US2680218A - Audio transformer - Google Patents

Description

June 1, 1954 KEROES 2,680,218

AUDIO TRANSFORMER Filed Oct. 26, 1950 INVENTOR HERBERT l. KEROES ATTORNEY Patented June I, 1954 AUDIO TRANSFORMER Herbert I. Keroes, Philadelphia,

Acro Products Company,

Pa., assignor to Philadelphia, Pa.

Application October 26, 1950, Serial No. 192,265

9 Claims. 1

The present invention relates generally to push-pull audio transformers, and more particularly to systems of relatively arranging the windings of audio transformers, to enable change of output impedance without change in performance.

The conventional push-pull audio transformer comprises three windings, two of which are connected in balanced relation, and which are substantially identical in respect to impedance. These windings are mounted on a magnetic core, on which is also mounted a secondary winding, thereby coupled to the primary windings. The primary windings are normally coupled in pushpull relation to a pair of push-pull audio amplifier tubes, and the secondary winding may be connected either in balanced or unbalanced fashion, i. e., with one end grounded.

The high frequency response of an audio transformer is limited by leakage reactance between the various windings, as well as by the di tributed capacity across the primary winding.

At low power level of output, high frequency response is limited primarily by the leakage reactance between primary and secondary windings. At high power levels high frequency response is further limited by leakage reactance between primary windings, because that leakage reactance relatively decouples the audio amplifier tubes.

Audio output transformers may be e: .ployed in amplifiers of the negative feed-back type, in :hich voltages derived from the audio output transformer are applied to an input circuit of the amplifier. In such amplifiers, the audio output transformer may be designed to introduce substantially zero phase shift, which requires that leakage reactance be minimized, and that the transformer operate in truly balanced fashion, i. e., without longitudinal effects, without spurious signal transmission, whether through inter-winding capacity, unequal coupling between primary windings and the secondary winding, or otherwise.

A problem arises'in commercial transformers, because these transformers may be required to drive speakers of various impedance, and to provide an impedance match. In particular, the output impedance of the transformer may be required to equal 4, 8 or 16 ohms, selectively, in

ctual transformer designs. vision for selective variation in impedance of the secondary winding, without appreciable change in transformer performance at the various impedances. The solution of providing taps on a conventional secondary winding, to establish This requires prowidely different output impedances, while superficially simple, is not in fact feasible in wide band, high quality transformers, i. e., transformers capable of transferring a wide band of audio fre quencies without appreciable phase or amplitude distortion, since in fact, those desiclerata of a high quality transformer referred to hereinabove, if present when the transformer load is connected to one of the taps, will not be present for any other of the taps.

In accordance with the present invention, a push-pull audio transformer capable of providing wide band response without appreciable distortion, and yet capable of matching selectively loads of widely varying impedance, is provided, by means of a novel system of secondary winding arrangement. More specifically, the primary windings are arranged as four concentric sections, two of which in series provide one of the primary halves, and the remaining two of which in series provide the other of the primary halves. The secondary winding is composed of three axially concentric sections, two of which are of equal numbers of turns. The latter sections are connected in parallel, and interleaved between the primary sections taken in pairs, i. e., one between the two innermost primary sections, the other between the remaining two. The parallel connected sections provide a low impedance secondary winding which is symmetrically arranged with respect to the primary windings, and evenly distributed thereamong, and which therefore is electrically in balance with respect to the primary winding, i. e., possesses equal leakage reactance with respect to each half of the primary. Capacitive coupling from the secondary to each primary half is approximately the same, and since voltage on one side of the section is of opposite phase to that on the other side, capacitive coupling is effectively balanced out.

The remaining section of the secondary winding is formed of two separate windings, parallel wound, and connected to form a re-entrant series connection, the term re-entrant meaning parallel wound mutually insulated conductors, each conductor connected in series with the succeeding winding by connecting its termination to the initial point of the succeeding winding. One of these windings is connected in series with the low impedance secondary to provide an intermediate impedance secondary. Both windings taken in series, and in series with the low impedance winding, then provides a high impedance secondary. The secondary windings so formed each remains symmetrical with respect to the primary winding or sections. Moreover, the winding layers of which the various secondaries are formed each extend over the entire length of each primary section, which reduces leakage reactance to a minimum.

By virtue of the construction above briefly eX- plaincd, leakage reactance as seen from the primary of the transformer is equal for all values of secondary impedance, thereby providing equivalent frequency response for all values of secondary impedance. Longitudinal current flow is minimized, for any value of secondary impedance, and whether the secondary is operated balanced or unbalanced. Leakage reactance between each primary half and the secondary is uniform, for all values of secondary impedance, thereby equally loading both primary halves. The secondary copper loss is uniform, and the insertion loss of the transformer the same, for all values of secondary impedance.

