D.W. Fearn VT-12 User manual

VT-12
Vacuum Tube
70dB
Microphone reamplifier
Operating Instructions
D.W. FEARN

D.W. FEARN VT-12 Microphone reamplifer
How to Contact us:
Telephone: [phone]
Fax: [phone]
Mail: P.O. Box 57, Pocopson, PA 19366 U.S.A.
Shipping Address: 182 Bragg ill Road
West Chester, PA 19382 U.S.A.
e-mail: [e-mail]
www.dwfearn.com

D.W. FEARN HAND-CRAFTED
PROFESSIONAL
RECORDING EQUIPMENT
www.dwfearn.com
P.O. Box 57
Pocopson, PA 19366 U.S.A.
Tel: [phone]
Fax: [phone]
Certificate of RoHS Compliance
D.W. Fearn is committed to manufacturing products that are fully-compliant with
the EU Ro S Directive.
The following products are compliant:
VT-1
VT-12
VT-3
VT-4
VT-7
VT-12
VT-15
LP-1
PDB
This declaration is based on our understanding of the current Ro S 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

4

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VT-12 Vacuum Tube Microphone reamplifier
Final Test Report
Model _______________ Serial Number_______________Mains Voltage: 100 120 240
Date ___________________ Tested by ________ Assembled by _____________________
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
Equivalent Input Noise _______ dB
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 ______________
hase Reverse __________
+48V ___________________
Listening Test ____________

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T a b l e o f C o n t e n t s
Final Test Report
CE Certification Data
Warranty .........................................................................................7
istory of the VT-series Mic Preamps .............................................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
List of Illustrations
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
materi al.
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 ©1995-2004 D.W. Fearn & Associates

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VT-12 Microphone reamplifer
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Limited 5-Year Warranty
During the warranty period, D.W. Fearn will, at no addi tional charge, repair or replace
defective parts with new parts.
This warranty does not extend to any VT-12 that has been damaged or rendered
defective as a result of accident, misuse, or abuse; by the use of parts not manufac-
tured or supplied by D.W. Fearn; or by unauthorized modification of the VT-12.
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 war-
ranties, express or implied, including any implied warranty of merchantability and fit-
ness for a par ticular purpose.

D.W. FEARN
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History of the VT-series
Vacuum Tube Microphone reamplifiers
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 remem-
bered to take it. That old record ing 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 record ings. 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 sal vaged 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 broad-
cast type (tube) limiter.
These preamps were 1940s vintage. They used octal metal tubes with a shielded grid cap, a
cylindrical output trans former the size of a coffee can, and a huge power supply on a sepa-
rate chassis. They were a lot of trouble — the tubes were microphonic and the output was
often noisy. In addi tion to the hums and hisses, occasionally a take would be ruined by crack-
les 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 pan-
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. it them with a bit
of excessive level and the sound became real fat. it them with just the right level and they
sounded warm and intimate.
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 con firmed that tubes were still easy
to obtain.

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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 poly-
styrene and polypropylene capacitors were preferable to the old paper types. The power sup-
plies could be solid state — and easily regulated. Electrolytic filter capacitors were smaller.
The only parts that remained to be found were top quality audio transform ers 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 avana coming through weakly but clearly.) Profes sional 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 bet-
ter shielding and a better layout. This one became the testing ground for additional experi-
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 iter-
ations. 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 record ing. 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 mix-
ers, 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 distor-
tion throughout their operating amplitude range. The difference in sound is particularly evi-
dent when the circuit runs out of headroom. Solid state devices tend to abruptly transition
from low distortion to extreme distortion (clipping). This is a good trait, since when operat-
ed 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 incremental-
ly. 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

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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 instru ments 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 rea -
sonable 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. ere’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 possi bilities
of this device, so it was easy to get volunteers to test, evaluate, and give me additional input
on the perfor mance. 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 perfor mance 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 musi cality 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 connec-
tions 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.
To avoid this, the VT-1 design eliminates as many of these trouble-prone connections as pos-
sible. 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 chemical-
ly treated. And should they ever need cleaning, it is very easy to do a thorough job.
SO DOES T E 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, elec tric instruments, classical — and it works well with
them all.

