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What is the ASF-20 ?




The Analog Spec Resonant Filter ASF-20 is a analog voltage controlled 24 dB Low Pass filter synth module with a sub-sonic bass-resonance frequency of as low as 10 Hz and with some very useful additional bells and whistles like:



- modulation generator (including Sample and Hold) for cutoff frequency

- Low frequency audio oscillator (saw wave and square wave) with glide feature

- a unique noise generator with "noise color" control (!) from crackle to waterfall



- Keyboard input with KB tracking for cutoff frequency and for audio oscillator pitch

- trigger output from the KB section, to control ADSR or other modules

- A fully functioning mono synth when combined with a ADSR/VCA module and a keyboard



- Can also be used as a fat sub-oscillator when linked to a synth







The hardware in a nut shell:


- Handmade by humans for humans

- one analog input and one analog output for audio signals (line level) compatible with common effect pedals

- massive audio analog "input overdrive" capability ( exceeding 30Vpp !)

- four analog CV inputs for filter frequency, resonance, osc pitch and noise colour

- 10 V (Volt/Octave) control voltage (CV)

- keyboard input with a very special "glide feature" and filter cutoff frequency tracking

- trigger output (5V) to feed into other modules (envelope generator, VCA etc.)

- 12 Volt DC power supply input (center positive)

- 16 knobs for "working the sound"





So ... What does the filter module sound like ?     I hear you ask ...



Here is a collection of sound files for you to download and listen to:

The sound is un-edited and in it's original raw state. No additional effects, reverb etc have been added. The sounds where created using the ASF-20 only. A square wave generator was sometimes used to make it a bit more interesting.



Sound example 01 - (27 Mbyte) just talking

Sound example 02 - (16 Mbyte) osc glide (plus square wave into audio input)

Sound example 03 - (19 Mbyte) the maschine

Sound example 04 - (31 Mbyte) mouth harp (plus square wave into frequ CV ?)

Sound example 05 - (22 Mbyte) S/H computer says no

Sound example 06 - (38 Mbyte) mod sweep (plus square wave into frequ CV)

Sound example 07 - (04 Mbyte) Ultrasound embryonic heart beat ?

Sound example 08 - (09 Mbyte) texture

Sound example 09 - (36 Mbyte) sequencer into keyboard input




resonant filter synth module



The various aspects of the Analog Spec Resonant Filter ASF20:



- The filter can be used as a very accurate oscillator, when self resonating

- it is very rich in harmonics when driven hard, due to the finely tuned component-non-linearities of the filter design

- low bass-resonance frequency of 10 Hz, excellent for kick-drum and perfect in combination with the noise generator !



The original design of the filter has been taken (unmodified) from the Midi Wave Theremin resonant filter (MWT-01) . Because we like this filter so much, we decided to issue it as a stand alone synth module. Not content with just putting a filter in a box, we wanted to make it a bit more interesting by adding more nice features and a lot of KNOBS to be creative. To push it even further, many knobs have dual functionality (left/right side from the middle position)


Filter section (24 dB LP):


- drive: adjust the total amount of audio (summed) going into the filter

- resonance: set the amount of feedback of the filter ... up to self resonance

- frequency: controls the idle cutoff frequency (without mod/CV/KB)


The modulator (mod):


- amount CV (left): adjust the mod amount that will be added to the idle filter cutoff frequency, or

- amount S/H (right): adjusts the randomly sampled amount that will be added ...

- Speed triangle wave (left): mod oscillator speed (triangle wave or S/H trigger) , or

- Speed square wave (right): mod oscillator speed (square wave or S/H trigger)


The low frequency audio oscillator (LFO):


- amount Saw (left): adjust the saw wave amount that will be added to the analog audio input signal, or

- amount Square (right): adjust the square wave amount ...

- pitch: low frequency oscillator speed from 0 to approx. 300 Hz (min = OFF) ... lowest frequency ca 0.5 Hz

- tune knob: plus/minus 12 semitones (one octave).

- glide: LFO pitch glide (min = OFF)


Color Shaping Noise generator:


- amount CV (left): adjust the noise amount that will be added to the idle filter cutoff frequency, or

- amount inp (right): adjust the noise amount that will be added to the filter audio input

- color: set noise color from nearly white noise, to pink noise, brown noise etc. (min = OFF)


CV amount knobs:

For all CV inputs, the CV can either be added (right) or subtracted (left) from the "idle position" of the respective control parameter (freq, res, pitch or color). CV for the filter cutoff frequency and for the oscillator pitch, is V/octave. For res and colour , 0V is zero, 10V is 100%


KB tracking knob:

The keyboard can be used to either control the cutoff frequency of the filter (hard left) or the pitch of the Low Frequency Oscillator (hard right) ... or, if the knob is at the 12 o'clock position, both control inputs will receive 100% of the KB control signal.


The left side of the knob will send 100% of the KB control to the filter and an increasing amount to the oscillator pitch, when turning clockwise. The same is mirrored on the right side of the knob: 100% of the KB control goes to the oscillator pitch and an increasing amount to the filter cutoff, when turning anti-clockwise.





Contact Horst on    "info  [at]  AnalogSpec.com"    for more info






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