Crown Audio Stereo Amplifier VT 15 User Manual

D.W. FEARN  
VT-15  
Vacuum Tube  
Recording Channel  
Operating Instructions  
 
P.O. Box 57  
HAND-CRAFTED  
PROFESSIONAL  
Pocopson, PA 19366 U.S.A.  
Tel: 610-793-2526  
Fax: 610-793-1479  
D.W. FEARN  
RECORDING EQUIPMENT  
Certificate of RoHS Compliance  
D.W. Fearn is committed to manufacturing products that are fully-compliant with  
the EU RoHS Directive.  
The following products are compliant:  
VT-1  
VT-2  
VT-3  
VT-4  
VT-7  
VT-15  
LP-1  
PDB  
This declaration is based on our understanding of the current RoHS Directive and  
from information provided by the supplier material declarations with regard to  
materials contained in the component that make up our products.  
Douglas W. Fearn  
President  
 
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5
VT-15 Vacuum Tube Recording Channel  
Final Test Report  
Serial Number_______________ Mains Voltage 120/240 set to __________________  
Date ________________ Tested by ________ VU Calibrated to _______________ dBm  
Test Equipment ____________________________ Microphone ________________________  
Line Input __________________  
Instrument Input ____________  
C->E Switching ______________  
Compressor:  
Frequency Response:  
20 cps to 20 kc/s +/- ___________ dB  
THD+Noise:  
20 cps ______________ %  
200 cps _____________ %  
2 kc/s _______________ %  
20 kc/s ______________ %  
Noise:  
Threshold ___________________  
Attack ______________________  
Release _____________________  
Harder/Softer ________________  
VU GR Calibration ____________  
VU Output Calibration ________  
Sidechain HPF ________________  
Equalizer:  
______________ dB below +4 dBm out  
Equivalent Input Noise _______ dB  
Operational Tests  
LFB __________  
Microphone Preamplifier:  
-20 pad ________________  
Lo-Z input ______________  
Phase Reverse __________  
+48V ___________________  
LFC __________  
HFB __________  
HF Q _________  
HF C _________  
Listening Test ____________  
VT-15 Recording Channel  
D.W. FEARN  
 
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Table of Contents  
RoHS Compliance Data ..................................................................3  
Final Test Report ............................................................................5  
Warranty ........................................................................................9  
History of the VT-15 .....................................................................11  
1. Specifications ..........................................................................13  
2. Description...............................................................................15  
3. Installation .............................................................................17  
4. Operation .................................................................................19  
5. Theory of Operation ................................................................27  
6. Maintenance ............................................................................31  
7. Warranty and Repair ................................................................33  
List of Illustrations  
1. Front Panel Controls and Indicators ........................................19  
D.W. FEARN  
VT-15 Recording Channel  
 
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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 ©2007 D.W. Fearn & Associates  
D.W. FEARN  
VT-15 Recording Channel  
 
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Limited 5-Year Warranty  
During the warranty period, D.W. Fearn will, at no additional charge, repair or replace  
defective parts with new parts.  
This warranty does not extend to any VT-15 that has been damaged or rendered  
defective as a result of accident, misuse, or abuse; by the use of parts not manufac-  
tured or supplied by D.W. Fearn; or by unauthorized modification of the VT-15.  
Vacuum tubes are excepted from the 5-year warranty, but are warranted for 90 days  
from date of purchase.  
Except as expressly set forth in this Warranty, D.W. Fearn makes no other war-  
ranties, express or implied, including any implied warranty of merchantability and fit-  
ness for a particular purpose.  
D.W. FEARN  
VT-15 Recording Channel  
 
11  
History of the VT-15 Vacuum Tube Recording Channel  
With the success of the VT-1/VT-2 Vacuum Tube Microphone Preamplifiers, the VT-  
4 LC Equalizer, the VT-7 Stereo Compressor, and the VT-I/F and VT-3 Vacuum Tube DIs,  
there was a great temptation to combine the features of all of these products into one record-  
ing channel.  
But I had a problem with the concept. I did not want to be tied into having the com-  
pression come before the equalization, or vice-versa, so a prime design goal was to make the  
order of the processing switchable. That way the user has a choice of compression before  
equalization or equalization before compression. Different circumstances require different  
approaches to processing.  
Achieving this switching with a minimum of noise and level change was solved quick-  
ly, but all the switching in the audio path could result in a degradation of the noise perform-  
ance of the VT-15.  
By definition, an “all-in-one” box has many compromises over using separate units. I  
was not satisfied with the VT-15 until the overall performance was as close as possible to using  
a VT-1/VT-7/VT-4. The final result is a box that has about 4dB more noise than the separate  
units under most circumstances -- a negligible difference for most applications.  
Because the VT-15 is capable of a tremendous amount of gain (about 90dB in the first  
prototype), it became a bit tricky to use properly. As long as you were familiar with optimiz-  
ing gain structure in a studio environment, it worked great. But not everyone has that back-  
ground, and in the heat of a session, it may get neglected. The result could be distortion --  
and not the kind you would normally find pleasing. Optimizing the gain structure and head-  
room was an ongoing project, complicated by a period of 18 months when we could not keep  
up with the orders for our VT-7 Stereo Compressor. In fact, I put the VT-15 project on hold  
entirely for six months. When I returned to it over the summer of 2007, the pieces fell into  
place and the VT-15 was ready to meet the world.  
The mic preamp is identical to the VT-1/VT-2 circuit with one exception: an instru-  
ment input has been added. The compressor is identical to one channel of the VT-7 Stereo  
Compressor. The equalization is a highly-simplified version of the VT-4 LC Equalizer, with  
just one frequency for the low-frequency boost, low-frequency cut, high-frequency boost, and  
the high-frequency cut. The chosen frequencies were based on my experience using the VT-  
4 and feedback from many users.  
Can the VT-15 replace the VT-1/VT-2, VT-7, and VT-4? Not really. The separate units  
have features that are just not practical to duplicate in the VT-15. But for many applications,  
the VT-15 provides the next best thing.  
November 2007.  
D.W. FEARN  
VT-15 Recording Channel  
 
