Synthi AKS KS Expander
Pictures below were very kindly produced by Zeitdehner
Production Unit Features
Case measures 273 x 167 x 35 mm made from satin black abs.
The Synthi AKS KS Expander is a new 'Expansion' unit for the Synthi
AKS Keyboard Sequencer (KS). It could work equally with an EMS TKS
(the version of the KS EMS made as a standalone unit to be used with
a VCS3 mk2). In fact the prototype was tested using a KS connected
to my VCS3(mk2). The KS is a great feature of the Synthi AKS and
while its no shakes compared to modern hardware sequencers its what
makes the Synthi AKS arguably the most desirable of all the portable
Synthi's. What the KS offers is 3 channels of CV into the Synthi
Matrix: two pitch CV channels via Input Ch1 and the Seq Input (the
latter recordable via the sequencer the former real time via the
touch sensitive keyboard) in addition to a dynamic envelope CV via
Input Ch2 whose amplitude varies with how hard the touch keys are
What is lacking from the KS compared to the EMS DK1/2 is the additional VCO that is a great feature of these keyboards. Whilst comparing a KS with a DK1/2 is hardly fair, I thought it would be great to be able to add additional oscillator(s) to the KS, available to the matrix via a switchable option on Input ch2. This was the starting point of the project but as is often the case the project quickly develops into something bigger and one quickly thinks of other features such a "KS Expander' might have! Of course there are many voltage processing effects besides additional VCO's one can imagine adding like Sample and Hold, Slew Limiters, Inverters, Octave Shift etc etc. In order to constrain things a bit and focus the project, I decided to see what I could cram into the standard 'trapezoidal' case used for the various Synthi A(KS) interface modules I make. One can imagine much larger expander units that can interface with the Synthi's (both AKS and VCS3) via the prestopatch socket with a host of features like additional filters, oscillators, EG's etc. That is something currently in development, but I thought that a compact (if less featured) expansion unit would be attractive to those lucky enough to own a Synthi AKS.
KS EXPANDER FEATURES
Two VCO's each built around the high spec Exar XR-2206 ANALOGUE waveform generator IC. Each VCO features Sine and Triangle waves and 'wave-folding' (voltage controlled) that adds higher harmonics/distortion to each waveform for amazingly rich and varied timbres ..even unfiltered! Each VCO's frequency is controllable via CV from either the Keyboard (which is also available to the matrix via Input Ch1) or the Sequencer CV (also available via matrix row 16) of the KS or 'free running', selectable via a 3-way toggle switch. Frequency range of the VCO's is very wide, from LFO to high frequency audio.
High spec matched transistors are used (MAT02) for the exponential converters in each VCO circuit. Synch of VCO1 to VCO2 is also implemented in a unique fashion based on a clever property of the XR-2206, that of 'phase sensitive amplitude modulation'. Effectively this gives a synching of the wavefolding of VCO1 by VCO2 rather than hard synching the core oscillator in VCO1. Front panel pots control initial frequency, amount of wave-folding ('shape') and level of each VCO.
A modulating triangle waveform is available (speed controlled by the S&H clock speed pot..see below) via toggle switches, to modulate the wave-folding of VCO1 and VCO2. Furthermore VCO1 can also act as a source of Vibrato which can be applied independently to either keyboard or Sequencer pitch CV from the KS. An additional new feature allows any Synthi waveform or CV to modulate wave-folding via the special pin cable.
The mixed VCO1/2 combination is available via Input ch2 of the matrix. These additional VCO's mean that 4-oscillator (if we include Synthi OSC1/2) patches tracking chromatically over several octaves can be created for extremely fat sounds! Even 5-oscillator patches are possible if your Synthi has the hi/lo mod on OSC3 ! Or the additional VCO's can be used as LFO modulation source(s) via input ch2. The unit allows, for example, the KS keyboard to control the pitch of Synthi Osc1 and 2 via Input ch1 and at the same time play CV sequences back just through the additional Expander VCO's 1 and 2. These are just some obvious new possibilities offered by the Expander VCO's..there are many more if you experiment.
Slew Limiter (SL) with separate control over the amount of slewing applied to a rising or falling input voltage ('attack' / 'decay') by front panel pots. Slew limiters like this are very versatile processors. Input/output to the SL is via a (blue ) E.M.S. pin on the special pin cable that plugs directly into the matrix, the matrix row being the input source and matrix column being the output destination. Thus any source voltage/waveform from the matrix can be slewed before being routed to an output.
The slew rate is switchable between linear/logarithmic response. The latter allows for long slew times. Obvious applications of the SL are adding portamento (glide) to pitch CV, or to an LFO to get long attack/decay modulation envelopes, slewing of the trapezoid envelope generated by the Synthi Envelope Shaper etc. The SL also has an invert toggle switch that inverts as well as slews the input. With attack/decay levels set at zero the SL can also thus act as a standard voltage inverter.
There is also a toggle switch that disconnects input into the SL via the pin cable and instead feeds a gate pulse triggered by the KS into the SL input. The subsequent slewing of the pulse gives a basic variable attack/decay envelope (with inversion option) as a useful additional modulation source for the Synthi. This envelope is available via the tip of the blue patch pin and can be applied to any destination matrix column.
