US2846503A - Output stages for low-frequency amplifiers comprising two loudspeakers - Google Patents

Description

Aug. 5, 1958 J. KUMP OUTPUT STAGES FOR LOW-FREQUENCY AMPLIFIERS COMPRISING TWO LOUDSPEAKERS Filed April 9, 1956 INVENTOR JOHANN KUMP AGENT United States Patent OUTPUT STAGES FOR LOW-FREQUENllY AMPLI- FIERS COMPRISING TWO 'LOUDSPEAK ERS Inhann-Kump, Vienna, Austria,; assignor, by mesne as.-

signments, to. N orth. American Philips. Company, Inc., New York, N; Y.,. a corporationof Delaware Application April 9, 1956, Serial'No. 577,113.

Claims priority, application. Austria May: 5, 1955 5 Claims. (Cl. 179-1)" The present invention. relates: toan amplifier output stage circuit arrangement. More particularly, the invention relates to output stages of' low-frequency amplifiers comprising two loudspeakers, which are preferably intended for the reproductionof low fequenciesand high frequencies, respectively, of the acoustic spectrum with the use ofone or more screen-grid tubes as output tubes.

The objectv of'the invention is. in such output stages to obtain a considerable decrease in distortion in the highfrequency range. It is commonknowledge that distor tion inthe high-frequency range is. subjectively very troublesome to the ear. Consequently, inthe. reproduction of an. acoustic. frequency-band, which. must comprise the whole acoustic. region. up. to about. kiljocycl'es. per second, the requirements in regard to freedom of distortion. must be much higher than in. the reproduction of an. acousticv frequency-band which is. limited in the direction of high frequencies.

On. account. of their high output, the present types. of output. stages. of low-frequency amplifiers. substantially utilize screen-grid tubes and more particularly pentodes. As appears from the characteristic curves. of such tubes, the new tones experienced therein. as distortion. substam tially comprise the third harmonicsand combination tones of higher order and hence high tones. Insofar as-loudspeakers. are used for the acoustic reproduction, which cannot or substantially cannot reproduce. the high-frequency range, the loudspeaker suppresses part of this distortion. If use is made of aloudspeaker reproducing the frequencies up to 15 kilocycles per second,. or if two or more. loudspeakers. are used, which are'each. preferably intendedfor the reproduction of the low-frequency ranges (low-tone loudspeaker) and the high-frequency rangesv (high-tone. loudspeaker), respectively, the. high. frequencies. produced in the amplifier are. no longer suppressed and thusbecome manifest. with maximum effect.

The behaviour of triodes ismore favorable. than that of screen-grid. tubes (pentodes) in. regard-v to distortion, but. their low. output constitutes a. great disadvantage.

A circuit. is already known, which has beenadopted literature as an ultra-linear circuit (of. British. Patent No. 496,883), in Whichit is endeavoured tozcombine the advantages of triodes. and screen. grid tubes. In this,circuit, the screen-grid ofthe output tube, instead; of being applied to cathode potential for alternating voltage, as isusually the case, has applied to it part of the alternating anode voltage. This may be accomplished, for example, by connecting; the screen-grid directly to a tapping oint on the primary winding of' the output transformer ineluded in. the anode. circuit of the output tube, so that the screen grid at the same time also receives the Tequired -nected to a high-tone loudspeaker 16.

2. direct voltage Duetothis circuit, whichinfact means, the introduction of a negative feedback. coupling, the. tube. acquiresproperties located: between those of. a.triode andof a. tube in screen grid connection. Thedistortionis. reduced at the expense of the outputv and evenlyfor any frequency.

The invention is basedonthe.consideration. that a decreased output. may be taken into the. bargain for/high. frequencies, if the output remains-unchangedin the low. frequency and. intermediate-frequency range, since thestrength of the subjective impression. of sound is determined substantially by the reproduction of this range. Ifthis range is reproduced by a. loudspeaker which is insensitive to high: frequencies and hence, by. a. low-tone loudspeaker, a higher distortion factor is permissible: for this.range, since, as: mentioned. before, the distortion which. consists of. very high tones is not audible. However, the highefrequency range, which must of. course not be. absent. in. the: reproduction ofthe; whole acoustic spectrum, mustthen be reproduced by a separate. high tone loudspeaker, in which. the distortion factor mustbe as low a's possible. A decreased outputinthe reproduc-- tion ofthis. range may betaken into-the bargain, since it is substantially not audible.