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D.W. FEARN  
VT-15 Recording Channel  
 
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1.  
SPECIFICATIONS  
Mic 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  
Instrument Input impedance  
1 Megohm  
Line Input  
Load Impedance  
40k ohms  
Maximum Line Input  
Level @ 20 cps  
+25dBm (+35dBm with pad)  
Gain  
<60 dB optimum  
(capable of 75dB)  
Frequency  
Response  
0.25 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  
SMPTE: <0.80%  
Intermodulation  
Distortion  
Signal to  
Noise Ratio  
72 dB typical operation  
Equivalent  
Input Noise  
-124 dbm typical operation  
low-Z, transformer balanced  
Output  
Maximum  
Output Level  
+22 dBm unterminated  
D.W. FEARN  
VT-15 Recording Channel  
 
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1.  
SPECIFICATIONS  
(CONT’D)  
Compression Range  
Compression method  
up to 30dB  
Pulse-width modulator  
Equalizer:  
Low-frequency Boost  
Low-frequency Cut  
High-frequency Boost  
High-frequency Q  
High-frequency Cut  
12 kc, shelving  
100 cps, shelving  
80 cps, shelving  
9.8 kc, peaking  
0.6 to 1.7  
Power  
Requirements  
100, 120, or 220 VAC  
50/60Hz, 50 W  
Dimensions  
Weight  
19” (48.26cm) W  
5.25” (13.34cm) H  
9” (22.86cm) D (VT-2 13” 22.9cm)  
18 lbs (8.16kg)  
(specifications are preliminary and subject to change)  
D.W. FEARN  
VT-15 Recording Channel  
 
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2.  
DESCRIPTION  
The D.W. Fearn VT-15 utilizes circuitry developed for the VT-1/VT-2 Vacuum Tube  
Microphone Preamplifiers, the VT-7 Stereo Compressor, and the VT-4 LC Equalizer.  
The Model VT-15 Recording Channel is designed to provide recording professionals with a  
sonically superior input device. It is typically used in sound recording studios for recording  
individual tracks. A quality microphone, electric instrument, or line-level signal is connected  
to a VT-15 input, and the VT-15 provides a line-level output. In most situations, the VT-15 will  
feed directly to the input of the recorder.  
The microphone preamplifier section is a recreation of the classic tube preamps of the 1960s,  
updated with improved modern passive components and computer-aided circuit optimiza-  
tion. Because of the unique qualities of vacuum tubes, the VT-15 has a clarity, transparency,  
and warmth that solid state preamps lack. Its modern design and construction allows the VT-  
15 to exceed the performance of vintage vacuum tube preamps. 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, a  
switchable input network to accommodate very low impedance microphones (typically trans-  
formerless types), and a true VU meter. The input can be switched to a front-panel quarter-  
inch jack for recording electric instruments.  
The compressor in the VT-15 is identical to a single channel of the D.W. Fearn VT-7 Stereo  
Compressor. In many ways, the VT-15 compressor recreates the sound of the classic tube  
compressors of the 1960s, but using modern gain control circuitry. The actual gain control  
element is a pulse-width modulator, driven by solid-state circuitry. However, the audio path is  
entirely class-A triode vacuum tubes. There are controls for compression threshold, gain,  
attack and release times, and compression characteristics.  
The equalization section of the VT-15 is based on the D.W. Fearn VT-4 Vacuum Tube LC  
Equalizer. It uses identical circuitry, but is limited to just one frequency each in the low boost,  
low cut, high boost, and high cut sections. These were the frequencies most often used on the  
VT-4. The low frequency boost and cut have shelving curves. The high-frequency boost is a  
peaking type, with adjustable Q. The high-frequency cut is shelving. All the controls are con-  
tinuously adjustable.  
The input and output transformers are custom-made for us by Jensen Transformers, Inc. All  
power supplies are solid state and fully regulated. All internal switching uses sealed, gold-con-  
tact relays for low noise and long, quiet operation. The relays never need cleaning.  
A combination of four 6072A and two 6N1P dual triodes are used in the VT-15.  
A total of 12 separate power supply voltages are required internally for the VT-15 circuitry.  
All are solid-state and fully regulated.  
The VT-15 is not mass-produced. Each one is hand-made and meticulously tested and lis-  
tened to before shipment to the customer. It is built with pride and precision. The VT-15 is  
designed and built to perform in your studio for decades to come.  
D.W. FEARN  
VT-15 Recording Channel  
 
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D.W. FEARN  
VT-15 Recording Channel  
 
