Gyraf Audio G14-S Parallel-Passive Stereo Tube EQ

The G14-S is a passive stereo equalizer, equipped with two different topologies for makeup gain and interfacing. The filter itself consists of five bands – each with 11 switchable frequencies, continuous variable “Q”, and individual boost/bypass/cut selection. The unit has an overall output level “TRIM”, and a hard relay bypass serving as reference for your work.

The stereo channels are linked – controlled with one set of knobs, so you won’t ever have to match settings of two channels. The “Q”s, filter sharpness, are rather low when boosting, but higher at cut – approaching notch behavior at extreme cut settings. This is both a side-effect of the applied passive technology – as well as quite desirable in situations where sound integrity is the paramount factor.

Frequencies for the five bands have been selected with special attention to controllability of the midrange area, with approximately half a frequency-range overlap to the adjacent bands.

This equalizer technology is a very extended version of the classic passive inductor/capacitor type filter. Although the classic versions are very useful tools in many situations, they often feel somewhat limited due to their simple construction and coverage. Our engineers wanted – as engineers tend to do – much more of everything. So we came up with the parallel-passive design, the G14.

G14-S is based on this truly passive design – switching a set of inductors and capacitors in and out of circuit. This passive filter set is then matched with input/output interfacing for trouble-free interfacing to the outer world, and a gain stage to make up for the approximate 20dB passive loss in the filters.

For this, we give you a selection of two different technologies – at a flick of the bypass switch: When set to “IN”, you have the traditional and much-loved selection of tube makeup gain based on 6H23n (E88CC/6922) tubes combined with transformer-balanced input and output interfacing.

If this gets to be too much of a good thing, you switch the bypass to “S” – and we replace everything but the filter and controls with entirely different technology: The inputs are now integrated balanced differential receivers, amplification is handled by a high-grade opamp, and output is driven by state-of-the-art active servo balancing circuits. Two entirely different personalities in one box.

An important feature – for us – is that the original signal path of the classic G14 is fully preserved in the G14-S: Only parts relating to the solid-state path has been added, in “tube mode”, the G14S.

The inputs are transformer balanced, ca. 5KOhm "bridging" type with the filters off. This impedance is lowered for the bands you select to boost. The input connectors are standard 3-pin XLR jacks, pin2 hot, pin3 cold, and pin1 Gnd.

The "bypass" function switch bypasses the unit for reference. A relay simply disables the unit's output, and shortens the input to the output XLR's. The power indicator light will dim a bit to show that the unit is now bypassed. If you wish to use the unit for straight line gain, but with bypassed equalisation, all eq is fully turned off (hard bypassed) when the mode select switches are in their centre position.

The G14 consists of five bands with selectable frequencies, 'Q', and boost/cut. Frequency is selected by the upper row of switches, and boost, bypass, or cut is selected for each individual band by the mode switch. The sharpness, 'Q', of each filter is selected by the Q-knob 'sharp' or 'high Q' is at the clockwise direction - towards the mark 'H'. The 'Level' control controls the amount of applied boost or cut - depending on what function is selected by the mode switch.

The Output trim pot controls the signal level from the filters to the output driver stage and the output. The trim range is ca. +/-3dB, and unity gain is somewhere around 12 O’clock at the output trim pot. The output impedance of this unit is 650 Ohm, and is - like the input - floating transformer balanced.

Note that the way we implement the passive filtering has a number of side effects that should be considered in use:

First of all, the range of maximum boost is limited to some 10-12dB at each band - depending on 'Q' setting - the maximum obtainable cut is larger at extreme settings of the 'level' controls.

Second, there is no 'adding up' of adjacent bands - if you boost two bands at the same (or close) frequency, you won’t end up with double the boost/cut range. This is true for all passive equalizer topologies.

Third, because of the parallel-passive architecture, the maximum available 'Q' is somewhat higher at the upper frequencies of each band than at the lower frequencies of that given band, and is sharper in cut mode than in boost mode. This distribution of filter bandwidth gives a very intuitive control of the equalizer, but also somewhat limits the maximum available boost sharpness of the individual filters.

Fourth, the amount of boost obtainable is dependent on the output (source) impedance of the previous unit - the passive filters will load the input at high levels of boost, which limits maximum available boost when driven from high source impedances. This should, however, not affect sound quality - only the amount of Control.

• Input impedance – around 5K Ohms, transformer balanced or 24K Ohms Servo/ differential receiver balanced input
• Output impedance – ~1K Ohms, transformer balanced or ~100 Ohms, active servo balanced
• Make-up gain Amplification by 6n23p/E88CC tube stage or contemporary opamp circuit
• Max boost/cut – ~10-12dB, depending on “Q” setting
• Gain trim ~+3 – -5dB – unity gain somewhere between 12:00 and 14:00, depending on tube matching point
• Each of the five bands are individually hard-bypassable (on the “MODE” control)
• Channel tracking better than ~1dB

Frequency markings are:
• Low: 35-48-60-70-80-100-120-160-190-220-270Hz
• 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
• High: 4K5-5K6-6K5-7K8-9K1-12K-14K-16K-18K-20K-22KHz