Finally there is a toggle switch that allows direct linking of the SL output to the Sample and Hold circuit which is described below.
Sample and Hold (S&H). Standard LF398 based S&H with variable clock (panel led shows rate). Input/Output is again via the pin cable (red pin) plugged anywhere into the matrix to select a source and destination.
In addition there is a toggle switch that links the output of the Slew Limiter into the input of the S&H. This effectively produces a S&H with variable slope slew/invertible input. Insertion of both the SL and S&H pins into the matrix is needed to operate in this mode, with the input source chosen by the row the blue SL pin is inserted and the output destination the column the red S&H pin is inserted into. This is a very useful combination to produce 'staircase' waveforms for modulation e.g. the filter or producing chromatic pitch steps, from e.g. pitch CV (up or down staircase using inversion) or Osc 3 triangle (up/down staircase) or joystick CV generated staircase etc.
The S&H rate pot also doubles as a frequency control for the modulating triangle wave that can be applied to VCO1 and VCO2 wave-folding, as discussed earlier.
Additional pin cable (red pin with white dot) that inverts the S&H output. Very useful feature e.g. it allows S&H of envelopes like the Envelope Shaper Trapezoid and then applies voltage inversion to the output.
Octave Shift via two front panel rotary switches giving octave shifts up to +/-3 octaves which greatly expands the pitch CV range of the KS. Can be applied independently to either Keyboard CV or Sequencer CV from the KS. A toggle switch allows slewing of either the KS Keyboard or Sequencer voltages in addition to octave shifts. This allows portamento to be easily applied to sequences in real time as well as fast octave shifting.
Inverter available via the white EMS pin cable. Straightforward operation of inserting the pin into the Synthi matrix. The voltage source from the matrix row where the pin is inserted is inverted before being routed to the destination column. Given that the Slew Limiter can also act as an inverter (see above), 2 inverters are available for use at any one time.
Interfacing with the Synthi AKS
The Expander interfaces with the Synthi AKS via the male/female
8-pin Jones connectors on the main Synthi unit and KS, and the
special EMS pin cable. The standard connecting cable is removed
between the KS and Synthi and the units own cables are used instead.
In the prototype unit all power was derived from the existing Synthi
internal psu. This is made possible through careful design and the
use of low-current JFET opamps to minimize the total current draw.
Soak testing for 24 hours continuous use and various other tests
show the psu in my VCS3(mk2) is beefy enough to power a KS and the
Expander without being over stretched (which would manifest itself
as voltage droop on the Synthi's main +12v,-9v power rails).
Nevertheless, since the Synthi psu (even the mk2) is not a great
design and also taking into account that there could be variations
between Synthi AKS units out there in how good their psu's are, I
have added the option to bypass the Synthi +12V power rail and use
instead a simple 12v AC-DC wall wart adapter (300ma maximum current
type being more than ample). An internal regulator in the Expander
gives stable +12V and provides current on the postive rail, with
current drain from the the -ve rail still being derived from the
Synthi. Since the Expander draws little current from the -9v power
rail, the mk2 power supply that is in all Synthi AKS's is not being
put under any strain.
Click on the above image to download a pdf of the userguide. This guide is work in progress and is not yet complete, but it will hopefully give you a good idea of the
many features available on the KS Expander.
Click on the image below to download a pdf version
A version for use with a VCS3 under DK1/2 control
I also plan to release a version of the Expander to be used with a
VCS3 (mk1 or mk2) and DK1/2 keyboard. It would be built into an
Afrormosia wooden case with VCS3 styling. This requires various
modifications to the current Expander design as the DK oscillator
appears to the matrix on Input ch1 and normal pitch CV on Input ch2.
The 'DK' version of the Expander would mix VCO1/2 with the DK
Oscillator on Input ch1. This could give you up to 5 oscillators
under DK control. The other features would work as in the KS
More information will be posted once a prototype has been
Audio Demo MP3 files
VCO and S&H demos
The demos below used an external KS clone unit and a VCS3(mk2) which is the complete equivalent setup as a Synthi AKS. Some reverb is added as the only effect.
New audio demos from Constantin
Constantin Papageorgiadis is well known in the E.M.S./ Synthi user
community as an avid fan of all things Synthi, an owner of both a
Synthi AKS and a Synthi A which he uses to great effect in his live
music sets, recordings etc. His Synthi blog
is well worth a visit for great ideas on implementing modifications
to the Synthi. Constantin has been beta-testing a KS Expander
production unit for several weeks now alongside his Synthi AKS and
has created a few demos:-
designing the KS Expander I have been inspired by circuit designs
from Harry Bissell and particularly Thomas Henry in his VCO circuits
based on the XR-2206 IC. The circuits used in my designs are not
direct copies and are modified to interface with Synthi waveforms
and 'odd' (+12v,-9V) supply voltages as well as featuring novel
design elements such as VCO synching via wave-folding described
above. Also many thanks to Constantin Papageorgiadis for his testing
of the original production unit and for his creative suggestions on
improving aspects of the unit.
are custom made to order and so there will be a turnaround time.
Contact me to be placed on a list.