D.W. Fearn VT-I-F User Manual

VT-I/F

Vacuum Tube

Instrument Interface

D.W

. FEARN

Operating Instructions

How to Contact us:

Telephone: [phone]

Fax: [phone]

Mail: P.O. Box 57, Pocopson, PA 19366 U.S.A.

Shipping Address:

West Chester, PA 19382 U.S.A.

e-mail: [e-mail]

World Wide Web: http://www.dwfearn.com

182 Bragg Hill Road

Table of Contents

History of the VT-I/F .............................................................................. 7

Specifications.......................................................................................... 9

Description..............................................................................................11

Installation ..............................................................................................13

Operation ................................................................................................17

Theory.....................................................................................................21

Maintenance............................................................................................23

List of Illustrations

Rear Panel Controls and Connections ....................................................14

Front Panel Controls and Connections ...................................................15

Block Diagram........................................................................................21

Notice

D.W. Fearn shall not be liable for technical or editorial errors or omissions in this manual, nor

for incidental or consequential damages resulting from the use of this material.

This instruction manual contains information protected by copyright. No part of this guide

may be photocopied or reproduced in any form without prior written consent from D.W. Fearn.

Copyright ©1995 D.W. Fearn & Associates

LIMITED 5-YEAR WARRANTY

During the warranty period, D.W. Fearn will, at no additional charge, repair or replace defective parts

with new parts.

This warranty does not extend to any VT-I/F that has been damaged or rendered defective as a result

of accident, misuse, or abuse; by the use of parts not manufactured or supplied by D.W. Fearn; or by

unauthorized modification of the VT-I/F. Vacuum tubes are excluded from the 5-year warranty, but are

warranted for a period of 90 days.

Except as expressly set forth in this Warranty, D.W. Fearn makes no other warranties, express or

implied, including any implied warranty of merchantability and fitness for a particular purpose.

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.

7

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.

8

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

VT-I/F Vacuum Tube Instrument Interface

D.W. FEARN