D.W. Fearn VT-7 Manuel utilisateur
VT-7
Vacuum Tube
Compression Amplifier
Operating Instructions
D.W. FEARN
D.W. FEARN VT-7 Compression Amplifier
How to Contact us:
Telephone: 610-793-2526
Fax: 610-793-1479
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: dwfearn@dwfearn.com
www.dwfearn.com
Certificate of RoHS Compliance
D.W. Fearn is committed to manufacturing products that are fully-compliant with
the EU RoHS Directive.
The following products are compliant:
VT-1
VT-2
VT-3
VT-4
VT-7
VT-15
LP-1
PDB
This declaration is based on our understanding of the current RoHS Directive and
from information provided by the supplier material declarations with regard to
materials contained in the component that make up our products.
Douglas W. Fearn
President
D.W. FEARN HAND-CRAFTED
PROFESSIONAL
RECORDINGEQUIPMENT
www.dwfearn.com
P.O. Box 57
Pocopson, PA 19366 U.S.A.
Tel: 610-793-2526
Fax: 610-793-1479
D.W. FEARN
VT-7 Compression Amplifier
5
VT-7 Vacuum Tube Compression Amplifier
Final Test Report
Model _______________ Serial Number_______________Mains Voltage ______________
Date ___________________ Tested by ________ VU Calibrated to _______________ dBm
Test Equipment _______________________________________________________________
Channel A
Frequency Response:
20 cps to 20 kc/s +/- ___________ dB
THD+Noise:
20 cps ______________ %
200 cps _____________ %
2 kc/s _______________ %
20 kc/s ______________ %
Noise:
______________ dB below +4 dBm out
Operational Tests:
Compression Range ________
Attack ____________________
Release __________________
Shape ___________________
Link _____________________
GR Meter ________________
Listening Test ____________
Channel B
Frequency Response:
20 cps to 20 kc/s +/- ___________ dB
THD+Noise:
20 cps ______________ %
200 cps _____________ %
2 kc/s _______________ %
20 kc/s ______________ %
Noise:
______________ dB below +4 dBm out
Operational Tests:
Compression Range ________
Attack ____________________
Release __________________
Shape ___________________
Link _____________________
GR Meter ________________
Listening Test ____________
D.W. FEARN
VT-7 Compression Amplifier
7
Table of Contents
Final Test Report ............................................................................3
Warranty .........................................................................................7
History of the VT-7 .........................................................................9
1. Specifications ..........................................................................11
2. Description...............................................................................13
3. Installation .............................................................................15
4. Operation .................................................................................17
5. Theory of Operation ................................................................23
6. Maintenance ...........................................................................25
List of Illustrations
1. Front Panel Controls and Indicators ........................................17
D.W. FEARN VT-7 Compression Amplifier
8
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
manual may be photocopied or reproduced in any form without prior written consent
from D.W. Fearn.
Copyright ©2005 D.W. Fearn & Associates
D.W. FEARN
VT-7 Compression Amplifier
9
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-7 that has been damaged or rendered
defective as a result of accident, misuse, or abuse; by the use of parts not manufac-
tured or supplied by D.W. Fearn; or by unauthorized modification of the VT-7.
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 fit-
ness for a particular purpose.
D.W. FEARN
VT-7 Compression Amplifier
11
History of the VT-7 Compression Amplifier
Compression (or limiting) is sometimes seen as a necessary evil in the recording process.
Often it is used to compensate for less than optimum performance or recording technique.
However, when used appropriately, there is no question that good-sounding compression can
add loudness, power, and improved impact to a recording.
Over many years of recording, I have had the opportunity to use and sometimes own some
of the best compressors in the world. For the D.W. Fearn line of products, a compressor was
obviously important. I began the search for the ultimate compressor design in 1995 and it
wasn’t until 2004 that I developed the sound I was looking for.
There are several ways to achieve compression, and all have their advantages and disadvan-
tages. The earliest, and still one of the best, designs uses the characteristic of vacuum tubes
that allows their gain to be varied by changing the grid bias. Only certain tube types are
appropriate for this (the variable-mu or remote cut-off types), and none are currently being
manufactured. Since I want my products to be used for a long, long time, I could not manu-
facture a product that used tubes that were increasingly rare.
