AU700459B2 - Dual single ended amplifier - Google Patents

Description

P/00/01I Vl Regulation 3.2

AUSTRALIA

Patents Act 1990

ORIGINAL

COMPLETE SPECIFICATION STANDARD PATENT Invention T itle: The following statement Is a full description of this invention, including the best method of performing it known to me:- 9 *9A VALVE RESEARCH PTY LTD 4* C 9 C C 9**t

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9. 9 9* 9, 9 9* COMMONWEALTH OF AUSTRALIA PATENTS ACT 1990 COMPLETE SPECIFICATION FOR THE INVENTION ENTITLED: "DUAL SINGLE ENDED AMPLIFIER" This invention is described in the following statement:- *999*C 9*09 99Cm 4-I I= ~Y I~CI L~lli- IYY -YPIII 1 This invention relates to single ended audio power amplifiers and in particular to a dual single ended audio power amplifier arrangement.

BACKGROUND

Most modern power amplifiers, valve and solid state, use a push-pull output stage. One half of a push-pull output stage passes increasing current (pushes) while the other half passes reducing current (pulls), and vice versa, while amplifying a signal waveform.

Push-pull (PP) output stages, as depicted in Fig i, have been the preferred design for audio power amplifiers since the 1930s for a number of reasons. A PP stage has low distortion because it can cancel even-order harmonics and their intermodulation products because of symmetric magnetic saturation of their output transformers. Cancellation occurs in successive half cycles of the signal waveform.

Other advantages include using clafs B or class AB operation, in which one half of the output stage does not conduct for part of the cycle thereby reducing the heat generated by the output stage of the amplifier.

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*S5 *3 4* 2i Use of valves in a PP design provides substantially zero magnetic flux in the core of the output transformer when the amplifier is idling which means that the transformer can be smaller and of less quality than a single ended equivalent.

However, a PP design canl exhibit high third order distortion since even though symmetrical magnetic saturation of the output transformers occurs as depicted in Fig 2 (no air gap), the symmetry does not substantially affect third order products nor all of their harmonics. This distortion characteristic will be described in greater detail later in the specification.

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i Also, rather than being an advantage some people believe that elimination,of all even-order harmonics provides an "unnatural" acoustic waveform.

Also, if there exists any unbalance of the DC currents in the primary of the PP transformer, high levels of distortion are possible. This characterictic is illustrated in Fig 3 and referred to in detail in Partridge, N.

Distortion in Transformer Cores, Wireless World, June-July 1939, parts I-IV, in particular part III.

All of the parts I to IV of the article are hereby incorporated by reference into this specification and will be referred to elsewhere by the mention of "the Partridgc reference".

It is also known that large variation of inductance and permeability in the transformer are due to the magnetic flux varying from zero tomaximum each half cycle, as illustrated L 20 in Fig 4. Variations of up to 5 to 1 are typical with a flux density peak being followed by flux density low at high amplifier output levels which can be accentuated at the resonant frequency of the speaker.

In contrast, Single Ended (SE) audio power amplifier design, for example the circuit depicted in Fig 5, requires class A operation, which unlike class AB has minimum distortion at low levels. The high standing current i requirement of a single ended circuit requires a large output :0 transformer usually with an air gap, relative to a PP transformer as depicted in Fig 6. A large output transformer has low distortion due to its core stability and smaller inductance changes occur with each change in flux density.

[L These characteristics are a result of unidirectional magnetic flux flow which also remains above zero during class A operation, as illustrated in Fig 7.

Although, single ended audio power amplifiers can be r constructed using valves or solid state transistors, triode 3 valves are typically used because of their low output impedance which results in low transformer distortion which may dominate if the triode is-operated in its linear range.

Thus even with a SE output stage, a non-linear valve stagp can result in dominant valve distortion as illustrated in Figs 8 and 9. There it can be seen that as the valve gain increases at high currents, the anode voltage waveform peaks in a negative direction as depicted in Fig 9.

However, in a low distortion valve stage, transformer distortion dominates due to flux saturation at high levels.

At low frequencies the variation in flux density also becomes non-linear above and below the point of peak permeability and as permeability reduces inductance reduces adding to the degree of non-linearity, Fig 4.

The net DC current flowing through the primary windings of a SE amplifier's output transformer ensures, that at Jlow signal waveform levels, the magnetic core is being operated at about midway along its magnetization curve rather than around zero-m~agnetization as in a PP design.

1. The inventor proposes that an audio power amplifier comprising two single ended amplifiers, as shown for example in Fig 10, driven in opposite phase provides added power output without the distortion characteristics of either a PP or SE amplifier.

