So this
circuit works better with the TLC555 and the VCO is more
precise with the 7555. Well, Professor Eaton at WPI in 1976 warned
me against using '555 timers, and I generally heed this advice.
However they are particularly appealing for quick and-dirty synth
circuits. Let the designer beware...
I needed a VCA right away to take the keyboard gate signal and vary
the output of a VCO. Since I don't know whether linear or
exponential is best, I found a Ray Wilson (MFOS) circuit that does
both. I built it up on a solder-less breadboard and am still using
it this way. I have a board design for a dual VCA but haven't built
it yet. My first VCA used the 5V gate signal as its control.
It works, but only does fast attack and decay. While it is better
than nothing, I need an ADSR!
I ordered the bare boards and hard-to-get parts for the MST
VC-LPF kit from Synthrotek. I like these guys since they also
have 3.5mm jacks and 9mm pots for good prices. I had many of the
parts on hand so it was a low cost project. It is quite flexible,
having a fixed 1V/Octave input as well as inputs and controls for
frequency and resonance. It has 12dB and 24dB per octave outputs. I
like it and it works well. My only complaint about MST kits is that
they do not provide a schematic. All kits should provide schematics,
not only to allow one to mess with the design, but also to allow
troubleshooting it if it doesn't work right away. A lot can go wrong
when building a kit.
I chose the circuit from Music from Outer Space (MFOS) by Ray
Wilson. Their boards are not Eurorack, and I wanted a few of these,
so I laid out a PC module with two parallel boards. With an
ExpressPCB mini board (3.8" x 2.5") I built a front panel board and
a circuitry board, each about 1.2 x 3.8", and then cut the two
boards apart lengthwise. This worked out pretty well.
But when I built them up, They would work sometimes, and not others.
I checked out the circuit, and discovered that the TL074 op-amp was
getting hot when it stopped working. This corresponded to an
increase in the -12V power supply current to the system. Turned out
that the CMOS 4066B switch doesn't like any negative voltage on its
input pins, and would latch-up. This only occurred when the -12V
supply came up at the same time or before the +12V. My system, like
many Euroracks, uses similar + and - 12V supplies, and since the
negative supply is generally less loaded than the +, it comes up a
millisecond or so before the plus. You could make it occur or not
occur by plugging in the 10 pin power connector to the ADSR tilted
towards the + or the - end first. I sent Ray Wilson a note with this
problem, and he said that hundreds of his ADSRs have been built
without any reports of this bug. I suspect it may be due to the
specific 4066B manufacturer parts that I used vs. the ones he uses.
I fixed the problem by changing the TL074 to a +12V CMOS single
supply part. This required a cut and jump on the opamp's - supply
pin. It works fine this way and I have built 3 of these so far.All
three failed when I used the original TL074. In general,
circuits that use multiple sourced parts should not depend on a
specific manufacturer's part.
Here are some shots of the ADSR board. Note the stacked board
construction, the use of Earthernvar 3.5mm jacks, and 9mm Pots. No
hand wiring, Yay!
The board is an ExpressPCB mini, sliced down the middle to make the
two boards. PCBs for 3 ADSRs for $80, not bad.
Building Front Panels
Most Synth builders use 6061 grade Aluminum, 0.062" thick for front
panels. At first I would mark it out and cut it on a band-saw with a
fine-tooth bi-metal blade, and file the edges smoother. Then I
discovered a 100 tooth carbide table saw blade for soft metal, $25
on Amazon. Now I can cut panels faster, smoother, and more
accurately. I lay out the panels with Visio Technical, which has
decent measuring and marking tools. Any other 2D CAD tool would
work. The saw blade throws vast amount of metal chips, so good eye
and clothing protection are a must when cutting. I use either
a tri-square with a 1/100th inch scale, or my digital height and
marking gauge. The height gauge is a great tool for doing precision
work for about $30. Simply set the height gauge to the dimension
from the edge that you want, hold the panel vertically on a flat
surface, and use the height gauge to scribe a line at the dimension
For labels, I use laser printed paper and a low cost laminating
machine with 3mil pouches. Then contact cement the label to the
panel, sighting the holes and outline through a bright light. Once
you cement the overlay to the panel, cutting the label outline and
the holes with a sharp Exacto knife is easy.
