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D.W. Fearn VT-7, Operating Instructions Manual

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D.W. Fearn VT-7 Operating Instructions Manual Download Page 20

D.W. FEARN 

VT-7 Compression Amplifier

25

5 .

T H E O RY   O F   O P E R AT I O N  

Input section

Line level (low source impedance, balanced, +4 dBm nominal) audio enters through the XLR-
3 female INPUT connector  to a bridging input transformer. The load imposed on the source
is 32k ohms and is constant regardless of the frequency.  

Input transformer

The input transformer is made by Jensen Transformers, Inc. and represents the state of the
art in transformer design. It exhibits extremely flat frequency 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.

Input Amplifier

First stage

The first stage is a 6N1P configured as a Class A voltage amplifier with a gain of approximate-
ly 20. 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 polpropylene
capacitor. This stage operates as a Class A voltage amplifier with a gain of approximately 15. 

Output Stage

The output stage operates as a cathode follower, presenting a comparatively low output
impedance (approximately 450 ohms). The cathode output is coupled through a polypropy-
lene capacitor to the Pulse Width Modulator board.

Pulse Width Modulator

These boards, mounted underneath the chassis, contains the pulse generation and control
circuitry, and the gain reduction element. The loss through the board is about 20dB with no
gain reduction, rising to about 50dB with maximum gain reduction (30dB standard on seri-
al numbers 19 and above). A second pair of printed circuit boards on the top of the chassis
shapes the audio signal for the PWM circuit.

Output Amplifier

First stage

The first stage is a 6072A configured as a Class A voltage amplifier with a gain of approxi-
mately 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 individ-
ual vacuum tube characteristics.

Summary of Contents for VT-7

Page 1: ...VT 7 Vacuum Tube Compression Amplifier Operating Instructions D W FEARN ...

Page 2: ...ession Amplifier 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: ...5 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 D W FEARN HAND CRAFTED PROFESSIONAL RECORDING EQUIPMENT www dwfearn com P O Box 57 Pocopson PA 19366 U S A Tel 610 793 2526 Fax 610 793 1479...

Page 4: ...0 kc s ______________ Noise ______________ dB below 4 dBm out Operational Tests Compression Range ________ Attack ____________________ Release __________________ Shape ___________________ Link _____________________ GR Meter ________________ Listening Test ____________ Channel B Frequency Response 20 cps to 20 kc s ___________ dB THD Noise 20 cps ______________ 200 cps _____________ 2 kc s ________...

Page 5: ...ble of Contents Final Test Report 3 Warranty 7 History of the VT 7 9 1 Specifications 11 2 Description 13 3 Installation 15 4 Operation 17 5 Theory of Operation 23 6 Maintenance 25 List of Illustrations 1 Front Panel Controls and Indicators 17 ...

Page 6: ... 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 2005 D W Fearn Associates ...

Page 7: ...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 7 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 ...

Page 8: ...uit The world has a good supply of old and new LA 2 style compressors and although I did some experimenting with this circuit I felt that there were enough of them and I didn t need to add another Also this design lacks the versatility I thought was necessary A more modern approach uses a voltage controlled amplifier VCA which is quite versatile and can combine several additional functions de essi...

Page 9: ... hearing The VT 7 has this kind of natural sounding compression that sounds just right to me The ultimate test for all my products is this Would I use it in preference to any other prod uct out there old or new The VT 7 passes this test There are not a lot of controls on the VT 7 Basically there is just a knob to control the amount of compression Threshold the amount of gain the attack and release...

Page 10: ... to 28 kc 3 dB 0 5 cps 65 kc THD Noise 0 05 20 cps to 20 kc Signal to Noise Ratio 78 dB minimum Output low Z transformer balanced Output source impedance 115 ohms Maximum Output Level 22 dBm into bridging input balanced or unbalanced Gain Reduction Range 0 to 20dB Power Requirements 100 120 or 220 VAC 50 60Hz 85 W Dimensions 19 48 26cm W 5 25 13 34cm H 13 22 9cm D Weight 22 lbs 9 55 kg Shipping We...

Page 11: ...mited to 15dB of compression to provide greater resolution in the Threshold control for set ting moderate amounts of compression The VT 7 can be special ordered with the full 30dB range Although a VT 7 could be modified after construction it is rather involved It is designed for use in the professional recording environment It accepts a low impedance 600 ohm nominal source impedance balanced or un...

Page 12: ...D W FEARN VT 7 Compression Amplifier 16 ...

Page 13: ...ide panel The fan outlet should not be blocked There are eight vacuum tubes and seven printed circuit boards loaded with solid state com ponents in the VT 7 It generates a lot of heat You need to think through your mounting scheme to provide the VT 7 with sufficient air flow for cooling It will eventually be damaged if the top and bottom cooling slots are covered Forced air cooling in your equipme...

Page 14: ...g 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 5x20mm type slow blow 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 Belde...

Page 15: ...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 7 has an input ter mination switch that switch should be in the off position The VT 7 can feed balanced or unbalanced inputs with no need for any modific...

Page 16: ...se times add more energy to the sound but can 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 con...

Page 17: ...chnical user should adjust The VU meter will always indicate an average amount of level or gain reduction not the peak value This is a true VU meter and conforms to ASA Standard C16 5 1954 The VT 7 is capa ble of a wider range of gain reduction than a standard VU meter can accurately display so it is possible that the reading could be fully to the left end of the scale greater than 20dB This will ...

Page 18: ...tputs connect ed to an appropriate destination typically to audio recorder inputs or as a mixer insert con figure the studio to monitor the VT 7 output Apply power and wait about twenty seconds for the tube filaments to get up to temperature Check for hum buzz or other noise For the first few minutes after a cold start it is not unusual for the VT 7 to produce hiss pops and micro phonic clanks as ...

Page 19: ...e superior sound it provides To gain the max imum benefit from your investment it is important that you hook up the VT 7 so that other factors do not adversely affect the sound quality Except for extremely short Attack and Release times the VT 7 will provide quality sound with any settings But like any instrument you will find the best adjustment of the controls by experimenting The starting adjus...

Page 20: ...econd stage The output of the first stage is coupled to the grid of the second stage through a polpropylene capacitor This stage operates as a Class A voltage amplifier with a gain of approximately 15 Output Stage The output stage operates as a cathode follower presenting a comparatively low output impedance approximately 450 ohms The cathode output is coupled through a polypropy lene capacitor to...

Page 21: ...M boards 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 7 through a standard IEC power input connector with an integrated power switch The Power switch energizes all five power supplies A mains voltage selector switch on the back panel allows the primary voltage of the VT 7 to be easily change...

Page 22: ...dict 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 7 and compare the results to previous measurements Tubes also sometimes dev...

Page 23: ... frequency is 50Hz the filter capacitors should be suspected They should be replaced with new capacitors of equivalent capacitance and voltage rating and the replace ments should be specified for a minimum ten year service life Electrolytic capacitors are also used as decouplers In choosing a replacement the same considerations as with the filter capacitors should be followed VU Meter Calibration ...

Page 24: ...ime base that can accurately resolve 40nS is required The circuitry of the VT 7 is quite stable and routine alignment is usually not required If the VT 7 requires alignment contact the factory for detailed instructions 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 ...

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