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
13
is gross distortion — the output becomes a square wave. Square waves are not found in sounds
e consider musical, so our ears’ r
hat w
t
esponse to them is negative.
he primar
When a tube cir
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
ponent to recorded sound. Fortunately, you can take your choice — keep the level rea-
com
sonable and obt
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 t
number of activ
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
he music business, both musicians and studio owners, were intrigued with the possibilities
t
of t
on the performance. This resulted in a few more changes that have been incorporated into
he final design.
t
The VT
cuit, but it will meet or ex
cir
designs, while still providing the clarity and musicality that characterizes vacuum tube
“sound.” N
the best parts are used, many of them Mil-spec.
heories to myself yet, but I think they have merit. Here’s one: perhaps the minimal
vice, so it was easy to get volunteers to test, evaluate, and give me additional input
his de
-1 represents, I believe, not only the best performance attainable with this type of
cuit distor
ain good clean audio, or r
e devices in the signal path makes a difference. My preamp has only four
ot only that, but it is built to last. Solid construction is used throughout, and only
ts, t
ceed the performance of today’s top-of-the-line solid state
y distortion product is even order harmonics. It so
un the circuit into distortion and generate some
e found over the years that one insidious source of distortion in the recording studio
v
I ha
comes from connectors, switches, relay contacts, patch jacks, etc. Although these connections ma
a little dir
a few of them and you will start to hear the degradation.
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
wit
at line le
ly treated. And should they ever need cleaning, it is very easy to do a thorough job.
SO DOES THE VT
Gone ar
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 instr
hem all.
t
y be perfectly adequate when new, after a period of time the contacts oxidize and get
. Individually they may not cause much of a problem, but put your audio through
ty
h ceramic insulation and massive silver contacts. The phase switch is in the output circuit,
vel, rather than on the input at very low level. In addition, the switches are chemical-
-1 DUPLICATE the sound of my old RCA tube preamps? No, not exactly.
e the hum, hiss, crackles, and bangs. And although I don’t have the old preamps to
uments, electric instr
uments, classical — and it works well with
VT-1 &
VT-2 Microphone Preamplifers
D.W. FEARN
14
The VT-2 was developed in 1995 in answer to requests from a number of our customers for
o-channel v
a tw
is four inches deeper than the VT-1.
ersion of t
he preamp. The VT-2 occupies the same amount of rack space, but
D.W. FEARN
VT-1 & VT-2 Microphone Preamplifers
1
SPECIFIC
.
TIONS
A
15
Input
Input Load
Impedance 1.5k ohms
Minimum Input
Level -65 dBm nominal
Maximum Input
Level @ 20 cps
Gain 53 dB minimum
Frequency
Response
THD + Noise
Intermodulation
Distor
Signal to
Noise Ratio 74 dB minimum
tion
150 ohms
-30 dBm without pad
-5 dBm with 20 dB pad
± 0.2 dB 20 cps to 20 kc
± 0.5 dB 11 cps to 28 kc
-3 dB @ 0.5 cps & 50 kc
<0.25% 20 cps to 20 kc
SMPTE: <0.80%
Equivalent
Input Noise
Output low-Z, transformer balanced
Maximum
Output Level +22 dBm unterminated
Power
Requirements 100, 120, or 220 VAC
Dimensions
Weight VT-1 14 lbs (6.35 kg)
VT-1 &
VT-2 Microphone Preamplifers
-124 dbm maximum
50/60Hz, 25 W
19” (48.26cm) W
5.25” (13.34cm) H
9” (22.86cm) D (VT-2 13” 22.9cm)
-2 18 lbs (8.16kg)
VT
D.W. FEARN
2.
DESCRIPTION
17
The Model VT
ding professionals with a sonically superior input device. (Unless there is a specific rea-
ecor
r
son to address a difference between the VT-1 and VT-2 preamplifiers, the device will be
referred to as the “VT-2.”) It is typically used in sound recording studios for recording individual tracks. A quality microphone is connected to a VT-2 input, and the VT-2 provides a
line-level output. In most situations, the VT-2 will feed directly to the input of the recorder.
The VT
moder
qualities of vacuum tubes, the VT-2 has a clarity, transparency, and warmth that solid state
preamps lack. Its modern design and construction allows the VT-2 to exceed the per-
or
f
It is designed f
balanced micr
denser microphones, a switchable 20 dB input pad, a phase (polarity) reversal switch, a
switchable in
formerless types), and a true VU meter. It is built to sound great for a long time, with top quality par
or t
f
All f
fully regulated. The Attenuation control potentiometer is a conductive plastic type for long,
noise-free operation.