Ithaspreviouslybeen suggested tosupply the low and high frequencies separately to two different output tubes, constituted-by a triode and a pentode, respectively. However, this arrangement requires the use of a, comparaa tively'large-number of circuit elements.

According to the invention, inthe output stage of.-; a:. low-frequency amplifier having: two loudspeakers preferablyintended" for the reproduction of lowfrequencies andhiglr. frequencies, respectively, of-the acoustic spectrum with the useof one. or more-screen-grid tubes as output tubes, the impedances; which are effected: by the loudspeakersinthe anode circuits of the; tubesare connected in series; The anode of each tubeis connected; to the. impedance: constituted by'the: low-tone loudspeaker, which impedance is shuntedby a capacitorrof a' value such that theresistance of the capacitor for high frequencies. is lowwith respect to this impedance. The screen-grid for alternating voltage is. connected. between the: two im pedances.-

I-n. order'that the-invention may bez-readil y' carried! into effect it will now be. described, by way. ofexample, with reference; todhe accompanying drawing, wherein:

Fig. 1 is a schematic diagram of anembodiment of the circuit arrangement of the present: invention; and

Fig. 2 is a -schematic diagram of another" embodiment of the circuit arrangement" of the present invention.

In the circuit-arrangement of Fig; 1; the alternating voltage to beamplified is; applied. to. control. grid 25- of pentode. 1. The: control: gridv receives: the.- requiredbiasing potentialiin the usual manner by connecting: cathode; 3; via; a resistor 4 and. acapacitor. 5,, toground. potential; Suppressor grid.6. is connectedto. thecathode. The.-cir.- cuit:leading from. anode 7 to battery 8 is completedvia the primary winding. 9. ofan output transformer 10,, the secondarywinding 11015 which is.conne.cted to.-a.low-tone loudspeaker 12, and via the primary winding 13,. con: nected'in. series with the winding 9, of an output' transformer 14, the secondary winding 15; of which is con- The primary winding 9 is; shunted by a capacitor 17" having a value such that it constitutes a very low impedance forihigh frequencies and hence substantially short-circuits output transformer for these frequencies. The screen grid 18 of pentode 1 isconnected directly to a point 19 of the anode circuit located between the two primary windings 9 and 13. The primary impedance 13 may be comparatively low, since an output transformer for a high-tone loudspeaker is concerned. For low frequencies, the screen grid 18 is thus substantially connected to the constant positive potential determined by battery 8, so that tube 1 operates as a screen-grid tube for low frequencies. Its output is very satisfactory. However, the distortion, which is greater in screen grid tubes, is not completely heard in the loudspeaker, since the loudspeaker 12 is a low-tone loudspeaker. For high frequencies, the transformer 10 is substantially short-circuited by capacitor 17, which implies that the'anode 7 and the screen-grid 18 are substantially directly connected to one another for these frequencies. The tube 1 thus operates as a triode for these frequencies and, while having a lower output for these frequencies, leads to a reproduction with a very low distortion factor. It is common knowledge in engineering to choose the proportioning such that the cooperation of the two loudspeakers yields the desired substantially rectilinear reproduction curve of the whole arrangement. Of course, instead of utilizing the separate loudspeakers 12 and 16, use may be made of groups of low-tone and high-tone loudspeakers each comprising a plurality of loudspeakers; it naturally being possible only to combine loudspeakers of the same kind into one group.

In the embodiment shown in Fig. l, the i-mpedances are constituted by the transformers loaded with the loudspeakers. The situation remains unchanged, if use is made of loudspeakers having impedances such that they can be included in the circuit without the use of transformers. Neither does a connection of the impedances free from direct current change anything in the fundamental operation.

Fig. 2 shows the use of the arrangement according to the invention in push-pull connection. The two pushpull tubes 20 and 21 are shown only diagrammatically and the two halves 22 and 23 of the output transformer 24 for the low-tone loudspeaker 25 are not connected directly to one another. Each half is bridged by a capacitor 26, 27, respectively, for high frequencies. The center tap 28 of the primary winding of output transformer 31 for the high-tone loudspeaker 32, which winding consists of two halves 29 and 30, is connected to the positive terminal of the source of anode supply. The anodes 34 and 35 of the tubes 20 and 21 are each connected to one end of the primary windings 22 and 23, the other ends of these windings being connected to the primary winnings 29 and 30. The screen-grid 36 of tube 20 is connected to point 37 between the windings 22 and 29, the screen grid 38 of tube 21 being connected to point 39 between the windings 23 and 30. The operation of the circuit arrangement of Fig. 2 is similar to that of the circuit shown in Fig. 1.

