A circuit for limiting the output power of a power amplifier to a preselected value in order to prevent the amplifier from either clipping the input signal or applying potentially damaging signal levels to a loudspeaker connected to the output of the amplifier. The limiting and clipping circuit includes a voltage controlled attenuator through which the input signal passes before entering the amplifier. The amplifier output is compared to an output reference by a control circuit which supplies an attenuation command to the attenuator whenever the output level reaches the reference level. The value of the output reference can be easily adjusted from a fairly low percentage of the amplifier power capacity to the amplifier's maximum power capacity in which condition the circuit prevents amplifier clipping. The circuit is relatively quick to attenuate input signals responsive to excessive output levels, but is relatively slow to reduce the attenuation when the output level is subsequently lowered. Consequently, the circuitry quickly limits the output to safe and distortion free limits without itself introducing waveform distortion of signals in the audio range. Various embodiments of the invention may be employed depending upon specific circumstances.

4,233,566
Nestorovic
November 11, 1980

The present invention provides a preamplifier, by which it is possible to prevent influence of noise components when there are noise components in power line. There is provided a transformer having a primary winding with a center tap and a secondary winding on output side, and the primary winding of the transformer is connected between plates of two vacuum tubes for performing differential amplification, and positive voltage is applied on the center tap of the primary winding. A switch may be arranged in such manner that input signal and output signal can be separately set to balanced state or unbalanced state.

Inventors: Yamada; Kazutoshi (Sakai, JP)
Assignee: Zanden Audio System Co., Ltd. (Osaka Pref., JP)
Appl. No.:958482

Filed: October 27, 1997

6,111,460
Yamasita
August 29, 2000

A BIFET vacuum tube replacement structure includes a plurality of devices that replicate the characteristics of a vacuum tube. The vacuum tube replacement structure has the same pin-out as the vacuum tube being replaced and so can be exchanged directly for a vacuum tube in an audio amplifier. The vacuum tube replacement structure is suitable for use in a wide range of audio amplifier applications without modification to the audio amplifiers. Further, there is no noticeable degradation to the human ear in the sound quality when the vacuum tube replacement structure is used in an audio amplifier in place of a vacuum tube. A unitary device that is a combination of a high impedance bipolar like transistor and a unipolar junction field effect transistor, that is referred to as a BIFET, is used in the vacuum tube replacement structure. In one embodiment, the bipolar like transistor is formed in combination with the gate of the unipolar junction field effect transistor. The vacuum tube replacement structure faithfully replicates input signals at low levels and also slowly and uniformly compresses the input signals at higher drive levels and under overdrive conditions.

In a direct coupled type vacuum tube amplifying circuit, to stabilize the operation and simplify the feedback loop, a plate power source EB5 is connected to a plate of a vacuum tube V5 of a first stage through a constant current source 1. A grid of a vacuum tube V6 of a next stage is connected to the plate of the vacuum tube V5 through a first resistance R3. A second resistance R4 is connected between the grid and the cathode of the vacuum tube V6. A negative power source EC6 is connected to the resistance R4 through a constant current source 2. In this constitution, the grid bias voltage of the second vacuum tube V6 is defined by the voltage drop of the second res

5,912,585
Yamada
June 15, 1999

A BIFET vacuum tube replacement structure includes a plurality of devices that replicate the characteristics of a vacuum tube. The vacuum tube replacement structure has the same pin-out as the vacuum tube being replaced and so can be exchanged directly for a vacuum tube in an audio amplifier. The vacuum tube replacement structure is suitable for use in a wide range of audio amplifier applications without modification to the audio amplifiers. Further, there is no noticeable degradation to the human ear in the sound quality when the vacuum tube replacement structure is used in an audio amplifier in place of a vacuum tube. A unitary device that is a combination of a high impedance bipolar like transistor and a unipolar junction field effect transistor, that is referred to as a BIFET, is used in the vacuum tube replacement structure. In one embodiment, the bipolar like transistor is formed in combination with the gate of the unipolar junction field effect transistor. The vacuum tube replacement structure faithfully replicates input signals at low levels and also slowly and uniformly compresses the input signals at higher drive levels and under overdrive conditions.

United States Patent
5,648,664
Rough ,   et al.
July 15, 1997

Solid state emulations of vacuum tube power amplifier are shown for both high-power and low-power applications. The emulations include the screen grid and power supply effect of compression, the variable output impedance for reacting with a speaker load, power tube distortions, the limiting and bias shifting created by the power tube grids, and the bias shifting of the phase splitter. The high-power disclosure includes a current amplifier which may be linear or switching. The low-power disclosure simulates a vacuum tube amplifier with a speaker load in order to drive a master volume without diminishing the classic distorted tone. It has means to simulate the speaker impedance. The solid state emulation includes a gain control means which produce an unclipped region which may be divided into more or less distortion than one percent and two clipped regions having different output impedances or different frequency characteristics.

