Heritage Audio has successfully replaced the Baxandall topology with a functional equivalent, much more musical one, and has implemented it in a one rack space, master bus/ mastering type unit. The Stereo Asymptotic Equalizer, affectionately known as “SYMPH”, takes its name from the shape obtained by its shelf curves, where the boost or cut amplitude grows up until the asymptotic value is reached, keeping the ultra highs and sub lows always controlled.
Its sound can only be described as tight, sweet and natural, and its use of parallel equalizing to obtain the desired response curves makes it to have exceptional low phase deviation and artifacts.
Baxandall equalizers topology, introduced to the public by Peter Baxandall in 1952, was a very ingenious way of obtaining high and low shelves with boost and cut option without using expensive rotary switches, as it was common at the time. This affordable circuit was soon adopted by hi fi and console manufacturers for its simplicity and low component count, albeit for more high quality equalizing it has always been necessary to rely on inductor, Pulteq like topologies or more elaborate parametric options.
High and Low shelf bands with surgical but musical precision
A smart circuit layout allows for a maximum boost / cut of 10.5 dB in 0.5 dB steps. The main rotary switch sets 1dB steps whilst the 0.5 button adds half a dB to the main reading. The CUT button turns the boost amount into the same but cut. Six strategically selected frequency choices per band are available, being 8, 10, 12, 15, 20 and 24 Khz on the high shelf and 470, 360, 220, 110, 60 and 20 Hz on the low one.
2 pole high and low pass filters
In order to keep the sub low and ultra high frequency content well under control, further high and low pass filters have been added on dual concentric rotary switches, at 12dB/octave. 5 frequency choices plus OFF have been implemented on each one, being 10, 12, 18, 22 and 30 Khz on the low pass, and 160, 82, 47, 20 and 15 Hz on the high one.
This is where the magic lives
The Symph EQ would have been an awesome sculpting tool just as it has been described above, but it turns into a monster piece with the following 3 additions:
It features the same exclusive Carnhill input transformers made in Oxford found in the Successor.
Its outputs are driven ( as its common in all HA products) by 73 type class A output amplifiers, using the same Carnhill St´Ives output transformers as the rest of the family.
Most importantly and, here is where the Symph EQ really shines, it has built in Mid Side (M/S) capabilities