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3.  
INSTALLATION  
The VT-15 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 possible claim for  
damage with the shipper.  
Included in the box:  
1) The VT-15 Recording Channel  
2) Line cord  
3) This instruction manual  
Mounting  
The VT-15 is designed for installation in a standard 19 inch rack. It requires 5.25 inches of  
vertical space, but additional spacing between it and adjacent equipment is recommended for  
adequate cooling. Ideally, a ventilated panel at least 1 rack unit high (1.75 inches) should be  
installed above and below the VT-15 (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-15. 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 ben-  
efit all your equipment.  
The VT-15 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-15 should not cause any  
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.  
Although the vacuum tubes in the VT-15 are selected for minimum microphonic response, it  
is a good practice to avoid mounting locations that subject the VT-15 to very high sound or  
vibration levels.  
Power  
The VT-2 is designed to operate from 100, 120, or 220-240 volt, 50/60 Hz power. The unit  
will be shipped set for the voltage specified in the order, but may be changed in the field if  
necessary. Switching between 120V and 220V operation is simple: a recessed switch on the  
rear panel can be set to either voltage. The fuse value should be changed as well: 2A SB for  
100/120V operation, and 1A SB for 220V operation. (For 100V operation, a simple internal  
wiring change is required. Call the factory for detailed instructions). The ground pin of the  
power cord is internally connected to the chassis. This configuration is standard in profes-  
sional equipment and is required by most electrical codes. A grounding screw is provided on  
D.W. FEARN  
VT-15 Recording Channel  
 
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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-15 is less suscep-  
tible to grounding problems than many studio devices.  
Connections  
The INPUT connector is a XLR-3 female wired with pin 1 ground, pin 2 “+” or “high,” and  
pin 3 “-” or “low.” The same input is used for either mic input or line input. The input match-  
es 150 ohm microphones or 600-ohm line signals and is transformer balanced.  
The OUTPUT connectors are XLR-3 male wired with pin 1 ground, pin 2 “+” or “high,” and  
pin 3 “-” or “low.” The VT-15 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 slow-blow type, 2 amp for 100 or 120 VAC operation, and 1 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-15 is operated in close proximity to AM, FM, and TV broadcast transmitters.  
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.  
D.W. FEARN  
VT-15 Recording Channel  
 
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4.  
OPER ATION  
INITIAL SET-UP IS CRITICAL  
for getting the best performance from your VT-15!  
The VT-15 has tremendous versatility, but with that versatility comes the potential for for very  
unpleasant sound. Think of the VT-15 as a small studio chain in one box: mic preamp, com-  
pressor, and equalizer. In most studios, these would be three separate pieces of equipment  
and you would patch them together in a chain feeding your recorder. The VT-15 is the same  
concept, except that the “patching” is done inside the VT-15. And just like a bad gain struc-  
ture in your mic pre/comp/eq will seriously degrade the performance, the same is true with  
the VT-15. Improper gain structure, in the VT-15 or in separate units, will increase the  
noise, reduce the headroom, and increase distortion. So follow these steps for the initial  
set-up of the VT-15. They provide the proper starting point, and optimize the gain structure  
for best performance.  
Figure 1. VT-15 front panel controls and indicators  
A Note about Powering Down the VT-15  
Because the VT-15 is designed so that the most often used settings do not energize the cor-  
responding internal switching relay, when you turn off the power to the unit, it will revert to  
the Mic input, whether you were using Mic or Line. If you are feeding a line-level signal into  
the VT-15, removing the power will immediately switch it to Mic input. Since the tube fila-  
ments and power supplies will continue to operate for about 20 seconds after power is turned  
off, the level could increase significantly during that time. Be sure to turn down the Preamp  
Gain control before turning off the Power switch. Or make sure that the VT-15 will not be  
feeding your monitors on shut-down. This will not hurt the VT-15, but it could damage your  
hearing or your speakers if the level were to suddenly jump 50dB!  
D.W. FEARN  
VT-15 Recording Channel  
 
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Control Set-up  
Input toggle switches:  
Mic/Line switch - as appropriate for the source  
0/-20 switch - 0  
150/Lo-Z switch - 150 (unless you are using a non-standard impedance mic)  
+48 switch - on for phantom-powered condenser mics, otherwise off  
Rev/Phase switch - down (no phase reversal)  
Mic/Inst switch - Mic position  
Preamp Gain - 10 o’clock position  
Compressor Controls:  
Eq->Comp switch - in the Comp->Eq position (down)  
Threshold - full counterclockwise  
Harder/Softer - mid-point  
Attack - mid-point  
Release - mid-point  
Gain - full counterclockwise  
Bypass/Comp switch - Comp position  
HPF In switch - out (down)  
EQ Controls:  
All knobs fully counterclockwise (flat)  
In/Out switch - In (up)  
Other controls:  
GR/VU switch - VU position  
Power switch - On (up)  
Set-up Procedure  
1. After the controls are set, have the performer start and adjust the Preamp Gain control for  
a normal reading on the VU meter. Peaks should rarely go into the red (0VU).  
2. If the level is too high, switch in the -20 pad.  
3. Switch the VU meter to the GR position.  
4. Turn up the Threshold control until the the VU meter indicates compression. Typically you  
will use from 2 to 10dB of compression most of the time.  
5. Adjust the Attack, Release, and Harder/Softer controls for the desired sound.  
D.W. FEARN  
VT-15 Recording Channel  
 
