Note: Descriptions are shown in the official language in which they were submitted.
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FIELD OF THE INVENTION
High fidelity audio power amplification for sound re-
production and for musical instruments, principally electric
instruments, e.g., electric guitar, electric bass, electric
piano, synthesizer, and the like.
BAC~GROUND OF THE INVENTION
Class A Triode operation is well known to discriminating
audiophiles because of its characteristic musicality and
"warmth". This warmth of tonality can be ascribed to the
avoidance of crossover distortion (because in a push-pull
configuration neither device ever approaches cut-off) and to
the soft or gradual onset of clip when dynamic levels exceed
available undistorted power capability. Unfortunately these
virtues of Triode Class A operation carry with them severe
penalties of economy, efficiency and low power ~apability.
Operating Class A means that less than half the supply power
available can be converted to useful work, and that the
devices themselves must be de-rated due to high zero-signal
current draw. Connecting the screen grid of a Pentode to its
plate, causing it to operate as a Triode, further reduces
potentlal power gain by about half.
These penalties of price and power availability (not to
mention heat) have prevented Class A Triode operation from
ever gaining popularity in the Music Instrument field even
more than in sound reproduction applications. This is
particularly unfortunate for a couple of reasons: First, due
to the extreme dynamic levels produced by the plucked string
(e.g., electric guitar), amplifier output clip is virtually
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unavoidable. Second, musicians have done more than learn to
live with amplifier distortion, they have incorporated it
creatively into their musical expression and have become
connoisseurs of different distortion characteristics. In the
"heavy metal" idiom as well as the blues and popular milieu,
elements o amplifier distortion are so strongly part of the
guitar sound as to often comprise about 50 percent of actual
sound output. Further, circuits to generate "desirable"
distortion effects are well known and in use. These can be
used to simulate output circuit distortion independent of
output power level. If the history of professional choice
were used as a yardstick of "desirable distortion" then
clearly the circuit of Smith (U.S. Patent 4,211,893~ is a
first choice. Here the option of distortion is available via
remote switching and can be controlled to enhance solo or
"lead" playing. It is interesting to note that Smith's
circuit, which generates the distortion in the pre-amplifier
section, uses Class A Triodes throughout.
The present invention, however, deals with output power
amplification and the inherent distortion characteris-tics of
different circuit configurations. Music is by nature a seri~s
of transient and fleeting events. The attack of any given
musical note is of particular concern to the musician (as well
as the listener) and much of a player's learned technique and
expression revolves around the attack of the note. Such is
clearly the case with all stringed instruments--including
piano--as well as reed, brass and percussion instruments.
Further, it can be shown that the manner in which an audio
amplifier handles these transient attacks is the single most
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important factor that distinguishes an outstanding amplifier
Erom one which is merely acceptable in both reproduction and
live performance applications. The steady state distortion
characteristic of most modern amplifiers is excellent:
distortion of any type cannot be heard and competitive
measurements have become pointless. Unfortunately though,
there is no standard measurement of rating for distortion
performance under actual dynamic conditions of use. To
produce realistic sound levels in one's living room of a
symphony or piano, without some clipping of the transient
peaks would require an amplifier of at least 1000 watts and
preferably much more.
5O the demand for high power is obvious to the home user
as well as the musician, yet the occurance of amplifier
distortion--clipping of peaks--remains an unavoidable fact
of life to both. Whereas certain types of amplifier circuitry
produce high power efficiently, a high sonic penalty is paid:
their distortion is noticeable, harsh and disturbing. This is
the typical "odd order" harmonic distortion which occurs in
Class B type amplifiers and is the product of hard clip and
sharp current transfer from push to pull, causing "crossover"
or "notch" distortion. On the other hand, the Triode Class A
circuit produces no crossover distortion and has a "soft"
clip. The sound of amplifier distortion at clip and beyond
is almost unnoticeable because clip does not occur suddenly,
and when it does, it is characterized by the predominance of
even order products which are actually harmonious musically
(that is consonant, not dissonant) to the fundamental. But
the penalty is power. Such a circuit is expensive and
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inefficient.
