US2444676A - Sound recording and reproducing control system - Google Patents

Description

F. w. ROBERTS ET AL 2,444,676- somin nnconnme Am; REPRODUQCING CONTROL SYSTEM Filed Dec 29, 1944 AAAA- Patented July 6, 1948 SOUND RECORDING ANDREPRODUCING CONTROL SYSTEM Frederick W. Roberts, Fairfield, and Roger C.

Curtis, New Haven, Conn., assignors to Dictaphone Corporation, New York, N. Y., a corporation of New York Application December 29, 1944, Serial No. 570,280

1 -This invention relates to control circuits for sound recording and reproducing systems, and more especially to automatic volume control or compression circuits in electronic tube amplifiers for use with such systems.

In order to produce satisfactory'records on the usual recording mediums with normal types of recorder elements, the level of the signal applied to the recorder element must be maintained within a relatively narrow range regardless of the original range of the sound being recorded. This is especially difiicult when a single recording system must be used to record sounds of widely different levels from many sources,such as in the recording of telephone conversations, radio programs or communications, conferences, speeches, and the like. This reduction in the volume range of the sounds being recorded to the limited range required at the recorder is usually accomplished by the provision in an electronic tube amplifier of an automatic volume compression circuit, frequently referred to as an A. V. C. circuit. In the usual operation of such circuits a D. C. voltage is produced which is proportional to the in-' put signal applied to the amplifier if the A V. C. circuit is of the forward-acting" type, or to the output signal of the amplifier if the A. V. C. circuit is of the backward-acting type. This D. C. voltage is applied to the grid of one or more variable-gain amplifier tubes, which may be of the super-control or variable-mu type, to control their amplification factor and therefore'the gain of the amplifier in accordance with the strength of the A. V. C. circuit, i. e. its compression time,

should be about ten milli-seconds or less. Such a compression time has been almost impossible to realize practically in single-ended amplifiers, i. e. other than push-pull amplifiers, for two principal reasons. The first reason is that a fast acting A. V. C. circuit having a short compres-' sion time causes the magnitude of'the average level of the signal passing through the amplifier to vary at such a high rate as to produce audible tone or bump, as it is frequently called. For example, with a ten milli-second compression time, the amplifier gain varies at a rate comparable to that of a sound having a frequency of one 8 Claims. (Cl. 179-1004) hundred cycles per second. This sound or bump 1 occurs each time the A. V. C. circuit operates and is superimposed upon and interferes with the signal being amplified. In the past, in order to avoid. this undesirable'efiect, it was necessary to increase the compression time of the A. V. cir-f cuit until its rate of operation was reduced to some value comparableto a sub-audiblei j'fre quency. However, such a remedy is limited by the amount of blasting and 'overcutting, caused by the reduction in the speed of operation of the A. V. C. circuit, which can be tolerated in the recording system. The second reason is that it is impossibleto filter the A. V. C. controlvolt age satisfactorily in an'A. V. C. circuit having a short compression time because such a short action period requires circuits with small time constants which means circuits having small re-' sistance and capacity values and therefore limited filtering efiectiveness. In order to operate satisfactorily the A. V. C. control voltage shouldf be substantially pure D. C. With insufficient: filtering in the A. V. C. circuit such as results from shorttime-constant circuits, the A. V; C.

potential willbe a pulsating D. C. containingan' A. C. component corresponding to the signal bee ing amplified, and this component will be im#' pressed on the grids of theamplifier tubes along with the signal, thus causing considerable distortion in the amplified signal. Normally, this eflect'can be overcome only by increasing the compression time of-the A. V. 'C. circuit because an A. V. C. circuit which operates at a rate comparable to some audible frequency must of necessity transmit that frequency and therefore can'- not be designed to eliminate this distortionvoltage'by filtering." Thus, single-ended amplifiers having a sufliciently rapid A. V. C. action for high quality recording are subject to'the disadvantages of bumping and distortion. Further, rapid acting A. V. C. circuits may cause blocking of the amplifier tubes due to large swings in'the A. V. C. potential. Although these disadvantages-may be partially overcome by the use of a push-pull transformer-coupled amplifier in which the ing a short compression time.

In accordance with the present invention a' highfidelity single-ended amplifier may beoperated throughouta Wide range of signal levels with an A. V.'C. circuit having a very shortcom pression time without bumping, distorting, or=

in sound recording and reproducing apparatus? for recording sounds from sourcesfofi-widelyd vergent levels, and to provide; novel circuits-for controlling its operation.

These and other apparent objects andadvantages of this invention are obtained by the means described in the following specification: andimay' be more readily understood by reference to the accompanying drawing whichshows sc'hematicale ly, an 1. electric recording and. reproducing apparatus embodying the invention.

