US2186825A - Amplifier muting circuit - Google Patents

Description

Jan. 9, 1940. DOME 2,186,825

AMPLIFIER MUTING CIRCUIT Filed April 23, 1938 2 Sheets-Sheet l OUTPUT Inventor": Robert )5. Dome,

by W H i s Attorney.

Jan. 9, 1940.

R. B. DOME AMPLIFIER MUTING CIRCUIT Filed April 23, 1938 2 Sheets-Sheet 2 Fig. [.EANP. 83 8589 6 6 m N m R w w NTROL Fig. 7

Inventor Robert B. Dome, b9 ma 649M414 His ttorneg.

Patented Jan. 9, 1940 UNITED STATES AM?LIFIER DIUTING CIRCUIT Robert B. Dome, Bridgeport, Conn., assignor to General Electric Company, a corporation of New York Application April 23, 1938, Serial No. 203,935

15 Claims.

My invention relates to an improved control for a signal amplifier and more particularly to an improved control for muting or silencing the output of a signal amplifier. While not limited thereto, ly invention is particularly suited for muting the output of a radio receiver during the time when the receiver is being tuned between stations.

The tuning of a radio receiver from one signal frequency to another is often accompanied by a certain amount of undesirable noise and other disturbing sounds. This is especially true when the receiver is provided with an automatic volume control which operates to increase the sensitivity of t is receiver to a high value Whenever the receiver is not tuned to a reasonably strong signal. It is desirable that this noise, which occurs principally between received stations, be eliminated while effecting a tuning of the receiver from stato station over a frequency spectrum.

It is therefore an object of my invention to silence the output of a signal amplifier during a tuning operation by an arrangement wherein the routing circuit is completed by a person while asping a tuning knob to effect the tuning adjustment.

A further object of my invention is to provide muting arrangement in which a muting potent al is applied to a grid of an electron discharge signal anplifier during an adjusting operation of the er, the potential being obtained from the ca acltlve unbalance of a Wheatstone bridge resulting from the hand capacity of a person in gr oping a control knob to effect the desired ad- 35 justment.

Another object of my invention is to provide a radio iver a siiencing circuit arrangem nt wherei an alternating current Wheatstone unbalanced during a tuning operation of in w. s. l, 3, 4 and 5 illustrate emts of my mvcnton wherein the electrical. vi y of the human body is employed in an: LG? siiencirc' a radio receiver output; Fig. 2 shows a partially sectioned elevation of a tuning knob especially suitable for use with the foregoing embodiments of my invention,

6 and 7 illustrate modifications of my invention which utilise the el ctrical capacity of the human body to mute or silence a radio receiver output, and Fig. 8 shows in cross-section a tuning knob particularly suited for use with the latter modifications.

Referring to Fig. l of the drawings, my invention is illustrated as embodied in a superheterodyne type of radio receiver having a radio fre-- quency amplifier it connected to an antenna ground system H, a first detector l3, a local oscillator 82 and an intermediate frequency amplifier it. The several tunable circuits of the radio frequency amplifier $8, the first detector l3, and the local oscillator I2 are connected, as indicated by the broken line of the drawing, for uni-control operation from a manually actuable knob l5 which may be grasped by a person. operating the receiver to effect a desired tuning adjustment.

The output of the intermediate frequency amplifier M is coupled through a transformer l6, having primary and secondary windings respectively tuned by condensers ll, IE to the intermediate frequency, to the elements of a diode rectifier H3. The signal oscillations are rectified by the diode rectifier i9 and the modulation components of the signal oscillation appearing across the resistor 2%] are coupled through a condenser 2! to a voltage divider resistance 27-! and to the grid 23 of an electron discharge device 24. The electron discharge device 24 has an anode 25 which is energized from a source of high potential 26 through resistor 21. The amplified audio frequency signal appearing in. the anode circuit of the device 24 is coupled through a condenser 25! to an audio frequency amplifier 29 where the audio frequency signal is further amplified for reproduction by a translating device 3B.

The operation of the Fig. l circuit arrangement thus far considered is well understood in the art and will not be described in detail.

