D.W. Fearn VT-2 User Manual
D.W. FEARN
VT-1
VT-2
Vacuum Tube Microphone
Preamplifiers
Operating Instructions
Ho
w to Contact us:
elephone:610-793-2526
T
ax:610-793-1479
F
Mail: P
.O. Bo
Shipping A
x 57, Pocopson, PA 19366 U.S.A.
ddress:182 Bragg Hill R
est Chester, PA 19382 U.S.A.
W
e-mail: dwfearn@dwfearn.com
www
.dwfearn.com
oad
D.W. FEARN
VT-1 & VT-2 Microphone Preamplifers
D.W. FEARN
www.dwfearn.com
D.W. Fearn is committed to manufacturing products that are fully-compliant with
he EU R
t
The following products are compliant:
-1
VT
-2
VT
VT-3
VT-4
-7
VT
-15
VT
LP-1
PDB
oHS Directive.
HAND-CR AFTED
PROFESSIONAL
EQUIPMENT
RECORDIN
tificate of RoHS Compliance
Cer
G
P.O. Box 57
ocopson, PA 19366 U.S.A.
P
el: 610-793-2526
T
Fax: 610-793-1479
This declaration is based on our underst
anding 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
esident
Pr
4
VT-1 / VT-2 Vacuum Tube Microphone Preamplifier
Final Test Report
Model _______________ Serial Number_______________Mains Voltage ______________
5
Date ___________________ Tested by ________ VU
Test Equipment ____________________________ Microphone ________________________
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
%
Calibrated to _______________ dBm
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
Equivalent Input Noise _______ dB
Operational Tests:
-20 pad ________________
Lo-Z input ______________
Phase Reverse __________
+48V ___________________
Listening Test ____________
VT-1 &
VT-2 Microphone Preamplifers
Equivalent Input Noise _______ dB
Operational Tests:
-20 pad ________________
Lo-Z input ______________
Phase Reverse __________
+48V ___________________
Listening Test ____________
D.W. FEARN
Final Test Report
7
Table of Contents
CE Cer
Warranty .........................................................................................7
History of the VT-1 and VT-2 ..........................................................9
1. Specifications ..........................................................................13
2. Description...............................................................................15
3. Installation ......................................................................17 21
5. Theory of Operation ................................................................29
6. Maintenance ...........................................................................33
LP-1 Line Pad Instructions ............................................................37
tification Data
List of Illustrations
VT-1 &
. Rear Panel Connections .......................................................... 18
1
2. Front Panel Controls and Indicators ........................................21
3. Typical Studio Interconnections ............................................ 27
4. Block Diagram ........................................................................30
5. Location of VU Meter Calibration
VT-2 Microphone Preamplifers
...........................................35
D.W. FEARN
8
D.W. Fearn shall not be liable for technical or editorial errors or omissions in this
manual, nor f
or incidental or consequential damages resulting from the use of this
material.
This instr
uction manual contains information protected by copyright. No part of this
manual may be photocopied or reproduced in any form without prior written consent
om D.W. Fearn.
fr
D.W. FEARN
Copyright ©1995-2004 D.W. Fearn & Associates
VT-1 & VT-2 Microphone Preamplifers
Limited 5-Year Warranty
9
During t
defective parts with new parts.
This w
dered defective as a result of accident, misuse, or abuse; by the use of parts not man-
actured or supplied by D.W. Fearn; or by unauthorized modification of the VT-1 or
uf
VT-2. Vacuum tubes are excepted from the 5-year warranty, but are warranted for 90
da
Ex
ranties, express or implied, including any implied warranty of merchantability and fitness for a particular purpose.
he warranty period, D.W. Fearn will, at no additional charge, repair or replace
ranty does not extend to any VT-1 or VT-2 that has been damaged or ren-
ar
ys from date of purchase.
cept as expressly set forth in this Warranty, D.W. Fearn makes no other war-
VT-1 &
VT-2 Microphone Preamplifers
D.W. FEARN
History of the VT-1 and VT-2
Vacuum Tube Microphone Preamplifiers
11
ONE D
came acr
I pulled t
ed t
good, so I made a cassette to listen to in t
AY IN 1991 I was going through some old masters in a closet at home and
oss a reel from 1968. It was one of the first studio recordings I ever made.
he tape box off the shelf and thought about those days. Although I suspect-
hat the recording might be a bit crude, I remembered that the music was pretty
he car.
I kept forgetting to put the cassette in my pocket for a few days, but finally I remembered to take it. That old recording brought back memories of my first studio — and
how primitive a setup it was. But listening to that tape was a revelation; some of the
e really nice. The vocals were full and warm but still punched through.
sounds w
coustic guitars had a depth I don’t often hear in current recordings. And the sax
A
solo — w
That r
prize micr
er
ow! It ripped through with a grossly distorted but beautifully powerful sound.
ecording was done on a 4-track Scully 280 and mixed to a 2-track Scully. My
ophone was a Neumann U-87 and that’s what was probably on the featured
instrument or voice on each track. Nothing too unusual about that.
