Mad Science Lab

SSM 2040 Voltage Controlled Filter Unit with LFO

SSM 2040 VCF chip

used in early Sequential Circuits Prophet 5 and EMU analog synths

This product combines a classic 4 pole voltage controlled filter and a low frequency oscillator in a stand alone unit.

Fully resonates and can get quite nasty but it is a very low noise unit.

Beech wood sides with a cherry stain add to the classic looks of this unit.


This is serial #2003- 03 as are all the pictures on this page

This is an excellent filter with exceptional sound quality and is very musical. I chose this design from several different chip designs. This design is based around the SSM 2040 VCF chip designed by Dave Rossum and used in early versions of the Prophet 5 and some EMU analog products like the SSM evaluation board.

A look from the other side.

Quality switchcraft jacks are used

On this model the cutoff and resonance pots are high quality Bourns sealed pots and the knobs are the expensive alcoknob brand

A little green light indicates the power is on

The top row contains 2 separate inputs and volumes for mixing signals going to the filter.

There is also 2 control voltage inputs with volumes. CV A can also output the cutoff voltage, so you can control other analog devices.

CV B outputs the LFO voltage and the Vol B pot controls the amplitude of the modulation for the LFO waveforms.

Cutoff controls the frequency cutoff of the filter and resonance controls the resonance of the signal.

The LFO section is on the right. 2 switches turn on or off a triangle wave and/or a square wave. The LFO rate determines the frequency of the LFO from 1 cycle every 60 seconds to the audible range

Some design notes:


I made the 2 inputs so they have a little gain so that low volume inputs like a CD can be used and still produce a professional level output. Also these inputs can be overdriven to produce some subtle over drive. This works great with drum machines. Not a harsh distortion, very subtle. This means with a 4db signal going in you do not want the volumes all the way up, unless you want the unit to lightly distort the signal, a setting of about 3 o clock should be perfect. I spent a lot of time making this a useful feature that I like a lot.



!Almost dangerous levels of resonance possible, use restraint at upper settings!

The setting in the picture above shows the resonance at about 3 o' clock. Over this and things can get nasty depending on the Cutoff setting.. It is a very musical especially with hi hats. It turns them all crunchy. See mp3 files below for examples. This means you need to be careful because full resonation at high volume levels can be painful. Use with caution. I have included a mp3 file where I had nothing plugged into the unit and just used this feature to make some crazy noises.


Initial Frequency:

This may confuse people, and I actually left it off earlier units until I discovered how cool this feature is. Basically this allows you to set the lowest the filter cutoff frequency will go. Here is one way to use it. Play a sound into the unit, a drum machine is a good use, then set the Cutoff all the way down and adjust the Initial Frequency to the lowest you want the filter to go. For example set Initial Frequency to about 9 or 10 o clock so that the hi hats are cut but not most of the snare and the kick is still full volume (high frequencies are getting cut while the low frequencies are passed through). Now you can fiddle with the Cutoff cranking it all the way down for breakdowns (no hats) and then cranking it full to bring in the full sound (unfiltered, with hi hats). This makes it easy to always get to a specific setting when you crank the Cutoff all the way down and is very useful musically. Also great for live use



The low frequency oscillator can modulate the filter cutoff. The triangle and square waves can be used alone or together. The CV B/ LFO jack can be used to send this LFO voltage to other units. The VOL B pot on the right controls how much you want the LFO to modulate the frequency cutoff. A low setting will be barely audible while a high setting will cause the frequency to vary greatly. If you crank the VOL B all the way up and the LFO Rate all the way up you can mutilate sounds. Using a square wave with a low Cutoff setting and you can get gating of the input signal.







MP3 DEMO files (no effects used, except the filer)

SSM 2040 demo 1 - a hi hat and snare line being filtered. No filter at 1st, then slow moving cutoff, then LFO cranked up to show how it can mess up sounds

SSM 2040 demo 2 - A rev 3 Prophet 5 playing a single note held down , no filter at first then varying resonance and cutoff, then add LFO with lots of modulation, then just a little LFO

SSM 2040 demo 3 - a hi hat line being filtered with various amount of resonance

SSM 2040 demo 4 - CD input of a steady beat being crunched up and gated, no filter at 1st shows the beat that repeats the whole way, but is filtered in various ways

SSM 2040 demo 5 - no inputs, just the unit self oscillating and being modified by the LFO for whacky sound effects. Slow LFO at first then fast near end. This is a fully resonating filter









The back side, power switch above the standard 3 prong power jack.

Here are the boards in the unit unwired. A high quality Power One power supply is used that is fully tested by the manufacturer.

A sturdy metal case is used to house the unit.

The PCB boards were made by PCB Express and all parts are hand soldered by me.

The SSM VCF chip is new old stock NOS meaning it has never been used or pulled from an existing product, like a dead synth.

More info: Serial # 2003-03 Inside the monster


More info: SSM 2040 VCF History and past versions

SSM 2040 Manual page


questions, comments....-->> e-mail