D.W. Fearn VT-7 User manual

VT-7
Vacuum Tube
Compression Amplifier
Operating Instructions
D.W. FEARN

D.W. FEARN VT-7 Compression Amplifier
How to Contact us:
Telephone: [phone]
Fax: [phone]
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: [e-mail]
www.dwfearn.com

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
D.W. FEARN HAND-CRAFTED
PROFESSIONAL
RECORDINGEQUIPMENT
www.dwfearn.com
P.O. Box 57
Pocopson, PA 19366 U.S.A.
Tel: [phone]
Fax: [phone]

D.W. FEARN
VT-7 Compression Amplifier
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VT-7 Vacuum Tube Compression Amplifier
Final Test Report
Model _______________ Serial Number_______________Mains Voltage ______________
Date ___________________ Tested by ________ VU Calibrated to _______________ dBm
Test Equipment _______________________________________________________________
Channel A
Frequency Response:
20 cps to 20 kc/s +/- ___________ dB
THD+Noise:
20 cps ______________ %
200 cps _____________ %
2 kc/s _______________ %
20 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 _______________ %
20 kc/s ______________ %
Noise:
______________ dB below +4 dBm out
Operational Tests:
Compression Range ________
Attack ____________________
Release __________________
Shape ___________________
Link _____________________
GR Meter ________________
Listening Test ____________

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Table 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

D.W. FEARN VT-7 Compression Amplifier
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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 ©2005 D.W. Fearn & Associates

D.W. FEARN
VT-7 Compression Amplifier
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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-7 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-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 merchantability and fit-
ness for a particular purpose.

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VT-7 Compression Amplifier
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History of the VT-7 Compression Amplifier
Compression (or limiting) is sometimes seen as a necessary evil in the recording process.
Often it is used to compensate for less than optimum performance or recording technique.
However, when used appropriately, there is no question that good-sounding compression can
add loudness, power, and improved impact to a recording.
Over many years of recording, I have had the opportunity to use and sometimes own some
of the best compressors in the world. For the D.W. Fearn line of products, a compressor was
obviously important. I began the search for the ultimate compressor design in 1995 and it
wasn’t until 2004 that I developed the sound I was looking for.
There are several ways to achieve compression, and all have their advantages and disadvan-
tages. The earliest, and still one of the best, designs uses the characteristic of vacuum tubes
that allows their gain to be varied by changing the grid bias. Only certain tube types are
appropriate for this (the variable-mu or remote cut-off types), and none are currently being
manufactured. Since I want my products to be used for a long, long time, I could not manu-
facture a product that used tubes that were increasingly rare.
Another excellent approach uses a photoresistor illuminated by a light source to vary the gain
of an amplifier. This method was pioneered by Teletronics/Universal Audio in the LA-2. Many
LA-2s were built over the years, and more recently there have been several new versions of
this classic circuit. 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-essing, gating, etc.) in one simple unit.
However, I was never satisfied with the sound of VCA compressors and did not consider that
design for my use.
I experimented with a few off-the-wall approaches, but was not convinced that these designs
would really work to my standards.
Another approach, not often used, utilizes a pulse-width modulator to control the duty-cycle
of a gain reduction element. This approach has several advantages, including the ability to
offer a wide range of adjustment. In the VT-7, a solid-state pulse-width modulator controls a
special shunt element in the audio path to control the gain. Since no audio passes through
the element, there is no added distortion or other degradation of the sound. That’s the sim-
ple part. Much more complex is the surrounding circuitry that provides precision control of
the element in a way that conforms to what I wanted to hear.
Because the element is well-isolated from the vacuum tube circuitry before and after it, the
effect on the audio is entirely linear. It does not introduce any measurable distortion. And it
passes my test for sonic superiority.
The vacuum tube audio path uses circuitry developed for the D.W. Fearn VT-4 LC Equalizer.
In some of the circuit, entirely new designs were needed. The VT-7 uses a combination of
6072A and 6N1P tubes and has the basic sound of all my products.