I couldn’t afford the Electrodyne board of my dreams back then. In fact, I built the
“mixer” myself. It consisted of half a dozen RCA tube microphone preamplifiers that
I salvaged from the junk pile of the radio station where I worked, an equal number of
en rotary faders, and key switches that “panned” the output to left, center, or
old Da
v
right.
as the
It w
everb, but maybe just a touch of compression on some sounds, from an old broad-
no r
tube preamps t
hat made that recording sound so good. There was no EQ,
cast type (tube) limiter.
These pr
cylindrical output transf
rate chassis. They were a lot of trouble — the tubes were microphonic and the output was
often noisy. In addition to the hums and hisses, occasionally a take would be ruined by crackles and bangs from the tubes. I couldn’t wait to get rid of the things.
And so I did, not long after
pots, echo sends and returns, and EQ on every input. No more noisy tubes for me.
But now, 25 years later, I got to thinking about the sound of those tubes. Hit them with a bit
of ex
sounded w
Could that sound be duplicated today? I dug out my old RCA Receiving Tube Manual and
veral other old reference books and reviewed the vacuum tube theory I hadn’t thought
se
about for years. A quick check in the supplier’s catalogs confirmed that tubes were still easy
to obtain.
eamps were 1940s vintage. They used octal metal tubes with a shielded grid cap, a
ormer the size of a coffee can, and a huge power supply on a sepa-
. I got a beautiful console with IC op amps, linear faders, real pan-
cessive level and the sound became real fat. Hit them with just the right level and they
arm and intimate.
VT-1 &
VT-2 Microphone Preamplifers
D.W. FEARN
12
Over the years much has changed in the world of electronic components. Were the necessary
ailable? I found out that they were (though not necessarily cheap) and, in many
ts still a
par
cases, they were vastly better than the components available back in the age of vacuum tubes.
Carbon resistors could be replaced with quieter metal film types. Sonically superior polystyrene and polypropylene capacitors were preferable to the old paper types. The power supplies could be solid st
The only par
the tube input and output impedances.
A call to the great folks at legendary Jensen Transformers revealed that not only were the
necessary transformers still available but that they were orders of magnitude superior to the
technology of t
A couple of months research into the classic tube mic preamp designs gave me a good idea
of how to proceed. A breadboard prototype was constructed and tested, and it worked great!
(Although the open construction resulted in some RFI; while experimenting with different
ponent v
com
Radio Havana coming through weakly but clearly.) Professional quality specs on frequency
esponse, distortion, noise, phase shift, and so on were definitely attainable.
r
Now it was necessary to squeeze the last dB of performance out of the circuit. Computer
cuit analysis was one tool not available to the designers of the original equipment, and it
cir
was amazing how careful manipulation of values could make a significant improvement in
formance.
per
v
ate — and easily regulated. Electrolytic filter capacitors were smaller.
hat remained to be found were top quality audio transformers that matched
ts t
he ‘50s and ‘60s.
alues one night with a pair of headphones on the output, I heard a half hour of
The next prototype was built and its performance was even better, largely because of better shielding and a better la
mentation. There is not a single component in that prototype that hasn’t been changed in an
attempt to improve performance. Some parts of the circuit have been through dozens of iterations. Moder
discovered back in the heyday of “hollow state.”
This pr
one I built soon after) for a year of location recording, mostly of classical and choral music,
but also f
ers, after using the tube preamp, I just couldn’t bring myself to use the solid state preamps
anymore.
y do tubes sound better? All properly designed audio amplifier circuits exhibit low distor-
Wh
hroughout their operating amplitude range. The difference in sound is particularly evi-
tion t
dent when the circuit runs out of headroom. Solid state devices tend to abruptly transition
w distortion to extreme distortion (clipping). This is a good trait, since when operat-
om lo
fr
ed right up to t
al audio circuits have similar characteristics.
Digit
Vacuum tube circuits, on the other hand, show a gradual increase in distortion throughout
heir operating rang
t
. Until a level is reached where something in the circuit just completely falls apart (e.g. a
ly
ormer saturates), the sound retains most of its original quality.
transf
n test equipment can quantify and graph parameters t
ototype became m
or studio sessions. Although I have some very fine commercial and homebuilt mix-
heir maximum level solid state amplifiers can maintain excellent performance.
e. But instead of an abrupt break, the distortion increases incremental-
yout. This one became the testing ground for additional experi-
hat had not even been
y preamp of choice for all my recording. I used it (and another
It’s the nature of the distortion that makes a difference, too. Solid state circuits run out of
oom when t
headr
D.W. FEARN
he output voltage exceeds the power supply voltage. The result at this point
VT-1 & VT-2 Microphone Preamplifers
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