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D.W. Fearn VT-15, Operating Instructions Manual

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D.W. Fearn VT-15 Operating Instructions Manual Download Page 25

D.W. FEARN 

VT-15 Recording Channel

27

5 .

T H E O RY   O F   O P E R AT I O N  

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

Summary of Contents for VT-15

Page 1: ...VT 15 Vacuum Tube Recording Channel Operating Instructions D W FEARN ...

Page 2: ...ording Channel How to Contact us Telephone 610 793 2526 Fax 610 793 1479 Mail P O Box 57 Pocopson PA 19366 U S A Shipping Address 182 Bragg Hill Road West Chester PA 19382 U S A e mail dwfearn dwfearn com www dwfearn com ...

Page 3: ...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...

Page 4: ...4 ...

Page 5: ...c s ______________ Noise ______________ dB below 4 dBm out Equivalent Input Noise _______ dB Operational Tests Microphone Preamplifier 20 pad ________________ Lo Z input ______________ Phase Reverse __________ 48V ___________________ Line Input __________________ Instrument Input ____________ C E Switching ______________ Compressor Threshold ___________________ Attack ______________________ Releas...

Page 6: ...ance 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 ...

Page 7: ...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 ...

Page 8: ... 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 m...

Page 9: ...plications 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 ...

Page 10: ...D W FEARN VT 15 Recording Channel 12 ...

Page 11: ...pedance 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 Intermodulation Distortion SMPTE 0 80 Signal to Noise Ratio 72 dB typical operation Equivalent Input Noise 124 dbm typical operation Output low Z transformer balanced Maximum Outp...

Page 12: ...ving Low frequency Cut 80 cps shelving High frequency Boost 9 8 kc peaking High frequency Q 0 6 to 1 7 High frequency Cut 12 kc shelving Power Requirements 100 120 or 220 VAC 50 60Hz 50 W Dimensions 19 48 26cm W 5 25 13 34cm H 9 22 86cm D VT 2 13 22 9cm Weight 18 lbs 8 16kg specifications are preliminary and subject to change 1 SPECIFICATIONS CONT D ...

Page 13: ...n 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...

Page 14: ...D W FEARN VT 15 Recording Channel 16 ...

Page 15: ...ounter 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 t...

Page 16: ...0 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 wi...

Page 17: ... 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 re...

Page 18: ...d 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 0...

Page 19: ...e 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...

Page 20: ...e 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 h...

Page 21: ... 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 compressi...

Page 22: ... 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...

Page 23: ...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 shoul...

Page 24: ...hould 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 absolu...

Page 25: ...tation 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 mat...

Page 26: ...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...

Page 27: ...ies 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 regu...

Page 28: ...D W FEARN VT 15 Recording Channel 30 ...

Page 29: ...or 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 i...

Page 30: ...ically 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 t...

Page 31: ... 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 1...

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