It is, accordingly, an object of the invention to provide a novel and improved wide-band high quality audio transformer.

It is a further object of the invention to provide a push-pull audio transformer capable of providing selectively a range of output imped ances, while maintaining the operating characteristics of the transformer unchanged over the range.

Another object of the invention resides in the provision of a novel secondary winding for a push-pull audio transformer.

A further object of the invention resides in the provision of a tapped secondary in a wideband push-pull audio transformer, which provides equal performance at each of the taps, by suitable distribution arrangement and interconnection of the windings of the secondary.

The above and still further objects, features and advantages of the invention will become apparent upon consideration of the following detailed description of specific embodiments thereof, especially when taken in conjunction with the accompanying drawings, wherein:

Figure 1 is a view in cross longitudinal section taken through the transformer of the invention, and illustrating the physical arrangement or" the sections;

Figure 2 is a schematic wiring diagram showing one possible arrangement of the primary sections of the transformer;

Figure 3 is a schematic wiring diagram of a variation of the arrangement of Figure 2; and,

Figure i is a schematic wiring diagram of the secondary winding of the transformer.

Referring now more especially to the drawings, Figure 1 illustrates generally an audio transformer having four primary sections, P1, P2, P3 and P4, capable of interconnection to provide a push-pull or balanced primary winding. Interleaved with the primary sections are four secondary coils S1, S2, S3, S4, which are interconnected to form a secondary winding, balanced or unbalanced, and provided with taps to establish impedances as seen from the secondary of 4, 8 and 16 ohms, respectively. It is, of course, to be understood that the specific values recited are by way of example only, and are not to be taken as limiting.

The primary sections may be electrically arranged in two different ways, as illustrated in Figures 2 and 3 of the accompanying drawings. In Figure 2 sections P1 and P2 are connected in series to provide one primary half, and sections P3 and P1 are connected in series to provide the other primary half. In the embodiment illustrated in Figure 3, sections P1 and P3 are connected in series, and P2 and P4 in series to provide the primary halves. In each case the sections must be so arranged that their mutual inductance adds to the series inductance of the separate sections, to establish maximum inductance in each primary half for the amount of copper and iron utilized.

The secondary section S1, interposed between primary sections P1 and P2, is connected in parallel with secondary section S4, interposed between primary sections P3 and P4. Secondary sections S1 and S4 are duplicates, in respect to number of turns, and are each evenly distributed with respect to the primary sections, and co-extensive therewith. Thereby leakage reactance is reduced to a minimum, and such leakage reactance as is present is balanced with respect to the primary halves. The residual capacitive coupling from the secondary sections Si and S4 to the primary sections is balanced, and since the potential on each side of the secondary section is of opposite phase to that on the other side, capacitive coupling is essentially balanced out. Such capacitive currents as do exist may readily flow to ground through the secondary windings.

The center section of the secondary is composed of two conductors or layers S2, S3, parallel or bi-filarly wound, forming a single layer, and connected to form a re-entrant series connection. Additional taps may be provided by similarly providing and connecting more than two conductors. The conductor S2 is then connected in series with the sections S1 and S4, taken in parallel, to provide an intermediate impedance secondary tap. By winding the conductor or layer S2 with a reduced number of turns, as compared with sections S1 or S4, the total impedance may be arranged to be 89. The conductor or layer S3 taken in series with S2, and in series with S1 and S4 taken in parallel, then establishes the high impedance secondary winding.

Whether the secondary winding at its intermediate or high impedance tap be considered, the secondary is evenly distributed with respect to the primary, and symmetrical with respect to the primary halves.

It follows, by virtue of the construction described and illustrated, that leakage reactance referred to the primary side of the transformer is the same for any secondary tap, whereby equivalent frequency response is obtained on any tap. Longitudinal transmission paths of both the capacitive and inductive types are substantially eliminated, as between any tapped section of the secondary and the primary, and whether the secondary be connected to a load in balanced or unbalanced relation. Leakage reactance between the secondary and each primary half is the same, providing symmetrical loading of the primary halves, for any secondary tap. The insertion loss of the transformer is the same for any tapped secondary connection.