-2 is a recreation of the classic tube preamps of the 1960s, updated with improved
n passive components and computer-aided circuit optimization. Because of the unique
mance of vintage vacuum tube preamps.
ts used throughout; all the transformers and many other components are custom-made
he VT-2.
our power supplies (filament, phantom power, B+, and meter amp) are solid state and
-1 and VT-2 Vacuum Tube Microphone Preamplifiers are designed to provide
or use in the professional recording environment. It accepts all low impedance
ophones. It f
put network to accommodate very low impedance microphones (typically trans-
eatures a regulated +48 volt supply for phantom powering con-
The VT-2 is not mass-produced. Each one is hand-made and meticulously tested and listened
ore shipment to the customer.
to bef
VT-1 &
VT-2 Microphone Preamplifers
D.W. FEARN
19
3.
TION
INST
The VT-2 is carefully packed for shipment and it should survive all but the most brutal handling. If t
damag
Included in the box:
here is any damage, keep the shipping material for use during any possible claim for
e with the shipper.
1) The VT-2 Microphone Preamplifier
ALL
A
2) Line cor
3) This instruction manual
d
Mounting
The VT-2 is designed for installation in a standard 19 inch rack. It requires 5.25 inches of vertical space, but additional spacing betw
adequate cooling. Ideally, a ventilated panel at least 1 rack unit high (1.25 inches) should be
alled above and below the VT-2 (and around any other heat producing equipment for that
inst
he bottom vent slots are not blocked. It is essential that air can flow into
matter). Be sur
the bottom and out of the top of the VT-2. Equipment that runs cool can last for a very long
time.
In tight equipment enclosures, be sure there is adequate air flow. Forced air cooling will benefit all y
The VT
Optional r
Moderate electrical and magnetic fields in t
degradation in noise performance, due to the well-shielded construction, but proximity to
devices with motors or large power transformers (i.e. tape machines or power amps) should
be avoided.
our equipment.
-2 can also be used without a rack, placed on a table, counter, or even on the floor.
e t
ubber f
eet are available, when requested at the time of the order.
een it and adjacent equipment is recommended for
he vicinity of the VT-2 should not cause any
hough the vacuum tubes in the VT-2 are selected for minimum microphonic response, it
Alt
is a good practice to avoid mounting locations that subject the VT-2 to very high sound or
vibration levels.
ower
P
The VT
will be shipped wir
VT-1 &
-2 is designed to operate from 100, 120, or 220-240 volt, 50/60 Hz power. The unit
ed for the voltage specified in the order, but may be changed in the field
VT-2 Microphone Preamplifers
D.W. FEARN
20
if necessary. (Call the factory for detailed instructions). The ground pin of the power cord is
nally connected to t
inter
and is required by most electrical codes. A grounding screw is provided on the back panel for
allations t
inst
check of the studio grounding scheme is needed. The VT-2 is less susceptible to grounding
oblems than many studio devices.
pr
hat use separate chassis gr
he chassis. This configuration is st
ounding. If gr
andard in professional equipment
ound loop hum is detected, a careful
Connections
The INPUT connectors are XLR-3 females wired with pin 1 ground, pin 2 “+” or “high,” and
pin 3 “-” or “lo
balanced.
The OUTPUT connectors are XLR-3 male wired with pin 1 ground, pin 2 “+” or “high,” and
pin 3 “-” or “low.” The VT-2 is optimized for feeding balanced bridging inputs. (Virtually all
moder
The “GND” ter
The Fuse is a 3AG-type 1 amp for 100 or 120 VAC operation, and 0.5 amp for 220-240 volts.
The AC input connector is used with the mating line cord (supplied). For 120 VAC operation,
t
The unit does not utilize any RFI filtering, and no RFI has been experienced, even when the
VT
n audio equipment has bridging in
his cord is a Belden 17250 or equivalent.
-2 is operated in close proximity to AM, FM, and TV broadcast transmitters.
(see Figure 1)
w.” The input matches 150 ohm (nominal) microphones and is transformer
puts.) The output is transformer-balanced.
minal is for use when an external grounding scheme is utilized.
Input and Output Connections
. Gold-plated XLR connectors are used for inputs and outputs. The input connec-
See Figur
tors are female and the outputs male.
e 1
Figure 1. The VT-2 rear panel connectors
D.W. FEARN
VT-1 & VT-2 Microphone Preamplifers
21
All connectors are wired according to AES standard: pin 1 is ground (shield), pin 2 is “high”
oltage on pin 2 of the input will result in a posi-
or “+,” and pin 3 is “lo
tive voltage on pin 2 of the output (with the Phase Reverse switch set to Normal).
w” or “-.” A positiv
e v
Grounding and Shields
A full discussion of proper studio wiring schemes is beyond the scope of this manual, but, in
general, the Input mating XLR connector must have the cable shield connected to pin 1. With
most micr
cable.