As one of the two SE amplifiers is turning on, the other 30 is turning off which provides two unidirectional curved magnetic flux changes in accordance with the magnetizing 0 current changes. Since these changes are unidirectional and out of phase with each other the combined magnetic flux change path is linear as depicted in Fig 11.

These characteristics will be described in greater detail later in the specification with the use of an embodiment which has been designed for the purpose depicting 4 rthe distortion perfomiance of the invention in comparison to Ftypical PP and SE amplifiers.

BRIEF DESCRIPTION OF THE INVENTION In a broad aspect of the invention an amplifier Fcomprises two substantially identical single ended output stages each driven with the same waveform signal one out of phase with the other, each output stage being connected to a respective transformer wherein windings of said transformers are connected in parallel or series to provide the combined output power of said amplifier.

In a further aspect of the invention an audio amplifier speaker arrangement for combining an audio power waveform comprises an amplifier having two substantially identical single ended amplifier output stages each driven with the same waveform signal one out of phase with the other, wherein each output stage is connected to respective substantially identical loudspeakers so that the audio output of said speakers combines the output power of said amplifier.

F To provide a description of the invention an embodiment will now be provided. This embodiment is configured in a circuit which is designed to provide a measurable distortion characteristic of the invention rather than as a working I model of an actual audio power amplifier. Although it will be clearly apparent to one skilled in the art that the essential elements of such an amplifier are described.

The embodiment provides a voltage distortion measurement technique which is equivalent to the current distortion oe measurement technique disclosed in the Partridge reference.

S 35 BRIEF DESCRIPTION OF THE FIGURES Fig 1 depicts a circuit for a valve-type push-pull .o amplifier;

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Fig 2 depicts a magnetic flux characteristic of a pushpull amplifier output transformer having no air gap; Fig 3 depicts an impedance versus magnetic flux density of a push-pull amplifier output transformer; Fig 4 depicts the permeability change of an amplifier .outpuit transformer; Fig 5 depicts a circuit for a valve-type single ended amplifier; Fig 6 depicts the magnetic flux characteristics of an amplifier output transformer having an air gap; Fig 7 depicts the impedance versus magnetic flux density of two push-pull amplifier output transformers where one has an air gap and the other does not; Fig 8 depicts a single ended triode valve circuit; Fig 9 depicts the valve gain versus amplifier current drain of the circuit depicted in Fig 8; Fig 10 depicts a dual single ended amplifier circuit using a triode valve; Fig 11 depicts an ideal flux path characteristic of the combined transformer output of the dual single ended amplifier; Fig 12 depicts a linear output current distortion testing circuit using identical amplifier output stages and various trans formers which provide when connected appropriately either a push-pull, single ended or dual single ended circuit arrangement; Fig 13 depicts the transformer distortion due to flux changes with magnetizing current for a push-pull circuit; Fig 14 depicts the transformer distortion due to the 030 flux changes with magnetizing current for a single ended circuit; Fig 15 depicts the voltage waveform recorded at each anode of the triodes used in the push-pull amplifier depicted in Fig 12 using a push-pull transformer (one waveform. has been inverted to assist visual comparison); Fig 16 depicts the combined voltage waveform recorded at the anodes of the push-pull amplifier of Fig 12, using a PP transformer; 6

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seeg 1 Fig 17 depicts the combined voltage waveform recorded at the secondary winding of the transformer of the push-pull amplifier of Fig 12, using a PP transformer; Fig 18 depicts the voltage waveform recorded at each anode of the triodes used in the dual SE amplifier of Fig 12 using two SE transformers (one waveform has been inverted to assist visual comparison); Fig 19 depicts the combined voltage waveform recorded at the anodes of the dual single ended amplifier of Fig 12 using two SE transformers; Fig 20 depicts the combined voltage waveform recorded at the secondary winding of the transformer of the dual single ended amplifier of Fig 12; and Fig 21 depicts an arrangement includina one way of combining the output of a dual single en, amplifier in the acoustic domain rather than in the electrical domain.

DETAILED DESCRIPTION OF THE INVENTION The invention is described herein with the assistance of a circuit as depicted in Fig 10. The circuit uses valves as amplifier elements. However, it will be apparent to those skilled in the art that solid state equivalent elements could be used. However, some listeners perceive a difference to the quality of the listening when solid state amplifiers are used in comparison to a valve amplifier.

The test circuit is also configured to allow voltage S distortion characteristics to be measured. However, it will also be apparent that current distortion characteristics could also have been measured by using a modified circuit.

The ase of a triode in the circuit of Fig 10 is also a preferred approach since a pentode or a tetrode valve connected as a partial triode (sometimes referred to as Ultra Linear or Distributed Loading) can be used by connecting the screen grid to a tap on the transformer primary winding. The S relatively high efficiency of these valves is retained while 00 S also having the known benefit of triode-like operation.