Referring to-thedrawing, the recording.- and reproducing apparatus shown therein generally comprises as-source of signal to be recorded, gens erally- .indicated bythe-blocked area lflm-suciras aimicrophone. telephone, radio, orthe like; a. pre-. amplifier,- stage, generally indicated at I02, in.- cl-uding. a voltageamplifier tube-V.l; an amplifier-A. V. C. stage; generally indicatedat Hi l, including. A. V. C; controlled.v amplifier. tubes Vw2.and;:V3-; an A. V. ,6. potential amplifying andmeotifying circuit; generally indicated at I 05,-.including an amplifier tube V.-4.. and a. rectifier tube V5.; a phase-inverter cathodefollower" circuit, generally indicatedatlOB, including-vacuumtubes V6 and V'l; a power amplifying stage, generally indicatedat H0, including. a. power amplifier tube VB.; a. power supply for allof these circuits generally indicatedat. ll2,.including arectifier'tube V9; arecorder and. reproducer mechanism, generallyv indicated at l 6 and: an output circuit, generally indicated .at I M; which may be selectively connected-.feither'to an electrical recorder element l3llror, a loudspeaker unit 1 l8 dependent. upon whether. the apparatus is conditioned for recording-.01 reproducing.

The signal: to be recorded fromethe signal: source 4 U0- is impressed-throughan input transformer.T-el-to.th e grid I2 ofthe-pre-amplifier triode -V--l .ofthe pre-amplifier stage 102.: Thepre amplifiertube Vl is provided withthecustomarycathode bias. resistor Bl-4 with its bypass ;=condenser C-l 9,. and -a -plate. loadresistor Ri--.5 connected .to.a suitable positive-highwoltage terminal. |26.-iin.the powersupply 1 l2; The amplified.-signa1 appearing, across the plate resistor Ree-5. of-tube,-Vl is applied through a-cou- 911. 1c0ndenser-:C 4 eto the-control grid 22 ofthe remote-cut-ofi, screen-grid pentode V2 in the amplifienA. V. Chstage'lfldnwhere it isamplifiectzinv a normal manner "so that a the -ampli-.

fiedtsignaiaappears across its :plate. load resistor Ry -l Lwhich is connected to apositive'high voltagerterminaltwl 24 in' power-supply l l2. The suppressor grid25 of the pentodeV- Ziis connected tothe cathode-Ql-in theusual manner, and V--2- is provided with the.customarysscreen-gridaresistor-R-F-S and bypass condenser C-8.= The controlfgrid: 22 oft. tube- V2 :is.. arranged *to be: grounded ithrough arelay contact-K-B to} preventthe appearance of any parasitic signalssonr the grid; eniwhen. --the apparatus is conditioned.-

for reproducing, as will be described hereinafter.

The amplified signal from pre-amplifier stage I02 is also fed to the A. V. C. circuit I06 through a condenser 0-5 to the control grid 42 of a remote-cut-ofi screen-grid pentode V-4 which amplifies-the A. V. C. potentiahbeforeitis rectified'ibytheA. V. C. rectifier V+5 This A. V. C. potential amplifying tube V-4 is provided with the usual grid return resistor R28, cathode bias resistor R2l with its bypass condenser C-Il, screen grid resistor- R,25 with its bypass condenserC-lS, and plate load resistor R-24 connectedtto 'aspositivahigh voltage tap I24 in the power supply; H2. Its suppressor grid 45 is connected-to its cathode-'41 in the usual manner. The amplifierhA. V. :6. potential which appears across the.p1ate.resistor. R-24 of amplifier tube V4 is app1i'edthrougli a coupling condenser C-l5 This rectifier V5 is a double diode connected in a voltagedoubler.- arangement as shown in thediagram so-thattherectified A.. V. C. pulsatingrDzCh potential-- appears across resistors R,2-2- and- R23; shunted by a'condenser Cl4-= Briefiy this voltage-doubler rectifier arrangement oper ates as follows: during. the portion of the: cycle of the signal impressed on" grid 42-". of: A-.-- V.. 0.- potential: amplifying tube V'--4--when the-grid" is. going. negative, the" current through this tube decreases,thusdecreasing the voltage .dropxacross i plate resistor R-24-andincreasing the positive: potentialabove ground of plate l3 andfthe'side" Ofi couplingcondenser GI 5 connected (thereto; This causes condenser Cl5 to be charged-toavoltage equal tor the peak value of 3 thisvoltage change through cathode 53 land anode St -of zone.

of the diodes of rectifier" tube--V'53 During; the

next. half tcycle of l the signal voltage wherr thegridf lfiroftube V'4 :is going positive, the current through tube. V4 increases; thus increasingthe voltage: drop-across plateresistor. Ila-24 and: thereby-decreasing the potential above'ground" ofzthe plate of 'condenser'C'l5 c0nneCted=t0 this resistor". and theplate 4'3 of'tubewV- l: This,

:- in". turn, causes condenser GI55 to discharge" through: cathode 51 and anode 52: of the other diode of rectifier 2 tube V+5 and"'condenser' (3-! 4" thereby. causing: condenser C-l4 to be'fcharged to-a=:vo1tage: substantially-equal to'the voltage change across plate resistor Re-Z 4" of tube D'4'- during one complete cycle of 1 the signal voltage;