5,636,284
Pritchard
June 3, 1997

An audio amplifier including a series connected vacuum tube output stage for driving an audio loudspeaker. The preferred amplifier uses the output signal as a floating reference for an intermediate stage to prevent output stage degeneration for producing balanced push-pull drive signals, thereby eliminating the need for an output transformer or AC coupling.

5,604,461
Rozenblit
February 18, 1997

A power output amplifier for use with electric guitar wherein the individual pairs of power tubes are of two or more differing types which may be selected by the user to operate alternately or simultaneously.
5,559,469
Smith
September 24, 1996

Other References

Hust, Lloyd B., "Extended Class A Amplifier", Radio and Television News, Sep. 1953, pp. 40-42 & 146-148.

An electronic amplifying apparatus intended for sound reproduction and for music instrument amplification wherein two parallel amplifiers, working simultaneously, each in a different class of operation, are fed from a single driver or signal source and whose outputs are combined. In the preferred embodiment at least two pairs of push-pull vacuum tubes are required and they are arranged so that one pair operates Triode Class A while the other pair (or pairs) operates Pentode Class AB (or Class B). The circuit can then be optimized so that the desirable sonic characteristics of Class A Triode operation are imparted into the Class AB (or Class B) Pentodes which actually produce all or nearly all of the power. The output power waveform has the high power and efficiency typical of Class AB (or Class B) Pentode operation but without the detrimental sonic side effects, namely, there is the complete absence of crossover or "notch" distortion, and there is a "soft" gradual onset of clip. Both of these traits are highly desirable and characteristic of the notoriously inefficient Class A Triode operation. On/Off switching of various pairs of output tubes may further be employed to add flexibility to the system with regard to power availability and sonic performance. Single ended configurations as well as solid state devices or hybrid combinations could be arranged to operate in simultaneous different classes of operation and enjoy its attendent benefits.

Smith; Randall C. (1317 Ross St., Petaluma, CA 94952)

4,593,251

BIFET vacuum tube replacement structure 
A BIFET vacuum tube replacement structure includes a plurality of devices that replicate the characteristics of a vacuum tube. 
The vacuum tube replacement structure has the same pin-out as the vacuum tube being replaced 
and so can be exchanged directly for a vacuum tube in an audio amplifier. The vacuum tube
replacement structure is suitable for use in a wide range of audio amplifier 
applications without modification to the audio amplifiers. Further, there is no noticeable degradation 
to the human ear in the sound quality when the vacuum tube replacement structure is used in an audio 
amplifier in place of a vacuum tube. A unitary device that is a combination of a high impedance bipolar
like transistor and a unipolar junction field effect transistor, that is referred to as a BIFET, is 
used in the vacuum tube replacement structure. In one embodiment, the bipolar like transistor is formed
in combination with the gate of the unipolar junction field effect transistor. The vacuum tube replacement
structure faithfully replicates input signals at low levels and also slowly and uniformly compresses the
 input signals at higher drive levels and under overdrive conditions. 

Inventors:  Rough; J. Kirkwood H. (264 S. 14th St., San Jose, CA 95112); Cogan; Adrian I. 
(1174 Hyde Ave., San Jose, CA 95129); Thornton; Neill R. (39450 Albany Common #W, Fremont, CA 94538)  

U.S. Patent Documents
3742261 Jun., 1973 Schneider et al. 257/272.  
3953808 Apr., 1976 Clark et al. 327/581.  
5321283 Jun., 1994 Cogan et al. 257/256.  
Other References 
Chiu & Ghosh, "Characteristics of Junction-Gate Field Effect Transistor with Short Channel Length", 
Solid State Electronics, vol. 14, pp. 1307-1317, 1971. C. Kim, E. Yang, "Carrier Accumulation and 
Space-Charge-Limited Current Flow in Field Effect Transistors", Solid State Electronics, vol. 13, pp. 1577-1589, 1970. 
J. Nishizawa, T. Terasaki, J. Shibata, "Field-Effect Transistor Versus Analog Transistor (Static Induction Transistor)"
 IEEE Transactions on Electron Devices, vol. ED-22, No. 4, Apr. 1975. A. S. Wang, C.J. Dell'Oca, "A Compatible 
Bipolar and JFET Process" IEDM Proc., pp. 45-47, Dec. 1976. J. Nishizawa, Semiconductor Technology in 
Japan, Chapter 15, North Holland, Publisher, NY, pp. 201-219, 1982, "Static Induction Transistor". 
M.G. Kane, R. Frey, "The PSIFET Emerges as a New Contender", MSN, pp. 46-58, Sep. 1984. 
A. Cogan et al., "Progress Toward the Ultimate Semiconductor Switch", Powertechniques Magazine, pp. 35-39, Sep. 1986. 
J. Browne, "Solid-State Triodes Boost High Voltages at Broad Bandwidths", Microwaves & RF, pp. 221-224, May, 1989. 
B.J. Baliga, "Bipolar Operation of Power Junction Field Effect Transistors.", 
Electronic Letters, vol. 10, No. 2, Feb. 1980.  

