US3171891A - Amplifier output circuit - Google Patents

Description

March 2, 1965 5, AMOS ETAL I 3,171,891

AMPLIFIER OUTPUT CIRCUIT Filed Dec. 15, 1960 INVENTORS Bernard Amos Jesse M Lafzm NEY United States Patent 3,171,891 AMPLIFIER OUTPUT CIRCUIT Bernard Amos and Jesse H. Lofton, Batavia, N.Y.,

assignors to Sylvania Electric Products Inc., a corporation of Delaware Filed Dec. 15, 1960, Ser. No. 75,995 2 Claims. (Cl. 1791) This invention relates generally to multi-channel reproduction and more specifically to a stereo amplifier output circuit.

Public acceptance of multi-channel reproduction systems in the form of two channel stereo circuits has increased the interest of manufacturers in seeking truly realistic sound reproduction. At first stereo reproducers were limited to an output system which utilized two spaced audio transducers or speaker systems, each of which was intended to reproduce the whole gamut of audio frequencies. Problems which arose in part from transcription errors and in part from speaker system spacings pointed up the fact that reliance upon two spaced sound sources sometimes produced a noticeable and undesirable sound void in the center position. One suggested prior art solution was to insert a relatively low frequency transducer in the center position between the two extended stereo transducers and feed this center transducer through low frequency filters from both channels. In such units, the center transducer reproduces all low frequencies, which apparently are relatively nondirectional, and the two side spaced transducers reproduce the mid and high audio frequency portion of the reproduction range. Further development lead to the realization that the center transducer or speaker, under some circumstances, should be allowed to reproduce some mid range frequency audio signals even though such frequencies are normally considered more directional in character than the lower portion of the audio range.

Though complicated circuits have been developed for feeding mid and high range audio frequencies to the center transducer in a multi-channel output circuit, it would be desirable to provide means for accomplishing this result Without expensive amplifiers, filters or crossover networks.

Thus it is an object of this invention to eliminate the need for expensive circuit arrangements for feeding blendable portions of the mid and high frequency portions of the audio range to a center transducer in a multi-channel transducer array.

It is a further object of this invention to blend at least portions of the mid audio range from both channels through a center transducer in a stereo transducer array, under control of the feedback circuits in both channels.

Briefly, at least one aspect of the invention comprises feeding the spaced transducers through a common current path into which is inserted a relatively low frequency transducer connected in parallel with a resistance capacitance controllable bypass circuit.

For a better understanding of the present invention, together with other and further objects, advantages and capabilities thereof, reference is made to the following disclosure and appended claims in connection with the accompanying drawing which shows a multi-channel output circuit utilizing a specific embodiment of our invention.

In the embodiment of the invention shown in the drawing, it may be seen that tubes 12 and 14 each comprise a single-ended final amplification stage of a separate channel in a stereo amplifier. Though single-ended outputs are shown for each channel, it is to be understood that push-pull outputs or any other output configiration, which occurs to one skilled in the art, may be used.

Thus in the upper channel, audio signals are fed from the penultimate stage, not shown, through a coupling capacitor 16 to the control grid of the output tube 12. Also in the lower channel audio signals are fed from the lower channel penultimate stage, not shown, through coupling capacitor 18 to the control grid of tube 14. A grid leak resistor 20 may be coupled between the control grid of tube 12 and ground, and grid leak resistor 22 may be coupled between the control grid 15 and ground. Though tubes 12 and 14 may each have individual selfbiasing cathode circuits, we have shown a resistance 28 which is bypassed by capacitor 30 as a common selfbiasing circuit in the cathode circuits of both tubes 12 and 14. Other types of biasing circuits may be provided where desired.

Output transformer 34 which passes the output signal from the upper channel comprises a primary winding 36 coupled in the output circuit of tube 12 and a secondary 38 which is coupled to the transducer circuitry to be described. Likewise output transformer 40 in the lower amplifier channel comprises a primary winding 42 in the output circuit of tube 14, and a secondary winding 44 which is also coupled in the transducer circuit to be described.