In a specific construction embodying secondary sections each of which is composed of a single layer, the wire size may be the same for every section of the secondary, and the total turns on layers S2 and S3 taken together, may be almost the same as on layer or section S1 or S4. No taps need be taken from the secondary except at the end of a layer, which provides economy of production. Since the layers or sections S1 and S4 are connected in parallel any unbalanced leakage flux between the primary halves tends to set up circulating currents in the layers S1 and S; which tend to cancel the unbalance. This is of great importance in amplifiers operating class AB-2 Or B.

The transformers windings hereinabove disclosed may be applied to transformers of the shell magnetic circuit type, using a single coil. The extension of the principle to core type units utilizing two similar coils is believed obvious, and to come within the scope of the invention.

While I have described and illustrated specific forms of the invention it will be clear that variations thereof may be resorted to without departing from the true scope of the invention as defined in the appended claims.

What I claim and desire to secure by Letters Patent of the United States is:

1. An audio transformer, comprising, four concentrically arranged primary sections, means connecting said primary sections in series by pairs to provide a balanced primary winding, a secondary winding comprising two concentric sections symmetrically arranged with respect to said primary sections and connected in parallel, said secondary winding further comprising two similar multifilarly Wound layers, means connecting said two multifilarly wound layers in reentrant series connection with each other, and in series with said parallel connected first-mentioned sections of said secondary winding, said two multifilarly wound layers arranged physically symmetrically with respect to said four concentrically arranged primary sections and said first-mentioned two sections of said secondary winding.

2. The combination in accordance with claim 1 wherein physically adjacent ones of said primary sections are connected in series by pairs to provide said balanced primary winding.

3. The combination in accordance with claim 1 wherein physically separated ones of said primary sections are connected in series by pairs to provide said balanced primary winding.

4. An audio transformer, comprising, an even number of concentrically arranged longitudinally co-extensive primary sections, means connecting said primary sections in two relatively symmetrical primary halves to provide a balanced primary winding, a secondary winding for said transformer, said secondary winding comprising an odd number of concentric secondary sections, each co-extensive longitudinally with said primary sections and symmetrically located with respect to said primary sections, means connecting the outermost and innermost of said secondary sections in parallel, said last named sections being of substantially equal numbers of turns, and means connecting the remainder of said secondary sections in series with said secondary sections in parallel.

5. The combination in accordance with claim 4 wherein said remainder of said secondary sections comprises a plurality of multifilarly wound conductors connected re-entrantly in series.

6. An audio transformer, comprising, a first primary winding section, a second primary winding section, a third primary winding section, a fourth primary winding section, all said primary winding sections wound as co-axial cylinders, a plurality of secondary sections interleaved with said primary sections, at least one of said secondary sections comprising a plurality of mutually insulated multifilarly wound conductors com nected re-entrantly in series, means connecting the remainder of said secondary sections in parallel, and means for interconnecting one of said multifilarly wound conductors, and all said multifilarly wound conductors taken in series re-entrantly, in series with said remainder of said secondary sections in parallel, said remainder of said secondary sections having substantially equal numbers of turns.

7. An audio transformer, comprising an even number of concentrically arranged longitudinally co-extensive primary sections, means connecting said primary sections in two relatively symmetrical primary halves to provide a balanced primary winding, a secondary winding comprising a plurality of concentric sections symmetrically arranged each with one of said primary sections, means for connecting said last named sections in parallel, said secondary winding further com prising a plurality of similar multifilarly wound layers, means for connecting said multifilarly wound layers in re-entrant series connection with each other, means for connecting said multifilarly winding, said multifilarly wound layers arranged physically symmetrically with respect to said concentrically arranged primary sections and said first mentioned sections of said secondary wind- 1 8. An audio transformer, comprising, an even number, at least four, of concentrically arranged longitudinally co-extensive primary sections, means for connecting said primary sections in two relatively symmetrical primary halves to provide a balanced primary winding, a secondary winding comprising a plurality of sections symmetrically arranged each with one of said primary sections, means for connecting said last named sections in parallel, said secondary winding further comprising a plurality of similar closely coupled layers, means for connecting said closely coupled layers in re-entrant series connection with each other, means for connecting said closely coupled layers in series with said first mentioned sections of said secondary winding, said closely coupled layers arranged physically symmetrically with respect to said primary sections and said first mentioned sections of said secondary windmg.

9. An audio transformer comprising a secondary winding comprising a first section, a second section, a third section, said third section comprising two multifilarly wound conductors reentrantly connected in series, means connecting said first and second sections in parallel, said first and second sections having substantially equal numbers of turns, and means connecting one end of said third section to one end of said first section.

References Cited in the file of this patent UNITED STATES PATENTS

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