You can also just tape a paper template to the metal and
center-punch the holes through the paper. This is a bit less
accurate but is quick and easy for building one panel. When
drilling, I start with all 1/8" holes and then enlarge the holes
with the appropriate bits. If you are making 3/8" or larger, it is
best to make the next-to-last hole just a bit smaller than the
final. the 1/3 - 2/3 - 3/3 rule works well to reduce burrs. Start
with a bit about 1/3 of the final size, then drill 2/3, then the
final size. This only takes a minute longer to change bits, and does
a nice job on soft aluminum.
Another trick for building more than one panel, is to mark and drill
all the 1/8" pilot holes on one panel, then use that as a drill
guide to build more. It's a good idea to use a thicker metal like
1/8" or more for the guide, depending on how many you plan to build.
This requires a way to keep the drill guide and the panel perfectly
aligned during drilling or errors will accumulate. You can use
screws and nuts, or clamps. I haven't perfected this yet.
Current Status, October 2015
So now my Eurorack system can play some decent sounds. It consists
What's Next? The plan is to add a second and someday a third rack
and fill it with these modules:
- One Rack, Homemade chassis, Vector rails and Synthrotek Power
- Midi2CV , Hexinverter kit
- Dual VCO, modified Thomas Henry design, my PCB
- Sequencer, half-built, based on SimpleSeq from HexInverter
- VCA, still on a solderless breadboard, based on MFOS VCA
- ADSR, MFOS ADSR, but my PCB layouts
- VC-LPF, Synthrotek
- Passive mixer: Just jacks and 100K resistors
- Power Supply. I'm still using a dual lab supply to power this.
- Sequencer: Wire up all the gate logic and design a variable
width gate. Maybe bring out all the individual outputs to drive
a drum machine.
- Dual VCA board, constructed like the ADSR on an ExpressPCB
mini board, cut in half
- Noise Source, maybe a random sequencer like Turing Machine
- Two more VCO/LFOs: another dual just like the other
- Proper Mixer with amplifiers.
- Full function filter: High, Low and Band-pass
- Copy of the Moog Ladder filter? I Want to see what the magic
is all about.
- Play with advanced Midi2CV features like keyboard split,
Prototyping and Design:
In building boards, I use a mix of prototyping, simulation, and
design analysis. For simulation, I mostly use Linear Technology's
excellent LTSpice. At various web sites, I found spice models
for commonly used synthesizer parts such as TL074, LM13700, and
555s. Based on these models I am able to simulate the VCO, VCA, and
VCFs. Follow this link for a .zip file containing these spice
models. Unzip the files, install LTSpice, and open the .asc files.
Then you can simulate the time and frequency domain circuits.
A warning though, precision circuits that require low offset
voltages, good matching, tight gain control, etc., all simulate
wonderfully, but the reality is that transistors don't match,
amplifiers have offsets, and gains (particularly the LM13700) vary
from part-to-part. This is the reason there are so many trimpots in
For prototyping, I use solderless breadboards. These perform nicely
for low frequency analog designs like those used in synthesizers. I
built up a small bracket to hold a few 3.5mm jacks and
potentiometers for convenience and reliability.
You've got to have a mixer or two. Or more. Ideally you should mix
VCOs or other sound sources together before filtering and VCA-ing.
This is a key capability of the Moog Mini and the Arp, and allows
you to turn knobs instead of moving cables. Then if you have
multiple voices, you will want to select them individually or
mix them together. A utility mixer for control voltages is
useful too. I wired up a simple passive, fixed gain mixer: four
inputs wired to 100K resistors, and a couple of outputs. This
worked, but is not at all flexible.
While I was contemplating a mixer design and looking at various
commercial ones, I came across Synthrotek's
MST 4 Input Audio / CV mixer. It is DC coupled and works with
CV's as well as audio. It has a switch per input, handy to isolate
signals without having to adjust and readjust knobs. It has an
inverted output, also handy for processing control voltages. An
inverted LFO ramp sounds very different from the non-inverted ramp.
Same with ADSRs. It has overload LEDs, too. Also a jumper-able x1 /
x2 gain. Best of all, the PC boards and front panel cost a mere $25.
It is a very simple, one IC design and I had the other parts in
stock: resistors, caps, TL074, connectors, pots and switches. Built
it in an hour and it works very well.
My only complaint is that I prefer screws and spacers to superglue
for holding the boards together. And since the connectors
between the board are symmetrical, it is easy to connect the boards
backwards. As usual, keep the power connector on the bottom. And of
course my usual complaint that Synthrotek doesn't provide schematics
for kits. In this case the design is so simple to reverse-engineer
that it seems just an inconvenience to not publish a schematic.
