D.W. Fearn VT4 User manual

Operating Instructions
D.W. FEARN
VT-4 Vacuum Tube LCEqualizer

How to Contact us:
Telephone: [phone]
Fax: [phone]
Mail: P.O. Box 57, Pocopson, PA 19366
U.S.A.
Shipping Address: 182 Bragg Hill Road
West Chester, PA 19382 U.S.A.
e-mail: [email protected]
Web: www.dwfearn.com

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 manu-
al may be photocopied or reproduced in any form without prior written consent from D.W.
Fearn.
Copyright © 2000 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-4 that has been damaged or rendered
defective as a result of accident, misuse, or abuse; by the use of parts not manu-
factured or supplied by D.W. Fearn; or by unauthorized modification of the VT-4.
Vacuum tubes are excepted from the 5-year warranty, but are warranted for 90
days from date of purchase.
Except as expressly set forth in this Warranty, D.W. Fearn makes no other war-
ranties, express or implied, including any implied warranty of merchantability and
fitness for a particular purpose.

History of the VT-4
Throughout my career in recording, there have always been a few equalizers that stood
out as being exceptional. Among my favorites are the 1970s-era Neve input-strip eqs, and the
Trident CB9066 parametric. I began my equalizer development project by first building a
series of test circuits, using all the various tone-modification techniques. After listening to a
wide variety of equalization circuits, it was obvious to me that the passive inductor-capacitor
(LC) circuit was the one that sounded the most musical and natural to me.
In thinking about how I use equalization, I realized that having simultaneous boost and
cut at the low and end frequencies was often very useful. For mid-frequencies, I found that I
always cut, usually around 400 Hz, and never had any reason to boost in that range. If I were
using a parametric equalizer, I invariably tended to use the low-Q (broadest) settings, and if I
had a choice between shelving or peaking on the high and low end, I almost always prefered
the shelving curve.
So the VT-4 was designed around those preferences -- low-Q curves, shelving, with
simultaneous boost and cut, mid-range cut but not boost, and using passive LC circuitry.
The amplification stages would be vacuum tube, and since the VT-1/VT-2 mic preamps
have had such a gratifying acceptance in the world of recording, it was important to preserve
the same sonic characteristics that distinquished the preamps. I decided to try the Svetlana
6N1P dual triode, and was pleasantly surprised to find that it is a wonderful-sounding tube,
with many of the same sonic characteristics as the 6072A used in my preamps. The active tube
circuitry fell into place with relatively little effort. Now it was time to make the equalization
circuitry work the way I wanted.
To start, I used the filter design tables developed by Bell Labs in the 1930s. That got
the project off the ground and it was starting to sound pretty good. For several months, I lis-
tened to a variety of music through a prototype equalizer while I was working on other things,
and gradually narrowed-in on what sounded really good and what didn’t. I would frequently
have a box of capacitors and clip leads next to the prototype and often clipped-in a different
value here or there and continued listening. Eventually, the final frequencies, curves, control
operation, etc. was determined. To this day, I have only a vague idea of what the actual curves
look like. Equalizers, like all audio equipment, should please your ears, not your test equip-
ment.
My experience with Jensen Transformers Inc. was so positive that I knew from the
beginning that I would utilize their products. The first couple of prototypes used inductors that
I wound myself, but for production units more-consistent inductors would be necessary. Jensen
agreed to manufacture the necessary inductors to my specifications, and the quality of the parts
is astounding.

Input 600 ohms
Input Load
Impedance 40k ohms
Minimum Input
Level -20 dBm nominal
Maximum Input
Level @ 20 cps+25 dB
Gain -4 dB
Frequency
Response ± 0.5 dB 20 cps to 20 kc
-3 dB @ 0.5 cps & 82 kc
THD + Noise <0.20% 20 cps to 20 kc
Intermodulation
Distortion SMPTE: <0.80%
Signal to
Noise Ratio 78 dB minimum
Outputlow-Z, transformer balanced
Maximum
Output Level +22 dBm unterminated
Power
Requirements100, 120, or 220 VAC
50/60Hz, 25 W
Dimensions 19” (48.26cm) W
5.25” (13.34cm) H
9” (22.86cm) D (VT-2 13” 22.9cm)
Weight 14 lbs (6.35 kg)
1.
SPECIFICATIONS
Note:Throughout this manual, frequency is specified in cps (cycles per second) or
kc (kilocycles per second). These units of measurement correspond to Hz and kHz
respectively. Specifications subject to change without notice
(All measurements made with +4dBm in and out, Eq switched in but all
controls set to flat.)

