The Synthi AKS KS Expander is an expansion add-on for the Synthi AKS Keyboard Sequencer (KS) that is under development.

The Synthi AKS KS 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 unit’s own cables are used instead.

Pricing is TBD. Technical details below.

KS Expander Features:

Two VCO’s each built around the high spec Exar XR-2206 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 wavefolding (‘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. 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 fat sounds. Even 5-oscillator patches are possible if your Synthi has the hi/lo mod on OSC3. The additional VCO’s can be used as LFO modulation source(s) via input ch2. The unit allows, eg, 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.

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 pin) cable pluggable 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. Two bi-coloured leds monitor the attack/decay voltage levels of the SL with colour change triggered by voltage inversions.

Sample and Hold (S&H). Standard LF398 based S&H with variable clock (panel led shows rate). Input/Output is again via a 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 comination to produce ‘staircase’ waveforms for modulation eg the filter or producing chromatic pitch steps, from eg pitch CV (up or down staircase using inversion) or OSC 3 triangle (up/down staircase) or joystick CV generated stairacse 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 eg 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.