This discharge: of condenser. 'Cl 5 prepares it I.

for the =next negative half cycle of the signal voltage and at the same time produces across resistors fir-22. and R23the D. C. potential to be used as'the A. V. C. control potential and \VhiCh':lS proportional to the magnitude of'the signaltpotentialt Part of this" potential is applied: asra negative bias'to the control grid 42 of the*A-.. V. Crpotential amplifying 'pentode V-4 by connecting the juncture between resistors R'22and'R-23 through the grid return resistor R 28;to-the control grid 42 of tube V-4." With this novelwarrangement of applying a portion: of

the A: V. C. D.'C. potential back to the grid-of th.e.--A.-..V. apotent-iaramplifying tube V-4l, an A: V. C.'effect is superimposed upon the original A. V. C. effect so as to greatly increase "the signalrange overwhich the A. V. C. circuit is effective.

By choice of suitable'values for resistors R-22 and-:R-23 determining: the proportion ofthe- A-.-- V'. C; D.'C- potential fed back to'tube V4;

theslope-ofthe A. V. C. operating characteristic curve-can be: predetermined as desired:

ThefA; V. C- D. .C. potential-developed.iacross resistors R'22 and Rr-23 is applied through a connection from the negative end of resistor lit-22 through a grid resistance R6 to the control grid 22 of the amplifier A. V. C. tube V2 above-described and, in the same phase, through a similar resistance R-8 and a relay contact K-l, hereinafter to be described, to the control grid 32 of the otherremote-cut-off pentode tube V3 of the amplifier A. V. C. stage I04. Tube V3 is identical to tube V2 and is similarly provided with the usual plate load resistor Rr-l2 connected to high voltage terminal I24, and screen grid resistor R l and bypass condenser C-'|. A series circuit comprising a resistor R'l and a condenser C-6 connected from the control grid 32 of tube V3 to ground so that the impedance across the input of tube V-3 is substantially equal to the impedance across the input'of tube V2 due to the plate circuit impedance of Vl and the grid circuit impedance of V4,so that the response of tubes V--2 and V3 to changes in the A. V. C. potential applied thereto through resistors R6 and R--8, respectively, is substantially identical. The control grid 32 of tube V-3 may also be connected through a relay contact K--2, alternate to the above-mentioned contact K-l, to a grid return resistor R-30 and, through a filter formed of a resistor R,26 and a shunting condenser C3,

to the reproducing element I32 of the recorderreproducer'mechanism H6 when the apparatus is conditioned for reproducing'and tube V'--3 is operated as the input amplifier tube. At the same time the A. V. C.- potential is grounded by another relay contact K'l connected to resistor R8. The A. V. C. amplifier tube V-4, the components of its associated circuits, and the A. V. C. rectifier tube V5 are all suitably selected so that condenser C--l4 can be charged rapidly to the full A. V. C. potential, 1. e. in a period of the order of ten milliseconds. Inasmuch as the charging time of condenser C-|4 determines the compression period of the A. V. C.

circuit this permits the A. V. C. response of the amplifier to be made sufiiciently rapid to permit high quality recording without blasting or overcutting.

With the circuit thus iar described, it is evident that the voltage which appears across the plate resistor Rl l of tube V2 is a combination of the signal from the source I00 amplified by tubes Vl and V2, plus any bump produced by operation of the A. V. C. circuit, plus any distorting voltage which may come from the A. V. C. circuit due to the presence in the A. V. C. potential of an A. C. component resulting from some of the signal voltage leaking through the A. V. C. circuit caused by lack of sufiicient filtering in the A. V. C. circuit. On the other hand, the voltage which appears across the plate resistor R-IZ of tube V3 is merely a combination of any bump voltage produced by operation of the A. V. C. circuit plus any distortion voltage appearing in the A. V. C. potential due to the presence of an unfiltered A. C. component of the signal. These bump and distortion voltages appearing across resistor R-I2 are in phase with, and of substantially equal magnitude to, the two similar voltage components appearing across plate resistor Rr-' I of tube V2.