EQ Features
(All controls are independent and can be used in any combination)
Low Cut at 30, 40, 100, or 400 Hz,
0 to -18 dB shelving in 2 dB steps
Low Boost at 20, 40, 60, or 140 Hz,
0 to 12 dB shelving in 2 dB steps
Mid Cut at 200, 300, 400, 500, 600,
or 700 Hz, 0 to -16 dB in 2 dB steps
High Boost at 2, 3, 4, 5, 8, 10, 12, or 16 kHz,
0 to 14 dB in 2 dB steps
High Bandwidth Q of 0.6, 0.8, 1.0, 1.4, or 1.7
High Frequency Cut at 1.7, 4, 10, or 28 kHz,
0 to -14 dB shelving in 2 dB steps
Gain adjustable from -9 to +9 dB,
referenced to +4 dBm, in 3 dB steps
Note:Throughout this manual, frequency is specified in cps (cycles per second) or kc (kilocy-
cles per second). These units of measurement correspond to Hz and kHz respectively.
Specifications subject to change without notice

2.
DESCRIPTION
The Model VT-4 Vacuum Tube LC Equalizer is designed to provide recording
professionals with a sonically superior outboard processing device. It is typically
used in sound recording studios for modifying the frequency response character-
isics of an individual track. A pair of VT-4s can be used to process a stereo mix
with precise matching between units.
It is designed for use in the professional recording environment. It accepts all
standard low-impedance (nominally 600 ohm) line-level (nominally +4 dBm) sig-
nals. The output is line-level (+4 dBm) low-impedance, transformer-isolated and
designed to feed bridging inputs. It is built to sound great for a long time, with top
quality parts used throughout; all the transformers, inductors and many other com-
ponents are custom-made for the VT-4.
All power supplies are solid state and fully regulated. All rotary controls use
high-quality silver-contact switches for precise repeatability and matching
between units.
Much of the circuitry for the VT-4 is based on the highly-successful D.W. Fearn
VT-1 and VT-2 Vacuum Tube Microphone Preamplifiers. Since line-level signals
are utilized in the VT-4, Class-A Svetlana 6N1P dual triodes are used for all
amplifier stages. The input transformer, inductors in the passive equalization cir-
cuitry, and the output transformer are custom-made for us by Jensen Transformers
Inc.
The VT-4 is not mass-produced. Each one is hand-made and meticulously test-
ed and listened to before shipment to the customer.
The philosophy behind the VT-4 is: use only the best components in an opti-
mized circuit, and build it with pride and precision. The VT-4 is designed and built
to perform in your studio for decades to come.

3.
INSTALLATION
The VT-4 is carefully packed for shipment and it should survive all but the
most brutal handling. If there is any damage, keep the shipping material for use
during any possible claim for damage with the shipper.
Included in the box:
1) The VT-4 Equalizer
2) Line cord
3) This instruction manual
Mounting
The VT-4 is designed for installation in a standard 19 inch rack. It requires 5.25
inches of vertical space, but additional spacing between it and adjacent equipment
is recommended for adequate cooling. Ideally, a ventilated panel at least 1 rack
unit high (1.25 inches) should be installed above and below the VT-4 (and around
any other heat producing equipment for that matter). Be sure the bottom vent slots
are not blocked. It is essential that air can flow into the bottom and out of the top
of the VT-4. Equipment that runs cool can last for a very long time.
In tight equipment enclosures, be sure there is adequate air flow. Forced air
cooling will benefit all your equipment.
The VT-4 can also be used without a rack, placed on a table, counter, or even
on the floor. Optional rubber feet are available, when requested at the time of the
order.
Moderate electrical and magnetic fields in the vicinity of the VT-4 should not
cause any degradation in noise performance, due to the well-shielded construc-
tion, 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-4 are selected for minimum microphon-
ic response, it is a good practice to avoid mounting locations that subject the VT-
4 to very high sound or vibration levels.

Input and Output Connections
Gold-plated XLR connectors are used for inputs and outputs. The input con-
nector is female and the output male.
All connectors are wired according to AES standard: pin 1 is ground (shield),
pin 2 is “high” or “+,” and pin 3 is “low” or “-.” A positive voltage on pin 2 of
the input will result in a positive voltage on pin 2 of the output
Grounding and Shields
A full discussion of proper studio wiring schemes is beyond the scope of this
manual, but, in general, the Input mating XLR connector must have the cable
shield connected to pin 1.
Whether the shield is connected to pin 1 of the output connector depends on
the standard in your studio. The shield should be connected to ground at only one
end of the output cable; however, although not recommended, the shields can
often be connected at both ends without a problem.

Input
Since the input cable will be carrying very high-quality audio, it is important
that a well-shielded cable is used.
Output
The output of the VT-4 is line level, transformer balanced. Note that vacuum
tube equipment is more sensitive to load impedance than solid state units. The
VT-4 design was optimized for feeding a balanced bridging input (20k ohms or
greater). When feeding a 600 ohm load, there may be a slight degradation of some
of the specifications. In modern studio equipment, bridging line inputs are uni-
versal. If the device being fed by the VT-4 has an input termination switch, that
switch should be in the “off” position.
The VT-4 can feed balanced or unbalanced inputs with no need for any modifi-
cation in output wiring. Either pin 2 or 3 can be grounded, although pin 2 is nor-
mally used as the ”hot” and pin 3 grounded in unbalanced configurations.