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6. Adjust the Eq controls for the desired sound. (Remember to use the Boost and Cut togeth-  
er to shape the sound.)  
7. Switch the VU meter back to the VU position.  
8. Adjust the Gain control for a normal indication. Peaks should rarely go into the red (0VU).  
This procedure will provide you with the optimum gain structure in the VT-15. Headroom will  
be maximum and noise will be minimum.  
After the initial set-up, you can fine-tune the controls for exactly the sound you are looking  
for. However, you will rarely have to change the Preamp Amp gain after the initial set-up.  
Here is detailed information on the VT-15 controls:  
Mic 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-15 input. This can be achieved with a dedicated line  
from an XLR connector in the studio to each VT-15 in the control room. Although long input  
cable runs have little effect on the performance of the VT-15, it is preferable to keep the input  
line as short as possible. Avoid locating the VT-15 where it will be subjected to high sound lev-  
els or excessive vibration (such as on a drum riser).  
Line Input  
The Input connector can also accept a line-level (+4dBm, balanced, 600-ohm nominal) signal. The  
front panel “Mic/Line” switch should be in the “Line” position. A line-level signal into the VT-15 while  
the unit is set for “Mic” will result in very high level and extreme distortion. Although this will not harm  
the VT-15, it could be dangerous to your speakers and your ears! So make sure the switch is set appro-  
priately before making the connections.  
Many of the other switches are usable while in the “Line” position. The “-20” pad in particular may be  
useful if the input signal has a higher-than-normal level. The “150/Lo-Z” and “+48” switches are locked  
out in the “Line” position. The “Phase” switch is still active and may be useful.  
Instrument Input  
The “Mic/Inst” switch changes the input source from the read-panel XLR connector to the front panel  
one-quarter inch “Inst” input. This jack is used for recording electric instruments such as electric bass,  
electric guitar, electronic keyboards, etc. Any device that would normally be plugged into a guitar amp  
or DI box will properly match the VT-15 “Inst” input.  
Output  
The output of the VT-15 is line level, transformer balanced. Note that vacuum tube equip-  
ment is more sensitive to load impedance than solid state units. The VT-15 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 specifications. In modern studio equip-  
ment, bridging line inputs are universal. If the device being fed by the VT-15 has an input termi-  
nation switch, that switch should be in the “off” position.  
The VT-15 can feed balanced or unbalanced inputs with no need for any modification in out-  
put wiring. Either pin 2 or 3 can be grounded, although pin 2 is normally used as the ”hot”  
and pin 3 grounded in unbalanced configurations.  
D.W. FEARN  
VT-15 Recording Channel  
 
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CONTROLS (see Figure 1.)  
The VT-15 front panel is divided into three main areas of controls. On the left is the Input sec-  
tion, to its right is the Compressor section, further right is the Equalizer section, and far right  
is the output and power section.  
Input Toggle Switch Section  
The group of 6 toggle switches in the upper left sets up the VT-15 input configuration.  
1. Mic/Line switch: In the Mic position, the XLR input connector on the rear panel accepts  
mic level signals: nominally 150-ohm, balanced, at a -50dBm level. The Line position is for  
+4dBm, balanced, 600-ohm signals.  
2. 0/-20 switch: This is a 20dB pad that can be inserted on either the mic or line signal. In  
the “0” position, microphone or line audio is connected directly to the input transformer.  
This provides the proper amplification for most condenser microphones and line-level sig-  
nals, and will be used in most situations. In the “0” position, the VT-15 can accept up to about  
a -30 dBm input signal at 20 cps with full gain (53 dB) in the Mic position, and approximate-  
ly +14dBm in the Line position, without an increase in distortion.  
In the “-20” position, a pad is inserted between the input connector and the input trans-  
former. This position would be used when the level is too high for the “0” position. On con-  
denser microphones that have a switchable pad, it will usually be necessary to use a -10 or -  
20 dB pad in the mic when recording very high sound levels to prevent overload of the micro-  
phone electronics. Whether this is used in conjunction with or as a substitute for the VT-15  
pad should be determined by experimentation. For the cleanest sound it is generally prefer-  
able to pad at the microphone first, then at the VT-15 if necessary. The sound of some micro-  
phones will change slightly from the “0” to “-20” position. This is a function of the interac-  
tion between the microphone transformer and the VT-15 input transformer.  
Sometimes a line-level signal will be too high for the VT-15 Line input. The -20 position can  
be useful in this situation.  
3. 150/Lo-z: Some microphones, particularly transformerless-output condenser mics, have  
a non-standard output impedance, typically 50 to 80 ohms. (The AES standard is 150 ohms.)  
The Lo-Z position often provides a better match to these microphones. The level is not  
changed, but the microphone loading is improved for these low-impedance mics. Use your  
ears to decide which position sounds best to you.  
4.+48 volt phantom power on/off switch: Supplies 48 volts for phantom powered con-  
denser microphones. Switch the 48 volts off for dynamic and ribbon microphones, or con-  
denser microphones with their own power supplies (e.g. vacuum tube condensers). This  
function is locked-out when in the Line position. The phantom powering circuit used in the  
VT-15 is suitable for use with all Neumann microphones, AKG 12 and 48 volt microphones,  
B&K phantom powered mics, all Schoeps mics, Shure SM81 and 85 mics, Crown PZM mics,  
and virtually all other phantom powered mics that require any voltage between 12 and 48.  
When turned off, the phantom-power resistors are completely disconnected from the circuit  
in the VT-15.  
5. Rev/Phase switch: Usually in the Normal position except when it is necessary to reverse the  
polarity of a microphone or line signal. It does not function on the Inst input. This switch revers-  
D.W. FEARN  
VT-15 Recording Channel  
 