These voltages appearing across resistors R.'| l and R,-l2 are impressed, respectively, upon the control grids 62 and 12 of the two triodes V--6 and VI connected in the phase-inversion cathode-follower circuit I08 in such a manner, hereinafter to be described, that the A. V. C. bump and distortion voltages appearing across resistor RH are cancelledby the corresponding voltages appearing across resistor R,-I2, thereby leaving only the signal voltage to pass on to the power amplifier stage H0. This is accomplished by the novel circuit connections shown in the drawing. The amplified voltage appearing across resistor R-ll comprising components resulting from the signal voltage, the A. V. C. bump voltage, and the A. V. C. distortion voltage, is reduced by a voltage divider formed of a resistor R-l3 and a resistor R,29, and this reduced voltage is applied through a coupling condenser C-Hl to the control grid 62 of one of the triodes VB of the cathode-follower phase-inverter circuit I118. The amplified voltage appearing across the plate resistor R.-l 2 of tube V3 comprising components resulting from the A. V. C. bump and distortion voltages is fed through an attenuator R48, connected in parallel with resistor lit-12, and a coupling condenser C-I0 to the control grid I2 of the other triode V---! in the cathode-follower phase-inverter circuit )8. The cathodes GI of tube V-'6 and H of tube V-'| are connected together and-to ground through a grid biasing resistor R-I 8 and a cathode-follower load resistor R|'l. The grid returns of the tubes V--6 and V! are made through grid resistors R--l5 and R.l6, respectively, connected togather and to the junction of resistors R.l 1 and Rl8. Resistance R.-l8 is of such value as to provide a normal operating grid bias for the tubes VB and VI, whereas cathode follower re-' sistor R-l'l is a much higher resistance than R,-l8 and forms part of the plate load of these tubes, as is usual in normal cathode-follower operation. The remainder of the plate load of these tubes is provided by two resistors R-l9 and R20 connected from the anodes 63 and 73 of tubes V-6 and Vl, respectively, to a suitable high voltage terminal I22 in the power supply H2. The operational characteristics of the cathode-follower phase-inverter circuit just described are such that any voltage impressed on the control grid of one of the tubes, e. g. tube VB, not only appears across the plate resistor of that tube, e. g. plate resistor 'Rl9, but also appears across the plate resistor of the other tube, e. g. plate resistor Rr-20 of tube V l, in approximate- 1y equal magnitude but in opposite phase. Without going into a rigorous analysis of the operational characteristics of this circuit which bring about these results, the cause of these effects may be understood from the following brief explanation. When the control grid 62 of tube V--6 becomes more positive its increased cathode-anode current causes the voltage drop across cathodefollower resistor R--l'l to increase, thus causing the cathode 6| likewise to become more positive with respect to ground, thus following the potential applied to the control grid 62 while the voltage drop across plate resistor Rl 9 increases simultaneously. At the same time, assuming that no signal is impressed on the control grid 12 of tube V-! so that its grid remains at a fixed potential with respect to ground, the cathode 'H of tube Vl, being connected to the cathode 6| of tube V6, becomes more positive with respect to its control grid 12, i. e. its control grid becomes more negative with respect to its cathode, thus reducing its cathode-anode current and hence reducing the voltage drop across its plate resistor R.2ll at the same time that the voltage. drop across the plate resistor Rr-S of tube;

The novel and advantageous results achieved t by thecircuit just described are now apparent.

The voltage appearing across plate resistor RFI I of tube V-2, comprising the signal voltage plus the =A. V. C. bump'voltage plus the A. V. C.

distortion voltage, is impressed on the control grid62 of tube V6 so that this voltage appears across the plate resistor R-I9 f tube V.6 and at the same time it appears in inverse phase across the plate resistor R- r of tube V-7.

Simultaneously, the voltage appearing across plate resistor R-I 2 of tube V-:3, comprising the- A..V. C. bump voltageplus the A.'V. C.ndistortion voltage, is impressed upon the control grid I2 of tube-V- I so that it appears across the plate resistor 3-20 of tube V-'I inphase with the corresponding voltage appearing across R-I9 as a result of the bump? and distortion voltages im= pressed on grid 02. .However, due to. the novel operation of this circuit, it also appears in inverse phase across resistor R-I9. Thus, the, total voltage appearing across the-plate resistor R-I9 of tube V6 is equal to. the sum of the signalvoltage plus the A. V. C. bump voltage plusthe A. V. C. distortion voltage, due to the voltage applied to the grid 62 of tubeV -B; plus;v thetwo last-named voltages in a negative sense,

their phase being inverted, due to the voltage applied to .grid I2 of tube V':', so that the net voltage appearing across resistor RI9 is the signal voltage alone, the A. V. C. bump and distortion voltages being cancelled out.

In actual operation, it is an inherent characteristic of the cathode-follower phase-inverter circuit above-described that when a voltage is impressed on the grid of one tube the voltage appearing across the output resistor of that tube is slightly larger than the inverse phase voltage appearing across the output resistor of the other tube; One reason for this effect is that the effective cathode to grid voltage applied to the first T tube is slightly greater than that applied to the second tube because of the feedback action of the common cathode-follower resistor R-I I. Because of this circuit characteristic just described, in order to achieve complete cancellation of the A.V.C. bump and distortion voltages, the abovedescrib'ed voltage-divider circuits, formed of the two resistors Rr-l3 and :R-ZSI, and of the attenuator R-48, are provided in parallel with the plate resistors RF-ll and R-I2, respectively, of tubes V2 and V--3. This permits the ratio of the. input voltages to tubes V-6 and V'I to be setby 'a suitable adjustment of the attenuator R48 so that the A.V.C. bump plus distortion voltage components impressed on the control grid 'IZ'ofthetube VI from plate resistor RI2 of tube V-3 are slightly greater than the corresponding voltage components impressed on control grid 62 of the tube V6 from the plate resistor R -II of tube V2, as above described.