In the transducer circuit, terminal 48 is coupled to one side of transformer secondary 38 and one side of transformer secondary 44 to provide a common current return path for transducer current flow. The remaining terminal of transformer secondary 38 is coupled through a mid-range transducer or speaker 50 to a given potential plane, shown in the drawing as ground. Coupled across speaker 50 through capacitance 52 is a high range transducer or tweeter type speaker 54. In similar manner the remaining terminal of transformer secondary 44 is coupled through a mid range speaker 56 to ground and through capacitance 60 and high range speaker or tweeter 58 to ground.

Low range transducer or woofer '62 is coupled between ground and the common transformer secondary terminal 48. Coupled across low range woofer 62 is a variable blend control comprising a variable resistor 64 and capacitor 66. v

In use, the speaker unit 68 comprising mid range speakers 50 and 54 is positioned in spaced relationship with the speaker unit 70, comprising mid-range speaker 56 and tweeter 58. In order to provide an acceptable stereo effect, minimum spacing between speaker units should be at least 8 or 9 feet with a maximum spacing distance governed by the wattage of the amplifiers and the power handling capacity of the particular speakers selected. Woofer 62 may be positioned between speaker units 68 and 70.so as to be in the middle, i.e., substantially the same distance from each side speaker unit.

Speakers 50 and 56, which reproduce mid-range audio frequencies, preferably should be selected so as to have a relatively low impedance to audio frequencies below 200 cycles per second. In commercial applications this can be accomplished by selecting speakers having a resonant frequency above 200 cycles per second. Woofer 62 should be selected to reproduce the low frequency portion of the audio range preferably between 50 cycles per second or slightly lower and approximately 400 cycles per second at the upper end of the range. Usually such speakers also will reproduce higher frequencies with reasonable efficiency. Capacitor 66 is selected to have a value which will provide relatively low impedance to mid range and high frequency audio signals, and variable resistance 64 should be capable of inserting a resistance value somewhat higher than the impedance of speaker 62 in the mid range audio frequencies.

'68, but also from speaker 70.

as ablend control to allow the listener to either fill in V or take out mid-range audio outputv from speaker 62.

"by the appended claims.

r 3 p .s r In operation, variable resistor 64 functions as a blend control, allowing the listener to either blend in or remove mid range audio output from speaker 62. 3 In other words, considering the audio output from transformer 34, it can be seen that full range audio signal current passes through speaker unit 68 to ground and then back to the other side of secondary 38 through common terminal 48. Current flow from ground to terminal 48- divides in two paths. Some of the current flows through speaker 62, and some of the current flows through variable resistor 64 and capacitance 66. If variable resistor 64 is adjusted to include a relatively high resistance in the circuit between ground and. capacitance 66, the majority of the return current flow tends to go through and drive the a coil of speaker As variable resistor 64 is adjusted to decrease the resistance between ground and capacitance 66, more and more of this return current flow tends to bypass the coil of speaker 62 and flow toward terminal 48 through capacitance 66. In similar manner, audio output signals impressed on speaker unit 70 from transformer secondary 40,,flow back to the other side of transformer secondary 44 through terminal 48 after passing through the parallel circuit arrangement of speaker 62 andvthe series circuit comprising variable resistor 64 and capacitor 66.

In other words, as variable resistance 64 is adjusted to increase the resistance between ground and capacitance 66, speaker 62 is driven by an increased mid-range audio signal contribution, not only from speaker unit Thus, resistance 64 acts Though there is a slight change in the mid-range and high frequency voltages impressedacross. speaker units 68 and 70, as resistance 64 is adjusted it is extremely difficult for the ear to detect any change in the output volume of these speakers and extremely easy to notice change in the output of speaker 62. p a 7 Feed back connections 72 and 74 which are taken from the upper sides of transformer secondaries 38 and, 44 are negative feed back paths connected to earlier amplifier stages, not shown. It is to be noted that feed back path 72 utilizes a signal taken from across secondary 44 and speaker 62 as well as speaker unit 70-. Likewise, feed back path 74 utilizes a signal taken from across secondary 38 and speaker 62 as well as speaker unit 68. Thus speaker units 68 and. 70 as well as speaker 62 are in the 1 feed back loops so as to minimize output distortion. 7

