D.W. Fearn VT-4 Vacuum Tube LC Equalizer

D.W. Fearn VT-4
Vacuum Tube LC Equalizer

by Barry Rudolph

With an uncompromising approach to audio processing, D.W. Fearn’s VT-4 Vacuum Tube LC EQ follows in the steps of the company’s single-channel VT-1 and 2-channel VT-2 Vacuum Tube microphone preamplifiers. In fact, much of the amplifier circuit design in this unit is based on those two mic pre’s. I feel privileged to write about the VT-4, because it’s immediately apparent that this handmade “labor of love” is in its own special class, and company owner Douglas Fearn individually inspects, tests and signs each unit.

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The VT-4 is a single-channel equalizer in a three-rackspace cabinet. Everything about the construction is first-class and built to last. The chassis is fabricated of heavy-gauge, anodized aluminum plate, and the 1/4-inch-thick front panel is finished in D.W. Fearn red to match the VT-1/VT-2 units. The large, mil-spec control knobs are from original Raytheon designs that are now manufactured by Electronic Hardware Inc. The heavy-duty AC power on/off switch is located on the back panel, away from sensitive audio lines and accidental “use” during a recording. The internal power supply fully regulates both the high voltages and DC-filament voltage. No pots are used: All controls are semisealed, silver contact-type, with rotary switches for repeatability. All audio capacitors are polystyrene or polypropylene, and resistors are 1% metal film. The inductors and input/output transformers are custom-made by Jensen Transformers.

Signal Path
The VT-4’s passive EQ circuit is surrounded by Class-A mode input and output amplifiers using Svetlana 6N1P dual triodes. Both amps are similar with single-ended triode sections coupled to cathode follower circuits. After the input transformer, the input amplifier isolates the passive equalizer circuit, whose output is then fed to the output amplifier and output transformer. The unit is designed to accept balanced or unbalanced signals and output line-level, balanced +4dBm signals. LC refers to using inductors or coils (L) and capacitors (C) in the series/parallel filter circuitry, rather than the typical RC equalizers that use resistors (R) and capacitors to keep costs and physical size down. Passive equalizers using LC networks sound noticeably more “open”
and smooth.

Getting In and Out
A large In/Out switch toggles the EQ in and out of circuit by substituting a resistive attenuator equaled to the level drop of the passive EQ circuit when all controls are set to flat. This is not a hard-wired bypass and works better, because you’ll hear just the sound of the tube input/output amplifiers sans EQ. It would be crazy to leave an EQ this good patched in Bypass mode. I did notice an occasional soft pop when I operated this switch, and since receiving my unit (serial #008), a new “make-before-break” switch was added to the newer units, eliminating all possibility of noise.

The Input Level control adjusts gain of the first amp stage in 3dB steps from -9 dB to +9 dB. For the most part, I kept this switch at unity or the center position. Fearn says: “this control is for maintaining proper headroom within the unit.” This control worked fine for adding level when I was able, by grounding pin 3 of the input XLR connector, to process -10dBv unbalanced signals. If you boost several frequencies at once, then you may have to dial back the input level to keep from overloading the output amplifier. Of course, if you wanted the sound of an overloaded tube stage, then you can crank the Input Level up and get lots of wonderful (albeit expensive) distortion. For this purpose, it would be nice to have an Output Level control, because the unit will put out up to +22 dBm.

Reprinted with permission from

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