Another excellent approach uses a photoresistor illuminated by a light source to vary the gain
of an amplifier. This method was pioneered by Teletronics/Universal Audio in the LA-2. Many
LA-2s were built over the years, and more recently there have been several new versions of
this classic circuit. The world has a good supply of old and new LA-2-style compressors, and
although I did some experimenting with this circuit, I felt that there were enough of them and
I didn’t need to add another. Also, this design lacks the versatility I thought was necessary.
A more modern approach uses a voltage-controlled amplifier (VCA), which is quite versatile
and can combine several additional functions (de-essing, gating, etc.) in one simple unit.
However, I was never satisfied with the sound of VCA compressors and did not consider that
design for my use.
I experimented with a few off-the-wall approaches, but was not convinced that these designs
would really work to my standards.
Another approach, not often used, utilizes a pulse-width modulator to control the duty-cycle
of a gain reduction element. This approach has several advantages, including the ability to
offer a wide range of adjustment. In the VT-7, a solid-state pulse-width modulator controls a
special shunt element in the audio path to control the gain. Since no audio passes through
the element, there is no added distortion or other degradation of the sound. That’s the sim-
ple part. Much more complex is the surrounding circuitry that provides precision control of
the element in a way that conforms to what I wanted to hear.
Because the element is well-isolated from the vacuum tube circuitry before and after it, the
effect on the audio is entirely linear. It does not introduce any measurable distortion. And it
passes my test for sonic superiority.
The vacuum tube audio path uses circuitry developed for the D.W. Fearn VT-4 LC Equalizer.
In some of the circuit, entirely new designs were needed. The VT-7 uses a combination of
6072A and 6N1P tubes and has the basic sound of all my products.
D.W. FEARN VT-7 Compression Amplifier
12
A look inside the VT-7 reveals a very full box with several densely-packed circuit boards for
the gain control driver circuitry, interspersed with eight vacuum tubes. Because of the very
high frequencies involved in the control circuitry, there are many surface-mount (SMT) com-
ponents. Thus, the VT-7 is a combination of the latest technology, where needed, with a clas-
sic great-sounding vacuum tube audio path.
The VT-7 was designed from the beginning to be a stereo compressor. The two channels can
be used separately or linked together for stereo.
So what does the VT-7 sound like? In many respects, it is like the classic vacuum tube com-
pressors with the same musical sound. I have always believed that for most applications the
compression characteristics that sound best are those that approximate the natural compres-
sion inherent in our hearing. The VT-7 has this kind of natural-sounding compression that
sounds just right to me.
The ultimate test for all my products is this: Would I use it in preference to any other prod-
uct out there, old or new? The VT-7 passes this test.
There are not a lot of controls on the VT-7. Basically, there is just a knob to control the
amount of compression (Threshold), the amount of gain, the attack and release times, and a
knob that smoothly varies the compression characteristic from a “softer” to a “harder”
sound. At first, some users may find the controls do not have the range of adjustment that
they are used to in more heavy-handed limiters. However, true to philosophy of design in all
my products, the VT-7 is intended to take good sound and make it sound great. Although its
effect can be made to sound obvious, I like the way the VT-7 sounds like what a really good
engineer would do if he had excellent taste and infinitely fast hands on a fader. In other
words, the VT-7 controls the levels without drawing attention to itself. To me, it just sounds
right.
Douglas W. Fearn
November 2004
D.W. FEARN
VT-7 Compression Amplifier
13
1 .
SPECIFICATIONS
Input 600 ohm source (nominal)
balanced or unbalanced
Input Load
Impedance 32k ohms, transformer
balanced bridging
Nominal Input
Level +4 dBm
Maximum Input
Level @ 20 cps +25 dBm
Gain unity to +15dB
Frequency
Response ± 0.2 dB 20 cps to 20 kc
± 0.5 dB 11 cps to 28 kc
-3 dB @ 0.5 cps & 65 kc
THD + Noise <0.05% 20 cps to 20 kc
Signal to
Noise Ratio 78 dB minimum
Output low-Z, transformer balanced
Output source impedance 115 ohms
Maximum
Output Level +22 dBm into bridging input
balanced or unbalanced
Gain Reduction Range 0 to 20dB
Power
Requirements 100, 120, or 220 VAC
50/60Hz, 85 W
Dimensions 19” (48.26cm) W
5.25” (13.34cm) H
13” (22.9cm) D
Weight 22 lbs (9.55 kg)
Shipping Weight: 28 lbs (12.27 kg)
(measurements are made with +4dBm input and output, no compression, into a bridging load)
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