23  
es the polarity of the output of the VT-15. A detailed discussion of the application of phase  
reversal of individual microphones is beyond the scope of this manual.  
Even when there is only one microphone being recorded, it may be useful to try the  
“Reverse” position of the Phase control. Although there is supposed to be standardization in  
polarity throughout the professional audio equipment industry, it is possible that a wiring  
error or the use of vintage equipment built before standardization may reverse the polarity in  
the recording/monitoring chain. The effect of reversed absolute polarity is subtle, but signif-  
icant with some sounds. If the “Reverse” position sounds better, use it.  
With more than one microphone on the same sound source (or picking up leakage from  
another sound source), the Phase switch may have a profound effect on the audio quality.  
Whichever position sounds best is correct. A check of monaural compatibility (by summing  
the various mics) should also be performed.  
For a vocalist monitoring his or her voice in the headphones, the position of the Phase switch  
will drastically alter the performer’s perception of their own voice. If there is no other reason  
not to, try both positions of the Phase switch to see which is preferred.  
6. Mic/Inst switch: Switches between the rear panel input connector (for mic or line  
input) and the front panel 1/4” Inst jack.  
7. Preamp Gain: Adjusts the input level, mic, instrument, or line.  
Compressor Controls  
Threshold The Threshold control adjusts the point where compression begins. With the con-  
trol all the way down, there is no compression and the VT-15 operates as a straight amplifier.  
As the control is turned clockwise, the amount of compression increases. This can be moni-  
tored on the VU meter (in the GR position), or by ear. The input level will also affect the  
range of the Threshold control, so adjusting the Preamp Gain control will also change the  
amount of compression. However, for best performance, use the set-up procedure on  
page 19.  
Gain Whenever compression is applied, the signal is reduced in level by the amount of gain  
reduction. The Gain control allows you to make up the lost gain as necessary. At the full  
counter-clockwise position, the gain of the VT-15 (with no compression) is about -3dB. The  
maximum gain available is about 15dB.  
Attack This control adjusts the time it takes the VT-7 gain reduction circuitry to react to a  
signal. It might seem that the faster the attack time, the better, but very fast attack times will  
result in significant distortion on material with a lot of low frequency content (this is true of  
all compressors). As the control is turned clockwise, the attack time becomes longer. Often  
with percussive sounds it is advantageous to have fairly long attack times to allow the initial  
transient of the sound to pass through the VT-15 without gain reduction. Experiment with the  
Attack control on percussive material to see how it changes the sound. Generally speaking,  
a fast attack time is best when complete control of the maximum instantaneous level is  
required. This might be desirable to protect a digital input from overload. However, a more  
natural sound is usually obtained with a slightly slower attack time. Long attack times can be  
useful as an effect, adding power to percussive instruments.  
Release The Release control adjusts how long it takes for the gain to return to normal after  
a sound ceases (or drops in level). Fast release times add more energy to the sound, but can  
D.W. FEARN  
VT-15 Recording Channel  
 