Whet
in y
however, although not recommended, the shields can often be connected at both ends without a problem.
ophones, this shield must also be connected to pin 1 at the microphone end of the
her the shield is connected to pin 1 of the output connector depends on the standard
our studio. The shield should be connected to ground at only one end of the output cable;
VT-1 &
VT-2 Microphone Preamplifers
D.W. FEARN
22
D.W. FEARN
VT-1 & VT-2 Microphone Preamplifers
Input
OPER
4.
TION
A
23
Since t
ed cable is used. Ther
between the microphone and the VT-2 input. This can be achieved with a dedicated line from
an XLR connector in t
runs have little effect on the performance of the VT-2, it is preferable to keep the input line
he input cable will be carrying very low level audio, it is important that a well-shield-
e should be no additional connectors, patch jacks, switches, etc.
he studio to each VT-2 in the control room. Although long input cable
2
6
1
3
5
3
4
6
5
8
7
1
Figure 2. VT-2 front panel controls and indicators
as short as possible. One successful method is to place the VT-2 in the studio with only a
short cable to the microphone. Line level from the VT-2 output is then fed back to the con-
oom. Avoid locating the VT-2 where it will be subjected to high sound levels or excessive
ol r
tr
vibration (such as on a drum riser).
Output
The output of the VT-2 is line level, transformer balanced. Note that vacuum tube equipment
e sensitive to load impedance than solid state units. The VT-2 design was optimized for
is mor
feeding a balanced bridging input (20k ohms or greater). When feeding a 600 ohm load,
there may be a slight degradation of some of the specifications. In modern studio equip-
-2 has an input
ment, bridging line inputs are universal. If the device being fed by the
VT
termination switch, that switch should be in the “off” position.
VT-1 &
VT-2 Microphone Preamplifers
D.W. FEARN
24
The VT-2 can feed balanced or unbalanced inputs with no need for any modification in output wiring. Eit
and pin 3 grounded in unbalanced configurations.
C
ONTROLS (see Figure 2.)
+48 volt phantom power on/off switch. Supplies 48 volts for phantom powered condenser
microphones. Switch the 48 volts off for dynamic and ribbon microphones, or condenser
ophones with their own power supplies (e.g. vacuum tube condensers).
micr
her pin 2 or 3 can be gr
ounded, alt
hough pin 2 is normally used as the ”hot”
Attenuation contr
Phase switch
ol. Adjusts the output level.
.
Usually in the Normal position except when it is necessary to reverse
the polarity of a microphone.
Meter
VU meter
Pilot lamp
Power
ower switch and indicator
P
Primary power is applied to the VT-2 circuits when the Power switch (8) is in the up position.
The amber pilot lamp (7) indicates that the unit is on. It takes about twenty seconds for the
preamp to start working, but it is suggested that you turn on the power at least five minutes
prior to use. The tubes ar
ing tem
switch.Turns on or off the VU meter. Normally ‘on’ except during some test
VT
VT-2.
-2.
procedures or when purposely over
.
Monitors the output level of the
-driving the
. Illuminates when AC power is applied and the Power switch is on.
switch. Controls primary AC power to the VT-2.
e often noisy until all the internal elements reach a stable operat-
perature.
Input “0” position.
he “0” position, microphone audio is connected directly to the input transformer. This
In t
provides the proper amplification for most condenser microphones, and will be used in many
micing situations. In the “0” position, the VT-2 can accept up to about a -30 dBm input signal at 20 cps wit
D.W. FEARN
h full g
ain (53 dB) without an increase in distortion.
VT-1 & VT-2 Microphone Preamplifers
25
Input “-20” position.
In the “-20” position, a pad is inserted between the input connector and the input
ormer. This position would be used when the level is too high for the “0” position. On
transf
condenser microphones that have a switchable pad, it will usually be necessary to use a -10
or -20 dB pad in t
ophone electronics. Whether this is used in conjunction with or as a substitute for the
micr
VT-2 pad should be determined by experimentation. For the cleanest sound it is generally
preferable to pad at the microphone first, then at the VT-2 if necessary. The sound of some
microphones will change slightly from the “0” to “-20” position. This is a function of the
interaction between the microphone transformer and the VT-2 input transformer.
he mic when recording very high sound levels to prevent overload of the
Input Lo-Z position
tain microphones (e.g. the Neumann TLM-
The Lo-Z position should be used only wit
series, some newer Schoeps) that exhibit an extremely low output impedance. These microphones typically f
ing netw
ance with these microphones. The best position can be determined by listening, although the
diff
ork is inser
erence may be subtle on many mics.