The well known amplifier Class AB offers some little
improvement. Although the output devices operate Class A
at low power, they become more and more Class B when
driven harder and a somewhat harsh sounding distortion
with an abrupt onset and visible crossover occurs at the
crucial time: at clipo The power output and efficiency
with Pentodes în an AB arrangement is fairly high however.
BRIEF DESCRIPTION OF THE INVENTION
In accordance with an aspect of the invention there
is provided an audio amplifier, primarily for audio power
for musical instruments and sound reproduction systems
including a plurality of electron discharge devices, each
having a plurality of electrodes including an input
electrode and an output electrode, at least two pairs
of said electron discharge devices being connected in a
push-pull parallel configuration, first means for simul-
taneously applying an electrical signal to each of said
input electrodes in common, second means for connecting
each of said output electrodes in common to a utilizaaion
device, means biasing the electrodes of a first pair of
said at least two pairs of electron discharge devices for
Class A operation thereof, and means biasing the electrodes
of the second pair of said at least two pairs of electron
discharge devices for operation in a class other than
Class A.
In my improved amplifier circuit, benefits are derived
continuously from both classes because the amplifier
actually operates simultaneously in both Triode Class A
and Pentode Class AB (or B). The Figure shows four output
devices in a push-pull parallel configuration. Electron
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vacuum tubes are shown but the principles underlying my
invention of simultaneous operation in two classes also
apply to solid state devices of all types. A pair of
Pentode tubes (V5 & V6) are configured as Triodes in a
push-pull arrangement. They are biased in such a way that
they run fully Class A, that is, they draw substantial
plate current at all times throughout the duty cycle and
never approach cutoff. At the same time a second pair of
output devices may be switched on in parallel to the Class
A Triodes for simultaneous different classes of operation
when higher output power is called for. This second pair
of tubes tV7 & V8) operates as push-pull Pentodes Class AB
tor B). This means that the fixed bias, being substantially
greater than that for the Triode pair, allows use of much
higher Plate voltage while still observing safe levels of
dissipation.
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Further, the fact that these devices are Pentodes, renders
the available power gain even greater. Signal drive and grid
bias for the two pairs are distributed through a divider
network adjusted for optimum balance. Such a balance can be
achieved so that the net composite operation retains the
virtues of both types of device (Triode and Pentode) and of
both classes of operation (Class A and Class AB). Since
abundant current is always flowing through both halves of the
output transformer to supply the Class A Triodes, the smooth-
ness of their transfer characteristic effects and smoothsthe apparent transfer characteristic through the transformer
of the AB Pentode pair also Even at maximum power no cross-
over distortion is evident. Furthermore, the Triode factor
contributes the characteristic of soft, gradual clip which
; can be made to occur in advance of and to predominate over
the clip of the Pentodes at all power levels.
So the circuit enjoys the benefits of high power and
efficiency contributed by the Pentodes running Class AB (or
even Class B) but has the preferred distortion characteristics--
clip and crossover-- of Triodes run Class A. Usable tube life
is also extended in an amplifier operating simultaneously in
different classes because the Class AB Pentodes can be biased
very high--into Class B-- for cool low current, high power
operation, while deterioration of the Class A Triodes results
in very little sonic degradation since their power contribution
; is small.
Part of the present disclosure is the preferred driver
amplifier circuitry which maximizes the sonic benefits of
simultaneous different classes of operation. A pair of Class A
s
Triode input devices serves the twofold purposes of amplifying
the incoming signal and splitting it into two phase inverted
components. Excellent linearity and accurate phase inversion
are accomplished by using a differentiating amplifier pair
whose cathodes are biased through a constant current device.