24  
add distortion to low frequency sounds. (This is true of all compressors.) A fast release time  
adds density to the sound, often with the compression becoming more obvious. Long release  
times make the compression less obvious and more natural, but can “punch holes” in the  
lower level audio under certain conditions of high percussive levels.  
Harder/Softer This control adjusts the nature of the compression curve. Toward the  
“Harder” end of the control, the compression ratio is higher and the levels more tightly con-  
trolled. At the “Softer” end, the compression ratio is lower and the compression is more gen-  
tle. This is not merely a ratio control, however, as other parameters also change as the con-  
trol is adjusted. More than any other control, this one needs to be adjusted by ear. This con-  
trol interacts with the Threshold, Attack, and Release controls, so after changing the  
Softer/Harder control, you should experiment with others to obtain the effect you desire. Its  
technical operation is complicated to explain, so it is much easier to understand it simply by  
experimenting.  
Sidechain HPF In the HPF In position, a high-pass (low cut) filter is inserted in the sidechain.  
This makes the compression less bass-sensitive and may be useful on mixes that have a very  
heavy bass content. It will reduce “pumping” of the mix from bass or bass drum hits, and  
increase the amount of low-end in the mix. The roll-off of bass sensitivity is a very gentle  
curve (about 6dB per octave) and has little effect above 150Hz. Using the HPF will also per-  
mit shorter Attack and Release times without adding any low-frequency distortion.  
Equalization Controls:  
The equalizer section of the VT-15 is derived from the D.W. Fearn VT-4 LC Equalizer. It is a  
much simplified version of the VT-4, but it retains the same basic circuitry and sound of the  
VT-4. The most commonly-used frequencies from the VT-4 were chosen for the VT-15.  
A key technique in using the VT-15 equalization controls is to use the Boost and Cut controls  
together. It may seem counterintuitive to boost and cut at the same time, but doing it that  
way is what adds some of the “magic” of the eq.  
There is no formula for setting the VT-15 eq controls. Simply experiment and find what works  
best for your application.  
The HF Q control adjusts the shape of the High Boost curve, from very broad to moderately  
sharp.  
All the curves on the VT-15 are shelving type with the exception of the High Boost, which has  
a peaking type curve. All curves are rather gentle and will affect a wide range of frequencies.  
Use the Boost and Cut controls together to modify the shape of the curve.  
Keep in mind that the VT-15 is not designed to be a “corrective” equalizer. It is designed to  
enhance sound that is already pretty good. If you need to correct a frequency response prob-  
lem, you will find many other outboard equalizers that are better suited. That said, the VT-15  
can add wonderfully power to the bass and a beautiful high shimmer to sounds. Often that is  
all that is needed to make the track perfect.  
Meter switch Switches the VU meter between monitoring the amount of compression (GR)  
and the output level (VU).  
The VU Meter measures the output level through an isolation amplifier. It is calibrated so  
that a +4 dBm output will indicate 0 on the meter. This is the standard “0 VU” level for all  
professional audio recording equipment built since the early 1970s. “0 VU” on the VT-15  
D.W. FEARN  
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25  
should result in “0 VU” on a properly aligned recorder. (This reference level can be changed;  
see the Maintenance Section.) This is a true VU meter, and conforms to ASA Standard  
C16.5-1954.  
In the GR position, the VU meter monitors the amount of gain reduction (compression). As  
the VT-15 warms up, the normal “0VU” position of the meter needle (with no compression)  
may drift slightly. This has no effect on the sound of the VT-15 nor the accuracy of the gain  
reduction measurement.  
Power switch and indicator Primary power is applied to the VT-15 circuits when the Power  
switch is in the up position. The amber pilot lamp and VU meter back-lighting indicate that  
the unit is on. It takes about twenty seconds for the VT-15 to start working, but it is suggest-  
ed that you turn on the power at least five minutes prior to use. The tubes are often noisy  
until all the internal elements reach a stable operating temperature.  
Bench Test  
If desired, test the VT-15 before installation. The source generator should be set to -50 dBm,  
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 the factory test results. Compare your measurements with the test data supplied with VT-  
15. Keep the results for comparison in future maintenance tests.  
Initial Set-Up  
The VT-15 should be installed as detailed in the Installation section. With the outputs con-  
nected to an appropriate destination (typically to audio recorder inputs), configure the stu-  
dio to monitor the VT-15 output. Apply power and wait about twenty seconds for the tube fil-  
aments to get up to temperature. Check for hum, buzz, or other noise. For the first few min-  
utes after a cold start it is not unusual for the VT-15 to produce hiss, pops, and microphonic  
“clanks” as the internal elements of the tubes expand from the heat. Correct any ground loop  
problems before proceeding.  
SUGGESTIONS:  
You have chosen to use the VT-15 because of the superior sound it provides. To gain the max-  
imum benefit from your investment, it is important that you hook up the VT-15 so that other  
factors do not adversely affect the sound quality.  
D.W. FEARN  
VT-15 Recording Channel  
 
26  
1. Use the best quality mic cable you can. We don’t believe you have to use esoteric  
wire, but do use a good cable designed for low impedance microphones. A quality  
cable with gold-contact connectors is best.  
2. There should be no additional cables, connectors, junction boxes, patch jacks, etc.  
between the mic and the VT-15 input.  
3. The output of the VT-15 should be fed directly to the recorder through the shortest  
practical length of quality cable. Avoid additional cables, connectors, junction boxes,  
punch blocks, or patch jacks. Use gold contact connectors if possible. Do not go  
through the mixing console unless you absolutely need its features for the track you  
are cutting.  
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VT-15 Recording Channel  
 
27  
5.  
THEORY OF OPER ATION  
Preamp Section  
Input section  
Microphone level (150 ohm source impedance, balanced, -50 dBm nominal) or line level  
(600-ohm source, +4dBm nominal) audio enters through the XLR-3 female INPUT connec-  
tor to the Input selector switch bank. In the 0 position, the input is connected directly to  
the input transformer. The load imposed on the microphone is 1500 ohms, but varies slight-  
ly with frequency but is never lower than 1100 ohms. In the -20 position, the input passes  
through a balanced 20 dB pad. This pad is designed to maintain approximately the same load  
on the microphone as the input transformer.  
In the Line position, a balanced pad of approximately 50dB attenuation is inserted on the  
input. In the Line position, the 48V phantom power and Lo-Z switches are locked-out.  
The Mic/Inst switch changes the input from the rear panel XLR Input connector to the one-  
quarter inch front panel Inst jack. The audio from the Inst jack is fed directly to the grid of  
the first amplifier stage, bypassing all the input switching. The load impedance on the Inst  
input is 1 megohm.  
All switching is through sealed gold-contact instrumentation relays with bifurcated contacts.  
Phantom powering  
+51 from the phantom power supply is switched on and off by the front panel +48 switch. A  
resistor drops the voltage as required depending on the current being drawn by the con-  
denser microphone electronics. This makes the phantom power supply universal for most 12  
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. The switching is through sealed gold-contact instrumentation relays with bifurcat-  
ed contacts. In the “Off” position, the phantom power resistors are completely disconnected  
from the Input connector. When the VT-15 is in the Line position, the 48V switch is locked  
out.  
Phase reversal switch  
In the Normal position, input/output phase (polarity) is maintained (which must be inverted  
due to the design of the circuit). The balanced output is polarity reversed in the Reverse posi-  
tion. Switching is accomplished with a sealed, gold-contact instrumentation relay with bifur-  
cated contacts.  
Input transformer  
The input transformer is custom-made for D.W. Fearn by Jensen Transformers, Inc. and rep-  
resents the state of the art in transformer design. This is the same transformer that we use  
on the D.W. Fearn VT-1/VT-2 microphone preamplifiers. It exhibits extremely flat frequency  
D.W. FEARN  
VT-15 Recording Channel  
 
