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 The SF-2
Super Filter contains two tunable active
filters of exceptional dynamic range. The filters can be configured individually as
low-pass, band-pass, and high-pass. In the stereo mode, two channels are provided, each
with a two channel mixer for combining the filtered and unfiltered signals. In an alternate
mono mode, a three channel mixer combines the two filtered and one unfiltered signal paths,
together with an input gain control. An instrument preamplifier with front panel input jack
is also available for stand-alone use. |
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Circuit detail
The SF-2 filter circuit is a resistively tuned two-pole universal active filters, which
yields 12 dB/Oct ultimate slopes in the high-pass and low-pass modes, and 6 dB/Oct ultimate
slopes in the band-pass mode. The unconventional filter design was optimized for wide dynamic
range, and achieves 20 dB better signal to noise performance compared with standard circuits.
This increased dynamic range makes use practical with guitar level signals as well as line
level signals. Additionally, the absolute polarity of the signals in the pass band of the filter
in all three modes of operation is preserved, allowing the additive mixing of the unfiltered
original signal with the filtered output without phase cancellation. A control calibrated
in Reciprocal Damping Ratio units adjusts the response of the filter in the vicinity of
cutoff, controlling the size of the resonant peak at the tuned frequency.
In the stereo mode, each channel is provided with a
two channel mixer. The direct, or unfiltered, signal is adjustable from zero level to unity
gain. The filtered signal is adjustable from zero to a gain of 3 times or 10 dB. The additional
gain in the filtered section is provided for those settings when the filter is used to bring out softer
spectral components of the signal. As mentioned above, in the passband of the filter, the
polarity is preserved so that in combination with the direct signal, the resultant will always be
additive, no matter whether in low- band- or high-pass mode. The amplitude of the filtered signal
in the passband of the filter is also held constant when the damping ratio control is
adjusted. The result is an easy to adjust unit, each control logically responding to its
designated function.
In the optional mono mode, the output is derived from a
three channel mixer, one channel for the direct or unfiltered signal, and a channel for
each of the two filter circuits. As in the previous paragraph, the direct signal adjusts to
a maximum of unity gain, and each filter can be mixed up to three times (10 dB) gain, needed when
enhancing weak spectral components. The left-most control on the front panel serves as an input
gain control (or master gain control) and provides unity gain when fully clockwise when the
signal is connected to the mono line input jack on the rear panel.
An instrument preamplifier with 1 MegOhm input impedance
is provided with an input jack on the front panel. The instrument preamplifier is automatically
connected thru 'normal' springs when no jack is inserted in the line level input on the
rear panel. A jumper block inside the unit provides signal amplification of 0 dB, 10 dB, and 20 dB
for the preamplifier circuit by selection of feedback resistors. The unit is set for 10 dB
amplification when shipped from the factory.
The Super Filter has many applications. As an effector for
guitar and bass, an unlimited field of both dramatic and subtle spectral effects are available.
And when the direct volume control is set fully counterclockwise, the filtered signal alone directs
the ear's attention to the natural-sounding filter effect, unlike an equalizer where the input
signal is always supplied to the output at unity gain.
Interesting vocal effects including band limiting can be
dialed in. And the opposite of the last effect can be used to extend the frequency response
of closed box speaker enclosures. The Super Filter is a very useful tool.
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