Wait, that's 3 complaints.
I bought a second MST mixer and they have added holes for board
spacers. Kudos! I may drill holes in my first unit to add
I have pretty much filled up the first 19" case, and foresee filing
another with a couple of VCOs, VCAs, a better sequencer, not to
mention some cool commercial modules. Three 19" racks should last me
a few years. Plan A was to buy or build 3U rack sides, but I don't
really want to mount it in a 19" rack. Instead wooden sides for now,
maybe a handle on top, designed for small size and
portability. I don't have any other rack stuff, so wood it is. Since
this will regularly sit behind
That means the rack sides should be simple metal strips to hold the
Vector rails. These will have a couple of holes so they can be
screwed into the wooden sides. A few caveats though: First, you need
to remove most the modules to access the screws. I don't foresee
removing the racks many times, but I may eat those words.
Next, Power distribution boards need separate mounting. To ease the
pain, I plan to use a single board (Synthrotek, but apparently
obsolete??) with 17 16 pin connectors, located between the two 3U
racks so the cables can reach modules in 2 racks. Finally, I
discovered that the screws that mount the rack to the wood need to
be flat heads and countersunk. Pan head screws interfered with some
Here it is new rack. The power distribution board is obscured behind
the center rails. Note the new Synthrotek Mixer. I built up
the second Dual VCO this weekend. The blank panel will be the
third VCO, someday. Even with the Dual VCO PC board 99% built,
it took about 7 hours to machine the panel and hand wire the 18
jacks, 2 switches, and 5 pots. Still have only the one
proto-board VCA, off screen. That's next.
I went a little crazy this weekend. While waiting for the Dual VCA
boards to show up, and on a rainy cold Sunday, I took apart the new
case, added the power supply, a bit of sanding, and a coat of
Then I finished the prototype sequencer, something that was on the
list for months. The mode switches that control the reset and enable
had not been connected to logic. The clock output had not been wired
up, and the control voltage out was hard-wired (hacked) to the pots,
and not buffered. All these are now addressed and I updated the
Last week I went to a synth show at "La Labratoire" near Kendall
Square. Great show with two synth performers, and one performer who
made music all from sampled sounds. On the way I stopped at Steve's
to play with his stuff. Steve loves the Turing Machine random
sequencer. I had ordered the bare board and had some of the parts.
While I was ordering the DAC0800 and other parts, I figured, why not
get another Synthrotek module, the MST Sample and Hold / Noise.. And
heck, while I'm there might just pick up the DS-M drum machine I've
been lusting after. Think of all the money I'll save in shipping
buying two mods at the same time. So now I have 3 Dual VCAs, a
Turing machine, a sample and hold and a drum machine all in process.
Yikes! Gonna be a fun thanksgiving! As they say on Muffwiggler, "I'm
not an addict, I can quit any time."
Finally the VCA boards arrived and I built up the first one. The
linear gain control works fine, but I got no output in Exponential
mode. After a bit of debug, I found that I had bought the wrong
matched PNPs. There are two versions of the DMMT3606 and I bought
the wrong one. Worse, the only matched 2N3906 in a SOT23 case has
the bases in common, not OK for an exponential source, and the
correct part only comes in a SOT363, which is the tiny package of
the PN5201s. I need to re-lay out the VCA and VCO to use this tiny
package. I hate to use such a fine-pitch part.
The initial output waveform of the VCA was a bit distorted. When 10V
p-p (+/- 5V) triangle input was applied, there is too much curve on
the slopes of the triangle (PHOTO!!). This is from overdriving the
low-voltage inputs of the LM13700 with a 25K/500 ohm 50:1 divider,
so +/- 100mV at the inputs. They are only linear to about +/- 50mV
or less. A 100:1 divider worked well.
I am getting better and quicker at building panels. With careful
setup, the table saw is able to hold about 10 mils accuracy on three
panels. Then I was able to mark one panel, drill all the holes to
1/8", then clamp all 3 panels together and using the first one as a
template, drill the other two to 1/8". Then it is a simple matter to
enlarge all the holes to 1/4", the jack and switch size, then
enlarge the Pot holes to 5/16". All 3 panels came out perfect, and
in record time.
