SF-2 picThe 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.

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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