eature a transformerless output. In the Lo-Z position, an impedance-match-
ted ahead of the input transformer and this optimizes the VT-2 perform-
h cer
he 20 dB pad is needed with a very low impedance microphone, simply select the -20 posi-
If t
tion of t
eep in mind that many musical instruments and vocalists produce peaks that are as much
K
as 20 dB above the average level. Often the “peak clipping” that occurs is of such short duration that it is not easily noticed and may be acceptable in many situations. This is particularly tr
ly gradual increase in harmonic distortion. Over-driving the VT-2 will increase second harmonic content, adding fullness and richness to t
best position f
+48 V
Solid-st
er
plished by feeding the positive side of a 48 volt power supply to both pin 2 and 3 of the input
connector (through precision matched resistors), and the negative to pin 1 (ground). The DC
voltage is recovered at the microphone with negligible effect on the audio signal.
acuum tube condenser mics and dynamic microphones do not require this power and the
V
+48 switch (1) should be turned off when using non-phantom powered mics. Although leaving t
eportedly been damaged when connected or disconnected from phantom powering. Also,
r
some engineers f
with the phantom power on.
he INPUT switch. The im
h the VT-2 because there is no hard threshold of clipping but rather a comparative-
ue wit
he Input control.
or t
olt Swit
ate condenser micr
ed: the microphone electronics are DC powered through the audio cable. This is accom-
he +48 on will not damag
ch (1)
eel that the performance of some dynamic mics may be subtly degraded
pedance match will be correct.
he sound. Use your ears to determine the
ophones in professional environments are usually phantom pow-
e any properly wired balanced mic, some ribbon mics have
VT-1 &
VT-2 Microphone Preamplifers
D.W. FEARN
26
The phantom powering circuit used in the VT-2 is suitable for use with all Neumann micro-
2 and 48 v
equir
G 1
y voltage between 12 and 48.
e an
phones, AK
Shure SM81 and 85 mics, Crown PZM mics, and virtually all other phantom powered mics
hat r
t
When turned off, the phantom-power resistors are completely disconnected from the circuit
he VT-2.
in t
olt microphones, B&K phantom powered mics, all Schoeps mics,
enuation
Att
ttenuation control (3) is between the second and third amplification stages of the VT-
The A
2. It should be adjusted to pr
In some situations, the Attenuation control can have an effect on VT-2 distortion, but at +4
dBm output the input stages will overload before the output. If you find that you need to run
ttenuation control near the bottom of its range, the input level may be too high and you
he A
t
might want to experiment with the microphone pad (if it has one), the VT-2 Input switch
position, or bot
adds “edg
If it is necessar
is too low and any padding at the mic or on the VT-2 should be reduced or removed.
Phase
This switch reverses the polarity of the output of the VT-2. A detailed discussion of the application of phase r
en when there is only one microphone being recorded, it may be useful to try the
Ev
e
“R
polarity t
ror or the use of vintage equipment built before standardization may reverse the polarity in
er
the recording/monitoring chain. The effect of reversed absolute polarity is subtle, but significant with some sounds. If the “Reverse” position sounds better, use it.
e,” “po
(4)
verse” position of the Phase control. Although there is supposed to be standardization in
hroughout the professional audio equipment industry, it is possible that a wiring
(3)
ovide the desired output level to the console or recorder.
h. Let your ears be the judge. Slight to moderate overdriving of the VT-2 often
wer,” or “excitement” to the sound.
y to operate the Attenuation control near the top of its range, the input level
versal of individual microphones is beyond the scope of this manual.
e
h more than one microphone on the same sound source (or picking up leakage from
it
W
her sound source), the Phase switch may have a profound effect on the audio quality.
anot
Whichever position sounds best is correct. A check of monaural compatibility (by summing
the various mics) should also be performed.
VU Met
The VU meter (6) measures the output level directly across the secondary of the output
transf
D.W. FEARN
er and VU Meter ON/OFF (5 & 6)
mer, through an isolation amplifier. It is calibrated so that a +4 dBm output will indi-
or
VT-1 & VT-2 Microphone Preamplifers
27
cate 0 on the meter. This is the standard “0 VU” level for all professional audio recording
equipment built since t
erly aligned recorder. (This reference level can be changed; see the Maintenance Section.)
This is a tr
In some applications, or while testing the VT-2, the output level may be considerably higher
han 0 VU. To prevent damage to the meter, it should be turned off under these conditions,
t
using t
Occasional meter pinning will not damage the meter.