The signal voltage, with the A.V.C. "bump and distortion voltage components cancelled out, appearing'across plate resistor RI'9 of tube V'-6 is impressed on the power amplifier stage H0 through a'coupling condenser (L -I2," a recording' volumecontrol potentiometer R-3'I, and a relay contact K-3, hereinafter to be described, on=the control grid 82 of a beam-power type power When the apparatus is conditioned for repr'o-.= ducing,.the signal is fed from reproducer element i32sthrough tube V--3 to the grid of tube V-''I.=- where it appearsacross the plate resistors RI9. and Rr-20 of tubes V6 and V-'I. In order tov permitthe volume of: reproduction to be-set at a different value fromthe recording volume, a" separate reproducing volume control is provided;

by utilizing the signal appearing across plate resistor R 20 instead of that appearing across R-I9. This is accomplished by impressing this signal through a coupling condenser C'-I8, a re.-.

producing volume control potentiometer fir-36;-

and a relay contact K I, alternate to contact -K--3, on to the grid 82 of power amplifier tubefier circuit, the cathode lead from resistor R-2I is connected to one side of the secondary of outa put transformer T-2, the other side of the sec- 1 ondary being connected to the ground circuit.

'The high side of the secondary of the output transformer T2 also is connected through a relay: contactv K-5 to the recorder element I30, the; other side of which is connected to the grounded-1 side of the secondary of transformer T-2. Byv means of a relay contact K-B, alternate to con-c. tact K5, the output of the power amplifier stage I10 from the secondary of transformer T-2 can be connected to the loudspeaker H8.

The recorder element I30 and reproducer ele--.

ment I32, which may be of any suitable typesuch as thoseshown in U. S. Patents No. 2,181,437 to Norton or No. 2,318,828 to Yerkovich, of the recorder-reproducer mechanism III; are mounted on a mechanical control mechanism, generally indicated at ISfl, operated by control leverI36,

whereby either the recorder element or the reproducer element may be operatively positionedwith respect to a suitable recording medium such as the wax cylinder I38. The control lever I36 is adjustable to three positions, a .recording position in which the recorder I30 is movedinto operative position with respect to the record I30,

a reproducing position in which the reproducer element I32 is moved into operative position with respect to record I38, and a neutral position in which neither the recorder nor reproducer ele-- ments are in operative relationship to the record.

Four contact points I40, .I42,'.I4 I'and I46 are associated with the mechanical control mechanism I34 so that contact is made between points.v

I40 and I42 whenthe control lever I36 is in .re-

cording position, and between contacts I44 and I46 when the control-lever I36 is in reproducing? position, and in which the circuits between contacts I40 and I02 and between I44 and I46 are open when the control lever I36 is in neutral position. The function of these contacts will be hereinafter described.

The power supply H2 is of the conventional full-wave rectifier type comprising a power supply transformer T3 connected to an alternating current supplyline I50 through a fuse FI and a double-pole single throw off-on switch S -I; a

of all the tubes.

a. full-wave rectifier tube V-9 having a cathode SI and two anodes 92 connected in the usual manner to the transformers T3 and to a filter system comprising a plurality of filtering resistors R-42, Fir-43, R44, R-45, a bleeder resistor R46, and filter condensers C--2 I, C-23, C'-25, C--26, and C-28 connected in the usual manner, providing D. C. operating potentials between a negative terminal I28 and a plurality of positive high voltage terminals I20, I22, I24 and I26 connected to the various circuits as above-described.

This power supply circuit is of the usual type except for the provision of a relay coil MI connected in series with the filter resistance R42 in the positive side of the output of the power supply between a terminal II9 connected to the cathode 9I of rectifier V9 and terminal I20, as shown in the drawings. This relay coil MI operates the various contact points KI through K8, above-described, to close contact points KI, K3, and K when it is deenergized, and to close contacts K2, K I, K-6, K--'I and KB when it is energized. The filter resistor R--42 has a resistance approximately equal to the resistance of relay coil MI. With this arrangement, when the off-on switch S-I is closed and the apparatus is in operative condition, the plate currents of all of the tubes pass through resistor R42 and relay coil MI. This relay coil MI and resistor R-42 are connected to the above-mentioned control contacts I40, I42, I44 and I46 associated with the recorderreproducer mechanical control mechanism I34. Terminal II9 of resistor R-42 is connected to contact point I46, terminal I20 of relay coil MI is connected to contact I40, and a terminal I2I between relay coil MI and resistor lit-42 is connected to contact points I42 and I44. Thus, when recording-producing lever I36 is in recording position, thus closing the circuit between contacts I40 and I42, relay coil MI is short circuited and therefore deenergized; whereas, when control lever I36 is in reproducing position, thus closing the circuit between contacts I44 and I46, resistor Rr-42 is short circuited, thus increasing the current through relay coil MI so that it is fully energized. When recording-reproducing control lever I36 is in neutral position both relay coil MI and resistor Rr-42 are in series with the plate supply This introduces a higher than normal resistance in this circuit and thus decreases the plate current to all the tubes. This arrangement tends to decrease the load on the tubes when the apparatus is in neutral or nonoperating condition and thus tends to prolong tube life and reduce servicing requirements.