During the time when. the tunable circuits of the receiver are being adjusted to receive a selected station, it is desirable that the output of the receiver be muted to avoid the reception of interstation noise, undesired signals. distorted caused by an improper tuning, etc., and to this end my invention contemplates the provision on the periphery of the manually actuable tuning knob 15 of a pair of current conducting rings 3 l 32 arranged to be grasped by an operator in efifecting the desired adjustment of the tunable circuits. The ring 3! may be connected by a conductor 36 to one end of a voltage divider comprised by the resistors 33. 34 and 355 While the ring 32 may be connected through a conductor 3! and a resistor 38 to point on the voltage divider between the resistors 34 and 35. A. nor

mal biasing potential for the grid 23 is obtained by connecting the lower end of the voltage divider 22 through the resistor 38 to the lower end of the resistor and by connecting the cathode 39 to the junction between the resistors 33 and 34, the magnitude of the bias being determined by the voltage drop across the resistor 34. The cathode 39 of the electron discharge device M is maintained at ground potential for alternating currents by the condenser 40 while the lower terminal of the voltage divider 22 is maintained at ground potential for alternating currents by the condenser 4|.

It will now be evident that a person in grasping theknob l5 completes an electrical circuit from the ring 3| to the ring 32 through one or more fingers of his hand. The completion of this circuit connects the conductor 31 through the rings 3! and 52 with the conductor 35 and thereby connects the left hand end ofthe resistor 38 to the lower end of the voltage divider resistor 35. Current now flows from the high voltage source 26 through a series circuit comprised by the resistors 33, 34 and 38 to the negative terminal of the high voltage source, not shown, which is at ground potential. This causes a voltage drop to appear across the resistor 38 with a polarity such that the grid 23 of the electron discharge device 26 is driven substantially to cut-01f thereby to reduce the gain of the amplifier by rendering the discharge device 24 unresponsive to signal potentials impressed through the condenser 2! upon the grid 23. The output of the receiver is thus effectively silenced and remains muted as long as the operators fingers complete a circuit from the ring 3! to the ring 32.

For the purpose of silencing the receiver at other times than during a tuning operation, a switch 42 may be connected as indicated by the broken lines with the conductors 36, 3'! to provide when the switch is closed a completion of the cir-' cuit between the conductors 36, 31 at a point other than at the rings 3|, 32.

A more detailed drawing of 'my tuning knob construction is shown in Fig. 2 wherein elements corresponding'to like elements of Fig. l are designated by like reference characters. The manually actuable tuning knob l5, with the current conducting rings 3! and 32 mounted on the periphery thereof, is supported" on theright-hand end of a condenser shaft 43 which extends through an aperture 44 provided in a panel 45. The current conducting ring 3| is connected by a conductor 46, the condenser shaft 43 and a flexible conductor 36 to ground. The current conducting ring 32 is connected to a collector ring 4'! arranged to be contacted by a collector brush 48 which is secured to the panel by a pin 49. A person in grasping the knob I5 to effect an adjustment of the tuning condenser silences the output of the receiver by completing through one or more fingers of his hand an electrical connection between the r ng 3| and the ring 32 thereby to change the bias on the grid of a signal amplifier as explained with reference to Fig. 1.

Fig. 3 illustrates a modification of theFig. 1 muting circuit. Like reference characters are used to designate elements of this figure corresponding to like elements of Fig. 1, and the rectangle 50 represents conventionally the combined apparatus represented by the rectangles iii, l2,

23 of the electron discharge device 24 is connected through the voltage divider resistance 22 and the resistor to a source of biasing potentialiil which provides the grid with a normal biasing potential. The manually actuable tuning control knob I 5 is supported on the right-hand end of a tuning condenser shaft 53, the knob having onthe periphery thereof a pair of current conducting rings 54, 55 which are respectively connected to current collecting rings 56, 5'! secured to the inner face of the knob. A pair of brushes 58, 59 engage the collector rings 56, 51 respectively and complete a circuit for the ring 55 through a conductor 3'5 to they junction between the voltage divider 22 and the resistor 51 and from the ring 54 through a resistor 60 and a source of potential 6| to the grounded cathode 39 of the discharge device 24. The potential of the source BI is sufiicient to b as the grid 23 of the discharge device 24 substantially to cut-on, and thereby silence the output of the receiver, whenever an electrical circuit is completed between the current conducting rings 54 and 55 as by an operator in grasping the knob Hi to tune the receiver to a desired station. A condenser 62 maintains the lower end of the voltage divider 22 at ground potential for alternating currents. The purpose of the resistor (it is to limit the current flow from the potential source iil when an electrical circuit is completed from the ring 54 to the ring 55. The operation of this modification is very similar to that of Fig. l as will be readily apparent from the above description.

