D.W. Fearn VT-7 User Manual

D.W. FEARN

VT-7

Vacuum Tube

Compression Amplifier

Operating Instructions

How to Contact us:

Telephone: 61

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: [e-mail]

www.dwfearn.com

0-793-2526

D.W. FEARN

VT-7 Compression Amplifier

D.W. FEARN

www.dwfearn.com

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

HAND-CR AFTED

PROFESSIONAL
RECORDING EQUIPMENT

Certificate of RoHS Compliance

P.O. Box 57
Pocopson, PA 19366 U.S.A.
Tel: [phone]
Fax: [phone]

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

VT-7 Vacuum Tube Compression Amplifier

Final Test Report

Model _______________ Serial Number_______________Mains Voltage ______________

Date ___________________ Tested by ________ VU Calibrated to _______________ dBm

Test Equipment _______________________________________________________________

5

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:

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 ____________

VT-7 Compression Amplifier

Compression Range ________

Attack ____________________

Release __________________

Shape ___________________

Link _____________________

GR Meter ________________

Listening Test ____________

D.W. FEARN

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

7

5. Theory of Operation ................................................................23

6. Maintenance ...........................................................................25

List of Illustrations

1. Front Panel Controls and Indicators ........................................17

VT-7 Compression Amplifier

D.W. FEARN

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.

D.W. FEARN

Copyright ©2005 D.W. Fearn & Associates

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 manufac­tured 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 war­ranties, express or implied, including any implied warranty of merchantability and fit­ness for a particular purpose.

VT-7 Compression Amplifier

D.W. FEARN

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 disadvan­tages. 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 manu­facture 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.

11

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 sim­ple 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.

VT-7 Compression Amplifier

D.W. FEARN