7 a a *at 71 ee Such a configuration has a medium output impedance and the transformer distortion is'a combination of voltage and current, resulting in a lower distortion power match to the loudspeaker load. A high level of feedback applied to the .inptkt valve cathode from the secondary winding of the output transformer is typical for pentode and tetrode valve output stages.

However, if ero or a lower level of feedback is desired, then triodes are used in preference and inherently provide low output impedance and an adequate damping factor.

In a zero feedback circuit, inherently linear output stages using triode valves require physically large transformers with an air gap (ie low distortion transformers) in which transformer saturation is not experienced.

When a triode output valve is used, the transformer is driven from a low impedance, producing linear voltage across the transformer such that the primary distortion mechanism is detected in the transformer current.

L When a pentode or tetrode output valve is used, the transformer is driven from a high impedance, producing a linear current in the transformer such that the primary distortion characteristic is detected in the transformer voltage.

Thus, a class A PP amplifier such as that depicted in Fig 1 is assumed by most to provide the lowest distortion.

This assumption is based on the understanding that a PP configuration will eliminate both second order distortion and 1 second order harmonic distortion which typically is predominant. It is to be noted that in class B and class AB the second order cancellation characteristic is provided by way of the symimetry of the waveform and actual cancellation 00 of the second order harmonics of the first half cycle by the second order harmonics of the second half cycle of the waveform. This is clearly the case for a given power output II U- I C 7 .;Y1

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compared to a class A SE amplifier operating at the same power output level.

It is known though that a PP amplifier can have perceptible and certainly measurable third order harmonic (ie third, fifth, etc) distortion, due to the valve or the transformer.

In this regard a SE amplifier circuit will also have third order distortion and since it is not symmetrical also has second order harmonic distortion.

However as prolused in the invention, a dual SE amplifier as depicted in Fig 10 and as in the test circuit of Fig 12, when configured, is such that each half cycle of the amplified waveform when added at the output to provide its combined output power cancels both second and third order components. Second harmonic distortion reduction also occurs instantaneously rather than in the second half of each of the respective cycles of the waveform.

Put most succinctly, the rounding of one of the dual SE circuits due to an increase in both valve and transformer currents tends to be cancelled by the decrease of valve and transformer current in the other SE circuit.

a a a at L e a a It is understood that since both SE circuits are identical but operating with opposite phases of the same signal waveform it is possible that all forms of distortion are reduced in each half cycle as well as reducing or eliminating distortion mechanisms in both the valves and transformers. A person skilled in the art will recognise that there are numerous equivalent circuits to replace the transformers while maintaining the functionality of the invention. Furthermore, it will be apparent that there are many ways of connecting the windings of transformers including connections to the anodes and cathodes of a valve output, for example, use of an auto transformer circuit, separate anode and cathode windings, bifilar windings, etc.

31~ I- Fig 13 shows transformer distortion due to the flux changes with magnetization current for a PP amplifier and exhibits the cancellation of second harmonic distortion in successive half cycles. It will also be apparent that potentially high levels of third harmonic distortion are possible (eg a square wave is symmetrical having low second order distortion but has very high levels of third order distortion caused not only by the sharp transitions of the waveform but also the nominal curving of the waveform at an angle which promotes the generation of these third order distortion components).

Fig 14 shows transformer distortion due to flux changes with magnetization current for a SE amplifier and exhibits both second and third harmonic distortion.

Fig 11 shows pictorially the resultant transformer distortion performance of a dual SE output stage. Trace A is j the curve of Fig 14 while trace B is the opposite phase curve equivalent to trace A and trace C is the additive ideal result exhibiting zero curvature and also being symmetrical such that it will exhibit the characteristics inherent in the elimination of both second and also third order harmonic distortion.

i. To demonstrate the different distortion characteristics ~of PP, SE to a dual SE valve output amplifier, a test circuit S having a low distortion high impedance characteristic was 4.: developed, refer to Fig 12. As described previously this was S 30 done primarily so that voltage distortion characteristics Gag# could be observed. However, it would be understood by one L-o skilled in the art that it is normally desirable to have a low output impedance amplifier and that the dual SE amplifier design can be easily incorporated into such a configuration 0'35 to provide this characteristic for application in the audio amplification application.

a1 r* r Referring to Fig 12, the output triode valves 10 and 12 have their cathodes connected to the input stage of the amplifier chain with resistors 14 and 16 respectively which result in a linear current drive to the output transformer I with little or no feedback from the transformer in the amplifier circuits.

A Trimax TA1044 transformer 18 was used in the PP test fand two Trimax TA851A single ended transformers (20, 22) were used for the SE and dual SE test.

The secondary windings of the SE transformers were combined to provide maximum output power.

