FEEDBACK

3,094,587

Patented June 18, 1963

3,094,587

IMPROVED DUAL CHANNEL AMPLIFIES
SYSTEM

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 3

crystal stereophonic phonograph transducer. The resistor 25 together with the capacitance of one crystal of the transducer functions as a low pass filter corresponding to filter 22 of FIG. 1. The resistor 25 and the capacitance of the other crystal function as a low pass filter corresponding to filter 24 of FIG. 1.

Amplifier 10' includes suitable circuits for supplying the mid-frequency and high frequency components of the output signal to a speaker 26 by way of connection 28. The low frequency components of the signals amplified by channel 10 are supplied by way of connection 32 to one input of adder circuit 34. The separation of the high frequency and low frequency components may be achieved by a conventional crossover network in the output circuit of amplifier channel 10. Similarly, amplifier channel 12 supplies the high frequency components of the program signal amplified therein to a high and mid-range speaker 38 by way of output connection 36 and the low frequency components to a second input of adder 34 by way of connection 42. The output of adder 34 is supplied to a third speaker 44.

The operation of the system is as follows. Stereophonically related signals from source 18 are supplied to the inputs 14 and 16 of amplifiers 10 and 12. A portion of the low frequency component of the signals supplied to input 16 is supplied to 'the input of amplifier 10 by way of low pass filter 24. Similarly, a portion of the low frequency component of the signal supplied to terminal 14 is supplied to the input of amplifier 12 by way of low pass filter 22. The signal transfer characteristics of filters 22 and 24 are preferably chosen so that, at frequencies below the selected upper limit of the bass frequency signals, the net amplitudes of the bass frequency components supplied to amplifier channels 10 and 12 are nearly equal regardless of the degree of unbalance of the bass frequency components supplied by source 18. In the circuit of FIG. 1A this can be accomplished by making the impedance of resistor 25 equal in magnitude to the capacitive impedance of one of the crystals of the phonontransducer at a selected frequency in the bass frequency range. In certain cross-coupling arrangements the signals supplied by way of filters 22 and 24 may tend to increase the amplitude of the net bass signal components supplied to the inputs of the two channels 10 and 12. This increase in amplitude is unobjectionable for, as will be explained presently, the low frequency components are supplied to a separate speaker. Therefore the level of the bass frequency components may be controlled independently of the mid-frequency and high frequency components in the coupling network to the separate speaker.

Since the effect of the filters 22 and 24 is to equalize the amplitudes of the bass frequency components supplied to channels 10 and 12, these two channels, in effect, operate in parallel for all bass frequency signals regardless of an unbalance in the bass frequency signals supplied to inputs 14 and 16 by source 18. As a result, the entire combined power handling capacity of amplifier channels 10 and 12 is available for amplifying the bass frequency components even though source 18 supplies a signal to only one of these channels.

The bass frequency components of the signals supplied by the two channels 10 and 12 are combined by adder 34 and supplied to speaker 44. Adder 34 may be either an active or a passive adder network. For example, it may be an active adder network which includes two amplifier stages having separate inputs but a common load impedance or it may be a passive network which comprises a transformer having two primary windings and a single secondary winding.

Preferably the upper frequency limit of the signals supplied by way of connections 32 and 42 corresponds closely to the upper cutoff frequency of filters 22 and 24 at the input of amplifiers 10 and 12. However there need not be an exact correspondence.

4

As mentioned above, the mid-frequency and high frequency components of the signal at the output of amplifiers 10 and 12 are supplied to speakers 26 and 38. These speakers may be appropriately placed in the room

3 to give the desired stereophonic effect to the total audio signal produced by the system. Speaker 44 may be placed midway between the speakers 26 and 38.

The amplification of the mid-frequency and high frequency components of the signals supplied by source 18

10 is not materially affected either by the cross-connection of the inputs or the cross-connection of the outputs of the two amplifier channels. As shown in FIG. 2 the average power level of typical musical passages represented by curve 45 falls off quite rapidly above about 400 cycles.

15 Also, speakers together with their enclosures are generally more efficient in converting electrical energy to acoustical energy at higher frequencies than they are at low bass frequencies. Therefore each of the channels 10 and 12 will be able to handle the entire mid-range or

20 high range signal, if necessary, without overloading. Speech signals tend to have a power peak at a frequency above 1000 cycles but the average power level of speech is usually so far below that of typical musical selections that there is little danger of the amplifier channels 10

25 and 12 overloading due to speech frequency components. In FIG. 1 the signal supplied from the first channel to the input of the second channel is derived from the input of the first channel. However it is to be understood that this signal may be derived from any appro

30 priate point in the .first channel. FIG. 3 illustrates a preferred embodiment of the invention in which the signal supplied to the input of each channel is derived from the output of the opposite channel. In FIG. 3 the source of stereophonic signals is again represented in broken

35 lines at 52. The source 52, illustrated in FIG. 3, is a stereophonic phonograph transducer similar to the transducer shown in FIG. 1A except that no cross-coupling resistor 25 is employed. One output of source 52 is connected to the ungrounded side 54A of a volume

40 control potentiometer 56A. The tap of 58A on potentiometer 56A is coupled to the control grid of a first amplifier tube -60A by way of an isolating resistor 62A. Tube 60A is provided with the usual anode load resistor 64A. It is also provided with an unbypassed cathode

45 resistor 66A. Cathode load resistor 66A provides a form of negative feedback which tends to 'Stabilize the operation of the amplifier. A capacitor 68A and a variable resistor 72A are connected in series between ground and the anode of tube 60A. Resistor 72A and capacitor

50 68A together form a tone control circuit which provides an adjustable rolloff characteristic at high frequencies.

The anode of tube 60A is coupled to the grid of a pentode power amplifier tube 74A by way of resistor capacitor coupling network 76A. Again tube 74A is pro

5o vided with an unbypassed cathode resistor 78A which provides a form of negative feedback or degeneration which stabilizes the operation of the amplifier stage. The use of unbypassed cathode resistors is optional and is shown by way of illustration only. The anode load impedance for tube 74A comprises one primary winding 82A of a transformer 84 and the single primary winding 86A of a second transformer 88A. The anode of tube 74A is bypassed to ground by a relatively small capacitor

6g 92A. Capacitor 92A is a tone control capacitor which reduces the gain of the power amplifier stage for high frequencies.

The signal supplied by the second output of source 52 is amplified in a second channel which is identical to

70 the one just described. Therefore the various components of the second channel have been given the same reference numeral as the corresponding part in the channel just described except that the reference numeral is followed by the letter "B" rather than the letter "A." The anode

75 supply source for tube 74A is connected to the comr