The SPL Passive Equalizer is a classic design comprised of passive coil filters for audio processing in recording, mixing, mastering, and Hi-Fi playback applications. The device features a total of 144 passive filters (72 per channel, 36 boost and 36 cut)) with individual single-core wound coils per filter, with each filter optimized for the frequency assigned.
This design excludes sonic degradation from mutual influences while also improving total harmonic-distortion values. The coils for the critical mid-frequency ranges are custom made, and boost and cut crossovers are designed to mesh perfectly so that the high number of frequencies one can dependably command is the most elaborate set of response curves to date. Each channel is divided into three cut and three boost bands, each offering 12 switchable frequency ranges. Makeup gain is provided by 120V makeup amplifiers based upon the company's SUPRA-OPs with a dynamic range of 150 dB and a slew rate of 200v/ms.
The Lundahl transformers are matched to the sonic characteristics of the EQ to provide balanced I/O stages. The XLR contacts are provided by Switchcraft, and the switches and potentiometers are from Elma and ALPS (including the ALPS "Big Blue" potentiometer with 41 distinct steps.) Power is supplied to the EQ via an internal linear power supply featuring a generously proportioned toroidal transformer and voltage selection (110 to 120V, 60 Hz or 220 to 240V, 50 Hz.)