D.W. Fearn VT-I 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, PA19366 U.S.A.
Shipping Address: 182 Bragg Hill Road
West Chester, PA19382 U.S.A.
e-mail: [email protected]
World Wide Web: http://www.dwfearn.com

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.

7
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.

VT-I/F Vacuum Tube Instrument Interface D.W. FEARN
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

D.W. FEARN VT-I/F Vacuum Tube Instrument Interface
9
Specifications
(with 200 mV input)
Input 200 mV nominal
unbalanced
Input Load
Impedance 1 megohm minimum
Minimum
Input Level 5 mV nominal
Maximum
Input Level 2.3 volts P-P for
1% THD
Gain -10 dB
Frequency
Response +/- 0.2 dB 10 cps to 20 kc
-3 dB @ 0.5 cps and 95 kc
THD + Noise <0.3% 20 cps to 20 kc
Intermodulation
Distortion <0.01% SMPTE
Noise minimum 90 dB below output
(22 cps to 22 kc bandwidth)
Output Transformer Balanced
(matches 150 ohm input)
Output Level -30 dBm nominal
Maximum
Output Level -12 dBm
Crosstalk maximum -90 dB @ 20 kc
Power
Requirements 100/120/220 VAC 50/60Hz
25W
Dimensions 8”W x 5”H x 12”D
(20.3cm x 12.7cm x 30.5cm)
Weight 15 lbs (6.82 kg)
NOTE: Throughout this manual, frequency is specified in cps (cycles per second) and kc (kilocycles per second). These units correspond
to Hz and kHz. Specifications and price subject to change without notice.

VT-I/F Vacuum Tube Instrument Interface D.W. FEARN
10

D.W. FEARN VT-I/F Vacuum Tube Instrument Interface
11
Description
The D.W. Fearn model VT-I/F Vacuum Tube Instrument Interface is designed to
provide recording professionals with a sonically superior method for recording
electric and electronic musical instruments by direct injection (DI). Any instru-
ment designed to operate into a “guitar amp” will work perfectly with the VT-I/F.
Typical instruments include: electric bass, electric guitar, electric piano, acoustic
instruments with a pickup (piano, acoustic guitar, electric violin, etc.), synthesiz-
ers, and samplers.
The VT-I/F is a two-channel device that provides the optimum load impedance
for these instruments. It is capable of quality reproduction of a wide range of
instrument levels. The unit does not have the limitations of passive transformer-
type DIs, and has greater headroom and warmth when compared to similar solid-
state devices.
It is designed to operate in the professional recording or performing environ-
ment. The output level is “hot” mic level (-30 dBm nominal). The output is trans-
former-balanced, using a custom transformer built for us by Jensen Transformers,
Inc., and is designed to match 150 ohm professional mic inputs. Top-quality parts
are used throughout. The case is machined from solid quarter-inch thick aluminum
plate for ruggedness and durability. Both the filament and plate power supplies are
fully regulated.
The VT-I/F is not mass-produced. Each one is hand-made and meticulously test-
ed before shipment to the customer.

13
Installation
The VT-I/F is carefully packed for shipment and should survive all but the most
brutal handling. If there is any damage, keep the shipping material for use during
any claim for damage with the shipper.
Included in the box:
1) The VT-I/F Instrument Interface
2) Line cord
3) This instruction manual
Mounting
The VT-I/F is designed to be placed on the floor, counter, or table-top. In most
cases, cooling will not be a problem, but avoid placing the unit where it is tightly
confined. Do not block the cooling holes on the top. Be sure that heavy carpeting
does not interfere with air circulation through the cooling holes on the bottom. The
VT-I/F runs cool, cooler, in fact, than many solid-state devices.
Rack mounting is possible if a shelf is provided. If the unit must be mounted in
a rack, be certain that there is adequate support for both the front and the back of
the unit. It is heavy and could be damaged if not securely bolted in place and prop-
erly supported.
Moderate electrical and magnetic fields in the vicinity of the VT-I/F should not
cause any degradation in noise performance, due to the well-shielded construction,
but proximity to devices with motors or large power transformers (i.e. tape
machines or power amps) should be avoided.
Although the vacuum tubes in the VT-I/F are selected for minimum micro-
phonic response, it is a good practice to avoid mounting locations that subject the
unit to very high sound or vibration levels.