Summarizing the operation of this control system above-described, when the recording-reproducing control lever I36 is placed in recording position, relay coil MI is short circuited and thus deenergized. This connects the A. V. C. control voltage from rectifier V5 through resistor R-8 and contact KI to the grid 32 of tube V-3. It also connects the recorder volume control potentiometer R-3'I through contact K3 to the grid 82 of the power amplifier tube V8. Finally, it connects the high side of the secondary of the output transformer 'T-2 through contact K5 to the recorder element I30. Simultaneously the recorder element I30 is positioned in operative relationship to the record I38. When a signal from the signal source I00 is introduced to the input of the apparatus this signal is amplified by tubes VI, V2, and V-8 1 0 and applied to the recorder element I30. At the same time a portion of the signal is amplified and rectified by circuit I06 into an A.- V. C. control potential which controls the amount of amplification of the signal by tube V-+'-2 in the am"- plifier A. V. C. stage I04. Through the action of tube V '3 and the cathode-follower phase; inverter stage I08, formed of tubes V-=6 and V--I, any "bump or'distortion voltage caused by the operation of the A. V. C. circuit is cancelled out in the output of tube V6, in the manner above-described, so that the signal amplified by tube V8 and applied to recorder element I30 .corresponds to the original signal undistorted by the operation of the A. V. C. circuit and by any A. V. C. bump. The average volume of the signal being recorded is determined by the setting of the recording volume control R--3'I. When the recording-reproducing control lever I36 is placed in reproducing position, relay coil MI is energized, thereby opening contacts KI, K3 and K5 and closing contacts K-2, K--4, K6, K--'I and K8. This grounds the control grid 22 of amplifier tube V2 through contact K-8 to obviate the possibility of a signal or other interfering voltage being impressed upon the grid of this tube during reproduction. The A. V. C. control voltage from the output of rectifier V-5 is rendered inoperative by being disconnected from the control grid 32 of tube V-3 by opening of the contact K-.I and by being grounded through the closing ofcontact KI.

At the same time the reproducing element I32 is connected through contact.K2 to the control grid 32 of tube V-3, whence it is amplified and appears across the plate load resistor R-20 of tube VI from which it is fed through the reproducing volume control R 36 and contact K4 to the control grid 82 of power amplifier .tube V--8. It is further amplified by tube VB and passed through the output transformer T-2 and contact K6 to the reproducing loudspeaker I I8. As above mentioned, the volume during reproduction is determined by the setting of reproducing volume control R-'-36..

As many embodiments may be made in the above invention and as many .changes may be made in the embodiment above described, it is to be understood that all matter hereinbefore set forth or shownin the accompanying drawing is to be interpreted as illustrative only and not in a limiting sense.

Weclaim:

1. In sound recording and reproducing apparatus, the combination of, a source of signal to be recorded, an electronic tube amplifier, recorder means, a record medium, reproducer means, loudspeaker means, positioning means for putting said recorder means in operative relation to said record medium when in one position, for putting said reproducer means in operative relation to said record medium when in another position, and for moving both said recorder andsaid reproducer means out of operative relation to said record medium when in a neutral position, relay means connecting said signal source to the input of said amplifier and connecting the output of said amplifier to said -recorder means during recording when in one position, and connecting said reproducer means to the input "ofsaid amplifier and connecting theoutput of said amplifier to said loudspeaker 'meanswhen in an alternate position during reproducing, biasing means for holding said relay means in one position, solenoid means for mov- 1.1 "ing s'aid relay means to its alternate position when energized, a power supply for energizing Isaidamplifier, said solenoid and an impedance of' approximately equal value connected in series in one supply line from 'said power supply to :said amplifier, switch means for shortcircuiting'said solenoid in response to movement of 'said positioning means to on'e position, for ShOIt-Cll' ciiiting said impedance in response to movement f said positioning means to its other position, 'and'for leaving both said solenoid and said re-v si'st'or in said power supply circuit. when said positioning means is in its neutral position, whereby the power supplied to said amplifier is reduced during standby periods'when the appara'tus is'neither recording nor reproducing.

*Sound recording and reproducing equipment, comprising, in combination, a' record medium, asource of signal to be recorded, a recorderelementya reproducer element, 'a loudspeaker element,-an electronic tube A. V.- C. amplifierincluding circuit means'i'or producinga CfApV. C. potential which is a function of the sig'nal to be 'r'ecorded'first and second 'amplifier "stages each including a remote cut-off pentode tube, means applying said A. V. C. potential' to both of said pento'de amplifier "stages, twoe vacuum tubes connected in a cathode-follower phase-inverter circuit, means connecting the output of 'said' first amplifier stage to the input of one of said phase-inverter tubes, means connecting the output of said second amplifier stage to theinputpfthe other of said phaseinverter tubes, and switching" means responsive tor the'positioning of said recordenelement in operative relationship to said record medium for connectingsaid signal source to said'first amfplifinwst'age and for connecting the outputof oneiof said phase inverter tubes to said recorder unit, arid responsive 'to the "positioning of said reproducer in operative relationship to said rec,- ord' me'dium' for applying thesignal from said reproducer element tothe input of said second amplifier. stage, for rendering said' A'. "V: C.- potential means inoperative and for applying the output of saideother phase inverter tube tosaid loudspeaker element.