While the invention has been described by means of specific examples and in specific embodiments, I do not wish to be limited thereto for obvious modifications will occur to those skilled in the art without departing from the spirit and scope-of the invention.

What is claimed is:

1. A low'frequency amplifier output circuit arrangement, comprising an electron discharge device having an anode, a cathode and a plurality of grids including a screen grid, a first loudspeaker system for reproducing sound in a substantially low audio frequency range, a second loudspeaker system for reproducing sound in a substantially high audio frequency range, means for applying an input signal to one of said grids, means connecting said first and second loudspeaker systems in series circuit arrangement and to said anode, said first loudspeaker system being interposed between said anode and IT; p I 2,846,503

the impedance of said first loudspeaker system at said high audio frequency range, means connecting said capacitor in shunt across said first loudspeaker system, and means connecting said screen grid to a common point between said first and second loudspeaker systems in said series circuit arrangement.

2. A low frequency amplifier output circuit arrangement, comprising an electron discharge tube having an anode, a cathode and a plurality of grids including a screen grid, a first loudspeaker for reproducing sound in a substantially low audio frequency range, a second loudspeaker for reproducing sound in a substantially high audio frequency range, means for applying an input signal to one of said grids, first and second coupling transformerseach comprising primary and secondary windings, means connecting said first andsecond primary windings in series circuit arrangement and to said anode, said first primary winding being interposed between said anode and said second primary-winding, means coupling said first loudspeaker to said first secondary. winding, means coupling said second loudspeaker to said second secondary winding, a capacitor having a reactance value which is substantially. low with respect to the impedance of said first primary winding at said high audio frequency range, means connecting said capacitor in shunt across said first primary winding, and means connecting said screen grid to a common point between said first and second primary windings in said series circuit arrange ment.

3. A low frequency amplifier output circuit arrange- I ment, comprising a pair of discharge tubes connected in push-pull arrangement each having an anode, a cathode and a plurality of grids including a screen grid, a first loudspeaker system for reproducing sound in a substantially low audio-frequency range, a second loudspeaker system for reproducing sound in a substantially high audio frequency range, means for applying an input signal to selected grids of said tubes, means connecting said first and second loudspeaker systems in series circuit arrangement between the anodes of said tubes, said first loudspeaker system being interposed between said anodes and said second loudspeaker system, a capacitor having a reactance value which is substantially low with respect to the impedance of said first loudspeaker system at said high audio frequency range, means connecting said capacitor in shunt across said first loudspeaker system, and means connecting said screen grid to a common point between said first and second loudspeaker systems in said series circuit arrangement.

4. A low frequency amplifier output circuit arrangement, comprising a pair of discharge tubes connected in push-pull arrangement each having an anode, a'cathode and a plurality of grids including a screen grid, a first loudspeaker for reproducing sound in a substantially low audio frequency range, a second loudspeaker for reproducing sound in a substantially high audio fre quency range, means for applying an input signal to selected grids of said tubes, a first coupling transformer comprising first and second primary windings and a secondary winding, a second coupling transformer comprising primary and secondary windings, means connecting the primary windings of said first and second transformers in series circuit arrangement between the anodes of said tubes, the primary winding of said second'transformer being interposed between the first and second primary windings of said first transformer, means coupling said first loudspeaker to said first secondary winding and means coupling said second loudspeaker to said second secondary winding, a first capacitor having a reactance value which is substantially low with respect to the impedance of the first primary winding of said first transformer at said high audio frequency range, a second capacitor having a reactance value which is substantially low with respect to the impedance of the second primary winding of said first transformer at said high audio frequency range, means connecting said first capacitor in shunt across said first primary winding of said first transformer, means connecting said second capacitor in shunt across said second primary winding of said first transformer, means connecting the screen grid of said first tube to a common point between said first primary winding of said first transformer and the primary winding of said second transformer in said series circuit arrangement, and means connecting the screen grid of said second tube to a common point between said second primary winding of said 10 5. A circuit arrangement as claimed in claim 4, further comprising a center tap on the primary winding of said second transformer and means for applying a supply voltage to said tap.

References Cited in the file of this patent UNITED STATES PATENTS

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