Benc
ue VU meter
he Meter switch (5). This switch has no ot
h Test
he early 1
, and conf
970s. “0 VU” on t
ms to ASA Standard C16.5-1954.
or
he VT-2 should result in “0 VU” on a prop-
her effect on the operation of the VT-2.
If desir
150 ohms impedance, balanced, and the output should feed a balanced bridging input of the
audio analyzer. Measured bandwidth should be 22 cps to 22 kc to obtain the same readings
as t
2. Keep the results for comparison in future maintenance tests.
ed, test the VT-2 before installation. The source generator should be set to -50 dBm,
actory test results. Compare your measurements with the test data supplied with VT-
he f
Initial Set-Up
The VT-2 should be installed as detailed in the Installation section. With the outputs connected to an appr
monitor the VT-2 output. Apply power and wait about twenty seconds for the tube filaments
et up to temperature. Check for hum, buzz, or other noise. For the first few minutes after
to g
a cold start it is not unusual for the VT-2 to produce hiss, pops, and microphonic “clanks” as
the internal elements of the tubes expand from the heat. Correct any ground loop problems
e proceeding.
or
bef
opriate destination (typically to audio r
ecorder inputs), configure the studio to
VT-1 &
VT-2 Microphone Preamplifers
D.W. FEARN
28
The controls should be set as follows. The numbers refer to Figure 2 on page 21.
ower (8). . . . . . . . . . . . . . . . . . . . . . On
• P
• Input (2). . . . . . . . . . . . . . . . . . . . . . . 0
• Att
enuation (3) . . . . . . . . . . . . . . . . . mid-point
• Phase (4) . . . . . . . . . . . . . . . . . . . . . . Normal
• Met
er (5)
• +48 (1)
, use the Phase reverse switch. The +48 switch should be off except when need-
If necessar
ed for phantom powered microphones. Turn the VU Meter off if it is pinning.
The best indication of pr
wide operating range and quite often the exact position of the controls is relatively non-critical.
Be cer
put. +4 dBm is t
older equipment may be designed for 0 dBm or +8 dBm, either of which can be easily accommodated b
Semi-professional equipment frequently uses a reference level of -10 dBv (roughly 14 dB
er than pro equipment). Although the VT-2 can fed the unbalanced, -10 dBv inputs of
w
lo
semi-pro gear with no difficulty, the VT-2 VU meter will have to be recalibrated in order to be
useful. (See Section 6 - Maintenance.)
y
oper operation of t
tain that the output level of the VT-2 is appropriate for the device connected to the out-
he accepted st
y the VT-2.
. . . . . . . . . . . . . . . . . . . . . . On
. . . . . . . . . . . . . . . . . . . . . . . . as required for mic
he VT-2 is how it sounds. This preamplifier has a
andard level for all professional recording equipment. Some
SUGGESTIONS:
You have chosen to use the VT-2 because of the superior sound it provides. To gain the maximum benefit fr
actors do not adversely affect the sound quality.
f
1. The VT-2 can be located in the studio or in the control room, but use the shortest
possible cable between the mic and the VT-2.
2. Use t
e, but do use a good cable designed for low impedance microphones. A quality
wir
cable with gold-contact connectors is best.
3. Ther
between the mic and the VT-2 input.
D.W. FEARN
om your investment, it is important that you hook up the VT-2 so that other
he best quality mic cable you can. We don’t believe you have to use esoteric
e should be no additional cables, connectors, junction boxes, patch jacks, etc.
VT-1 & VT-2 Microphone Preamplifers
4. The output of the VT-2 should be fed directly to the recorder through the shortest
oid additional cables, connectors, junction boxes,
practical lengt
h of quality cable. A
v
punch blocks, or patch jacks. Use gold contact connectors if possible. Do not go
hr
t
he mixing console unless y
ough t
ou absolutely need its features for the track you
are cutting.
29
5. In g
eneral, for superior sound, we recommend recording directly to the recorder with
ocessing (com
no pr
pression, equalization, gating, etc). Any processing can be added
in the mix, if necessary. You may find that far less processing is required when using
-2. If processing is required while recording the track, insert the processing
he VT
t
device after the VT-2 and before the recorder..
VT
VT
-2
-2
VT
-2
(RECOMMENDED)
(Use this configuration only
if the console functions are
absolutlely required)
VT-1 &
VT-2
Figure 3. T
ypical Studio Interconnections with the VT-2
VT-2 Microphone Preamplifers
(Recommended only if pro
cessing is necessar
cutting the track)
y while
D.W. FEARN
-
30
D.W. FEARN
VT-1 & VT-2 Microphone Preamplifers
31
5.
