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D.W. Fearn VT-1 Скачать руководство пользователя страница 37

D.W. FEARN

VT-1 & VT-2 Microphone Preamplifers

37

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-2 top cover (see above).

2. Terminate the output in the same impedance as the VT-2 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 a fifteen minute warm-up. Feed a 1 kc tone at about -

45 dBm into the VT-2 input.

4. Measure the output and set the Attenuation control for +4 dBm (or whatever refer-

ence is desired).

5. Set the Meter Calibration trimmer potentiometer for 0 VU on the VT-2 meter(s). Be

sure the meter is turned on. This control is a 25-turn pot so the adjustment is easy. A
small screwdriver or alignment tool is necessary. The controls are located on the
meter amplifier printed circuit board on the top of the chassis and they are the only
trim pot(s) on the VT-1/VT-2. See Figure 5 on page 38.

6. Replace the top cover. Be sure the ventillation slots are over the tubes (towards the

back).

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-2 to the factory for repair. Call first, however, for shipping information.

Fig. 5 VU Meter Amplifier PCB

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Содержание VT-1

Страница 1: ...VT 1 VT 2 Vacuum Tube Microphone Preamplifiers Operating Instructions D W FEARN ...

Страница 2: ...ophone Preamplifers 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 ...

Страница 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...

Страница 4: ...4 ...

Страница 5: ...__________ 2 kc s _______________ 20 kc s ______________ Noise ______________ dB below 4 dBm out Equivalent Input Noise _______ dB Operational Tests 20 pad ________________ Lo Z input ______________ Phase Reverse __________ 48V ___________________ Listening Test ____________ Channel b Frequency Response 20 cps to 20 kc s ___________ dB THD Noise 20 cps ______________ 200 cps _____________ 2 kc s _...

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Страница 7: ... VT 1 and VT 2 9 1 Specifications 13 2 Description 15 3 Installation 17 21 5 Theory of Operation 29 6 Maintenance 33 LP 1 Line Pad Instructions 37 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 ...

Страница 8: ...s 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 1995 2004 D W Fearn Associates ...

Страница 9: ... dered defective as a result of accident misuse or abuse by the use of parts not man ufactured or supplied by D W Fearn or by unauthorized modification of the VT 1 or VT 2 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 ...

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Страница 11: ...lf It consisted of half a dozen RCA tube microphone preamplifiers that I salvaged from the junk pile of the radio station where I worked an equal number of 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 br...

Страница 12: ...nt 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 equ...

Страница 13: ...e best 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 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 ...

Страница 14: ...ne Preamplifers 14 The VT 2 was developed in 1995 in answer to requests from a number of our customers for a two channel version of the preamp The VT 2 occupies the same amount of rack space but is four inches deeper than the VT 1 ...

Страница 15: ...se 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 Level 22 dBm unterminated Power Requirements 100 120 or 220 VAC 50 60Hz 25 W Dimensions 19 48 26cm W 5 25 13 34cm H 9 22 86cm D VT 2 13 22 ...

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Страница 17: ...transparency and warmth that solid state preamps lack Its modern design and construction allows the VT 2 to exceed the per formance of vintage vacuum tube preamps It is designed for use in the professional recording environment 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 po...

Страница 18: ......

Страница 19: ...o the bottom and out of the top of the VT 2 Equipment that runs cool can last for a very long time In tight equipment enclosures be sure there is adequate air flow Forced air cooling will ben efit all your equipment The VT 2 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 a...

Страница 20: ...ransformer balanced The OUTPUT connectors are XLR 3 male wired with pin 1 ground pin 2 or high and pin 3 or low The VT 2 is optimized for feeding balanced bridging inputs Virtually all 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...

Страница 21: ... 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 g...

Страница 22: ...D W FEARN VT 1 VT 2 Microphone Preamplifers 22 ...