A power output amplifier for use with electric guitar wherein the individual pairs of power tubes are of two or more differing types which may be selected by the user to operate alternately or simultaneously.

5,559,469
Smith
September 24, 1996

Light Dependent Resistors (LDR's) are used together with a differential amplifier and a plurality of reference resistors to replace conventional analog potentiometer controls and thus offer digital activation and programmability to a traditional style vacuum tube preamplifier.

5,208,548
Van Riezen
May 4, 1993

An audio power amplifier for general audio and sound reinforcement use, or musical amplification, stringed or otherwise, which combines the sonic properties of vacuum tube amplification with the modern technology of solid state electronics, with its attendant advantages.
5,148,116
Robinson
September 15, 1992

5,912,585  
Yamada  June 15, 1999  

A.R. Pearlman

5,144,246
Stelloh ,   et al.
September 1, 1992

Preamplifier 

In a circuit including an amplifier for amplifying a small signal from a signal
source wherein the inductance of a transducer and the capacitance of a capacitor 
form a resonance circuit, the output from the amplifier is negatively fed back through
a resistor or a combination of resistors to the input of the amplifier, whereby the
noise in the demodulated signal may be suppressed. 
 4,240,038  
Hasegawa ,   et al.  December 16, 1980  

Assignee: AdventUnited States Patent  4,117,412  
Holman, II  September 26, 1978  

A playback preamplifier for use on a recording machine utilizing a magnetic recording medium to provide a substantially flat response between a general range of 200 to 6000 Hz. and having a substantially improved signal-to-noise ratio. The circuit includes a three stage amplifier having an overall DC feedback loop. The magnetic transducer is connected in the feedback loop so that the DC feedback current biases the transducer with the DC feedback current.

An audio power amplifier can be operated in either class A, class B or class AB amplifier modes by selectively switching bias values set in a bias setting circuitry provided for a driver stage which drives the main power amplifier stage.

4,087,761

Inventors:  Fukumoto; Nobuo (Hamamatsu, JA); Ohiwa; Mitsugo (Hamamatsu, JA); Matsui; Teruyoshi (Hamamatsu, JA); Ohta; Hajime (Hamamatsu, JA)  

United States Patent  4,240,038  
Hasegawa ,   et al.  December 16, 1980  
Preamplifier 
Abstract
In a circuit including an amplifier for amplifying a small signal from a signal source wherein the inductance of a 

transducer and the capacitance of a capacitor form a resonance circuit, the output from the amplifier is negatively 

fed back through a resistor or a combination of resistors to the input of the amplifier, whereby the noise in the 

demodulated signal may be suppressed. 

A phonograph preamplifier network with infrasonic cutoff response. The preamplifier network is an active filter configuration with a high gain, high input impedance differential amplifier provided with an RIAA equalization feedback filter network coupled between the amplifier output and inverting input. An RC filter network is coupled between the inverting amplifier input and ground potential to provide a single, low frequency real axis pole. In addition, an input filter feedback network is coupled between the preamplifier input terminal and the non-inverting input of the amplifier, and further bootstrapped to the amplifier inverting input. The network provides a pair of low frequency off-axis complex poles. The RC filter and the input filter feedback networks are adapted to control the infrasonic frequency response of the active filter preamplifier network so that the preamplifier network is characterized by an 18 dB per octave roll-off response at frequencies below a predetermined infrasonic frequency, and a relatively flat response at frequencies in a range adjoining and above that infrasonic frequency.

A switch connected between two voltage sources and responsive to a sensing circuit. The switch includes a transistor which operates to interchange the electrical connection of the two voltage sources from parallel to series upon the sensing circuit signals the need for more voltage. The two sources are normally in parallel to increase the cost efficiency and energy efficiency of the amplifier and power supply.

4,788,452  
Stanley  November 29, 1988  

A power converter which includes a bridged power amplifier powered from a sole or single DC input power source through a plurality of parallel connected PWM buck converters. The converters are modulated with reference signals which operate in time alternation such that in any conversion cycle there exists a plurality of modulating wave forms which are identical except for their time delays and which are evenly spaced in time over a single conversion cycle. A feedback control amplifier compares the voltage output of the parallel connected converters through the magnitude of the amplifier's required supply voltage and drives the modulating components of the circuit to control the switching duty cycle of each of the individual PWM converters.