The first triode (Vl) operates conventionally with its yrid
s~rving as the input element and its plate furnishing the
output. The second triode (V2) derives its signal input from
its shared cathode configuration with Vl. The grid of V2 is
grounded (or held slightly above ground to allow the injection
of negative feedback) and amplified phase inverted output
(with respect to output at the plate of Vl) is present at V2's
plate. The use of a constant current source in the common
cathode circuit greatly helps to insure linearity of Vl-V2
over a wide range of operating conditions. A user operable
switch means is provided to allow selection of full loop
feedback, local feedback only, or no feedback at all. Even
though judicious applications of negative feedback are
wonderously effective in producing impressive distortion
specifications under static testing conditions, many listeners
will prefer the greater dynamic realism and open, unrepressed
impact of less--or ultimately no--negative feedback. A dual
buffer amplifier (V3 & V4) is used to provide sufficient drive
amplitude for the amplifier output states. Tubes V3 and V4
are, as usual, operated as Class A Triodes but their common
cathodes are carefully biased to maximize one of the effects
of simultaneous operation in different classes: soft and gradual
clipping. In other words, thelplate loading and cathode
biasing of V3-V4 is carefully configured so that the onset of
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clip at V3-V4 is soft and occurs slightly beore clip occurs
in the amplifier output stages.
A novel configuration with regard to all coupling
capacitors is used throughout the driver/buffer circuit.
Capacitors with large values are often used in audio pre-
amplifier circuits to allow linear frequency response down
to 20 hz or below. Usually the circuit designer (who is aware
of the negative consequences of large capacitor values) must
make a compromise based on three trade-offs. If small value
capacitors are used, low frequency linearity can not be
obtained. If large value capacitors are used, phase shift
tor time delay) increases with increasing capacitor value and
improved low frequency linearity because each AC signal wave
must charge the capacitor against its time constant with the
attendent circuit resistance. This problem is remedied in
conventional practice by keeping a fairly high impedence (or
resistance) on the output side of the capacitor. The undesired
(and overlooked) consequence of this solution in a low feed-
back amplifier is that the zero DC reference state of the
capacitor output becomes unstable and gyrates with strong
signal fluctuations. This is apparently overlooked because
most circuit designers concentrate on the steady state per-
formance of their amplifiers where the DC gyrations on big
transients, which settle out after about one second, are
usually not discovered or dealt with. In my improved driver/
buffer circuit these three problem/trade-offs are virtually
eliminated by using large capacitors with a fairly low im-
pedance across their inputs and a very high impedance at
their outputs.
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So then, to summarize, some novelties about my amplifier
are:
1. A differential amplifier has a constant current cathode
source and acts as phase inverter/first amplifier followed
by 2; a dual buffer amplifier whose loading and biasing are
arranged to produce clip in a controlled fashion in harmony
with 3; a pair of parallel output power amplifiers working
simul-taneously and in differing classes of operation whose
outputs are combined. 4. Negative feedback may be applied
selectively: full loop, local or avoided entirely. 5. To
provide low frequency linearity while still ensuring faithful
AC and DC response in the time domain, large coupling
capacitors are arranged throughout in such a fashion that the
impedance at their inputs is fairly low while the impedance
at their outputs is very high.
BRIEF DESCRIPTION OF THE DRAWING
The drawing shows a schematic diagram of an amplifier
circuit in accordance with the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
In Fig. 1 the input 1 is connected to the grid 3 of Vl
via conductor 2. Grid leak resistor 14 is connected to ground
25. The cathode 5 of Vl is tied directly to the cathode 6
.l of V2 and both are energized from a negative B- source 12.
Use of a constant-current source device 11 in the cathode
circuit accomplishes vastly improved linearity of Vl and V2
under widely ranging operating conditions. As Vl amplifies,
signal fluctuations which appear on its cathode 5 also
appear on the cathode 6 of V2 and are used to drive V2.