D.W. FEARN VT-7 Compression Amplifier
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A look inside the VT-7 reveals a very full box with several densely-packed circuit boards for
the gain control driver circuitry, interspersed with eight vacuum tubes. Because of the very
high frequencies involved in the control circuitry, there are many surface-mount (SMT) com-
ponents. Thus, the VT-7 is a combination of the latest technology, where needed, with a clas-
sic great-sounding vacuum tube audio path.
The VT-7 was designed from the beginning to be a stereo compressor. The two channels can
be used separately or linked together for stereo.
So what does the VT-7 sound like? In many respects, it is like the classic vacuum tube com-
pressors with the same musical sound. I have always believed that for most applications the
compression characteristics that sound best are those that approximate the natural compres-
sion inherent in our 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 times, and a
knob that smoothly varies the compression characteristic from a “softer” to a “harder”
sound. At first, some users may find the controls do not have the range of adjustment that
they are used to in more heavy-handed limiters. However, true to philosophy of design in all
my products, the VT-7 is intended to take good sound and make it sound great. Although its
effect can be made to sound obvious, I like the way the VT-7 sounds like what a really good
engineer would do if he had excellent taste and infinitely fast hands on a fader. In other
words, the VT-7 controls the levels without drawing attention to itself. To me, it just sounds
right.
Douglas W. Fearn
November 2004

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1 .
SPECIFICATIONS
Input 600 ohm source (nominal)
balanced or unbalanced
Input Load
Impedance 32k ohms, transformer
balanced bridging
Nominal Input
Level +4 dBm
Maximum Input
Level @ 20 cps +25 dBm
Gain unity to +15dB
Frequency
Response ± 0.2 dB 20 cps to 20 kc
± 0.5 dB 11 cps 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 Weight: 28 lbs (12.27 kg)
(measurements are made with +4dBm input and output, no compression, into a bridging load)

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2.
DESCRIPTION
The Model VT-7 Vacuum Tube Compression Amplifier is designed to provide recording pro-
fessionals with a sonically superior signal processing device. It is typically used in sound
recording studios for processing individual tracks or, when the channels are linked, as a
stereo compressor. The inputs and outputs are at line level.
The VT-7 in many ways recreates the sound of the classic tube compressors of the 1960s, but
using modern gain control circuitry. The audio path is entirely Class-A triode vacuum tubes,
similar to classic designs but updated with improved modern passive components and com-
puter-aided circuit optimization. Because of the unique qualities of vacuum tubes, the VT-7
has a clarity, transparency, and warmth that solid state compressors lack. Its modern design
and construction allows the VT-7 to exceed the performance of vintage vacuum tube com-
pressors.
The VT-7 is capable of over 30dB of gain reduction, but all units above serial number 19 are
limited 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 unbalanced line level (nominal +4dBm)
signal. It features two independent channels, which can be linked to provide precise gain
reduction tracking of a stereo signal. There are controls for compression threshold, gain,
attack and release times, and compression curve. Additional controls are used to link the two
channels and to switch the true VU meter between measuring gain reduction and output
level. It is built to sound great for a long time, with top quality parts used throughout; the out-
put transformers and many other components are custom-made for the VT-7.
All five power supplies (filament, B+, and three voltages for the solid-state control circuitry
and meter amp) are solid state and fully regulated.
The VT-7 is not mass-produced. Each one is hand-made and meticulously tested and listened
to before shipment to the customer.

