US2455471A - Electrical apparatus for loudspeaker systems - Google Patents

Description

ec. 7, W48. w. o. CRANE 2,455,471

ELECTRICAL APPARATUS FOR LOUD srmpn SYSTEMS Fiid March 28, 1946 Patented Dec. 7, 1948 ELECTRICAL APPARATUS FOR LOUD- SPEAKER SYSTEMS Webster 0. Crane, St. Charles, 111., assignor to Operadio Manufacturing 00., St. Charles, 11].,

a corporation of Illinois Application March 28, 1946, Serial No. 657,881

9 Claims. I

This invention relates to an electrical apparatus and particularly to a speaker system. In public address systems, announcing systems or alarm systems, it is desirable to operate a number of speakers and to vary the number of such speakers in accordance with load requirements. If a speaker system is to be operated efi'iciently, it becomes important that proper matching between the output transformer of the system and the speakers be maintained. While a certain amount of mis-match may be tolerated, the eiiiciency of the systemis greatly reduced. if the magnitude of the speaker load varies through substantial limits. Unless means are provided for introducing some degree of regulation, the audio frequency potential available across speakers may rise sharply withthe removal of a substantial number of speakers from the load. The converse is also true.

It has hitherto been the practice to provide resistor as dummy loads for speakers. In addition to wasting power, the resistors frequently burn out.

To overcome the disadvantage incident to dummy resistor loads, the art has relied upon scalied parallel feedback transformers for endowing a speaker system with satisfactory regulation. This parallel feedback transformer comprises a three-winding transformer having one winding which may be arbitrarily considered as a primary and the remaining two windings which may be considered as secondaries. The primary winding in this system is connected in shunt to the speaker load. The secondary windings are connected in the input circuits of a push-pull amplifier. The phasing between primary and secondaries is such as to provide degeneration. Thus, as the potential across the speaker load increases above a predetermined normal value, degeneration will increase and there will be anincreasing tendency to cut down the signal strength at the input of the amplifiers. Conversely, if the potential across the speaker load tends to decrease toward a-. predetermined normal value, the degeneration becomes weaker; It is understood that the potentials referred to'are audio'frequency potentials. Thus, when the'syst'em is operating under what might be termed anarbitrary normal load condition, a predetermined amount of degeneration occurs in the feedback transformer. The regulation depends upon the variation in this degenerative action.

In the systemdescribed above, it has been found that the regulation permits substantial variation in the potential applied to-the speakers resulting in variation in the power output from individual speakers as the load changed. Asa rule, it has been found that a power variation of not less than three db. at each speaker would result from a change of speaker load in spite of the regulation. Thus, the regulating action of the parallel feedback transformer inherently requires a substantial potential variation across the speaker load for operation.

Since a three db. power variation represents a substantial change, as far as a human ear is concerned, it is desirable to improve the regulation. The invention herein provides a system wherein the potential across the speaker loads is maintained substantially constant over wide variations of speaker load. The invention provides, in one respect, a three-winding feedback transformer wherein the primary is connected in series with the entire speaker load rather than in shunt thereto. By virtue of such a connection, the variation of current through the load is relied upon for regulation. This three-winding transformer, which may be considered as a series feedback transformer, has the primary and secondaries so phased that the action is regenerative.

To control regeneration, minimize instability and control frequency non-discrimination, the primary may be by-passed with a suitable resistor. In order to reduce distortion introduced by a regenerative feedback transformer, the invention additionally provides for the inclusion in the system of a parallel feedback transformer connected as in the prior art. Thus, the parallel feedback transformer secondaries are connected in series with the secondaries of the series feedback transformer. By virtue of the degenerative action of the parallel feedback transformer, the amount of regeneration in the series feedback transformer may be increased above what might be considered desirable if a parallel feedback transformer is omitted. The combination of the two feedback transformers provides a compound feedback action which is particularly effective as far as regulation is concerned and provides a minimum of distortion.

Referring, therefore, to the drawing, the single figure shows a circuit diagram of a multi-speaker system embodying the present invention. l0 represents any suitable source of audio frequency as a microphone, phonograph pick-up or other sound reproducing system, or that portion of a radio receiver which provides audio frequencies. Source ID; which in itself may include as many stages of amplification" as desired, is connected to primary II of driver transformer l2. Transl4 prol5 and former l2 has secondary windings l3 and viding a common grounded connection outer terminals 8 and 9.