- 3.: Sound recording and reproducing equipment; comprising; in combination, a? record medium, asource of signal-to be'rec'orded, a recorder elementya -reproducerel'ement, aloudspeaker element, an electronic tube A. V. C. amplifierincluding circuit means for producing a D.-"C. "A. V: C. potentiai' which is afunction of the signal :toberecorded, first andseco ndamplifier stages each-e includingzia remote cut-off pentode tube,

means applying said A. 'V. 0. potential to both of said pentode amplifier stages, two vacuum tubes connected in cathode-follower phase-inverter circuit, means connecting the output of said firstamplifier stage to the-input-of one of said phase-inverter tubes, means connecting the out.- put ;of said i second amplifier stage to the input ofi-the' other of said phase-inverter, tubes, switching: means responsive to the positioning of said recorder element in operative relationship to said record medium for connecting 'said signal source to said first amplifier stage and for connecting the outputof Y onefioii said' phase-inverter tubes: 'tOSaidQI'CCOTdEI' unit, :and responsive to the 1 positioning of :"said; reproducer in operative relationshipto said record medium for applying thesignal from saidreproducerelement to the input of said second amplifier stage, for "rendersaid -A-. Vi. Cl-inpotential-meansinoperative and .for applying the output of said other phaseinverter tube to said loudspeaker element, first volume c'ontrolmeans in the output circuit of said first-phase inverter tube, and second volume control means in the output circuit of said second phase-inverter tube, whereby the amount of amplificationused when recording'and when reproducing may. be set independently at different levels-without interaction therebetween.

4'. Sound recording and reproducing equipment, comprisingrin combination, a record medium, a source of signal to be recorded, a-reeorder element, a reproducer element, a loudspeaker-element, an electronic tube A. V. 0. amplifier including circuit means for producing a D. C. A. V, C. potential'which is a function of the signal to be recorded, a first amplifier-stage including a first variable-mu vacuumtube, a second amplifier stage including a'second variablemu vacuum tube, the cathode'of said second tube being connected to that of "said first tubeand to acornmon ground circuit, means for applying said A. V. C. potential to the: control grids of both of said tubes to supply an operating'bias thereto, third and fourth vacuum tubes each having a cathode, anode; and control grid", means for: coup-ling the plate of said first tube to the control'grid of said third tube, means for coupling the plate of'said second 'tu'b'e to the control grid of said fourth tube, means connecting the cathodes of said third'and fourth tubes to"- gether and to said ground circuit through a common resistor forming; part of the anode load of said tubes, biasing means for supplying a this potential between the cathodes and the control grids of said third and fourth tubesf two plate resistors each connected" to the anode of one or said third and fourth tubes andeach'forming the remaining portion of the anode load of its re spective'tube, means connecting said two lastnamed" plate-resistors together and through a source of positive anode potential to said ground circuit, and 'switching means responsive to the positioning of said recorder element in operative relationship to said record medium for connecting said signal source'tothe grid of said first amplifier tube and for connecting the outputof said'third or Lfourth' tubes to said recorder element, and responsive to the positioning of said reproducer element, in operative relationship to said record" medium for applying thesignalfrom said reproducer element to the grid of'said second amplifier tube, for rendering said A. C. potential means inoperative'and for applying theoutput ofthe otherrof said third or fourth tubes; to-said loudspeaker element.

5. Sound *recording and reproducing equipment, comprising, incombination, airecordi inedi-um, a source of signal to be recorded-a; recorder: element; a reproducer' element, a loudspeaker 'element-an electronic tu be'A; V; cfamplifier includin'g 'circuit' means for producing a D. C; V L C. potentialhvhich is a function of the signal td be recorded, a first amplifier stage in- 13 troi grid of said third tube, means for coupling the plate of said second tube to the control grid of said fourth tube, means connecting the cathodes of said third and fourth tubes: together and to said ground circuit through a common resistor forming part of the anode load of'said tubes, biasing means for supplying a bias potential between the cathodes and the control grids of said third and fourth tubes, two plate resistors each connected to the anode of one of said third and fourth tubes and each forming the remaining portion of the anode load of its respective tube, means connecting said two last-named plate resistors together and through a source of positive anode potential to said ground circuit, and switching means responsive to the positioning of said recorder element in operative relationship to said record medium for connecting said signal source to the grid of said first amplifier tube and for connecting the output of said third or fourth tubes to said recorder unit, and responsive to the positioning of said reproducer in operative relationship to said record medium for applying the signal from said reproducer element to the grid of said second amplifier v tube, for connecting said A. V. C. potential producing means to said common ground circuit, and for applying the output of the other of said third or fourth tubes to said loudspeaker element.