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VT-7 Compression Amplifier
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3.
INSTALLATION
The VT-7 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. You might also want to keep the box and packaging material in case
you ever have to ship the VT-7.
Included in the box:
1) The VT-7 Compression Amplifier
2) Line cord
3) This instruction manual
Mounting
The VT-7 is designed for installation in a standard 19 inch rack. It requires 5.25 inches of ver-
tical space, but additional spacing between it and adjacent equipment is recommended for
adequate cooling. Ideally, a ventilated panel at least 1 rack unit high (1.25 inches) should be
installed above and below the VT-7 (and around any other heat producing equipment for that
matter). Be sure the top and bottom vent slots are not blocked. It is essential that air can
flow into the bottom and out of the top of the VT-7. Equipment that runs cool can last for a
very long time.
A small (45mm) fan is mounted on the right side 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 equipment rack
will benefit all your equipment.
The VT-7 can also be used without a rack, placed on a table, counter, or even on the floor if
there is room for air to enter the bottom of the unit. Optional rubber feet are available, when
requested at the time of the order.
Moderate electrical and magnetic fields in the vicinity of the VT-7 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.
The VT-7 uses some high frequency signals in the control circuitry and although the VT-7
itself is designed to be immune to these signals, other sensitive equipment mounted above or
below the VT-7 may not be designed as well as the VT-7. You may have to move equipment
that has poor RFI rejection to a location farther away from the VT-7.

D.W. FEARN VT-7 Compression Amplifier
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Power
The VT-7 is designed to operate from 100, 120, or 220-240 volt, 50/60 Hz power. The unit
will be shipped for the voltage specified in the order, but may be changed in the field if nec-
essary. (See the Maintenance section for detailed instructions on changing the primary volt-
age). The ground pin of the power cord is internally connected to the chassis. This configu-
ration is standard in professional equipment and is required by most electrical codes. A
grounding screw is provided on 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-7 is less susceptible to grounding problems than many studio devices.
Connections
The INPUT connectors are XLR-3 females wired with pin 1 ground, pin 2 “+” or “high,” and
pin 3 “-” or “low.” The input matches 600 ohm (nominal) line level (+4dBm nominal) bal-
anced or unbalanced signals. The input uses a top-quality Jensen line bridging transformer.
The OUTPUT connectors are XLR-3 male wired with pin 1 ground, pin 2 “+” or “high,” and
pin 3 “-” or “low.” The VT-7 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 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 Belden 17250 or equivalent.
The unit does not utilize any RFI filtering, and no RFI has been experienced, even when the
VT-7 is operated in close proximity to AM, FM, and TV broadcast transmitters.
Input and Output Connections
Gold-plated XLR connectors are used for inputs and outputs. The input connectors 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.
Grounding and Shields
A full discussion of proper studio wiring schemes is beyond the scope of this manual, but, in
general, 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 out-
put cable; however, although not recommended, the shields can often be connected at both
ends without a problem.

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4.
OPERATION
Input
Since the input cable will be carrying high quality audio, it is important that a well-shielded
cable is used. You should strive to minimize the number connectors, patch jacks, switches,
etc. between the source and the VT-7 input.
Output
The output of the VT-7 is line level, transformer balanced. Note that vacuum tube equipment
is more sensitive to load impedance than solid state units. The VT-7 design was optimized 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 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.
Figure 2. VT-7 front panel controls and indicators
Threshold
Harder/Softer
Release
Attack
Gain
Link Meter switch
Pilot lamp

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CONTROLS (see Figure 2.)
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-7 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.
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-7 (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-7 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
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.
Separate/Link/Link HPF This is a three-position switch that links the two channels togeth-
er for stereo operation. In the Separate position, the two channels operate completely inde-
pendently, and can be used on totally different sources without any interaction. In the Link
position, the two channels track each other precisely, so that the stereo image will remain
constant even if one channel has much more level than the other. In the Link HPF 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 HPF operates on both channels, in a linked configuration. The roll-off of bass sen-
sitivity is a very gentle curve (about 6dB per octave) and has little effect above 150Hz.