S
The grid 4 of V2 is for all intents and purposes grounded
although a small resistor 13 may be used to allow the
injection of negative feedback to the grid 4 via an RC net-
work lS, 16. A switch means 91 is then pxovided to select
negative feedback originating locally from the output of
buffer stage V3 through a blocking capacitor 96 and a series
resistor 93. Or as alternatives to suit program material,
choice of speaker and listener preference~ switch 91 can also
be used to select overall negative feedback, which signal
would derive from a point 95 at the amplifier output and be
buffered through resistor 92. As the third alternate, the
switch 91 offers the user the selection of no negative feed-
back whatever, which listening tests demonstrate is most often
: preferred. Plates 7, 8 of triodes Vl and V2 are connected to
the high voltage B+ supply 80 through high impedence load
resistors 9 and 10. Amplified, split phase signals are
conducted from the plates 7, 8 of Vl and V2 to the grids 29,
30 of V3 and V4 respectively via coupling capacitors 19 and
20 which are large in value. Grid leak resistors 23, 24
represent a very high impedance while much lower impedance
resistors 21, 22 provide DC stability to the large capacitors
19, 20 in the very high impedance grid circuit of V3 and V4
inclusive. Cathodes 27, 28 of V3 and V4 respectively are
energized through resistor 26 whose vaiue is selected in
conjunction with the values of loading resistors 35, 36 to
cause a soft and gradual clip of V3, V4 to occur just prior
to the onset of clip in the Class A Triode section of the
amplifier output state. The plates 31, 32 of V3 and V~
are energized from the B+ high voltage supply 80 through
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resistors 33, 34. Output from the plates 31, 32 of V3 and
V~ i.s coupled to the output stages through capacitors 37
and 38, both of which have a large value, and resistors
83 and 84 which are used to present a very high impedance to
the output of capacitors 37 and 38. Negative bias for the
output stage comprised of V5, V6, V7 and V8 derives from a
supply represented at 71. This negative DC bias flows through
an output tube balancing network comprised of potentiometer
69 and drain resistors 81 and 82. Bias for the Pentodes V7
and V8 is obtained through low value resistors 43 and 44, and
is maintained at a level that provides Class AB (or ~lass B)
operation for these Pentodes. Screen grids 61, 62 of these
Pentodes (V7~ V8) are fed from a high voItage supply 74
through low value resistors 63 and 6~. Pentode plates 59, 60
; are impedance matched to the full primary winding 72 of the
: output transformer with one plate at each of the ends 87, 88.
The cathodes 67, 68 of V7 and V8 are selectively energized
from ground 25 through a user operable switch means 70. When
the switch is open, no current flows, V7 and V8 are effectively
in a "standby" mode and the amplifier comprises functionally
of the pair of Class A Triodes V5 and V6. With the switch 70
closed, the amplifier circuit comprising V7 and V8 (and any
other number of Pentode AB output pairs in parallel) and their
inclusive parts becomes operational and in fact contribute
the vast majority of power to the common output 75 as the plates
59 and 60 of V7 and V8 are connected to the output transformer
primary 72 through conductors 76 and 77. Pentodes shown as
V5 and V6 are made to function as Triodes because their plates
45, 46 arè tied to their respective screen grids 51, 52
s
through low value resistors 53, 54. Cathodes 57, 58 are
energized from ground 25. Grid bias on V5 and V6 is reduced
to a low value compared to the AB Pentode pair by the
divider networks comprislng resistors 39 and 41 for V5 and
resistors 40 and 42 for V6. Furthermore, the values
selected for these divider networks and other resistors in
the bias network--as well as in the signal path--are chosen
to cause the onset of clip in the Triode Class A pair to
occur slightly before the Pentode AB pair(s) beings to clip.
Taps 89, 90 can be provided at lower impedance points on the
output transformer primary 72 to properly match the plate
loads imposed by the Class A Triode pair. Voltage dropping
resistors 47 and 48 can be used to reduce dissipation of the
plates 45, 46 of the Class A Triodes V5 and V6 to safe
levels while still allowing the maintenance of full DC
voltage on the Pentode pair V7, V8. Bypass capacitors 49,
50 may be used to res-tore signal gain lost through the
dropping resistors 47, 48.
Though the invention has been described with respect to
a specific preferred embodiment thereof, many variations and
: modifications will immediately become apparent to those
skilled in the art. It is therefore the intention that
:~ the appended claims be interpreted as broadly as possible
; in view of the prior art to include all such variations and
modifications.
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