5,513,094  
Stanley  April 30, 1996  

6,178,102  
Stanley  January 23, 2001  

6,023,193  
Ierymenko  February 8, 2000  

5,767,744  
Irwin ,   et al.  June 16, 1998 

J. Futterman

4,240,038  
Hasegawa ,   et al.  December 16, 1980  

4,107,619

Pass, Nelson

Pass, Nelson

4,899,387

Pass, Nelson

4,752,745

Pass, Nelson

4,638,260

Hamley, James

4,121,168

Inventors: Stitt; Robert M. (Tucson, AZ)

Assignee: Burr-Brown Research Corporation (Tucson, AZ)

5,343,166

Pass, Nelson

4,115,739

Inventors: Sano; Nobuya (Nara, JP); Goto; Hiroshi (Katano, JP); Hirota; Yutaka (Toyonaka, JP) Assignee: Matsushita Electric Industrial Co., Ltd. (JP)

Abstract A power amplifier comprises: a complementary symmetry push-pull circuit formed with two complementary transistors mutually coupled at their emitters and having an operating point for Class-A mode operation; a floating power supply having a neutral terminal serving as the output terminal of the power amplifier, a positive and a negative terminal connected respectively to the collectors of the respective transistors; a bootstrapping circuit for driving the mutually coupled emitters in proportion to the potential at the output terminal; and a circuit for negative feedback of signal from the output terminal to the bases of the transistors.

4,206,419

Inventors: Yokoyama; Kenji (Hamamatsu, JP)

Assignee: Nippon Gakki Seizo Kabushiki Kaisha (Hamamatsu, JP)

A buffer amplifier configuration simultaneously reduces d.c. voltage offsets through the signal flow path between its input and output and maintains a high input impedance and a low output impedance. In a preferred embodiment, high input impedance is achieved by coupling the input transistor's collector to a high impedance current source, which is coupled to one of the buffer's power supply rails. The emitter of the input transistor is coupled to the input terminal and its base of the base of a like polarity bipolar output transistor, the emitter of which is coupled to an output terminal and the collector of which is coupled to one supply rail. Since both the input and output transistors are of the same polarity type (so that they can be reasonably well matched during manufacture) and have their base-emitter junctions connected back-to-back between the input and output terminals, they impart effectively no Vbe-based d.c. offset voltage through the buffer. To ensure a high input impedance regardless of output load and output stage gain an isolating emitter-follower transistor stage is preferably coupled between the collector of the input transistor stage and the base of the output transistor stage

5,218,321

Inventors: Jost; Steven R. (Palm Bay, FL)

Assignee: Harris Corporation (Melbourne, FL)

A low impedance class AB buffer stage in complementary transistor technology has its quiescient current stabilized and its operation thereby made more reliable by means of suitable error op-amps that are supplied with transistor feedback loops which are connected to the stage's output terminal through resistors. In addition, for full rail-to-rail output voltage capability, transistor switching devices are added to turn off current through either of the resistors when the output voltage approaches the voltage of either power rail, and also a pair of serially rail-to-rail connected transistor is connected in parallel with the feedback loops.

Inventors: Khoury; John M. (New Providence, NJ)

Assignee: AT&T Bell Laboratories (Murray Hill, NJ)

The present invention is a single-ended Class A gain stage amplifier consisting of an active current source. A feedback path from an output node of the amplifier senses the direction and magnitude of the current going through a load coupled to the output node. The feedback path varies the output of the active current source in accordance with the sensed load current. By so doing, the efficiency of the amplifier of the present invention can be doubled as compared to prior art amplifiers operating in Class A mode. In addition, the present invention amplifier exhibits lower signal distortion.

Inventors: Pass; Nelson S. (Foresthill, CA)

Assignee: Pass Laboratories, Inc. (Foresthill, CA)

An audio amplifier with amplified feedback having a discrete component amplifier and an amplified negative feedback network. The amplified negative feedback network includes a feedback amplifier, a first resistor network, a second resistor network and a further feedback means for providing local negative feedback to the feedback amplifier and substantially removes a load associated with the output of the discrete component amplifier.

Inventors: Johnson; William Z. (Edina, MN)

Assignee: Audio Research Corporation (Minneapolis, MN)

In a high-fidelity sound reproducing system wherein the speaker is located remotely from the amplifier and therefore the electrical signals reaching the speaker are subject to distortion from the capacitive and inductive reactance of the conductors joining the amplifier to the speaker, and further subject to pickup of hum and other forms of interference, the distortion of the signal is substantially eliminated by the use of a negative feedback network connected between the input terminal of the load and the input terminal of the amplifier and by a positive feedback network connected between the input terminal of the amplifier and the return terminal of the load, the feedback loops operating on the amplifier to alter its output in such a way that the signal applied at the terminals of the remote speaker more nearly resembles the signal applied as an output to the amplifier.

4,236,118

Wheeler, Turner