The muting circuit of Fig. 4 differs from that of Figs. 1 and 3 in that the screen grid of an elcctron discharge device is used in the Fig. i arrangement for muting purposes rather than a control grid as in the Figs. 1 and 3 arrangements. Signal oscillations, which may be either of audio or radio frequency, are coupled from an input circuit 53, 645

through a condenser 65 to the grid 66 and cathode 67 of an electron discharge device 68. The electron discharge device 68 is provided with an anode 59, which is energized from a high potential source 1Q through a resistor H. provided with a suppressor grid 12, which is connected directly to the cathode El, and a screen grid 73 which is connected to an intermediate point on a'voltage divider comprised by the resistors 14, 75 connected across the high potental source 10. A normal biasing potential is impressed on the grid '12 by a cathode biasing resistor l6 and parallel connected condenser ll. The screen grid 13 is maintained at ground potential for alternating currents by a condenser Amplified s gnal oscillations appearing in the the screen grid 13 of the device 68 from a normal a.

positive bias, with the switch in open position, to a small negative bias, when the switch is in. closed circuit position. The negative bias is provided by the potential drop across the resistor '15 and appears on both the grid 66 and the screen grid 13. This has the result that the current flowing from the anode 69 to the cathode 67 is reduced substantially to zero thereby to render the device 68 unresponsive to signal oscillations impressed on the grid 66. The output of the signal amplifier The device 68 is also arrangement shown in Fig. 4 is thus effectively muted or silenced by closure of the switch 82 which, in practice, is preferably positioned to be operated by manual manipulation. My preferred embodiment replaces the switch 82 with a pair of current conducting rings arranged on a tuning knob in the manner of 2 whereby the positive potential on the creen grid Hi is reduced merely by the act of a person in grasping the tuning knob to effect a desired tuning adjustment of the receiver with which. the Fig. t arrangement may he used.

Fig. 5 illustrates a modification of my invention in which the muting oi the radio receiver output is accomplished in an intermediate frequency amplifier stage by theintroduction of a muting potential to the admission grid/of an electron discharge device. The intermediate frequency amplifier stage here shown includes an electron discharge device 83 provided with a cathode 84, a pair of anodes 85 and 85, sup-- pressor grids 31 and 88 positioned respectively adjacent the anodes 85, St, a screen grid 89 common to both anodes 85, 3 5, a control grid 9i common to the anodes 85, 85, and an admis sion grid ill positioned to control the electron stream to the anode 85. The control grid 90, which is connected to receive oscillations of intermediate frequency from a tuned intermediate frequency transformer Q2, is provided with a normal operating bias by the connection of the cathode 3-! and the control grid (through the transformer 92 and the resistor 3132s) to spaced apart points on a voltage divider comprised by the resistors and 95 connected across a high voltage source 95. A suitable potential is impressed on the screen grid. from a high potential source 91' The anode 85 circuit of the electron discharge device 83 is not shown completed, but it is understood that this circuit is connected to an intermediate frequency output transformer to which may be connected additional stages of intermediate frequency amplification, a second detector, an audio frequency amplifier connected to a speaker, a frequency discriminator network. supplying potential to an automatic frequency control circuit, etc, as is well understood in the art. The admission grid ill of the device 33 is connected through a resistor 98 to the junction between the voltage divider resistors 94, and is also connectible through a resistor 33 and a pair of circuit closing elements it to the negative terminal, not shown, of the high potential source Q5 which is at ground potential. By-pass condensers Elli, ma, Hi3 and 5524 are employed to maintain the elements to which they are connected at ground potential for currents of alternating frequency.

