Soma is a clear departure from the old "passive EQ:s are for general shaping only" paradigm. It is the only commercially available passive EQ with real Q adjustment, which makes it more usable and accurate than other products which use only a resistor based Q adjustment which affects boost/cut amount and does not offer a wide range.
It is not surprising that this type of ?real Q adjustment? passive EQs have not existed before Knif Soma. When passive circuits reigned the EQ world there was no feasible technology available for the rather complex requirements. And when engineers started to require more control, active circuits were already there and thus passive designs were abandoned until it was realized that they offer superb sonic results. It was taken for granted that they have their limitations and were used accordingly, until now.
Well implemented real Q adjustment in passive EQs requires logic circuits and a lot of subminiature relays (around 140 pieces in Soma). Signal path in Soma is short, because frequency and Q adjustment (switching different capacitors and inductances) is executed locally and signal is taken to the front panel for gain control only.
Because reliability and long time ease of repair is important, there are no programmable IC:s or microcontrollers. Only basic CMOS logic (in IC sockets) is used. Relays are rated for 100 million operations and thus it is likely that not a single one will ever break. Replacing logic circuits does not require soldering (if they actually break some day).
MS-matrix in Soma is passive, transformer based, and does not add one single component in the signal path and thus does not degrade sonic performance. Input and output transformers have multiple windings which are switched locally, with relays, to perform sum and difference operations.
Filter section is very simple. It should be mentioned that there is the possibility to switch S-channel high pass for 6dB/oct for ?elliptical? bass equalizing.
Filter caps are mostly polypropylene. Only in the lowest range it was necessary to use some polyester ones. Coils (enclosed in mu-metal) are hand wound by J. Knif and have maximum amount of taps to make the Q-adjustment possible. Make up gain amplifier is a simple two-stage affair with moderate amount of DC-coupled feedback, with tubes of course. Individual discrete regulators for anode supplies, mu-shielded signal transformers, all teflon/copper/silver wiring, no connectors on signal path, only solder joints, the list would go on and on. Read the paper if interested in more tech babble and info.
Objective performance is flawless, Soma is a mastering quality product in every respect. As an icing on the cake all knobs are from Elma's ?soft touch? collet knob range, offering substantially warmer feel and better grip than normal hard plastic knobs.
Note that front panel markings for gain are for every other position, switches are 23-position Elma rotary switches . Frequency (16 positions per range) and Q-value selection is done with binary logic switches, also from Elma.
There are no potentiometers. This is a product for mastering and made to last.
You can read a "paper" about quality factors in Soma and other Knif gear