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The VT-12 was developed in 1995 in answer to requests from a number of our customers for
a two-channel version of the preamp. The VT-12 occupies the same amount of rack space,
but is four inches deeper than the VT-1.
When the VT-1 and VT-12 preamps were designed, condenser mics were the standard for pro-
fessional recording. But after 2000, ribbon microphones started to make a comeback. The
smooth response, highly-defined pattern, and unique sound of ribbon mics makes them an
excellent choice for many recording situations.
owever, ribbon microphones have a very low output -- often 20dB lower than condenser
mics. The VT-1 and VT-12 were optimized for condenser mic level, and although the VT-
1/VT-12 were often used with ribbon mics, the 54dB of gain in the stock VT-1/VT-12 was not
quite enough for some situations.
We developed a simple mod that added 6dB of gain to the VT-1 or VT-12, and that was enough
to make ribbon mics useable in almost all situations.
In 2010 I decided to work on a new preamp design that would have higher gain than the VT-
1/VT-12. The new preamp, which became the VT-12, was designed around the 12AT7, a dual
triode vacuum tube that had more gain than the 6072A used in the VT-1/VT-12. A prototype
was built and tested, and it worked very well with ribbon mics. The new preamp had 70dB at
maximum gain.
After speaking with several ribbon microphone designers, I learned that they all agreed that
the load impedance presented by the mic preamp had to be higher than the standard 1500
ohms if I wanted to get the most out of the ribbon sound. Consequently, the VT-12 has an
input impedance of 10k ohms.
Although the VT-12 can be used with condenser mics, it is really a special-purpose preamp
designed with the requirements of the ribbon mic in mind. With a condenser mic, the high
gain of the VT-12 may make it very touchy to adjust (you will almost certainly need the 20dB
pad), and the non-standard input impedance will make many condenser mics sound a bit
harsh and brittle on the high end.

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1 .
S E C I F I C AT I O N S
Input Designed to match ribbon mics
Input oad
Impedance 10k ohms
Minimum Input
evel -70 dBm nominal
Maximum Input
evel @ 20 cps -30 dBm without pad
-5 dBm with 20 dB pad
Gain 70 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 evel +22 dBm unterminated
Power
Requirements 100, 120, or 220 VAC
50/60Hz, 25 W
Dimensions 19” (48.26cm) W
5.25” (13.34cm) H
13” (22.9cm) D
Weight VT-12 18 lbs (8.16kg)
Shipping Weight 24 lbs (10.9kg)

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2 .
D E S C R I T I O N
The Model VT-12 Vacuum Tube Microphone Preamplifier is de signed to provide recording
professionals with a sonically superior input device for ribbon microphones. It is typically
used in sound record ing studios for recording individual tracks. A quality ribbon microphone
is connected to a VT-12 input, and the VT-12 provides a line-level output. In most situations,
the VT-12 will feed directly to the input of the recorder.
The VT-12 is a recreation of the classic tube preamps of the 1960s, updated with improved
modern passive components and computer-aided circuit optimization. Because of the unique
qualities of vacuum tubes, the VT-12 has a clarity, transpar ency, and warmth that solid state
preamps lack. Its modern design and construction allows the VT-12 to exceed the per -
formance of vintage vacuum tube preamps.
It is designed for use in the professional recording envi ronment. It accepts all low impedance
balanced microphones. It features a regulated +48 volt supply for phantom powering con-
denser microphones, a switchable 20 dB input pad, a phase (polarity) reversal switch, and a
true VU meter. It is built to sound great for a long time, with top quality parts used through-
out; all the transformers and many other components are custom-made for the VT-12.
All four power supplies (filament, phantom power, B+, and meter amp) are solid state and
fully regulated. The Gain control potentiometer is a MIL-spec carbon type for long, noise-free
operation.
The VT-12 is not mass-produced. Each one is hand-made and meticulously tested and lis-
tened to before shipment to the customer.