TION
THEOR
Y OF OPER
Input section
ophone level (150 ohm source impedance, balanced, -50 dBm nominal) audio enters
Micr
hrough the XLR-3 female INPUT connector to the three-position Input selector switch. In
t
the 0 position, the input is connected directly to the input transformer. The load imposed on
he microphone is 1500 ohms, but varies slightly with frequency but is never lower than 1100
t
ohms.
In the -20 position, the input passes through a balanced 20 dB pad. This pad is designed to
maint
ain appr
oximately the same load on the microphone as the input transformer.
A
All switching is t
Phantom po
om the phantom power supply is switched on and off by the front panel +48 switch. A
1 fr
+5
resistor drops the voltage as required depending on the current being drawn by the condenser micr
and 48 volt condenser microphones. The phantom powering resistors are precision matched
to 0.10% or better. They provide exactly equal voltage to pins 2 and 3 respectively of the input
connector
cated contacts. In the “Off” position, the phantom power resistors are completely disconnected fr
om the microphone connector.
Phase re
In the Normal position, input/output phase (polarity) is maintained (which must be inverted
due to t
tion. Switching is accom
cated contacts.
he design of the circuit). The balanced output is polarity reversed in the Reverse posi-
hrough sealed gold-contact instrumentation relays with bifurcated contacts.
ering
w
ophone electronics. This makes the phantom power supply universal for most 12
. The switching is t
hrough sealed gold-contact instrumentation relays with bifur-
versal switc h
plished with a sealed, gold-contact instrumentation relay with bifur-
VT-1 &
VT-2 Microphone Preamplifers
D.W. FEARN
32
Input transformer
The input transformer is custom-made for D.W. Fearn by Jensen Transformers, Inc. and rep-
esents the state of the art in transformer design. It exhibits extremely flat frequency
r
esponse, low phase shift, excellent square wave response, low distortion, and high noise
r
immunity. The secondary of is connected directly to the grid of the first amplifier stage.
Figure 4. VT-1/VT-2 Block Diagram (one channel shown)
First st
The first stage is a selected 6072 configured as a Class A voltage amplifier with a gain of
appr
lattens the frequency response, and makes the gain of the first two stages less dependent on
f
individual v
Second st
The output of the first stage is coupled to the grid of the second stage through a polystyrene
capacitor. This stage operates as a Class A voltage amplifier with a gain of approximately 30.
The plate is coupled t
otary potentiometer (Attenuation).
r
D.W. FEARN
age
ximately 30. Negative feedback from the plate of the second stage reduces distortion,
o
acuum tube characteristics.
age
ough a polypropylene capacitor to the top of a conductive plastic
hr
VT-1 & VT-2 Microphone Preamplifers
Third Stage
33
The ar
also operates Class A wit
the grid of the output stage through a polystyrene capacitor.
Output St
The output st
impedance (approximately 800 ohms). The cathode output is coupled through a proprietary
polypropylene capacitor to the primary of the output transformer. This is a custom Jensen
transf
VU
A two-stage IC operational amplifier is used to isolate the VU meter from the VT-2 output. An
output sample is derived from th secondary of the output transformer. The VU meter amplifier and chassis.
m of the Attenuation potentiometer feeds the grid of the third stage (a 6072A), which
ain of approximately 30. This stage is capacitively-coupled to
h a g
age
e operates as a cathode follower, presenting a comparitively low output
ag
mer and is capable of low distortion output at levels exceeding +24dBm.
or
Meter and Meter Amplifier
2 volt regulator are constructed on a printed circuit board mounted above the main
1
The meter and meter amplifier may be totally disconnected from the circuit by the Meter
switch. The meter calibrationis set with a 20-turn trimmer potentiometer. Reference level (0
VU) can be set fr
forming to ASA Standard C16.5-1954.
ower Supplies
P
Primary power from the AC mains is connected to the VT-1/VT-2 through a standard IEC
power input connector. The Power switch energizes all four power supplies. A fuse, accessible on t
ar below its rated voltage of 28. The life of the bulb is lengthened, and the light output is
f
mor
unit custom-made for the VT-1/VT-2 and has primary taps for 100, 120, or 220-240 volt operation. The VT-2 may be rewired for different mains voltages.
VT-1 &
he r
e compatible with other modern studio equipment. The power transformer is a toroidal
VT-2 Microphone Preamplifers
om -20 to o
ear panel, protects the VT-1/VT-2. The Pilot lamp is a type 1819 bulb, operated
ver +20dBm. The level indictor is a custom true VU meter con-
D.W. FEARN
34
Filament Supply
The po
ulated to 12.0 volts by a three-terminal regulator. The negative output of this supply is
gr
eff
Met
A negative version of the filament supply provides regulated -12 volts for the IC operational
amplifiers in the meter amplifier circuit. This regulator is located on the Meter Amp PCB.