Here is the Dual VCA schematic. On my first board I installed no
trimpots and it works well, but I have not tested the important
parameter of feed-through. When the audio input is 0, and the
control voltage changes, there should be no output glitch. This is
what R13 and R33 are for. I'll let you know.
Synthrotek MST Noise / Sample and Hold
The board and panel arrived and I built this up. It works well as a
random CV generator. No output level adjust, so your inputs may
needs a level adjust. I found that the S/H input has a low 1K input
impedance. Since most CV outs have about 1K output impedance, you
will lose about 1/2 of the CV signal and any other CV inputs that
you also send to will lose signal. There is a spare op-amp on the
board that could have been used as an input buffer, so I'm not sure
why MST did it this way. I may modify it to use the spare
opamp. The board works well as a noise source and as a
Sample and hold.
Synthrotek DS-M Drum Machine
I built this one up also. It is based loosely on the widely cloned
Coron DS-8 drum design. Pretty happy with the way it works. It uses
the trigger input and a front panel pot to determine the drum
amplitude as well as the noise time, sweep time, etc. So the trigger
input pulse width has a strong effect on the drum sound. See, I knew
it would be important to have a duty cycle control on my sequencer
clock output. It makes an acceptable tom or bass drum. I haven't
found good snare or hi-hat settings yet. Now I could really use the
Turing Machine with Pulses output. Soon...
I saw some high-frequency oscillation on the bottom of the triangle
wave output. The output amplifier of th DS-M consists of an LM13700
VCA stage feeding an LM324 with a 510K feedback resistor. The
LM13700 output adds a bit of capacitance, and so the 510K
resistor causes a 'pole' in the response and marginal
instability. I changed the LM324 opamp to a TL074 and this
caused the oscillation to go away, but the correct fix is probably
to add a 10-22pF compensating cap across the 510K resistor. Changing
all the opamps from LM324s to TL074s seems to work fine.
Turing Machine Random Sequencer
I am so impressed with the simple Turing Machine sequencer design
and the amazing range of sounds it is capable of. Consisting of a
simple 16 bit shift register, a noise generator, 2 switches, a
knob and wow! Steve uses one with the optional Voltages and Pulses
Expanders as his main sequencer. He uses Pulses to drive drum
machines. I need to get one. The 4 boards and panels are
reasonably priced on at $120 on Synthcube. Speaking of which,
Synthcube, where have you been all my life! They offer hundreds of
DIY kits, boards and panels. Yikes! I am waiting for a couple of the
main boards from Thonk for cheap, but have also ordered the full set
of 4 PC boards and 3 panels from Synthcube since I really like their
panels and want at least the Pulses Expander.
The first board arrived from Thonk. I had ordered parts for it so
was able to build the board in a couple of hours. It worked right
away. Glad I ordered the other boards and front panels.
Then the other boards and panels arived. The expander and Pulses
modules are no-brainers. Very simple and low parts cost. Voltages
uses 8 Alpha slide pots with cute LEDs, but these are only $2.50
each from Mouser.
As usual, I am the bug finder! In looking at the Turing
machine schematic. I see that the designer built an XOR gate the
hardest way possible: using two transistors and two transmission
gates. And I spied the usual bug: An op-amp operating from +/-
12V feeding CMOS running from +12V. Sure enough, the op-amp
gets very warm when outputting - voltage, due to the CMOS protection
diode. At least it doesn't latch up and die like the MFOS ADSR did.
A simple series diode and a resistor to ground addresses this.
People, please don't do this.
It is still not working 100%. I accidentally ordered the SMT
version of the 4015 shift registers. And the ancient ones I had
since the early 80's don't like driving the LEDs, so they fail after
a few hours. New parts should fix this.
Here is my baby with 2 racks full. Time for the third rack and a
second power board.
With all the new modules, my power supply is drooping a bit. The
problem appears as annoying 120Hz FM on the VCO outputs. I used a
+/- 12V adjustable linear board I bought on Ebay. I feed it with a
2x12VAC 1.5A transformer, but 12VAC is not enough to prevent droop
at currents of 0.5A. I could use a 15VAC transformer, but then I
would be dropping several volts more across the regulators. Ideally
it would be 13VAC or 14 VAC per side, not common voltages.
Time for a switcher.
I had some old DC-DC boards at work and modified an obsolete one to
convert +12V to -12V. For the +12V I use the amazingly small and
fully enclosed TDK-Lambda LS25-12, 12V at 2.1A, $20 at Digikey.