While there has been shown and, described what is at 'present considered the preferred embodiment of the invention, it'will be obvious to those skilled in the art that various changes andmodifications maybe made therein without departing from the invention as defined Having thus disclosed our invention, weclaim; *1. In' a multi-channel amplifier the combination comprising, a series coupled pair of similar speaker units having an intermediate terminal, said speaker units pro output circuit coupled in series and having a second intermediate terminal, means coupling said series connected output circuits in parallel with said vseries connected speaker units, a third speaker providing audio signals over the low and middle portion of the audio range, a variable resistance-capacitance impedance coupled across said third speaker to form a parallel circuit, said capacitance having a relatively low impedance for middle audio frequencies, and means coupling said last mentioned parallel circuit between said two intermediate terminals.

2. In a two-channel amplifier'having a separate feed back loop in each channel the combination comprising a series coupled pair of audio transducers having an intermediate terminal, said audio transducers having a relatively low impedance to frequencies across the low end of the audio range, a first channel output circuitv and second channel-output circuit, said output circuits being coupled in series, and having a second intermediate terminal, means coupling said series connected output circuits in parallelwith said series connected audio transducers, means coupling said first channel output circuit in the feed back loop of the first amplifier channel, means coupling said second channel output circuit in the feed back loop of the second amplifier channel, a third audio transducer'having a given impedance providing audio signals over the low and middleportion of the audio range, a

variable resistance-capacitance impedance connected across said third audio transducer to form a parallel over'the middle portion of the audio range and means coupling said last mentioned parallel circuit between 'said two intermediate'terminals.

References Cited in the file of this patent UNITED- STATES PATENTS OTHER REFERENCES 7 Psychoa'coustic Applied to Stereophonic Reproduction System, by Goldmark et al., Journal of the Audio Engineering Society, April 1959, vol. 7, No. 2, pages 72-74.

Adding the Third Channelj High Fidelity Magazine,

April 1959, pages 109, 125 and-126.

' .Trimensional Stereo Speaker System, by Victor Brociner, Audio, June 1959, pages 21-23 and 74.

I 7 "Phantom Channel for'Stereo, by Herman Burstein,

Electronic Work, June1959, pages 46, 47 and 80.

. Extended Stereo System, Electronics'World, January 1960, pages-50, 156, 157,

Claims (1)

1. IN A MULTI-CHANNEL AMPLIFIER THE COMBINATION COMPRISING A SERIES COUPLED PAIR OF SIMILAR SPEAKER UNITS HAVING AN INTERMEDIATE TERMINAL, SAID SPEAKER UNITS PROVIDING MID AND HIGH RANGE AUDIO REPRODUCTION AND HAVING A RELATIVELY LOW IMPEDANCE TO FREQUENCIES ACROSS THE LOW END OF THE AUDIO RANGE, A FIRST AND SECOND CHANNEL OUTPUT CIRCUIT COUPLED IN SERIES AND HAVING A SECOND INTERMEDIATE TERMINAL, MEANS COUPLING SAID SERIES CONNECTED OUTPUT CIRCUITS IN PARALLEL WITH SAID SERIES CONNECTED SPEAKER UNITS, A THIRD SPEAKER PROVIDING AUDIO SIGNALS OVER THE LOW AND MIDDLE PORTION OF THE AUDIO RANGE, A VARIABLE RESISTANCE-CAPACITANCE IMPEDANCE COUPLED ACROSS SAID THIRD SPEAKER TO FORM A PARALLEL CIRCUIT, SAID CAPACITANCE HAVING A RELATIVELY LOW IMPEDANCE FOR MIDDLE AUDIO FREQUENCIES, AND MEANS COUPLING SAID LAST MENTIONED PARALLEL CIRCUIT BETWEEN SAID TWO INTERMEDIATE TERMINALS.

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