Gyraf Gyratec XIVS (G14) – Parallel-Passive Stereo Tube Equalizer

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 a reference for your work.

The stereo channels are linked – controlled with one set of knobs, so you won’t ever have to match the 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 the 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

• 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 is 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

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