28  
response, low phase shift, excellent square wave response, low distortion, and high noise  
immunity. The secondary of is connected directly to the grid of the first amplifier stage.  
First stage  
The first stage is a selected 6072 configured as a Class A voltage amplifier with a gain of  
approximately 30. Negative feedback from the plate of the second stage reduces distortion,  
flattens the frequency response, and makes the gain of the first two stages less dependent on  
individual vacuum tube characteristics.  
Second stage  
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 through a polypropylene capacitor to the top of a potentiometer  
(Preamp Gain).  
Third Stage  
The arm of the Preamp Gain potentiometer feeds the grid of the third stage (a 6072A), which  
also operates Class A with a gain of approximately 30. This stage is capacitively-coupled to  
the grid of the output stage through a polystyrene capacitor.  
Preamp Output Stage  
The output stage operates as a cathode follower, presenting a comparatively low output  
impedance (approximately 800 ohms). The cathode output is coupled through a proprietary  
polypropylene capacitor to either the Equalizer or Compressor section.  
Compression Section  
Audio from the mic preamp is fed to the input of the pulse-width modulator (PWM) board,  
which provides the gain reduction. The audio output of the PWM goes through sidechain cir-  
cuitry which determines the control voltage and hence the amount and characteristics of the  
compression.  
The compressor preamp consists of two stages of a 6072A dual triode, which then feeds  
either the Line Amp or the Equalizer circuitry.  
Equalization Section  
The passive LC (inductor/capacitor) equalization is inserted either at the mic preamp output  
or the compressor output. A 6N1P dual triode makes up the gain and then feeds either the  
Compressor input or the Line Amp.  
Line Amp Section  
Either the eq output or the compressor output feeds a 6N1P dual triode with both sections in  
parallel to provide line-level output. The output transformer is custom-made for us by Jensen  
Transformers, Inc. and is the same transformer we use on all of our products.  
D.W. FEARN  
VT-15 Recording Channel  
 
29  
VU Meter and Meter Amplifier  
A two-stage IC operational amplifier is used to isolate the VU meter from the VT-15 output.  
An output sample is derived from the primary of the output transformer. The meter calibra-  
tion is set with a 20-turn trimmer potentiometer. Reference level (0 VU) can be set from -20  
to over +20dBm. The level indictor is a custom true VU meter conforming to ASA Standard  
C16.5-1954.  
Power Supplies  
Primary power from the AC mains is connected to the VT-15 through a standard IEC power  
input connector. A rear-panel voltage selector slide switch selects either 120 or 220V opera-  
tion (the switch is labeled “115/230”). The VT-15 can also operate on 100V power with an  
internal wiring change. The mains fuse is slow-blow type 3AG, rated at 2A for 100/120V  
operation and 1A for 230V operation. The VT-15 can operate on either 50 or 60Hz power.  
The Power switch energizes all power supplies. The Pilot lamp is a type 1819 bulb, operated  
far below its rated voltage of 28. The life of the bulb is lengthened, and the light output is  
more compatible with other modern studio equipment. The power transformer is a toroidal  
unit custom-made for the VT-15.  
Filament Supply  
The power transformer output is rectified by a bridge rectifier and filtered before being reg-  
ulated by a three-terminal regulator. This supply also powers all the switching relays in the  
VT-15, as well as the pilot lamp and VU meter lamps.  
+/- 15V Supply  
The solid-state control circuitry for the VT-15 compressor requires regulated +15 and -15V  
supplies.  
B+ Supply  
Seven separate regulated voltages are required for the plates of the VT-15. The B+ is filtered  
with long-life, low-leakage, computer-grade filter capacitors before being regulated and exten-  
sively bypassed and decoupled. The negative side of the supply is grounded.  
+48 Supply  
The regulator circuitry is on the power supply printed circuit board. The actual output of this  
supply is 51 volts, which is reduced to 48 volts through the decoupling resistor at the micro-  
phone input (see Phantom Power, above).  
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VT-15 Recording Channel  
 