The anode 86 of the electron discharge device 83 is connected to the high potential source 9'3 through a resistor Hi5 and oscillations of intermediate frequency which appear in the circuit of anode 88 are coupled through a condenser N36 to the diode elements ml of the electron discharge device I98 which constitute a control tube for a tuning indicator, not shown. Resistors lllil and HS are connected in series across the diode elements iil'l'of the device Hi8 and a portion of the unidirectional pulsating potential which appears across resistors PM. M l as a result of the rectifying action of the diode elements lil'l. connected through a resistor ill to a control grid H2 provided in the dis charge device 38. A condenser H3 is connected between the left-hand end of resistor Ill and ground while a condenser H4 is connected between the right-hand'end of resistor HI and ground, the two condensers H3, H and the resistor ill acting as afilter to smooth out the pulsating uni-directional potential appearing across the resistors I 09, H0 before applying this potential to the grid H2. The anode N5 of electron discharge device Hill is connected in a circuit, not shown, which controls a suitable tuning indicator in a manner to indicate the relative intensity of a received signal oscillation. The voltage appearing across the resistor ill! may be supplied through a resistor lit to automatic volume control circuit Ill for the purpose of automatically controlling the amplification of preceding radio frequency amplifier stages, not shown.

The operation of the routing arrangement shown in Fig. 5 will now be explained. The circuit closing elements Hi6 may take the form of concentric metal rings mounted on a manually actuable control knob in the manner shown in Figs. 2 and 3. When an electrical circuit is completed across the elements M18, as by a direct metallic connection or by the fingers of the human body grasping the knob to effect a desired adjustment, the bias on the admission grid ill of the electron discharge device 83 i creases to a sufficient negative potential, by virtue of the connection of the admission grid ill through the resistor 99 and elements we to the lower end of the voltage divider resistor 55, to terminate the flow of current from the anode 85 to the cathode 84 and thereby reduce to zero the output of signal energy in the anode $5 circuit. The muting of an intermediate frequency amplifier stage in this manner is especially able the arrangement not only silences the output of the receiver, but simultaneously decnergizes the automatic frequency control circuit in radio receivers provided with automatic frequency control.

It may be noted that the admission grid ill controls only the current flow from the anode '5 to the cathode 84. Further, the bias for the control grid 99 is unaffected by the increased potential applied to the admission grid 3! since the resistor 98 has a high value of resistance in the order of several megohrns. The control of the grid 90 over the current in the anode circuit 86 is, therefore, unaffected by of potential on the admission grid ti and the elements 86, E8, 89, 9B and 84 of the electron discharge device 33 continue, after completion of the muting circuit at the contact e .ments Hill, to amplify the intermediate frequency cillations supplied through the inter-med quency transformer 92 and to supply the oscillations thus amplified to the diode elements ill? of the discharge device I [28. The potential which results from the rectification of the intermediate frequency oscillations by the diode laments l? is supplied to the automatic vol in ontrol circult ill and is impressed on the 552 of device 108 to control the current i the circuit of the anode H5 and thereby con; 01 eration of the tuning indicator, not shown. for purposes of providing an indication that desired tuning of the receiver has been accom plished.

The muting circuit shown in Fig. 6 is similar to that shown in Fig. 1 except that a normally balanced Wheatstone bridge is unbalanced during a tuning operation to provide the necessary potential for biasing the grid of a signal amplifier the receiver.

reference characters.

beyond cut-off thereby to silence the'output of Elements in Fig; 6 -corresponding to like elements of Fig. l are designated by like In this arrangement, a source of alternating current H8 supplies an alternating current potential to opposite arms of a capacitive Wheatstone' bridge comprised by the condensers H9, I29, I2I and I22. This bridge is normally balanced and no potential therefore appears across the bridge at points a and b. A manually actuable tuning control knob I23, constructed of a metallic material, is secured to the right-hand end of a metallic shaft I24 which in turn is supported from a condenser shaft represented generally at I25. The knob I23 and shaft I24, which are insulated from the shaft I25 by an insulating segment I26, are connected by a conductor I21 and brush I28 to a point on the bridge between the condensers I20 and I2I. Points a. and b of the bridge are connected across both the diode rectifier I29 and the series connected resistors I30, I3I. The grid 23 of the electron discharge device 24 is connected through a conductor I32 and a resistor I33 to the junction between the resistors I39 and I3I, a condenser I34 maintaining the lower end of the resistor I33 at ground potential for alternating currents. A switch I35, when closed, maintains the junction point between the resistors I31! and iti at ground potential for both direct and alter nating currents.

