Mad Science Lab

SSM 2040 Voltage Controlled Filter Manual


Mad Science Labs

SSM 2040 VCF with LFO


The SSM 2040 is an analog filter chip used in analog synthesizers. A filter can cutoff part of the frequency range of the audio that goes into the IN A and IN B jacks. This can alter the audio in many different ways. The filter can resonate at this frequency and it can also completely self oscillate. This unit is based on a design by Dave Rossum of Emu fame, and he is the one that designed the chip as well. He originally helped Sequential in the design of the first 2 versions of the Sequential Circuits Prophet 5. This SSM 2040 VCF is widely acknowledged as one of the best filter chips ever put in an analog synthesizer.

Back to Main SSM 2040 VCF page



The IN A and IN B jacks can accept signals like a CD player, drum machine or synth or any signal via a mixer.

Initially always set the VOL A and VOL B pots all the way down. Then adjust them upwards as needed. A CD input would sound good at 12 o"clock (straight up). You can use a RCA to 1/4" adapter to run any CD input into to the filter.

Each IN jack has a volume knob that is associated with it. Plug what you want to filter into these jacks and go from the Out jack to your mixer.

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. Basically the unit is designed to operate best at professional levels turned down to about 9 o' clock on the Inputs IN A and IN B. At this level you have the option to overdrive A or B input by cranking the VOL A or VOL B pot fully CW.

I really like the way hi hats and snare drums sound through this unit. Crank the Volume pot to upper settings and it makes hi hats really cut through a mix. Add resonance to taste.

I refer to knob positions using the hour hand of a clock. I also use

CCW to represent counter clockwise , fully turned down for the pot.

CW to represent clockwise , fully turned up for the pot.


Here is a good initial setting. The audio out sounds exactly like the original. No filtering is happening because the Cutoff knob is fully CW so all frequencies are passing. Both LFO's are off.



The Cutoff knob controls the frequency cutoff of the filter and resonance controls the resonance of the signal. These are the two main controls of the filter. The other knobs add modulation to the filter.


This is the frequency where the filter starts to cutoff hi frequencies. (or where the Low frequencies pass). If you put a full frequency signal in the hi frequencies can get filtered out at varying amounts . This cutoff can also resonant at that point selected by the cutoff and this amount is set by the Resonance knob.


!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 and can easily reach frequencies above levels that humans can hear. 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 LFO generates two different waveforms a triangle and/or a square wave. These waveforms can modulate the filter cutoff. The triangle and square waves can be used alone or together. The two switches turn the waveforms on or off. If you do not want the LFO to be varying the voltage and want to have full manual control turn both of these off. (down position) 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. Keep in mind that if the Cutoff or Initial Frequency knob is set to upper positions the LFO can not increase the filter frequency. So you need to set these 2 knobs to lower positions to hear the effects of the LFO. Usually it is best to set the Initial Frequency to fully CCW or turned all the way off. Then adjust the Cutoff and CV B/LFO VOL B knob to hear the many possibilities of alteration.

Ways to use the LFO

If you crank the CV B/LFO VOL B knob all the way up (fully CW) and the LFO Rate Knob all the way up you can mutilate sounds. Turn on one or both waveforms and turn the LFO knob up and down chop up signal. Vary

Using a square wave with a low Cutoff setting and you can get gating of the input signal. To do this set the Cutoff and Initial Frequency knobs fully CCW, then set the CV B/LFO VOL B knob all the way up (fully CW). The LFO Rate determines the speed of the gating. Make sure the Square waveform is turned on.

The CV B/ LFO jack can be used to send this LFO voltage to other units. It also creates a hearable audio that is very low frequency. Plug a cable from the CV B into IN A. The LFO Rate determines the frequency and past 9 'o clock you can hear the audio when one or both of the LFO waveforms is turned on.








The control voltages are used in analog synths to modulate or alter a setting. The voltage in this case is always low level DC voltage below 15 volts. Only use these jacks when interfacing with other synthesizer modules. The filter cutoff frequency has 2 CV jacks that can be used in a variety of ways.


If you are not sending CV or receiving CV always set the VOL A pot under the CV A jack to fully CW, or turned fully up. This insures the best performance of the cutoff pot.

The CV A and VOL A jack and knob can be used in 2 ways.

1. The CV A jack can send a control voltage of 1 to 15 volts to control another analog synthesizer. The voltage sent will match the voltage of the SSM 2040 filter cutoff frequency.

2. You can send voltage from an external LFO into this jack to control the Cutoff of the filter.


This jack can output the LFO for use in other Synthesizers, or can also operate as an input for control voltages as in #2 above.







Above is the circuit board for the SSM 2040. The unit is based on the board above which contains everything except the LFO. The SSM 2040 VCF chip is the longer one in the middle.




Additional Info:

The SSM chip has a tempco resistor installed on the top of it to make it more stable. A tempco resistor is a temperature compensating resistor. This helps the accuracy of the filter. This is a common way to stabilize oscillators when exact precision and stability is needed and was a technique used by all the great analog synthesizer companies. See picture below

This is a great patch to use with drums that have nice high end like loud high hats or snares. I use this patch to run the snare and hi hat from my X Base 09 drum machine into VOL A and the kick into Vol B.
Below is a blank patch sheet.  

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