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VT-7 Compression Amplifier
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You can run a stereo signal with the switch in the Separate position for an effect. On a stereo
pair such as a piano or other sub-mix stereo source, the image will tend to shift from left to
right as the signal becomes louder in one channel compared to the other. This would be dis-
concerting on a stereo bus, but it could be interesting on individual stereo tracks.
With very fast Attack and/or Release times, low-frequency distortion can often be reduced
significantly using the Link HPF position.
Meter Switch The VU meter can be switched to read either output (“VU”) or gain reduction
(“GR”). In the VU position, the meter will indicate the actual output level of the VT-7. The
“0” VU mark indicates +4dBm output
In the “GR” position, the meter indicates the amount of gain reduction (compression), in dB.
With no compression, the meter will rest at the “0” VU mark. As compression increases, the
meter will move more to the left. The meter reading indicates the average amount of com-
pression. The precise calibration of the no-signal 0VU indication may drift slightly as the VT-
7 warms up, and change somewhat over time as various components age. This does not affect
the sound of the VT-7 or the gain reduction range. There is a calibration control to set the
0VU indication but it is not something that a non-technical 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 cause no harm to the VT-7, although it is an extreme amount of compression.
Like all true VU meters, the reading is an average and does not indicate the peak level, either
in output nor in gain reduction.
Pilot light This yellow lamp lights whenever the VT-7 is turned on. The VU meters are also
illuminated.
Rear Panel Controls
Power switch Primary power is applied to the VT-7 circuits when the rear panel power
switch is in the “on” position (|). This switch is directly below the AC input connector.
Mains voltage selector This two-position switch is set to the appropriate mains power volt-
age in the location where the VT-7 is used. The only active positions are the “115” and “230”
positions. The “230” volt position will properly accommodate any mains voltage from 220 to
240 volts nominal. See the Maintenance section for instructions on changing the voltage.
When operating in the Link or Link HPF positions, only the left
side (Channel A) controls are operative. The right channel (B)
controls will have no effect on the operation. The only exception
is the Gain control, which will still adjust the level of the B chan-
nel in the Link position.

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For 100 volt operation, internal wiring changes must be made. Contact the factory if this
becomes necessary.
Units ordered for 100 volt operation will be wired for that voltage only.
Fuse The center part of the voltage selector is a fuseholder. For 100 and 120 volt operation,
the fuse is 2 amps. For 240 volt operation it is 1 amp. See the Maintenance section for fuse
replacement information.
Bench Test
If desired, test the VT-7 before installation. The source generator should be set to +4dBm,
600 ohms impedance, balanced, and the output should feed a balanced bridging input of the
audio analyzer. Measured bandwidth should be 20 cps to 20 kc to obtain the same readings
as the factory test results. Compare your measurements with the test data supplied with VT-
7. Keep the results for comparison in future maintenance tests.
Initial Set-Up
The VT-7 should be installed as detailed in the Installation section. With the outputs 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 the internal elements of the tubes expand from the heat. Correct any
ground loop problems before proceeding.
The controls should be set as follows.
• Power (back panel) . . . . . . . . . . . . . On
• Threshold . . . . . . . . . . . . . . . . . . . . . . 9 o’clock
• Gain . . . . . . . . . . . . . . . . . . . . . . . . . . 9 o’clock
• Attack . . . . . . . . . . . . . . . . . . . . . . . . 1 o’clock
• Release . . . . . . . . . . . . . . . . . . . . . . . 9 o’clock
• Softer/Harder . . . . . . . . . . . . . . . . . . mid-point
• Link/Separate . . . . . . . . . . . . . . . . . . Separate
The best indication of proper operation of the VT-7 is how it sounds. This compressor has a
wide operating range and many different settings can provide a wide variety of effects.
Be certain that the output level of the VT-7 is appropriate for the device connected to the out-
put. +4 dBm is the accepted standard level for all professional recording equipment. Some
older equipment may be designed for 0 dBm or +8 dBm, either of which can be easily accom-
modated by the VT-7.

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VT-7 Compression Amplifier
23
Semi-professional equipment frequently uses a reference level of -10 dBv (roughly 14 dB
lower than pro equipment). Although the VT-7 can feed the unbalanced, -10 dBv inputs of
semi-pro gear with no difficulty, the VT-7 VU meter will have to be recalibrated in order to be
useful. (See Section 6 - Maintenance.)
SUGGESTIONS:
You have chosen to use the VT-7 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-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 adjustments listed above will get you started. Now play with the
controls to find the sound that fits your project perfectly.
Use your ears to find the setting that sounds best to you for any given sound.

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VT-7 Compression Amplifier
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5.
THEORY OF OPERATION
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.