The capacitive bridge is initially balanced by adjusting the condenser I 2! with the hand ofi of the tuning knob I23. When a balance of the bridge has been established, no voltage is im pressed across the diode I29 and the electron discharge device 24 operates with a normal bias on its. grid 23 determined by the voltage drop a ross the cathode biasing resistor 34. If now a anger of the human body touches the knob I23, as during a tuning operation, the property of electrical conductivity of the human body causes the body to act as one electrode of a capacitor whose other electrode is ground and the bridge becomes unbalanced by the added capacity of the sand. The alternating potential appearing in the unbalanced bridge between the points a, b is impressed across the diode I29 and resistors tee, I3I. The positive half cycles of current produced by this potential flow substantially in their entirety through the diode I29; the negative half cycles cannot, however, flow through the diode but flow only through the resistors I30, I3! nd a uni-directional potential therefore appears across the resistors MW, I3I. When the switch see is open. the potential across the resistor I36 is impressed through the resistor I 33 and the conductor I 32 upon the grid 23 of the electron discharge device 24 to drive the grid beyond cuton and thereby silence the output of the receiver. When the tuning adjustment has been completed and the tuning knob I23 is released, the apacitive bridge becomes again balanced whereupon the muting potential is removed from the rid 23 of the electron discharge device 24 to allow the device 24 to operate once more with normal output. A time delay in the removal of the muting potential from the grid 23 may be eifected by varying the time constant of the circuit comprised by the resistor I3I and condenser lat. The closing of the switch I35 serves to maintain the grid 23 always at its normal bias irrespective of the condition of balance of the bridge and thus enables the receiver to be tuned without silencing the receiver output.

The knob I 23 preferably has. a construction like that of the knob I36 shown in Fig. 7. This knob is suitably mountable on a metal shaft I38 and consists of an insert I39, a thin insulating strip I40 and an outside metal band II. A knob constructed in this manner has the advantage that one finger tip or the whole hand of the human body causes the same capacity change so that a good capacity change is always efiected. The capacity of an adult person is estimated to be of the order of 50 microfarads, a capacity easily sufficient to unbalance the bridge even though the frequency of the alternating current supply source to the bridge is as low as 60 cycles per second.

An additional modification of the knob construction is shown in Fig. 8. A metal insert I42 is provided with a socket I43 in which may be inserted the end of a tuning condenser shaft, not shown, the insert I42 being secured to the condenser shaft by a set screw I44. Surrounding the insert I42 is a body of insulating material I45 which may be grasped by a person in effecting a desired adjustment of the tuning condenser to which the knob is secured.

The muting circuit of Fig. 7 is similar in arrangement and operation with that of Fig. 6 except that the purely capacitivev bridge of the Fig. 6 arrangement is replaced in the circuit of Fig. 7 with a combined capacitive and inductive Wheatstone bridge. Elements in this figure corresponding to like elements of Fig. 6 are designated by like reference characters. The input and output circuits of the electron discharge device 24 are shown incomplete but it is understood that the circuits are the same as those shown in the arrangement of Fig. 6. The power supply transformer I46 has a primary connected to the power supply lines I I 8 and a center tapped secondary winding 'I4'I which constitutes a pair of inductive arms in a brigde comprised by the transformer winding I4! and the condensers I2I and I22. The bridge is balanced by the condenser I2I while the hand is off of the tuning control knob I36. With the bridge balanced, no potential appears across the diode I29 and the electron discharge device 24 operates with a normal bias on its control grid 23. When, however, a finger of the human hand touches the metallic ring I4I of the knob I36, the balance of the bridge is destroyed and a muting potential is impressed upon the grid 23 of the electron dis charge device 24 to reduce the output of the device 24 to zero and thereby silence the output of the radio receiver. The normal balance of the bridge is restored when the knob I36 is released by the hand, the muting potential thereupon disappearing from the grid 23, and the electron dismal output.

While I have shown a particular embodiment of my invention, it will of course be understood that I do not wish to be limited thereto since many modifications may be made both in the mechanical arrangement and the instrumentalities employed, and I therefore contemplate by the appended claims to cover any such modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

l. The combination, in a signalling system, of means for receiving signal impulses. means for amplifying received signal impulses, means for reducing the amplification of said amplifying.

charge device 24 once more operates with normeans, and means responsive to the electrical conductivity of the human body for actuating said last named means.

2. In a signalling system, the combination of means for receiving signal impulses, means including an electron discharge device having a grid for amplifying received signal impulses, means for biasing said grid to selectable values of potential for controlling the amplification of said device, and means responsive to the electrical conductivity of the human body for actuating said last named means.

