Dw fearn VT2 User Manual
VT-1
VT-2
Vacuum Tube Microphone
Preamplifiers
Operating Instructions
D.W. FEARN
D.W. FEARN
VT-1 & VT-2 Microphone Preamplifers
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
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VT-1 / VT-2 Vacuum Tube Microphone Preamplifier
Final Test Report
Model _______________ Serial Number_______________Mains Voltage ______________
Date ___________________ Tested by ________ VU Calibrated to _______________ dBm
Test Equipment ____________________________ Microphone ________________________
Channel _______
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 ____________
Channel _______
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 ____________
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Table of Contents
Final Test Report
CE Certification Data
Warranty .........................................................................................7
History of the VT-1 and VT-2 ..........................................................9
1. Specifications ..........................................................................13
2. Description...............................................................................15
3. Installation .............................................................................17
4. Operation ................................................................................21
5. Theory of Operation ................................................................29
6. Maintenance ...........................................................................33
LP-1 Line Pad Instructions ............................................................37
LLiisstt ooff IIlllluussttrraattiioonnss
1. Rear Panel Connections .......................................................... 18
2. Front Panel Controls and Indicators ........................................21
3. Typical Studio Interconnections ............................................ 27
4. Block Diagram ........................................................................30
5. Location of VU Meter Calibration ...........................................35
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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.
Copyright ©1995-2003 D.W. Fearn & Associates
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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-1 or VT-2 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-1 or
VT-2. 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.
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History of the VT-1 and VT-2
Vacuum Tube Microphone Preamplifiers
ONE DAY IN 1991 I was going through some old masters in a closet at home and
came across a reel from 1968. It was one of the first studio recordings I ever made.
I pulled the tape box off the shelf and thought about those days. Although I suspect-
ed that the recording might be a bit crude, I remembered that the music was pretty
good, so I made a cassette to listen to in the 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
sounds were really nice. The vocals were full and warm but still punched through.
Acoustic guitars had a depth I don’t often hear in current recordings. And the sax
solo — wow! It ripped through with a grossly distorted but beautifully powerful sound.
That recording was done on a 4-track Scully 280 and mixed to a 2-track Scully. My
prize microphone 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
old Daven rotary faders, and key switches that “panned” the output to left, center, or
right.
It was the tube preamps that made that recording sound so good. There was no EQ,
no reverb, but maybe just a touch of compression on some sounds, from an old broadcast type (tube) limiter.
These preamps were 1940s vintage. They used octal metal tubes with a shielded grid cap, a
cylindrical output transformer the size of a coffee can, and a huge power supply on a separate 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. I got a beautiful console with IC op amps, linear faders, real panpots, 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 excessive level and the sound became real fat. Hit them with just the right level and they
sounded warm and intimate.
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Could that sound be duplicated today? I dug out my old RCA Receiving Tube Manual and
several other old reference books and reviewed the vacuum tube theory I hadn’t thought
about for years. A quick check in the supplier’s catalogs confirmed that tubes were still easy
to obtain.
Over the years much has changed in the world of electronic components. Were the necessary
parts still available? I found out that they were (though not necessarily cheap) and, in many
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 state — and easily regulated. Electrolytic filter capacitors were smaller.
The only parts that remained to be found were top quality audio transformers that matched
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 the ‘50s and ‘60s.
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
component values one night with a pair of headphones on the output, I heard a half hour of
Radio Havana coming through weakly but clearly.) Professional quality specs on frequency
response, distortion, noise, phase shift, and so on were definitely attainable.
Now it was necessary to squeeze the last dB of performance out of the circuit. Computer
circuit analysis was one tool not available to the designers of the original equipment, and it
was amazing how careful manipulation of values could make a significant improvement in
performance.
The next prototype was built and its performance was even better, largely because of better shielding and a better layout. This one became the testing ground for additional experimentation. 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. Modern test equipment can quantify and graph parameters that had not even been
discovered back in the heyday of “hollow state.”
This prototype became my preamp of choice for all my recording. I used it (and another
one I built soon after) for a year of location recording, mostly of classical and choral music,
but also for studio sessions. Although I have some very fine commercial and homebuilt mixers, after using the tube preamp, I just couldn’t bring myself to use the solid state preamps
anymore.
