Andromeda A6 - Alesis Part 4 - Filters, the post filter mixer.

This is part of an earlier series of posts on the Andromeda Synth.

The earlier posts are here:

+ Andromeda A6 - Part 1 - Basic intro

+ Andromeda A6 - Part 2 - Ribbon Controller & Modulation Wheel

+ Andromeda A6 - Alesis Part 3 - Oscillators, FM, & Pre-filter mixer

 

The A6 has two filters:

Filter 1 is a 12db multimode 2-pole.

It was designed to emulate the Oberheim® Synthesizer Expander Module™ (SEM) which was introduced in the mid 1970’s. It has four fully adjustable modes: low pass, high pass, band pass and notch, just like the original SEM

 

Filter 2 is a 24db 4-pole lowpass. It was designed to emulate the Moog® modular synthesizers introduced in the late 1960’s.

Each filter has four pot controls, 3 mod inputs and a view button.

The pots set the Cutoff frequency, Resonance, ENV 2 and Key tracking.

Key tracking will vary the filter frequency across the keyboard.

 

 

 

 

 

 

 

 

 

 

Notice that the two filters can be connected to each other. 

There are 4 settings: Off, Mix, Notch & BP.

 

Off

FILTER 2 receives no input (but it can generate tones on its own when in full resonance).

 

Mix

When filter 2 input is set to mix, both filters are working in parallel.

That is, both filters are independent.

Thus the LP,BP, HP of filter 1 & the LP of filter 2 are all available at the same time

 

Notch

FILTER 2 will filter the output of FILTER 1’s notch filter(Low Pass and High Pass).

 

BP

FILTER 2 will filter the output of FILTER 1’s band pass filter

 

The Post Filter Mix

Each of the filter outputs has it's own filter level input pot.

They each also have mod inputs  

 

Notice the BP invert and the extra Pre-filter inputs -

 the sines from OSC 1 & 2 and the ringmod.

BP invert will change the BP filter into a fixed width

band reject filter. 

Thus the Filter Cutoff frequency control will raise or lower 

the centre of this band.

 

 

 

 

 

 

Filter 2 self oscillation

 It's a classic Moog style filter so can self oscillate. (Filter 1 can't).

 FILTER 2 receives no input but it can generate tones on its own when in full resonance.

Set key track to 11O'clock and turn all outputs from filter 1 to off.

Resonance to full clockwise. 

The sound is a simple sinewave.

Press any key on the keyboard.

Filter Feedback

Filter feedback is a popular "trick" of many synths.

You plug the output of your headphone jack into the filter input.

This overloads the filter & gives a gritty, nasty sound 

The button is in the pre-mix section.

The noise level adjustment is the filter feedback.