B+
T
tered with long-life, low-leakage, computer-grade filter capacitors before being regulated and
extensiv
rate B+ supplies are used in the VT-2, one for each channel.
wer transformer output is rectified by a bridge rectifier and filtered before being reg-
ounded. Although the filaments are rated for 12.6 volts, utilization of 12.0 volts has no
ect on the operation of the VT-1/VT-2.
er Amplifier Supply
Supply
o separate regulated voltages are required for the plates of the VT-1/VT-2. The B+ is fil-
w
ely bypassed and decoupled. The negative side of the supply is grounded. Two sepa-
+48 Supply
The regulator circuitry is on the power supply printed circuit board. The actual output of this
olts, which is reduced to 48 volts through the decoupling resistor at the micro-
supply is 5
phone input (see Phantom Power, above).
1 v
D.W. FEARN
VT-1 & VT-2 Microphone Preamplifers
35
6.
MAINTENANCE
The VT-2 is built with only the highest quality parts and will prove to be extremely reliable.
acuum tubes and electrolytic capacitors, however, have a finite useful life and must be
V
eplaced eventually.
r
op Co
T
Removing the top cover allows access to the vacuum tubes, the VU meter calibration, and to
the rotary switches. 6-32 allen-head bolts must be removed (phillips-head machine screws are
used on t
are over the tubes (towards the back on the VT-2, to the left when facing the unit on the VT-
1).
acuum Tubes
V
our 6072A tubes are used in the VT-2. V101 and V201 are the input stage and V102 and
F
V202 are the output stages. There can be as much as a 15 dB difference in noise level among
an assortment of tubes, and the tubes used in the V101 and V201 positions should be carefully chosen to maint
V102 and V202 are far less critical and almost any quality off-the-shelf 6072A will perform
satisf
performance.
Tube lif
failure is rare with this type of device, but a gradual increase in noise, microphonics, distortion, or a reduction in headroom, should indicate the need for replacement. It is recommended that you periodically perform a quick noise and distortion check on the VT-2 and compare
he results to previous measurements.
t
ver Removal
he VT-1). When replacing the cover, position it so that the slotted ventillation holes
w noise. Selected low-noise tubes are available from D. W. Fearn.
ain lo
actorily. 12AY7 tubes may be substituted for the 6072A tubes, but with slightly reduced
e is difficult to pr
edict, but it will probably be measured in years. Catastrophic tube
Tubes also sometimes develop a microphonic response — they will respond to ambient noise
and vibration. This can be an insidious problem since measurements in a quiet room will indicate perfect performance. Gently tapping the tube shields while listening to the output at a
mal monitor le
nor
meter connected to t
dBm is excessive. Replacement is indicated unless the VT-2 always operates in a quiet and
vibration-free environment.
hough y
Alt
or V20
$56.00 for a selected low-noise 6072A for V101 or V201, and $24.00 for a tested but less rigorous noise-spec 6072A for V102 or V202. We test the tubes in a VT-2 after a burn-in period
VT-1 &
ou could purchase a batch of 6072A tubes and select the quietest one(s) for V101
1, it may be cost effective to buy a low-noise tube from the us. Current prices are
VT-2 Microphone Preamplifers
vel should reveal nothing more than a slight “clank.” On a peak-reading
he VT-2 output, with 50 dB gain, any microphonic response above -55
D.W. FEARN
36
and grade them according to noise, microphonic response, distortion, and other characteristics. A lo
The base pins of v
contact resistance and oxidation prevention. When handling these tubes, care should be
taken to avoid removing or contaminating the treatment. Use a lint-free cloth or paper towel
void direct contact between any part of the tube and your fingers.
to a
Remember that vacuum tubes may be quite hot during operation. Protect your fingers during tube replacement. The VT-2 should be turned off before removing tubes. Allow at least
one minute for the filter capacitors to discharge before tube removal or insertion.
w-noise tube fr
acuum tubes supplied b
om us will meet t
he original VT-2 specifications.
earn have been chemically treated for low
. F
y D.W
Tubes ar
wit
e made of glass and will break if dropped or even bumped in a critical area. Handle
h care.
Electrolytic Capacitors
The VT
ponents (e.g. electr
ment becomes obsolete. The electrolytic capacitors used in the VT-2 typically will last at least
tw
capacitors should be suspected. They should be replaced with new capacitors of equivalent
capacit
y
Electr
ment, the same considerations as with the filter capacitors should be followed.