Transformer l2 feeds its output to series feedback transformer I1 consisting of primary l8 and secondaries I9 and 20. Transformer i1 is a conventional iron core audio frequency transformer with the various windings closely coupled to each other. The phasing of the windings are regenerative as will be indicated later.

Outer terminals 8 and 9 of driver transformer I2 are connected respectively to secondary windings l9 and 2D. secondaries l9 and are connected to secondaries 22 and 23 respectively of transformer 24 similar to transformer 11. Windings 22 and 23 extend the circuit to control grids 26 and 21 of push-pull amplifiers 28 and 29 respectively. These amplifiers are power amplifiers and, for simplicity, are shown as three electrode tubes. It is understood, however, that other amplifiers, such as beam power tubes, may be used.

Amplifiers 28 and 29 have cathodes 3t and 3| grounded at 32. These tubes have anodes 34 and 35 connected to primaries 36 and 31 respectively of push-pull output transformer 38. Primary windings 36 and 31 have center tap 39 for connection to the usual source of B plus supply. Output transformer 38 has secondary 40 across which condenser 4| is connected. Condenser 4! serves to stabilize against oscillation. Secondary 4B and condenser 41 together have terminals 42 and 43 respectively. Terminal 42 is connected by line 44 back to grounded primary I8 of transformer I1. Across primary [8, there is preferably disposed low resistance 45 for controlling regeneration and frequency non-discrimination. This resistance may be omitted if desired under certain conditions. Terminal 43 is connected by lead 46 back to grounded primary 41 of transformer 24.

Connected between terminal 43 and ground are speakers 50 of any desired construction. Thus, if these speakers are of the permanent magnet type, the speaker input will be connected between terminal 43 and ground. If these speakers are of electro-dynamic type, it will be understood that the voice coils are connected between terminal 43 and ground and that other means for supplying current to the field windings will have to be provided. Whether or not the field circuit in a dynamic speaker is cut-out together with the voice coil makes little difference with regard to the invention. Suitable switching means are diagrammatically indicated by switches in the speaker leads.

It will be noted that primary l8 and resistor 45 is in series with the entire speaker load. Transformer l1 may be considered as a series-connected feedback transformer.

Transformer primary 41 of transformer 24, on the other hand, is connected across terminal 43 and ground so that it is effectively in shunt to all the speaker loads. This transformer may, therefore, be considered as a parallel feedback transformer.

Referring now to transformer l1, primary I8 is so phased with reference to the two secondaries that the transformer action is regenerative for the entire system. Thus, any signal going through primary l8 will induce in both secondaries l9 and 20 potentials tending to increase the signal strength already in the secondaries by virtue of the output from driver transformer windings l3 and 14. By shunting primary IS with a res tance, the amount of regeneration may be determined while keeping the over-all resistance of the output circuit for secondary 40 low.

Primary 41 on the other hand is phased with reference to secondaries 22 and 23 so that degenerative action occurs. Thus, a signal through primary 41 will induce potentials in secondaries 22 and 23 which tend to oppose the normal signal potentials in these secondaries by virtue of their connection in the output circuit of windings l3 and I4. The amount of degeneration in transformer 24 may be controlled by the transformer ratio or by disposing a resistance in series with primary winding 41. If transformer 24 is omitted, series feedback transformer I1 may provide good regulation but tends to introduce distortion. The distortion, as a rule, increases with the amount of regeneration. By decreasing regeneration in transformer I1, however, regulating action is undesirably affected.

By combining both transformer l1 and 24, as shown in the system, the distortion which may arise by virtue of regeneration is reduced greatly by the opposing degeneration introduced by transformer 24. In fact, by having both transformers, substantial regeneration in transformer l1 may be tolerated without rendering the entire system unstable and susceptible to oscillation.

The combination of transformer l1 and 24 is highly effective in providing a regulating characteristic which is effective for slight changes in output from secondary 40. As is well known, the essence of regulation resides in the utilization of slight variations in the quantity being regulated to maintain the quantity being regulated constant within limits. Thus, as speakers are added or subtracted from the load, little or no variation in potential across the speaker and large variation in load current suffice to introduce a regulating action in the amplifier circuit.

