So, some 70 years after its creation, the Baxandall EQ effectively gets a definitive update. Ultimately, Heritage Audio has successfully replaced the Baxandall shelving EQ topology to which it turned for inspiration with a functional equivalent — much more musical in its implementation as a single rack space-occupying master bus-/mastering-orientated unit fit for vintage sound-seeking 21st Century studios: STEREO ASYMPTOTIC EQUALIZER, affectionately known as SYMPH EQ, takes its name from the shape obtained by its shelving curves (where the boosted or cut amplitude increases until the asymptotic value is reached, so sub lows and ultra highs are always controlled). After all, an asymptotic line is a line that gets closer and closer to a curve as the distance gets closer to infinity. In the case of the SYMPH EQ, its sound can only be described as tight, sweet, and natural, while its use of parallel equalisation to obtain the desired response curves results in exceptionally low phase deviation and also aids audio artefact reduction.
Since shelving with surgical yet musical precision is really what the SYMPH EQ is all about, a smart circuit layout allows for a maximum boost/cut of 10.5 dB in 0.5 dB steps; the main rotary switch sets 1 dB steps whilst the +0.5 button adds half a dB to the main reading, while the CUT button turns the boost amount into cut. Six strategically-selected frequency choices per band are available — 8, 10, 12, 15, 20, and 24 KHz on the high shelf, and 470, 360, 220, 110, 60, and 20 Hz on the low shelf.
Above and beyond that, there are also high-pass and low-pass filters available on dual concentric rotary switches at 12dB/octave in order to keep the sub-low and ultra-high frequency content under control. Five frequency choices — 10, 12, 18, 22, and 30 KHz on the low-pass, and 160, 82, 47, 20, and 15 Hz on the high-pass — plus OFF are available on each one.