3. The combination, in a radio receiver, of means for tuning said receiver to a desired signal frequency, said means having an actuator ar ranged for manual manipulation, means for amplifying received signals, and means responsive to the electrical conductivity of the human hand in grasping said actuator to effect a desired tuning adjustment for reducing the amplification of said last named means.

4. The combination, in a radio receiver, of means for tuning said receiver to a desired signal frequency, said means having an actuator arranged for manual manipulation, means for amplifying received signals, said last named means including an electron discharge device having a grid, and means responsive to the electrical conductivity of the human hand in grasping said actuator to efiect a desired tuning adjustment for biasing said grid toward cut-off whereby the output of said receiver is reduced.

5. In a signalling system, the combination of means -lor receiving signal impulses, means for amplifying received signal impulses, a normally balanced Wheatstone bridge having capacitive arms, meansresponsive to the unbalancing of said bridge for reducing the amplification of said amplifying means, an electric current conducting member and means responsive to the proximity of the human body to said member for adding the electrical capacity of the body to one arm of said bridge whereby said bridge is unbalanced and a reduction in amplification is effected.

6. vIn a signalling system the combination of means for receiving signal impulses, means for amplifying received signal impulses, a normally balanced Wheatstone bridge having a capacitive arm, means responsive to the unbalancing of said bridge for reducing the amplification of said amplifying means, a circuit control element having an actuator arranged for manual manipulation, and means responsive to the electrical capacity of the human body in proximity to said actuator for unbalancing said bridge.

7. The combination, in a signalling system, of means for receiving signal impulses, means for amplifying received signal impulses, a circuit control element having an actuator arranged for manual manipulation, a pair of electrical current conducting members electrically insulated from each other and coaxially mounted in spaced apart relation on said actuator, means including said conducting members for reducing the amplification of said amplifying means, and means responsive to the completion of an electrical circuit between said members through the human hand while grasping said actuator for energizing said last named means.

8. The combination in a control for a radio receiver having a tunable element, of a manual knob for actuating said tunable element, an electrical current conducting member on said knob arranged to be grasped by an operator in using said knob, means for amplifying received signal impulses, said means including an electron discharge device having a grid, means for impressing a normal biasing potential on said grid, and means including said conducting memher for increasing the bias on said grid when said knob is grasped to efiect a tuning operation, said last named means biasing said grid toward cut-off for reducing the output of said amplifier.

9. A knob for manually actuating an adjustable circuit element of a radio receiver, said knob having a body of insulating material and a pair of current conducting members thereon arranged to be grasped by an operator in effecting an adjusting operation, and means carried by said knob for completing an electrical connection between each of said current conducting members and stationary electrical conductors.

10. A knob comprising, in combination, a metallic insert, means for securing said insert to a shaft, a metallic band concentrically positioned about said insert, and means for electrically insulating said band from said insert.

11. The combination, in a signaling system, of an adjustable circuit element for said system, means for manually adjusting said element, means including an electron discharge relay having a control electrode for controlling an.

operating condition of said system, and means responsive to the electrically conductive property of the human body in close proximity to said first named means for changing the potential on said control electrode thereby to effect a change by said relay in the operating condition of said system during the time that said first named means is manually grasped in effecting a manual adjustment of said circuit element.

12. The combination, in a signaling system, of means for controlling an operating condition of said system, an adjustable circuit element for said system, means for manually adjusting said element, and means responsive to the electrically conductive property of the human hand for effecting control of said system by said first named means while the human hand is at least in proximity to said second named means.

13. In a signaling system, the combination of an amplifier, means for controlling the gain of said amplifier, an adjustable circuit control element having a manual actuator, and means responsive to the immediate proximity of the human body to said manual actuator for energizing said first named means to control the gain of said amplifier.

14. In a signaling system, the combination of an amplifier, means for controlling the amplification of said amplifier, an adjustable circuit control element having a manual actuator therefor, and means responsive to the electrical conductivity of the human body for energizing said first named means to control the amplification of said amplifier while said manual actuator is grasped in effecting an adjustment of said circuit control element.

15. In combination, a receiver having a tuning device and an audible output device, a manual actuator for said tuning device, and means responsive to the conductivity of the human body ,during manipulation of the manual actuator to reduce the output from said output device.

ROBERT B. DOME.

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