Crown Audio VT-15 User Manual

D.W. FEARN

VT-15

Vacuum Tube

Recording Channel

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-15 Recording Channel

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

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VT-15 Vacuum Tube Recording Channel

Final Test Report

Serial Number_______________ Mains Voltage 120/240 set to __________________

Date ________________ Tested by ________ VU Calibrated to _______________ dBm

Test Equipment ____________________________ Microphone ________________________

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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

Equivalent Input Noise _______ dB

Operational Tests

Line Input __________________

Instrument Input ____________

C->E Switching ______________

Compressor:

Threshold ___________________

Attack ______________________

Release _____________________

Harder/Softer ________________

VU GR Calibration ____________

VU Output Calibration ________

Sidechain HPF ________________

Equalizer:

Microphone Preamplifier:

-20 pad ________________

Lo-Z input ______________

Phase Reverse __________

+48V ___________________

VT-15 Recording Channel

LFB __________

LFC __________

HFB __________

HF Q _________

HF C _________

Listening Test ____________

D.W. FEARN

Table of Contents

RoHS Compliance Data ..................................................................3

Final Test Report ............................................................................5

Warranty ........................................................................................9

History of the VT-15 .....................................................................11

1. Specifications ..........................................................................13

2. Description...............................................................................15

3. Installation .............................................................................17

4. Operation .................................................................................19

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5. Theory of Operation ................................................................27

6. Maintenance ............................................................................31

7. Warranty and Repair ................................................................33

List of Illustrations

1. Front Panel Controls and Indicators ........................................19

VT-15 Recording Channel

D.W. FEARN

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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 ©2007 D.W. Fearn & Associates

VT-15 Recording Channel

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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-15 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-15.
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-15 Recording Channel

D.W. FEARN

History of the VT-15 Vacuum Tube Recording Channel

With the success of the VT-1/VT-2 Vacuum Tube Microphone Preamplifiers, the VT-

4 LC Equalizer, the VT-7 Stereo Compressor, and the VT-I/F and VT-3 Vacuum Tube DIs,
there was a great temptation to combine the features of all of these products into one record­ing channel.

But I had a problem with the concept. I did not want to be tied into having the com-

pression come before the equalization, or vice-versa, so a prime design goal was to make the
order of the processing switchable. That way the user has a choice of compression before
equalization or equalization before compression. Different circumstances require different
approaches to processing.

Achieving this switching with a minimum of noise and level change was solved quick-

ly, but all the switching in the audio path could result in a degradation of the noise perform­ance of the VT-15.

By definition, an “all-in-one” box has many compromises over using separate units. I

was not satisfied with the VT-15 until the overall performance was as close as possible to using
a VT-1/VT-7/VT-4. The final result is a box that has about 4dB more noise than the separate
units under most circumstances -- a negligible difference for most applications.

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Because the VT-15 is capable of a tremendous amount of gain (about 90dB in the first

prototype), it became a bit tricky to use properly. As long as you were familiar with optimiz­ing gain structure in a studio environment, it worked great. But not everyone has that back­ground, and in the heat of a session, it may get neglected. The result could be distortion -­and not the kind you would normally find pleasing. Optimizing the gain structure and head­room was an ongoing project, complicated by a period of 18 months when we could not keep
up with the orders for our VT-7 Stereo Compressor. In fact, I put the VT-15 project on hold
entirely for six months. When I returned to it over the summer of 2007, the pieces fell into
place and the VT-15 was ready to meet the world.

The mic preamp is identical to the VT-1/VT-2 circuit with one exception: an instru-

ment input has been added. The compressor is identical to one channel of the VT-7 Stereo
Compressor. The equalization is a highly-simplified version of the VT-4 LC Equalizer, with
just one frequency for the low-frequency boost, low-frequency cut, high-frequency boost, and
the high-frequency cut. The chosen frequencies were based on my experience using the VT­4 and feedback from many users.

Can the VT-15 replace the VT-1/VT-2, VT-7, and VT-4? Not really. The separate units

have features that are just not practical to duplicate in the VT-15. But for many applications,
the VT-15 provides the next best thing.

November 2007.

VT-15 Recording Channel

D.W. FEARN

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D.W. FEARN

VT-15 Recording Channel