United States Patent Office
IMPROVED DUAL CHANNEL AMPLIFIES
Harrison E. Dow, Wycombe, Pa., assignor, by mesne assignments, to Phiico Corporation, Philadelphia, Pa., a corporation of Delaware
Filed July S, 1960, Ser. No. 40,815
15 Claims. (CI. 179—1)
The present invention relates to audio amplifier systems and more particularly to dual channel amplifier systems for the reproduction of stereophonically related program signals or the like.
It is well known that in most musical passages a large fraction of the total audio power lies in the bass frequency components of the signal—that is, components having frequencies in the frequency range up to approximately 300500 cycles per second. It has been discovered also that these bass frequencies contribute very little to the overall stereophonic separation effect. Since the bass frequency reproducer i.e. speaker and enclosure of an audio system, is usually relatively large, it has been proposed to employ in dual channel stereophonic audio reproducers only a single bass range speaker system but separate mid-range speaker systems. This is usually accomplished by providing a crossover network or the equivalent in the output of the two channels of the stereophonic system, supplying the low frequency outputs of the two* crossover networks to a linear signal adder network and then coupling the output of the adder network to the input of the common bass range speaker system. While this arrangement can result in a substantial reduction in the overall size of the audio system by eliminating one of the two large bass frequency reproducers it does nothing to relieve a second problem which is usually present in dual channel reproducers, namely, that it is usually uneconomical to construct the amplifiers in the two channels so that each is individually capable of supplying the total power required to reproduce properly the bass frequency components of the two stereophonically related signals. If a bass frequency signal is supplied equally to the two inputs of the dual channel system, the amplifiers in the two channels are effectively operating in parallel to amplify the bass frequency components. Therefore the total undiatorted power available in the bass frequency range is equal to the sum of the power outputs of the two amplifiers. However in many recordings the amplitudes of the bass frequency components in one channel may be several times that of the bass frequency components in the other channel. In this case the power supplied to the single bass frequency reproducer is supplied almost entirely by only one of the two channels. If the amplifiers are operated at or near their maximum power handling capacity with balanced bass frequency signals, the unbalancing of the signals in the bass frequency range reduces the total undistorted power available to approximately one-half that available for balanced bass frequency signls. This will produce a notable loss in audio power in the bass frequency range and/or an increase in the distortion level.
In certain audio amplifier systems, negative feedback is provided from the speaker terminals to an earlier point in the amplifier circuit to improve the linearity of the amplifier system. In amplifiers which derive the feedback signal from the combined bass frequency range signals of the two channels, evan a relatively small unbalance in the amplitudes of the bass frequency components of the signals in the two channels will cause the amplitude of the signals in both channels to increase rapidly to the point where overloading of one or both the amplifiers is likely to result. This too will bring about a loss in available bass range power and an increase in the amount of distortion present.
Therefore, it is an object of the present invention to provide a dual channel amplifier system which makes full use of the power handling capabilities of both channels for all signals in the bass frequency range.
5 Another object of the present invention is to provide a dual channel amplifier system having a single bass frequency range speaker system which is relatively free of distortions on unbalanced input signals. Still another object of the present invention is to pro
10 vide a dual channel, negative feedback stabilized, amplifier system which is relatively insensitive to unbalancing of the input signals to the two channels.
I have discovered that these and other objects of the present invention may be achieved, and the operation of
15 dual channel amplifier systems greatly improved, by cross-coupling the two channels for low frequency signals at a point prior to the final stage of each channel in addition to coupling the low frequency output signal components of the two channels to a single bass frequency repro
20 ducer. The cross-coupling of the inputs of the two channels is achieved by supplying to the inputs of the amplifier in each channel a bass frequency signal derived from an appropriate point in the opposite channel. This crossfeeding of signals to the inputs of the two amplifier chan
25 nels tends to equalize the amplitudes of the bass frequency components present in the two channels.
For a better understanding of the present invention together with other and further objects thereof reference should now be made to the following detailed description
30 which is to be read in conjunction with the accompanying drawings in which:
FIG. 1 is a block diagram of one preferred embodiment of the present invention; FIG. 1A is a diagrammatic representation of one form
35 of cross-coupling circuit which may be employed in FIG. 1;
FIG. 2 is a plot of average signal power versus frequency range for a typical signal to be reproduced;
FIG. 3 is a schematic drawing of a preferred embodi40 ment of the present invention which derives its cross-feed from the output of each amplifier channel;
FIG. 4 is a further embodiment of the present invention in which the cross-feed between the two channels is provided by a difference signal and is fed back to the input 45 of the two amplifier channels;
FIG. 4A is a partial schematic drawing showing a possible modification of the circuit of FIG. 4; and
FIG. 5 is a partial block diagram of a dual channel amplifier system which includes negative feedback con50 nections.
The system shown in block form in FIG. 1 illustrates the basic principles of the present invention. This system embodies two amplifier channels 1ft and 12. Each channel may include one or more amplifier stages. The
55 signal input connections to amplifier channels 10 and 12 are represented by input leads 14 and 16, respectively. A stereophonic source 18 is shown connected to these two leads. Source 18 is shown in broken lines since it does not form a part of the invention per se. The source
60 18 may be the output of a stereophonic tuner or the stereophonic transducer of a tape or disc reproducer. The source 18 may include, in addition, preamplifier stages, volume controls, tone controls and the like.
The two input leads 14 and 16 are cross-connected by
65 low pass filters 22 and 24. Preferably filters 22 and 24 pass only those bass frequency components which contribute little to the stereophonic separation effect. For example, they may have an upper cutoff frequency in the range of 300 to 500 cycles.
70 One economical form of cross-coupling circuit of the type required in FIG. 1 is shown in FIG. 1A. A resistor 25 is coupled between the ungrounded terminals of a