Synth Kit and DIY Building Tips
It occurs to me that non-EE synth people may struggle with building
kits. It is pretty straightforward if you have some basic tools
and supplies. Start with a simple kit such as the MST 4 Channel
Mixer to build your skills and confidence. Here are the basic tools,
which are good to have around for any electronics fun:
- A decent temperature controlled soldering iron with 0.060"
chisel tip and 0.020" or 0.025" lead solder
- Nice needle-nose pliers, diagonal cutters and wire strippers
- Nut-drivers for the front panel jack and pot nuts, typically
5/16" and a 10mm deep socket
- A bin of plastic drawers to keep parts in
- Illuminated magnifier
- SolderWick size 1 and 2
- Good fine tweezers
The kits I build use a lot of the same parts over and over, so
having a stock of the basics is good. You can probably build this
stock up for about $100:
- DIP ICs, a few of each: LM13700, TL072, TL074, LM324
- Transistors: 2N3904, 2N3906
- Resistor 1/4W 5% assortment: 5 or 10 of the common values,
check on Ebay and Amazon
- Resistor 1/4W 5% in multiples of x1.00, x2.00, x4.99, values
from 1.00K to 1.00M. Buy 50 each of the 1.00 values and 20 of
the others. You can always use a 1% in place of a 5%
- Ceramic caps, radial, 50V, 0.2" lead spacing, 100p, 1000p,
- Electrolytic caps 10uf 25V, radial, < 0.3" high
- DIP IC sockets, 8/14/16 pins
- Synthrotek or Earthenvar vertical 3.5mm jacks and hex nuts, a
bag of 50 will get you started
- 9mm knurled shaft pots, Linear taper, 50K, lots of 100K, 1M
- SPDT toggle switches, ON-ON, ON-OFF-ON
- 16mm knurled shaft pots, 100K and 1M for hand-built stuff
- 5, 6 and 10 pin single row female headers, 0.1", gold
- Break-apart male headers, 0.1", single and dual row, gold
- 10 pin Box headers for Eurorack power
- 10 and 16 pin ribbon cable connectors and 10 pin ribbon cable
if you build your own power cables.
Then when I build a kit, I go through the BOM and my stock to see
what parts are missing and do a Mouser or Digikey order for the
missing ones. As I build a board, I print the BOM and mark any
components that I am running low on. I generally keep an open
shopping cart on Digikey.com. If you go back and touch the cart
once a week or so, it seems to stay there. If you're worried about
your cart disappearing before you check out, just print out a
Of course you'll need synth cables and adapters:
- 6", 12", 18", 24", 3.5mm mono or stereo patch cables
- 2:1 adapters
- 3.5mm to 1/4" adapters or cables.
For hardware and panels:
- M2.5 x 6mm screws for Eurorack
- 4/40 x 7/16" or 11mm standoffs
For building panels:
- 6061 aluminum, 0.062"
- 100 tooth soft-metal carbide 10" blade (Amazon)
- Table saw for above
- Mill files
- 1/8" Drill bits
- 1/4" Drill bits
- 5/16" Drill bits
- Exacto Knife
- 3 mil Laminator pouches and a low cost Laminator
- DAP Contact Cement
To build SMT boards:
- Same ICs, resistors and cap values as above, but 0.050" pitch
ICs and 0805 discretes
Surface Mount (SMT) Building Tips
If you would like to get started building SMT boards, my ADSR and
VCA are probably a good place to begin. Install ExpressPCB, and then open the .zip file, and you
can click on any .SCH or .PCB file. Once you open a .PCB file, you
can order boards directly. While the boards are being fabricated,
open the BOM and order the SMT parts from Digikey or Mouser, and the
front panel parts from Earthernvar or Synthrotek.
When the boards arrive, you'll need to cut them apart with a band
saw, jig saw, or hack saw. Use a bi-metal blade for FR4 PCB
material. File the edges smooth.
To solder SMT, add a small dot of solder to only one pad of each
component footprint on the board. While melting the dot, place the
component on the and align with tweezers. Then go back and re-solder
the other pad(s). Clean up shorts or blobs with solderwick. Clean
the boards with Flux-Off or alcohol based cleaner. For good
soldering instructions, see EEVBLOG #180 on
Youtube. Dave has an excellent 3 part soldering tutorial
Blog for this project
Dave's Home Page