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Mad
Science Labs
SSM
2040 VCF with LFO
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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.
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Back to Main SSM 2040 VCF
page
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The IN A and IN
B jacks can accept signals like a CD player, drum machine or synth or
any signal via a mixer.
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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.
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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.
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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. |
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MAIN CONTROLS
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.
Cutoff
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.
Resonance:
!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.
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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
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LOW FREQUENCY
OSCILLATOR -LFO
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LFO:
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.
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CONTROL VOLTAGES
- CV
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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.
CV A:
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.
CV B:
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.
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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.
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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
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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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