Gyraf Audio Gyratec XIV:
Parallel-Passive Stereo Tube Equalizer...
The G14 is a true tube, all passive, stereo equalizer. It consist of five bands - each with 11 switchable frequencies, variable "Q", and boost/cut selection. It also has an output level trim, and a "hard" relay bypass control.
The two channels are linked - controlled with one set of knobs. The "Q"s (filter sharpnes'es) are rather low when boosting, but higher at cut - approaching a notch on extreme cut settings. This is both a side-effect of the applied passive technology - as well as quite desireable in situations where sound integrity is the paramount factor.
The frequencies are selected with special attention to controlability of the midrange area, and with half an frequency-range overlap to the adjacent bands.
This equalizer is a very extended version of the classic passive inductor/capacitor type filters, the most famous being the "Pultec" units. Even though the classic versions are usefull tools in most situations, they are somewhat limited in use due to their simple construction. Our engeineers wanted - as engeineers tend to do - more of everything.
So we came up with the parallel-passive design, the G14.
It is a truely passive design - based on switching a set of inductors and capacitors in and out of circuit - combined with an E88CC (6DJ8) tube-based makeup gain/output stage. Not a single IC's or semiconductor in the signal path - this is a real tube unit.
The inputs and outputs are - off course - transformer balanced, for trouble-free interfacing to the outer world.
Input impedance - around 5K Ohms, transformer balanced
Output impedance - ~1K Ohms, transformer balanced
Max boost/cut - ~10-12dB, depending on "Q" setting
Gain trim ~+6 - -20dB
Each band individually hard-bypassable
Channel tracking within ~1dB
Frequency markings are:
Low mid: 180-230-280-330-400-430-490-550-590-650-750Hz
Mid: 500-570-700-800-900-1K-1K1-1K3-1K5-1K8-2K1 Hz
High mid: 950-1K2-1K4-1K7-2K-2K5-3K-3K5-4K2-5K-6K3 Hz