Why do tubes sound better? All properly designed audio amplifier circuits exhibit low distortion throughout their operating amplitude range. The difference in sound is particularly evident when the circuit runs out of headroom. Solid state devices tend to abruptly transition
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from low distortion to extreme distortion (clipping). This is a good trait, since when operated right up to their maximum level solid state amplifiers can maintain excellent performance.
Digital audio circuits have similar characteristics.
Vacuum tube circuits, on the other hand, show a gradual increase in distortion throughout
their operating range. But instead of an abrupt break, the distortion increases incrementally. Until a level is reached where something in the circuit just completely falls apart (e.g. a
transformer saturates), the sound retains most of its original quality.
It’s the nature of the distortion that makes a difference, too. Solid state circuits run out of
headroom when the output voltage exceeds the power supply voltage. The result at this point
is gross distortion — the output becomes a square wave. Square waves are not found in sounds
that we consider musical, so our ears’ response to them is negative.
When a tube circuit distorts, the primary distortion product is even order harmonics. It so
happens that musical instruments also produce primarily even harmonics. By definition,
that’s what makes them “musical.” So you could say that tube circuits can add a musical
component to recorded sound. Fortunately, you can take your choice — keep the level reasonable and obtain good clean audio, or run the circuit into distortion and generate some
harmonics that weren’t there to begin with.
There may be other factors at work, too, that make vacuum tube amplifiers sound different,
even when operated in the low distortion part of their range. I have not proven any of these
esoteric theories to myself yet, but I think they have merit. Here’s one: perhaps the minimal
number of active devices in the signal path makes a difference. My preamp has only four
active devices, while even the simplest solid state op amp circuit may have dozens.
But back to the story ....
Finally the preamp was ready for some serious testing in the studio. A number of friends in
the music business, both musicians and studio owners, were intrigued with the possibilities
of this device, so it was easy to get volunteers to test, evaluate, and give me additional input
on the performance. This resulted in a few more changes that have been incorporated into
the final design.
The VT-1 represents, I believe, not only the best performance attainable with this type of
circuit, but it will meet or exceed the performance of today’s top-of-the-line solid state
designs, while still providing the clarity and musicality that characterizes vacuum tube
“sound.” Not only that, but it is built to last. Solid construction is used throughout, and only
the best parts are used, many of them Mil-spec.
I have found over the years that one insidious source of distortion in the recording studio
comes from connectors, switches, relay contacts, patch jacks, etc. Although these connections may be perfectly adequate when new, after a period of time the contacts oxidize and get
a little dirty. Individually they may not cause much of a problem, but put your audio through
a few of them and you will start to hear the degradation.
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To avoid this, the VT-1 design eliminates as many of these trouble-prone connections as possible. In fact, the only non-soldered connections from input to output are the input select and
phase reverse switches and the tube sockets. The input and phase switches are rotary types
with ceramic insulation and massive silver contacts. The phase switch is in the output circuit,
at line level, rather than on the input at very low level. In addition, the switches are chemically treated. And should they ever need cleaning, it is very easy to do a thorough job.
SO DOES THE VT-1 DUPLICATE the sound of my old RCA tube preamps? No, not exactly.
Gone are the hum, hiss, crackles, and bangs. And although I don’t have the old preamps to
do an AB comparison of the sound, I am certain that the better modern passive components
give the VT-1 a superior sound. The VT-1 has been used for all types of recording — vocals,
announcers, acoustic instruments, electric instruments, classical — and it works well with
them all.
The VT-2 was developed in 1995 in answer to requests from a number of our customers for
a two-channel version of the preamp. The VT-2 occupies the same amount of rack space, but
is four inches deeper than the VT-1.
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1.
SPECIFICATIONS
Input 150 ohms
Input Load
Impedance 1.5k ohms
Minimum Input
Level -65 dBm nominal
Maximum Input
Level @ 20 cps -30 dBm without pad
-5 dBm with 20 dB pad
Gain 53 dB minimum
Frequency
Response ± 0.2 dB 20 cps to 20 kc
± 0.5 dB 11 cps to 28 kc
-3 dB @ 0.5 cps & 50 kc
THD + Noise <0.25% 20 cps to 20 kc
Intermodulation
Distortion SMPTE: <0.80%
Signal to
Noise Ratio 74 dB minimum
Equivalent
Input Noise -124 dbm maximum
Output low-Z, transformer balanced
Maximum
Output Level +22 dBm unterminated
Power
Requirements 100, 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 VT-1 14 lbs (6.35 kg)
VT-2 18 lbs (8.16kg)
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