D.W. Fearn VT-12, Operating Instructions Manual

Page 1: ...VT 12 Vacuum Tube 70dB Microphone Preamplifier Operating Instructions D W FEARN...

Page 2: ...hone Preamplifer 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 dwfea...

Page 3: ...acturing products that are fully compliant with the EU RoHS 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...

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Page 5: ...0 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 Chan...

Page 6: ...es 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 F...

Page 7: ...n 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 phot...

Page 8: ...ed 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 ye...

Page 9: ...consisted of half a dozen RCA tube microphone preamplifiers that I salvaged from the junk pile of the radio station where I worked an equal number of old Daven rotary faders and key switches that pann...

Page 10: ...n 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 mentati...

Page 11: ...est performance attainable with this type of circuit but it will meet or exceed the performance of today s top of the line solid state designs while still providing the clarity and musicality that cha...

Page 12: ...2 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...

Page 13: ...y 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 No...

Page 14: ...rency 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...

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Page 16: ...cked 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 r...

Page 17: ...g scheme is needed The VT 12 is less susceptible 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 an...

Page 18: ...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 th...

Page 19: ...D W FEARN VT 12 Microphone Preamplifer 22...

Page 20: ...essful 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 wil...

Page 21: ...witch up ENABLED the front panel 48V switches are enabled No phantom power is supplied until the individual 48V switch is turned to the ON position With the rear panel switch in the Enable position th...

Page 22: ...r as a substitute for the VT 12 pad should be determined by experimentation For the cleanest sound it is generally preferable to pad at the microphone first then at the VT 12 if necessary The sound of...

Page 23: ...nt with the microphone pad if it has one the VT 12 Input switch position or both Let your ears be the judge Slight to moderate overdriving of the VT 12 often adds edge power or excitement to the sound...

Page 24: ...meter pinning will not damage the meter Bench Test If desired test the VT 12 before installation The source generator should be set to 60 dBm 150 ohms impedance balanced and the output should feed a b...

Page 25: ...ily accom modated by the VT 12 Semi professional equipment frequently uses a reference level of 10 dBv roughly 14 dB lower than pro equipment Although the VT 12 can fed the unbalanced 10 dBv inputs of...

Page 26: ...n general for superior sound we recommend recording directly to the recorder with no processing compression equalization gating etc Any processing can be added in the mix if necessary You may find tha...

Page 27: ...D W FEARN VT 12 Microphone Preamplifer 30...

Page 28: ...rent 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 match...

Page 29: ...e channel shown First stage The first stage is a selected 12AT7 configured as a Class A voltage amplifier with a gain of approximately 50 Negative feedback from the plate of the second stage reduces d...

Page 30: ...the output transformer The VU meter amplifier is constructed on a printed circuit board mounted above the main chassis The meter and meter amplifier may be totally disconnected from the circuit by the...

Page 31: ...operational amplifiers in the meter amplifier circuit This regulator is located on the Power Supply PCB B Supply Two separate regulated voltages are required for the plates of the VT 12 The B is filte...

Page 32: ...ophic 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...

Page 33: ...year service life Electrolytic capacitors are also used as plate and cathode decouplers In choosing a replace ment the same considerations as with the filter capacitors should be followed VU Meter Ca...

Page 34: ...ube equipment repair experience you may return the VT 12 to the factory for repair Call first however for shipping information An RMA form is available on our web site under Support Warranty Repair If...

Page 35: ...D W FEARN VT 12 Microphone Preamplifer 38...

Page 36: ...t for use with the LP 1 An exception would be if the instrument had a professional level low impedance balanced output for recording To use the LP 1 simply connect the 4 dBm balanced signal source suc...

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