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History of the VT-I/F Vacuum Tube Instrument Interface

As far back as I can remember, taking instruments “direct” has never sounded very good to me. There always seemed to be a lack of dynamics, and a sterile qual- ity to sounds recorded with a direct box (or DI).
Frankly, initially I was not very excited about designing the VT-I/F. I assumed that the lifeless sound I was accustomed to was the nature of direct sounds.
Before beginning the design of the circuit, I spoke to a number of engineers, producers, and musicians about what they felt was lacking in DI boxes. Almost without exception, they all said, “It’s got to have tons of headroom.” How much headroom was enough? I spoke to a number of instrument pickup manufacturers and got an idea of the peak output level of a variety of instruments. These figures were confirmed with an oscilloscope placed directly across the output of various electric guitars, basses, pianos, synthesizers, etc.

The first design goal was to accommodate the full dynamic range of sources likely to be connected to the VT-I/F. Secondly, the design had to be quiet. After that, it was just a matter of designing it to have the type of performance and pack- aging that audio professionals have come to expect from our VT-1/VT-2 Vacuum Tube Microphone Preamplifiers.

The decision was made early on that the output of the VT-I/F would be at micro- phone level. Although a line-level output is not difficult to design, it would increase the cost. Besides, everyone has mic preamps available. Although the VT- I/F will work with virtually any mic preamp, it was designed to complement the VT-1/VT-2 series of preamps.

We tried the first prototype on a solid-body electric guitar, and compared the sound to several other respected DI boxes. We were astonished at first listen! It sounded very close to the sound of the guitar through a good vacuum tube amp.
This prototype was evaluated by a number of studio friends, who made some useful suggestions. These suggestions were incorporated into the second proto- type, and the VT-I/F design was complete. By the way, our evaluators were very, very reluctant to return the prototype.

I have seen how direct boxes can be abused in the studio environment, so the VT-I/F was built to take rough treatment. The case is machined from solid quar- ter-inch thick aluminum plate, and finished with a tough polyurethane aircraft fin- ish.
Why does the VT-I/F sound so good? I’m not entirely sure, but I do know that it provides the proper load to the instrument. This is vital for an unrestricted sound. The frequency response is flat from 10 cps to 20 kc, with -3 dB points at 0.5 cps and 90 kc. The VT-I/F circuit is very similar to the circuit of the VT-1/VT-2, with a different input design. The output is identical to the VT-1/VT-2 but operates at a lower level. The power supply is virtually identical to that used in the VT-1/VT-2.

The VT-I/F has been used on electric and acoustic (with a pickup) guitars, elec- tric and acoustic (with a pickup) bass, electric pianos, synthesizers, samplers, etc. and it sounds great on all of them. It will not overload on any instrument, although when driven hard, the sound becomes fatter. It has enough gain, and it’s quiet enough, for use with very low level instruments, like finger-picked acoustic gui- tar.
The lifeless, restricted sound I thought was part of direct recording is gone. The VT-I/F has depth, fullness, dynamics, and excitement while remaining quiet and under control with any instrument.

Douglas W. Fearn
November 1994
  • Model: VT-IF
  • Manufactured by: D.W. FEARN