4.
OPERATION
The use of most controls on the VT-4 Equalizer is self-explanatory. One
feature to note is that controls may be used in any combination. For example,
use of both the Low Cut and Low Boost controls at the same time can result in
some extraordinary effects. This also applies to the High Boost and High Cut.
The Mid-Cut control has a relatively low Q at minimal sttenuation set-
tings. Thus, the band covered is quite broad. As the Mid Cut control is
advanced, the Q becomes sharper and effects the selected frequency more nar-
rowly. Low attenuation settings (-2 to -6) are effective for removing some of
the “mid-range muddle” than can degrade a recording. The higher settings are
useful to null-out an annoying resonance.
The High Cut control is useful for reducing noise. The highest frequen-
cy setting (28 kHz) is particulary useful for digital recording. Attenuation in
this range may help avoid effects common with anti-aliasing filters in A-D con-
verters.
The Input control allows processing of a wide range of input levels.
Also, extreme amounts of boost could overload the input of the device being
feed by the VT-4. Reducing the Input control setting may be helpful in this cir-
cumstance. It also allows the overall output level of the VT-4 to be reduced to
provide a better level to the recorder. Settings above the “0” position are useful
when processing signals from semi-pro gear that typically have a lower output
level.
Like all recording gear, there is no right or wrong when using the VT-4.
Use your ears and select settings that achieve your sonic goal. But avoid boost-
ing frequencies when there is no effect (such as boosting the extreme highs on a
bass guitar), since doing so may only increase noise.

5.
THEORY OF OPERATION
Input section
Line level (600 ohm source impedance, balanced, +4 dBm nominal) audio
enters through the XLR-3 female INPUT connector to the eight-position Input
level control. The input is balanced-bridging, with an impedance of approxi-
mately 40k ohms. The input transformer is made by Jensen Transformers Inc.
The input switch is between the secondary of the input transformer and the grid
of the first stage of the input amplifier. A two-stage Class-A triode amplifer uti-
lizing Svetlana 6N1P vacuum tubes provides isolation from the input of the VT-
4 to the passive LC equalization circuitry.
Equalization section
All equalization is accomplished with passive LC (inductor/capacitor) cir-
cuitry. The inductors are custom-made for the VT-4 by Jensen Transformers Inc.
All capcitors are polystyrene or polypropylene types. The rotary switches are
semi-sealed, silver-contact types selected for quiet operation and long life.
Output section
The output amplifier is similar to the Input section, using a two-stage, Class-A
6N1P triode amplifier with a cathode-follower output. The output is tranformer-
balanced, using a custom-made Jensen transformer. The output is optimized for
feeding bridging inputs, which are standard in the recording industry. Either a
balanced or unbalanced source may be fed.
Power Supplies
Both the filament and B+ power supplies are solid-state and fully regulated.
The filaments are operated on regulated DC. A custom-made toroidal power
transformer supplies the proper voltages and has primary windings for 100, 110-
130, or 220-240 VAC. The power switch and line fuse are on the back panel. The
voltage regulator for the filament supply is mounted on the back panel for
improved cooling. It is normal for this regulator to operate at a temperature that
is warm to the touch.

6.
MAINTENANCE
The VT-4 is built with only the highest quality parts and will prove to be
extremely reliable. Vacuum tubes and electrolytic capacitors, however, have a
finite useful life and must be replaced eventually.
Top Cover Removal
Removing the top cover allows access to the vacuum tubes. 6-32 allen-head
bolts must be removed. When replacing the cover, position it so that the slotted
ventillation holes are over the tubes (towards the back ).
Vacuum Tubes
Four 6N1P tubes are used in the VT-4. V101 and V201 are the input stage
and V102 and V202 are the output stages. Tube life is difficult to predict, but it
will probably be measured in years. Catastrophic tube failure is rare with this
type of device, but a gradual increase in noise, microphonics, distortion, or a
reduction in headroom, should indicate the need for replacement. It is recom-
mended that you periodically perform a quick noise and distortion check on the
VT-4 and compare the results to previous measurements. Replacement tubes
may be obtained from us (tested and graded for low-noise and microphonics), or
other suppliers.

Rotary Switches
The rotary switches in the VT-4 were chosen for their self-cleaning property. In
a clean environment (non-smoking, with frequent room and equipment vacuum
cleaning), and with regular use, the rotary switches should require little attention
for years. But if cleaning becomes necessary, we recommend using De-Oxit (Caig
Laboratories). Follow the instructions provided with the product.
If the switches are not used regularly, oxide and dirt can build up on the con-
tacts. Rotate the switches through their range a few times on a weekly basis to
keep them clean.
Warranty Repair
If the VT-4 should develop a problem during the five-year warranty period, call
the factory for return shipping instructions. We will repair and return your VT-4
quickly. Note that the warranty does not cover vacuum tubes, which must be peri-
odically replaced.