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VT-15 Recording Channel  
 
31  
6.  
MAINTENANCE  
The VT-15 is built with only the highest quality parts and will prove to be extremely reliable.  
Vacuum tubes and electrolytic capacitors, however, have a finite useful life and must be  
replaced eventually.  
Top Cover Removal  
Removing the top cover allows access to the vacuum tubes, the VU meter calibration, and to  
sidechain alignment trimpots. Eighteen 6-32 phillips-head screws must be removed. When  
replacing the cover, position it so that the slotted ventilation holes are over the tubes (towards  
the back).  
Bottom Cover Removal  
WARNING! High voltages are exposed when the bottom cover of the VT-15 is removed.  
Refer all servicing to the factory or to technicians who are experienced in working on  
vacuum tube equipment. Voltages over 400 are used in the VT-15. Contact with these  
voltages could result in serious injury or death.  
Removing the bottom cover allows access to the VT-15 circuitry, the pulse width modulator  
board adjustments, and the filament voltage adjustment. Eighteen 6-32 phillips-head screws  
must be removed.  
Alignment  
The VT-15 compressor is complicated to align and requires test equipment nor normally  
found in most studios. If alignment is required, the VT-15 should be returned to the factory,  
or contact the factory for detail alignment instructions. Mis-alignment of the VT-15 will result  
in very poor (and possibly bizarre) performance. Do not touch any adjustments!  
Vacuum Tubes  
Four 6072A tubes and two 6N1P tubes are used in the VT-15. If tube replacement is needed,  
contact the factory for instructions and tube layout. The 6072A mic preamp input tube and  
the compressor preamp tube are quite critical for proper performance. There can be as much  
as a 15 dB difference in noise level among an assortment of tubes, and the tubes used in these  
stages should be carefully chosen to maintain low noise. Selected low-noise tubes are avail-  
able from D. W. Fearn. Other tubes are far less critical and almost any quality off-the-shelf  
6N1P will perform satisfactorily. 12AY7 tubes may be substituted for the 6072A tubes, but  
with reduced performance.  
Tube life is difficult to predict, but it will probably be measured in years. Catastrophic tube  
failure is rare with this type of device, but a gradual increase in noise, microphonics, distor-  
tion, or a reduction in headroom, should indicate the need for replacement. It is recommend-  
ed that you periodically perform a quick noise and distortion check on the VT-15 and com-  
pare the results to previous measurements.  
D.W. FEARN  
VT-15 Recording Channel  
 
32  
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 indi-  
cate perfect performance. Gently tapping the tube shields while listening to the output at a  
normal monitor level should reveal nothing more than a slight “clank.” On a peak-reading  
meter connected to the VT-15 output, with 50 dB gain, any microphonic response above -55  
dBm is excessive. Replacement is indicated unless the VT-15 always operates in a quiet and  
vibration-free environment.  
Remember that vacuum tubes may be quite hot during operation. Protect your fingers dur-  
ing tube replacement. The VT-15 should be turned off before removing tubes. Allow at least  
one minute for the filter capacitors to discharge before tube removal or insertion.  
Tubes are made of glass and will break if dropped or even bumped in a critical area. Handle  
with care. There are only a limited number of NOS (new old-stock) 6072A tubes in the world.  
Treat each one carefully.  
Electrolytic Capacitors  
The VT-15 is designed and built to last for a long, long time, and it is possible that some com-  
ponents (e.g. electrolytic capacitors) may reach the end of their life long before the equip-  
ment becomes obsolete. The electrolytic capacitors used in the VT-15 typically will last at  
least twenty years. If there is a measurable and/or audible increase in 120 cps noise, the fil-  
ter capacitors should be suspected. They should be replaced with new capacitors of equiva-  
lent capacitance and voltage rating, and the replacements should be specified for a minimum  
ten-year service life.  
Electrolytic capacitors are also used as plate decouplers. In choosing a replacement, the  
same considerations as with the filter capacitors should be followed.  
VU Meter Calibration  
The meter amplifier circuit is stable and re-adjustment of the meter calibration is not normal-  
ly required. If the output reference level needs to be changed, here is the procedure:  
1. Remove the VT-15 top cover (see above).  
2. Terminate the output in the same impedance as the VT-15 will normally be feeding.  
This will almost always be a high-impedance bridging termination of 20k ohms or  
more.  
3. Apply AC power. Allow at least an hour warm-up. Feed a 1 kc tone at about -45 dBm  
into the VT-15 input.  
4. Measure the output and set the Preamp Gain control for +4 dBm (or whatever refer-  
ence is desired).  
5. Set the Meter Calibration trimmer potentiometer for 0 VU on the VT-15 meter. Be  
sure the meter is in the VU position. This control is a 25-turn pot so the adjustment  
is easy. A small screwdriver or alignment tool is necessary. The control is located on  
the relay printed circuit board on the top of the chassis, on the far left side and it is  
the only trim pot on this board.  
6. Replace the top cover. Be sure the ventilation slots are over the tubes (towards the  
back).  
D.W. FEARN  
VT-15 Recording Channel  
 
33  
7.  
Troubleshooting  
Factory Check-Out  
Warranty Repair  
Troubleshooting  
Most problems will be traced to defective vacuum tubes. However, if normal tests do not eas-  
ily reveal the problem, feel free to call the factory for assistance. If you lack access to a qual-  
ified service technician with vacuum tube equipment repair experience, you may return the  
VT-15 to the factory for repair. Call first, however, for shipping information.  
Factory Check-Out  
We will always provide free testing and alignment of any of our products. There is no charge  
except for shipping in and out, and any parts that are not covered by the warranty. Call for  
instructions. We can usually return a properly-performing VT-15 in a couple of days. If repairs  
are needed, we will discuss it with you before proceeding. We always operate any repaired VT-  
15 for at least 24 hours to make sure it is operating properly before return.  
Warranty Repair  
If the VT-15 should develop a problem during the five-year warranty period, call the factory  
for return shipping instructions. We will repair and return your VT-15 quickly. Note that the  
warranty does not cover vacuum tubes, which must be periodically replaced.  
D.W. FEARN  
VT-15 Recording Channel  
 

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