The impedance as seen by output winding 40 remains constant over a wide range of speaker loads. The impedances introduced into the output system by primary windings l8 and 21 are complex and variable and, in a properly designed system, tend to compensate for the variation of impedance due to adding or subtracting speakers. The regenerative action in transformer 11 results in the introduction of a negative resistance in the system. The reverse is true of transformer 7.4. Thus, as speakers are added or subtracted, the relative magnitudes of these resistances vary and maintain the efficiency of the entire system at a high level over large variations of load.

The large variation in current drawn by the speaker load causes substantial action in transformer l1. Any slight variation in potential across the speaker load causes some action in transformer 24.

By proper design of the system, various regulating characteristics may be obtained. Thus, a constant potential across the speaker load may be obtained with wide changes in the load. It is also possible to obtain a rising potential characteristic across the speaker load with increase in speaker load. The reverse may also be obtained.

It is possible to use ordinary single tube amplifiers instead of push-pull. In such case, the extra secondary windings for feedback would be unnecessary.

What is claimed is:

1. A speaker system comprising a driver transformer having a primary adapted to be energized with audio frequency potentials and having a secondary, a vacuum tube amplifier having input and output circuits, a feedback transformer having primary and secondary windings, means for connecting the secondary windings of said driver and feedback transformers in series in said amplifier input circuit, an output transformer having primary and secondary, said output primary being connected in said output amplifier circuit, a plurality of parallel connected speakers forming a load, means for connecting said load, feedback primary and output secondary in series, said feedback windings being so phased as to provide regenerative action whereby the potential across said speaker load is maintained substantially constant in spite of load variations.

2. The system of claim 1 wherein a by-pass resistor is connected across the feedback primary.

3. A speaker system comprising a driver transformer having a primary adapted to be energized with audio frequency potentials and having a secondary with a center tap and outer terminals for push-pull operation, a vacuum tube amplifier in push-pull having cathode, grid, and anode electrodes, a feedback transformer having one primary and two secondary windings, connections for disposing a secondary feedback winding between one control grid and the outer terminal of the driver secondary so that said two feedback secondaries are in the input circuits of said pushpull amplifier, said cathodes and center tap being connected, an output transformer having a primary connected to the anodes of said amplifiers, said output transformer having a secondary, a plurality of parallel connected speakers forming a load, connections for disposing said load and the primary of said feedback transformer in series across said output secondary, said feedback transformer having the primary and secondary windings so phased as to provide regeneration.

4. The system of claim 3 wherein a by-pass resistor is provided acrosssaid feedback primary to control said regeneration.

5. A speaker system comprising a driver transformer having a primary adapted to be fed by audio frequency potentials and having a secondary, an audio frequency amplifier having input and output circuits, a pair of feedback transformers having primary and secondary windings, means for connecting said driver secondary and the feedback secondaries in series in said amplifier input circuit, an output transformer having a primary in the amplifier output circuit, said output transformer having a secondary, a plurality of speakers in parallel forming a load, means for connecting one feedback primary in series with said load across said output secondary and means for connecting the other feedback primary across said load, said one seriesconnected feedback transformer having the windings phased to provide regeneration and the other parallel feedback transformer having the windings phased to provide degeneration.

6. The system of claim 5 wherein a by-pass resistor is connected across said one feedback primary in said series feedback transformer.

7. A speaker system comprising a driver transformer having a primary adapted to be energized with audio frequency potentials and having a secondary with two outer and one center terminal, a pair of vacuum tube amplifiers, each having cathode, grid and anode electrodes, means for connecting said cathodes to said center terminal of said driver secondary, circuits connecting the outer driver secondary terminals to said control grids for push-pull operation, two feedback transformers each having a primary and two secondaries, said grid circuits each including one secondary of each transformer in series between a grid and outer terminal of the driver secondary, an output transformer having a pushpull primary connected to said amplifier anodes, said output transformer having a secondary, a plurality of parallel connected speakers as a load, means for connecting the primary of one feedback transformer in series with said load across said output secondary and means for connecting the primary of the other feedback transformer in shunt across said load, said one feedback transformer having its windings phased for regeneration and the other feedback transformer having its windings phased for degeneration.

8. The system of claim 7 wherein a by-pass resistor is connected across the primary of said one series feedback transformer.

9. The system of claim 7 wherein a by-pass resistor is connected across the primary of the one series feedback transformer and wherein a condenser is connected across the output secondary.

WEBSTER O. CRANE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,107,817 Davis Feb. 8, 1938 2,162,369 Campbell June 13, 1939 2,166,517 Begun July 18. 1939

Images