VT-I/F Vacuum Tube Instrument Interface D.W. FEARN
14
REAR PANEL CONNECTIONS (See Figure 1)
AC (Mains) Power (2)
The VT-I/F is designed to operate from 100, 120, or 220 volt, 50/60 Hz power.
The unit will be shipped wired for the voltage specified in the order, but may be
changed in the field if necessary. The ground pin of the power cord is internally
connected to the chassis. This configuration is standard in professional equipment
and is required by most electrical codes. If ground loop hum is detected, a careful
check of the studio grounding scheme is needed. The VT-I/F is less susceptible to
grounding problems than many studio devices.
The Fuse is a 3AG-type 1 amp for 100/120 VAC operation, and 0.5 amp for 220
volts.
The AC input connector (2) is used with the mating line cord (supplied). For 120
VAC operation, this cord is a Belden 17250 or equivalent.
Figure 1. VT-I/F Rear Panel Connections and Controls
1
2
D.W. FEARN
VT-I/F Vacuum Tube Instrument Interface
ON
OFF
POWER
0.5A 100/120V
0.25A 220V
D.W. FEARN P.O. BOX 57 POCOPSON, PA 19366 U.S.A. 610-793-2526

D.W. FEARN VT-I/F Vacuum Tube Instrument Interface
15
FRONT PANEL CONNECTIONS (See Figure 2)
The VT-I/F has two identical channels, labeled Aand B. They may be used indi-
vidually or simultaneously. These instructions apply to either channel.
The musical instrument output connects to the VT-I/F INSTRUMENT input (2)
via a standard 1/4 inch single-conductor (monaural) phone jack. The input is
unbalanced. Since the input cables carry very low level audio, it is important that
well-shielded cables are used. There should be no additional connectors, patch
jacks, switches, etc. between the instrument and the VT-I/F inputs. It is important
to keep the input lines as short as possible. Avoid locating the VT-I/F where it will
be subjected to high sound levels or excessive vibration (such as on a drum riser).
If desired, the musical instrument may be simultaneously connected to the input
to an external instrument (guitar) amplifier. The AMP jack (1) is a 1/4” single-con-
ductor (monaural) phone jack wired directly in parallel with the INSTRUMENT
jack.
Figure 2. VT-I/F Front Panel Controls and Connections
D.W. FEARN VT-I/F Vacuum Tube Instrument Interface
INSTRUMENT AMP INSTRUMENT AMP
AB
OUT
A
OUT
B
GND
LIFT
GND
LIFT
1
2
3
45

VT-I/F Vacuum Tube Instrument Interface D.W. FEARN
16
The OUTPUT connectors (4) are XLR-3 male wired according to AES standard:
pin 1 is ground (shield), pin 2 is “high” or “+,” and pin 3 is “low” or “-.” The out-
put is transformer-balanced. The output level is a nominal -30 dBm. This is some-
what higher in level than a typical microphone.
Grounding and Shields
Afull discussion of proper studio wiring schemes is beyond the scope of this
manual, but, in general, the shield should be connected to pin 1 of the output con-
nector. The shield should be connected to ground at only one end; however,
although not recommended, the shields can often be connected at both ends with-
out a problem.

17
OPERATION
REAR PANEL CONTROLS (see Figure 1, page 14)
Power switch (1)
Primary power is applied to the VT-I/F circuits when the Power switch (1) is in
the up position. The amber pilot lamp (5) on the front panel indicates that the unit
is on. It takes about twenty seconds for the Interface to start working, but it is sug-
gested that you turn on the power at least five minutes prior to use. The tubes are
often noisy until all the internal elements reach a stable operating temperature.
FRONT PANEL CONTROLS (see Figure 2, page 15)
The only controls on the VT-I/F are the GND LIFT switches. These connect or
disconnect pin 1 of the OUTPUT connectors to the unit internal ground. In most
situations, the GND LIFT switch (3) will remain in the down (pin 1 to internal
ground) position. In some situations, particularly when an external instrument
amplifier is connected to the AMP jack, a ground loop may occur. This causes a
hum or buzz in the recording equipment. Placing the GND LIFT switch in the up
position may help alleviate this.
The musical instrument is connected to the INSTRUMENT jack (2), an option-
al guitar amp is connected to the AMP jack (1), and the external preamp (VT-1/VT-
2 or other) is connected to the OUT jack (4).
HUMS AND BUZZES
Unfortunately, it is the nature of high-impedance, unbalanced instrument output
to be highly susceptible to hum and buzz. Some of this is often unavoidable — the
instrument pickup(s) are very sensitive to the omnipresent AC fields in our envi-

