LOWPASS FILTER (6 POLE MODE),
SELF-OSCILLATION MODE, MODULATED by LFO/SAW
OUTPUT (Volts)
5.0
2.5
0.0
-2.5
-5.0
-7.5
100 200 300 400 500 600
Time (milliseconds)
7.5
LPF is characterized by a non-linear transfer function, so-called saturation mode that is affected by the
level of the incoming signal. In the audio mixer the red markings on the potentiometer scales show
the level at which the filter operates in the non-linear mode (saturation mode). The illustration below
shows the saturation effect.
5.0
2.5
0.0
-2.5
-5.0
-7.5
3.00 4.00 5.00 6.00 7.00 8.00
7.5
OUTPUT (Volts)
LOWPASS FILTER, SATURATION EFFECT
Time (milliseconds)
LINEAR MODE (SQR WAVE in, 3Vp-p)
EXTREME SATURATED MODE (SQR WAVE in, EXT AUDIO INPUT)
LOWPASS FILTER, SATURATION EFFECT
OUTPUT (Volts)
5.0
2.5
0.0
-2.5
-5.0
-7.5
3.00 4.00 5.00 6.00 7.00 8.00
Time (milliseconds)
7.5
LINEAR MODE (SQR WAVE in, 3Vp-p)
SATURATED MODE (SQR WAVE in, 6Vp-p)
A complicated saturation effect can be seen, amplitude, amount of resonance, and frequency corner
are affected and altered by the input signal strength.
Spectrum analysis in the illustration below shows that the filter effectively suppresses the square wave
overtones but also adds even harmonic overtones (the most dominant being the 2nd overtone – 2000
Hz). This brings to mind certain characteristics in electron tube amplifiers that from a musical point of
view can be advantageous. It is well known that even overtones often add a musical timbre and
electron tube amplifiers are valued for this, among other things.
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