6. Sound recording and reproducing equipment, comprising, in combination, a record medium, a source of signal to be recorded, a recorder element, a reproducer element, a loudspeaker element, an electronic tube A. V. C. amplifier including circuit means for producing a D. C. A. V. C. potential which is a function of the signal to be recorded, a first amplifier stage including a first variable-mu vacuum tube, a second amplifier stage including a second variable-mu vacuum tube, the cathode of said second tube being connected to that of said first tube and to a common ground circuit, means for applying said A. V. C. potential to the control grids of both of said tubes to supply an operating bias thereto, third and fourth vacuum tubes each having a cathode, anode, and control grid, means for coupling the plate of said first tube to the control grid of said third tube, means for coupling the plate of said second tube to the control grid of said fourth tulbe, means connecting the cathodes of said third and fourth tubes together and to said ground circuit through a common resistor forming part of the anode load of said tubes, biasing means for supplying a bias potential between the cathodes and the control grids of said third and fourth tubes, two plate resistors each connected to the anode of one of said third and fourth tubes and each forming the remaining portion of the anode load of its respective tube, means connecting said two lastnamed plate resistors together and through a source of positive anode potential to said ground circuit, and switching means responsive to the positioning of said recorder element in operative relationship to said record medium for connecting said signal source to the grid of said first amplifier tube and for connecting the output of said third or fourth tubes to said recorder unit, and responsive to the positioning of said reproducer in operative relationship to said record medium for applying the signal from said reproducer element to the grid of said second amplifier tube, for connecting said A. V. C. potential producing means and the grid circuit of said first 14 tube to,- said. commonground circuiuiand for applying the output of the other of said third or fourth tubes to said loudspeaker element.

.7. In sound recording and reproducing equipment having a recordmedium, a source of signal to be recorded, a recorder element, a reproducer element, and a loudspeaker element; the combinationwith said elements of an electronic tube A. V. C. amplifier including circuit means for producing a D. C. A. V. C. potential which is a function of the signal to be recorded comprising first and second amplifier stages each including a variable-gain tube; means applying said A. V. C. potential to both of said amplifier stages; two vacuum tubes connected in a cathode-follower phase-inverter circuit; means connecting the output of said first amplifier stage to the input of one of said phase-inverter tubes; means connecting the output of said second amplifier stage to the input of the other of said phase-inverter tubes; and connection means associated with the input circuits of said amplifier stages, the output circuits of said phase-inverter circuits, and said A. V. C. potential means, and adapted to be connected to a circuit switching means; whereby the signal source may be connected to the input of said first amplifier stage and the output of one of said phase-inverter tubes may be connected to the recorder unit by operation of the switching means in response to the positioning of the recorder element in operative relationship to the record medium, and a signal from the reproducer element may be connected to the input of said second amplifier stage, the output of said other phase-inverter stage may be connected to the loudspeaker element, and said A. V. C. potential means may be rendered inoperative by operation of the switching means in response to the positioning of the reproducer in operative relationship to said record medium.

8. For use in sound recording and reproducing equipment having a record medium, a source of signal to be recorded, a recorder element, a reproducer element, and a loudspeaker element; in combination, an electronic tube A. V. C. amplifier including circuit means for producing a D. C. A. V. C. potential which is a function of the signal to be recorded; first and second variable-gain stages; the cathode circuits of said stages being connected together to a common ground circuit; means for applying said A. V. C. potential to said stages to supply an operating gain control bias thereto; third and fourth vacuum tubes each having a cathode, anode, and control grid; means for coupling the plate circuit of said first amplifier stage to the controlgrid of said third tube; means for coupling the plate circuit of said second amplifier stage to the control grid of said fourth tube; means connecting the cathodes of said third and fourth tubes together and to said ground circuit through a common resistor forming part of the anode load of said tubes; biasing means for supplying a bias potential between the cathodes and the control grids of said third and fourth tubes; a resistor in the anode circuit of each of said third and fourth tubes forming the remaining portions of the anode loads of said tubes; means connecting said last-named resistors together through a source of positive anode potential to said ground circuit;. and switching means responsive to the positioning of a recorder element in operative relationship to a record medium for connecting a signal source to the input of said first amplifier stage and for connecting the output of said third or p QAQQBQB x M is i i furthtubes to a; recorder elementand resp'bn- REFERENCES CITED lsive to the positioning of a. reproducer element;

i r tiv relati shi 'flt c d di far The following references are of recordwin the applying the signalfrom s id'reproducer element file 0f h sfpatent:

xto ighe @input of said 'seeconl'li amplifier Stage, for 15 UNITED STATES PATENTS rendering said A. V. (3. potential means inoperative, and forapplying-the dutpurof said third-or Number Name Date f'ou'rth tubes to a loudspeaker-lemnt, 1 ,6 4 Petty May 11, 1943 ,:m E R ERT 2359389 Proctor Oct. 3,1944

ROGER C. CURTIS. -1

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