Rot
-2 is designed and built to last for a long, long time, and it is possible that some comolytic capacitors) ma
enty years. If there is a measurable and/or audible increase in 120 cps noise, the filter
ance and voltage rating, and the replacements should be specified for a minimum ten-
ear service lif
olytic capacitors are also used as plate and cathode decouplers. In choosing a replace-
e.
y reach the end of their life long before the equip-
ary Switches
otary switches in the VT-2 were chosen for their self-cleaning property. In a clean envi-
The r
onment (non-smoking, with frequent room and equipment vacuum cleaning), and with reg-
r
ular use, the rotary switches should require little attention for years. Audio is not switched in
the rotary switch itself -- the switches only control gold-contact relays. But if cleaning
becomes necessary (as evidenced by intermittent relay switching), we recommend using De-
w the instructions provided with the product.
Oxit (Caig Laboratories). F
If the switches are not used regularly, oxide and dirt can build up on the contacts. Rotate the
switches through their range a few times on a weekly basis to keep them clean.
The togg
le switches should ne
D.W. FEARN
ollo
ver require cleaning.
VT-1 & VT-2 Microphone Preamplifers
VU Meter Calibration
37
The meter am
ly required. If the output reference level needs to be changed, here is the procedure:
. Remove the VT-2 top cover (see above).
1
2. Terminate the output in the same impedance as the VT-2 will normally be feeding.
This will almost alw
mor
3. Apply A
45 dBm into the VT-2 input.
4. Measur
ence is desir
5. Set t
sure the meter is turned on. This control is a 25-turn pot so the adjustment is easy. A
small screwdriver or alignment tool is necessary. The controls are located on the
meter am
trim pot(s) on the VT-1/VT-2. See Figure 5 on page 38.
eplace t
6. R
back).
plifier circuit is stable and re-adjustment of the meter calibration is not normal-
ays be a high-impedance bridging termination of 20k ohms or
e.
C power. Allow at least a fifteen minute warm-up. Feed a 1 kc tone at about -
e the output and set the Attenuation control for +4 dBm (or whatever refer-
ed).
he Meter Calibration trimmer potentiometer for 0 VU on the VT-2 meter(s). Be
plifier printed cir
he top cover. Be sure the ventillation slots are over the tubes (towards the
cuit board on the top of the chassis and they are the only
Troubleshooting
Most pr
ily reveal the problem, feel free to call the factory for assistance. If you lack access to a qualified service technician with vacuum tube equipment repair experience, you may return the
VT
oblems will be traced to defective vacuum tubes. However, if normal tests do not eas-
he factory for repair. Call first, however, for shipping information.
-2 to t
Fig. 5 VU Meter Amplifier PCB
VT-1 &
VT-2 Microphone Preamplifers
D.W. FEARN
38
Warranty Repair
he VT-2 should develop a problem during the five-year warranty period, call the factory for
If t
return shipping instructions. We will repair and return your VT-2 quickly. Note that the warranty does not co
ver vacuum tubes, which must be periodically replaced.
D.W. FEARN
VT-1 & VT-2 Microphone Preamplifers
39
LP- 1 Line Pad Accessory
1 Line Pad is an accessory for the VT-1 and VT-2 Vacuum Tube Microphone
The LP
Preamplifiers that allows the preamp to accommodate a line-level signal of approximately +4
dBm. This is used when it is necessary for the VT-2 to process a line-level signal, such as in
final processing of a mix.
N
ments, t
if t
To use the LP-1, simply connect the +4 dBm, balanced signal source (such as the output of a
mix
2 preamp. The VT-2 output would then feed the recorder.
The loss t
high for the VT-2 in the “0” position of the Input switch. The “-20” position will provide a
better le
some circumstances, such as when the line level is particularly low, or when it is advanta-
eous to over-drive the VT-2 to obtain a “fatter” sound. As always, use your ears to determine
g
which position y
-
ote that the LP-1 is not designed to be used as a “direct box.”For most musical instru-
he level and impedance are not correct for use with the LP-1. An exception would be
he instrument had a professional level, low-impedance, balanced output for recording.
er) to t
he LP-1 “Input.” Then connect the LP-1 “Output” directly to the input of the VT-
ough the LP-1 is approximately 43 db. With a +4 dBm input, the output is a bit
hr
vel in most circumstances. However, it may be desirable to use the “0” position in
ou find most appr
opriate for the project.
The LP
and no routine maintenance is necessary.
-1 is a passive device and requires no power. There is little to go wrong with the LP-1,
VT-1 &
VT-2 Microphone Preamplifers
D.W. FEARN
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