In the dual SE test the two independent SE amplifiers were driven in balanced differential mode.

Using a 20Hz input signal the output signal waveforms recorded in Figs 15-20 were connected to a pure resistive f 20 load and the total output power was set to 5 watts which is less than half the rated power handling capability of the transformers and well within the class A valve current operation specifications.

Fig 15 depicts the voltage signal waveform at the junctions 24 and 26 of the anodes in the PP circuit to the PP transformer 18. This waveform is not unlike the waveforms 99.:o obtained by Partridge disclosed in parts III and IV. One of :the waveforms has been inverted for display purposes.

Fig 16 depicts the combination of the anode voltage 0.0* signal waveform in the oscilloscope and in particular the 0, characteristic voltage kink 32 is visible. This phenomenon is described in Partridge and is also visible (although not as clearly) in Fig 17.

Fig 17 shows the combined output across the resistive load feed from the secondary winding of the transformer. The kink 32 is shown just past the midway point of the signal hbm 11

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waveform. This kink is indicative of high level third order harmonic distortion products.

Fig 18 depicts the dual SE anode voltage signal waveforms at 28 anc' 30. One of the waveforms has been inverted for display purposes. The expected rounding characteristics of the SE amplifier output is clearly apparent.

However when the two anode voltage waveforms are combined in the oscilloscope, Fig 19, there is no visible kink of the waveform which is in effect a dual SE amplifier voltage waveform.

Fig 20 depicts a voltage signal waveform from the combined secondary windings of the two SE transformers. There is no visible kink in the voltage signal waveform.

Wave analyser measurements were made of the PP, SE and dual SE outputs and measured distortion was substantially reduced for the dual SE configuration and in particular for third harmonic distortion.

The test configuration described above may not be conclusive nor represent all tests that can be conducted for the purpose of describing the invention. However, these configurations clearly present theory and practical 9990 o~o implementations which would enable a person skilled in the .i art to design an amplifier incorporating the invention.

.°0.o'30 A yet further arrangement of components which provides I •D •the result of the invention is to acoustically combine the output of two speakers which are each driven by SE amplifiers which themselves are driven in balanced differential mode.

1.4 Fig 21 depicts a dual SE amplifier (valve or solid state) which with the indicated connection of either of the two output transformers or alternatively being directly r connected from the output of the SE amplifier to a typical 12 loudspeaker apparatus located adjacent to each other. The acoustic output from each loudspeaker provides the mechanism for the combination of the output power. Acoustic combination of the audio waveform will cancel second and third harmonic distortion.

V The isolating of the electrical power of each SE amplifier until only acoustic combination occurs may also have additional benefits in reducing intermodulation interference and the like.

Fig 21 depicts of course a stereo arrangement with the listener 34 located in the "sweet" spot for maximum stereo perception.

The invention has disclosed a novel method of using two substantially identical single ended output stages, suitably driven and combined, that can produce distortion levels lower than both push-pull and single ended type amplifiers.

It will be appreciated by those skilled in the art, that the invention is not restricted in its use to the particular application described and neither is the present invention restricted in its preferred embodiment with regard to the particular elements and (or) features described herein. It will be appreciated that various modifications can be made S without departing the principles of the invention, therefore, ~the invention should be understood to include all such *0 modifications within its scope.

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*0*4 S.Dated this day of 19/V 1991 O i VALVE RESEARCH PTY LTD 0 4 oA*.

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Claims (2)

1. An amplifier arrangement comprising two substantially identical single ended output amplifier stages, each stage driven with the same waveform signal one out of phase with the other, each output stage being connected to a respective transformer wherein windings of said transformers are connected in parallel or series to provide the combined output power of said amplifier.

2. An audio amplifier speaker arrangement for combining an audio power waveform comprises two substantially identical single ended amplifier output stages each driven with the same waveform signal one out of phase with the other, wherein each output stage is connected to respective substantially identical loudspeakers so that the audio output of said speakers combines the output power of said amplifier. e p o* :16 Dated this 10th day of September, 1998. VALVE RESEARCH PTY LTD p o ABSTRACT An amplifier comprising two substantially identical single ended output stages each driven with the same waveform signal one out of phase with the other is disclosed. Each output stage is connected to a respective transformer wherein windings of the transformers are connected in parallel or series to provide the combined output power of the amplifier. Also disclosed is an audio amplifier speaker arrangement for combining an audio power waveform. The arrangement comprises an amplifier having two substantially identical single ended amplifier output stages each driven with the same waveform signal one out of phase with the other. Each output stage is connected to respective substantially identical loudspeakers so that the audio output of the speakers combines the output power of the audio amplifier. *t .a09 a 9 *9e9 Sr> 6, t rn