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3 .
I N S TA L L AT I O N
The VT-12 is carefully packed for shipment and it should survive all but the most brutal han-
dling. If there is any damage, keep the shipping material for use during any possi ble claim for
damage with the shipper.
Included in the box:
1) The VT-12 Microphone Preamplifier
2) Line cord
3) This instruction manual
Mounting
The VT-12 is designed for installation in a standard 19 inch rack. It requires 5.25 inches of
vertical space, but addi tional spacing between it and adjacent equipment is recom mended for
adequate cooling. Ideal ly, a ventilated panel at least 1 rack unit high (1.25 inches) should be
installed above and below the VT-12 (and around any other heat producing equipment for
that matter). Be sure the bottom vent slots are not blocked. It is essential that air can flow
into the bottom and out of the top of the VT-12. Equipment that runs cool can last for a very
long time. A 1RU vented panel, the D.W. Fearn VRP painted in the same red as our products
is available.
In tight equipment enclosures, be sure there is adequate air flow. Forced air cooling will ben-
efit all your equipment.
The VT-12 can also be used without a rack, placed on a table, counter, or even on the floor.
Optional rubber feet are available, when requested at the time of the order.
Moderate electrical and magnetic fields in the vicinity of the VT-12 should not cause any
degradation in noise perfor mance, 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.
Although the vacuum tubes in the VT-12 are selected for minimum microphonic response, it
is a good practice to avoid mounting locations that subject the VT-12 to very high sound or
vibration levels.

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ower
The VT-12 is designed to operate from 100, 120, or 220-240 volt, 50/60 z power. The unit
will be shipped set for the voltage specified in the order, but may be changed in the field if
necessary. A rear-panel, recessed slide switch changes the input voltage for either 120V or
230V. When the voltage is changed, the value of the fuse on the back panel must be changed
as well. (100V operation is also possible. Call the factory for detailed instructions). The
ground pin of the power cord is internally connected to the chassis. This configuration is
standard in professional equipment and is required by most electrical codes. A grounding
screw is provided on the back panel for installations that use separate chassis grounding. If
ground loop hum is detected, a careful check of the studio grounding scheme is needed. The
VT-12 is less suscep tible to grounding problems than many studio devices.
Connections (see Figure 1)
The INPUT connectors are XLR-3 females wired with pin 1 ground, pin 2 “+” or “high,” and
pin 3 “-” or “low.” The input matches 150 ohm (nominal) microphones and is trans former
balanced.
The OUTPUT connectors are XLR-3 male wired with pin 1 ground, pin 2 “+” or “high,” and
pin 3 “-” or “low.” The VT-12 is optimized for feeding balanced bridging inputs. (Virtually all
modern audio equipment has bridging inputs.) The output is transformer-balanced.
The “GND” terminal is for use when an external grounding scheme is utilized.
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,
this cord is a Belden 17250 or equivalent.
The unit does not utilize any RFI filtering, and no RFI has been experienced, even when the
VT-12 is operated in close proximity to AM, FM, and TV broadcast transmitters.
Figure 1. The VT-12 rear panel connectors

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VT-12 Microphone reamplifer
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Input and Output Connections
See Figure 1. Gold-plated XLR connectors are used for inputs and outputs. The input connec-
tors are female and the outputs male.
All connectors are wired according to AES standard: pin 1 is ground (shield), pin 2 is “high”
or “+,” and pin 3 is “low” or “-.” A positive voltage on pin 2 of the input will result in a posi-
tive voltage on pin 2 of the output (with the Phase Reverse switch set to Normal).
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 microphones, this shield must also be connected to pin 1 at the microphone end of the
cable.
Whether the shield is connected to pin 1 of the output connector depends on the standard
in your studio. The shield should be connected to ground at only one end of the output cable;
however, although not recommended, the shields can often be connected at both ends with-
out a problem.

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4 .
O E R AT I O N
Input
Since the input cable will be carrying very low level audio, it is important that a well-shield-
ed cable is used. There should be no additional connectors, patch jacks, switches, etc.
between the microphone and the VT-12 input. This can be achieved with a dedicated line
from an XLR connector in the studio to each VT-12 in the control room. Although long input
cable runs have little effect on the performance of the VT-12, it is preferable to keep the input
Figure 2. VT-12 front panel controls and indicators
line as short as possible. One successful method is to place the VT-12 in the studio with only
a short cable to the microphone. Line level from the VT-12 output is then fed back to the con-
trol room. Avoid locating the VT-12 where it will be subjected to high sound levels or exces-
sive vibration (such as on a drum riser).
Output
The output of the VT-12 is line level, transformer balanced. Note that vacuum tube equip-
ment is more sensitive to load impedance than solid state units. The VT-12 design was opti-
mized for 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 pecification . In modern tudio
equipment, bridging line input are univer al. If the device being fed by the VT-12 ha
an input termination witch, that witch hould be in the “off” po ition.