VT-I/F Vacuum Tube Instrument Interface D.W. FEARN
18
ronment. Experiment with the orientation of the instrument to minimize this
source of noise. Most guitar and bass players will do this automatically, but with
keyboards it may be necessary to rotate and/or move the instrument to find the qui-
etest spot.
Fluorescent lights and SCR dimmers are terrible electrical noise generators.
Turn off the fluorescents. If SCR dimmers must be used, they will generally pro-
duce the least noise in their full-on (brightest) position.
Another source of these hums and buzzes is ground loops caused by the inter-
connection of various AC (mains) powered equipment. Aproperly-wired studio
should not create a ground loop between the VT-I/F and the studio equipment, but
often a loop is formed when the VT-I/F is connected to an instrument and guitar
amp simultaneously. In this case, use the GND LIFT (3) switch to find the proper
IMPORTANT
When using the VT-I/F with the D.W. Fearn VT-1 or VT-2 Vacuum
Tube Microphone Preamplifier, the VT-1/VT-2 Input switch must
be in the -20 dB position (unless the instrument level is extremely
low). With other preamps, use the input pad and/or reduce trhe
gain of the preamp.
position for minimum noise. If the guitar amp has a “Ground” switch, experiment
with it as well. Be sure to try all combinations of all switches.
The VT-I/F and the guitar amp (if used) should be on the same electrical circuit
(same circuit breaker or fuse) as the studio equipment to minimize ground loop
potential.
Suggestions
You have chosen to use the VT-I/F because of the superior sound it provides. To
gain the maximum benefit from your investment, it is important that you hook up
the VT-I/F so that other factors do not adversely affect the sound quality.

D.W. FEARN VT-I/F Vacuum Tube Instrument Interface
19
1. The VT-I/F must be located near the instrument. Ten feet of cable between
the instrument and the VT-I/F should be considered the maximum.
2. Use the best quality cables you can. We don’t believe you have to use eso-
teric wire, but do use good quality, well-shielded cables. The input cable
shielding is particularly important. The output cable should be a standard
balanced mic cable designed for use with low-Z microphones. Gold-contact
phone and XLR connectors are recommended.
3. There should be no additional cables, connectors, junction boxes, patch
jacks, punch blocks, etc. between the VT-I/F output and the VT-1/VT-2
Input.
4. The outputs of the VT-1/VT-2 should be fed directly to the recorder through
the shortest practical lengths of quality cables. Avoid additional cables, con-
nectors, junction boxes, punch blocks, or patch jacks. Use gold contact con-
nectors if possible. Do not go through the mixing console unless you
absolutely need its features for the track you are cutting.
4. In general, for superior sound, we recommend recording directly to the
recorder with no processing (compression, equalization, gating, etc). Any
processing can be added in the mix, if necessary. You may find that far less
processing is required when using the VT-I/F. If processing is required while
recording the track, insert the processing device after the VT-1/VT-2 and
before the recorder.
5. Feeding the VT-I/F into the studio console will negate some of the benefits
of the VT-I/F. Use a premium-quality microphone preamplifier (such as our
VT-1/VT-2). Go through the console only if it is absolutely necessary.

20

21
THEORY OF OPERATION
This circuit description refers to only one channel. Both channels are identical,
sharing only the power supply. (See Figure 3.)
Input section
The INSTRUMENT input jack is fed directly to the grid of the first amplifier
stage, a selected 6072 or selected 12AY7. The input impedance is very high (1
megohm) for proper loading of the musical instrument.
The first stage is capacitively-coupled to the grid of the output stage.
Figure 3. VT-I/F Block Diagram
Output Stage
The output tube is the second section of the 6072/12AY7. It is operated as a
cathode follower, capacitively-coupled to the custom output transformer. Aresis-
From Instrument
To Guitar Amp
Mic Level Output
to Preamplifier

VT-I/F Vacuum Tube Instrument Interface D.W. FEARN
22
tive network on the secondary of the transformer provides proper loading and
matching to the input of the external microphone preamplifier.
The GND LIFT switch disconnects pin 1 of the OUTPUT connector from the
internal ground when the switch is in the up position.
Power Supplies
Primary power from the AC mains is connected to the VT-I/F through a stan-
dard IEC power input connector. The Power switch energizes all power supplies.
Afuse, accessible on the rear panel, protects the VT-I/F.
The Pilot lamp is a type 1820 bulb, operated far below its rated voltage of 28.
The life of the bulb is lengthened, and the light output is more compatible with
other modern studio equipment.
The power transformer is custom made for the VT-I/F and has primary taps for
100, 120, or 220 volt operation. The VT-I/F may be rewired for different mains
voltages.
Filament supply
The power transformer output is rectified by a bridge rectifier and filtered
before being regulated to 12.0 volts by a three-terminal regulator. The negative
output of this supply is grounded. Although the tube filaments are rated for 12.6
volts, utilization of 12.0 volts has no effect on the operation of the VT-I/F.
B+ supply
Two separate regulated voltages are required for the plates of the VT-I/F. The
B+ is filtered with long-life, low-leakage computer-grade filter capacitors before
being regulated and extensively bypassed and decoupled. The negative side of the
supply is grounded.