Страница 23: ... possible One successful method is to place the VT 2 in the studio with only a short cable to the microphone Line level from the VT 2 output is then fed back to the con trol room Avoid locating the VT 2 where it will be subjected to high sound levels or excessive vibration such as on a drum riser Output The output of the VT 2 is line level transformer balanced Note that vacuum tube equipment is mo...

Страница 24: ...s or when purposely over driving the VT 2 VU meter Monitors the output level of the VT 2 Pilot lamp Illuminates when AC power is applied and the Power switch is on Power switch Controls primary AC power to the VT 2 Power switch and indicator Primary power is applied to the VT 2 circuits when the Power switch 8 is in the up position The amber pilot lamp 7 indicates that the unit is on It takes abou...

Страница 25: ...impedance microphone simply select the 20 posi tion of the INPUT switch The impedance match will be correct Keep in mind that many musical instruments and vocalists produce peaks that are as much as 20 dB above the average level Often the peak clipping that occurs is of such short dura tion that it is not easily noticed and may be acceptable in many situations This is particular ly true with the V...

Страница 26: ...he Attenuation control near the top of its range the input level is too low and any padding at the mic or on the VT 2 should be reduced or removed Phase 4 This switch reverses the polarity of the output of the VT 2 A detailed discussion of the appli cation 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 ...

Страница 27: ...e 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 2 Keep the results for comparison in future maintenance tests Initial Set Up The VT 2 shoul...

Страница 28: ... can be easily accom modated by the VT 2 Semi professional equipment frequently uses a reference level of 10 dBv roughly 14 dB lower than pro equipment Although the VT 2 can fed the unbalanced 10 dBv inputs of semi pro gear with no difficulty the VT 2 VU meter will have to be recalibrated in order to be useful See Section 6 Maintenance SUGGESTIONS You have chosen to use the VT 2 because of the sup...

Страница 29: ...ral 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 that far less processing is required when using the VT 2 If processing is required while recording the track insert the processing device after the VT 2 and before the recorder Figure 3 Typical Studio Intercon...

Страница 30: ...D W FEARN VT 1 VT 2 Microphone Preamplifers 30 ...

Страница 31: ...wer 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 resp...

Страница 32: ...el shown 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 ...

Страница 33: ...ransformer The VU meter ampli fier and 12 volt regulator are 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 Meter switch The meter calibrationis 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 con formin...

Страница 34: ...e IC operational amplifiers in the meter amplifier circuit This regulator is located on the Meter Amp PCB B Supply Two separate regulated voltages are required for the plates of the VT 1 VT 2 The B is fil tered with long life low leakage computer grade filter capacitors before being regulated and extensively bypassed and decoupled The negative side of the supply is grounded Two sepa rate B supplie...

Страница 35: ...tubes but with slightly 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 2 ...

Страница 36: ...p ment becomes obsolete The electrolytic capacitors used in the VT 2 typically will last at least twenty years If there is a measurable and or audible increase in 120 cps noise the filter capacitors should be suspected They should be replaced with new capacitors of equivalent capacitance and voltage rating and the replacements should be specified for a minimum ten year service life Electrolytic ca...

Страница 37: ...the Meter Calibration trimmer potentiometer for 0 VU on the VT 2 meter s Be sure the meter is turned on This control is a 25 turn pot so the adjustment is easy A small screwdriver or alignment tool is necessary The controls are located on the meter amplifier printed circuit board on the top of the chassis and they are the only trim pot s on the VT 1 VT 2 See Figure 5 on page 38 6 Replace the top c...

Страница 38: ...pair If the VT 2 should develop a problem during the five year warranty period call the factory for return shipping instructions We will repair and return your VT 2 quickly Note that the war ranty does not cover vacuum tubes which must be periodically replaced ...

Страница 39: ...imply connect the 4 dBm balanced signal source such as the output of a mixer to the LP 1 Input Then connect the LP 1 Output directly to the input of the VT 2 preamp The VT 2 output would then feed the recorder The loss through the LP 1 is approximately 43 db With a 4 dBm input the output is a bit high for the VT 2 in the 0 position of the Input switch The 20 position will provide a better level in...

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