US2072708A - Amplification control - Google Patents

Description

March 2, 1937. N. P. cAsE 2,072,708

AMPLIFICATION CONTROL Y Filed Aug. 4, 1934 3 SheetS-Sheet l INVENTOR Nfl 50N FiF/7) (,455

ATTO R N EYS N. rP.4 CASE' AMPLIFICATION COISTROL 3 Sheets-Sheet 2 Filed Aug. 4, `19:54

INVENTOR NELSON PERRY CASE @wmd ATTORNEYS Mamh 2, 1937.

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AMPLIFICATION GOTROL Filed Aug. 4, 1934 3 Sheets-$heet 3 www a6 52 64 ree 25p 512 m21. 20411111196819216318# FREU/VCY INVENTOR /VES/V Pff/Ff C462'- BY 79m, MJ

ATTORNEYS Patented Mar. 2, 1937 Unirse STAT-ss Parri-:NTA OFFICE AMPIJFIQATION CONTROL Nelson Perry Case, Bayside, N. Y., assigner to Hazeltine Corporation, Jersey Gity, N, J., a corporation ofDelaware application August 4, 1.934, serial No. '138,494

18 Ulaixns. (Cl. 179-171) 'Ihis invention relates to ampliner systems and more particularly to means for automatically varying the relative amplication of voltages at diierent frequencies within the frequency hand in a desired manner and as a function of the signal level at a selected point in the system.

In accordance with the general principles of this invention, signal voltage is applied to the input of an ampliiier system which includes one portion designedto amplify voltages within a substantially broad band of frequencies and another portion designed to amplify voltages within a narrow frequency band which may be within or adjacent to the wide band of frequencies. A

`i portion of the amplified signal voltage is rectied and applied to the control element of a vacuum tube which alters the transmission properties of that portion of the amplifier designed to amplify Within the narrow frequency band, thus autoor matically varying the output-vs.-frequency characteristic of the ampliiier system as a whole, depending upon the strength of signal voltage at a given point thereof. For illustration, this invention will be described 25 vin detail as applied to an audio-frequency amplifying system, such as might be employed in a radio broadcast receiver or in a phonograph, a1- though the same general principles may be applied to a system adapted to amplify radio-fre- A 30 quency currents or combined radioand audiofrequency currents. By means of the4 invention an audio-frequency amplier may be so arranged as to increase the ratio of low-frequency voltages' to voltages of higher frequency in the output, 35 whenever the output voltage level is reduced below a designated value.

It is a-wellknown characteristic of the human ear that it becomes relatively insensitive to sounds of very low frequency when the sound intensity o falls to a low level. The effect of this characteristic,-in relation to music, is as follows:

When a musical program, as from 'a radio broadcasting station, is being reproduced at an ,acoustic level nearly the same as that at the 5 microphone, ,the reproduction should sound most natural if the output--vs.frequency characteristie of the electro-acoustic system is at, i. e., if all the component frequencies are present in the reproduction in the same relative amplitude as in 50 the original rendition.

Due to the small size of the averageroom in which a radio receiver or phonograph is operated, it is not normally desirable to have the repro- -duced sound as loud as the original. However, 55 when me vo1ume level is reduced, toa satisfactorily low value without altering the relative amplitudes of the various frequency components, it

is found that, because of the above-mentioned characteristic of the ear, the music sounds-thin, j as though there were a serious deciency in the 5 reproduction of the low frequencies or bass notes. Therefore, in order to produce the psychological effect of true tone balance at low acoustic levels, it is necessary to introduce frequency discrimination in the ampliiier so that the actual low frequency response is much greater than that for high frequencies. I

An object of the present invention is, therefore, to provide a new4 and improved sound reproducing system 4in which the output volume 15 may be reduced to any audible level without an apparent loss or diminution of the low frequencies. Attempts have been made t'o achieve the desired result by connecting appropriate circuit eleacteristic of the amplier system varies in shape according to the setting of the manual audio-frequency volume level control. While this method results in an appreciable improvement over the performance of an uncompensated amplifier, it is not entirely satisfactory, since the actual output of the amplier Variesl over a wide range with a vfixed setting of thevolume control device, because of the normal variation of musical amplitude between pianssimo and fortissimo, or for other reasons.

It is, therefore, an additional object of this invention to augment the low-frequency response of an amplifier system relative to the response at high frequencies whenever the output level is low, regardless of Whether the level is low because of a low input level or because the volume level control device is set at a low value.

In one form which this invention may take, the audio-frequency voltages are amplied by an audio-frequency amplifier system including a supplementary low-frequency amplier. This e supplementary amplifier contains a frequencymentary amplifier in such a manner that whenever the output voltage of the amplifier is large, the degree of amplification or gain of the supplementary amplifier will be reduced and the audio- 5 frequency voltages will be amplified only in the main amplifier. Whenever the output voltage decreases to a sufficiently low level, the supplementary amplifier will operate to amplify the low audio frequencies disproportionately. l0

controlled by the integrated values of the various audio-frequency'voltages that are present in the output circuit, although it is not necessary, and

is often not desirable, to integrate uniformly over the entire audio-frequency band.. It is usually.

preferable to give the higherv frequency components relatively greater weight in controlling the gain of the supplementary amplier.

be controlled by the usual manual volume control arrangement provided inthe input of the special amplifier constructed in accordance with.` this invention. In a'radio receiver, the usual automatic radio-frequency amplication control and manual volume control may be provided, operative upon the signals before they are supplied to the amplifier system constituting the present invention. y 1

An alternative arrangement incorporating this .invention may be provided wherein the-amplification characteristic of an amplifier stage is caused to be selective in favor of the frequencies which it is desired to accentuate whenever the output voltage of the amplifier is low.

ing the present `invention may be provided wherein a portion of the voltages ofv medium and high audio frequency is bypassed and thus prevented 'from reaching the final amplifier, whenever the f 40 total output voltage is low.' In this arrangement,

- Whenever the output voltagefalls to a sumciently low level, for any reason whatever, the low-:fre-l Fig. 5 is a circuit diagram of an `audio-frequency amplifier constructed in accordance with a modification of the present invention;

d0 Fig. 6 is a circuit diagram of another modification ofthe present invention; and

Fig. 7 is a diagram showing thev percentage re' lationships, for high and 'low output, voltage levels, between the various frequencies in the c5 audio-frequency output of an amplifier constructed in accordance with this invention. A

Referring first to Fig. l, the ordinates represent the sound pressure and the abscissas represent audio frequencies. 'Ihe curve, which was taken 70 from Speech and Hearing by Fletcherygives the minimum audible value of the sound pressure for various audio frequencies. It can be seen from this figure that the average ear is most sensitive to frequencies lying between 1000 and 4000 cycles 75 and that the sound pressure necessary for audi- The action `of the supplementary amplifier is l The output volume of the reproducer or loud speaker may V Another'alternative arrangement incorporatao'zavoe bility at a frequency between 64 and 128 cycles is many times that necessary for audibility at 1000 cycles.

Fig. 2 shows a schematic diagram of an amplifier system constructed in accordance with a preferred embodiment of this invention. In this figure the signal to be amplified is supplied to the input amplifier il and the supplementary amplifier I2. The outputs of both the amplifiers II and I2 are supplied to the output amplifier I3 by' which the signals are further amplified. A portion of the output of the output amplifier is supplied to the frequency filter, rectifier, and time constant circuit indicated at` I4. Here the voltages of the various frequencies are integrated, the higher frequencies usually being favored over the lower, and rectified to produce a controlling bias for theA supplementary amplifier I2. The circuitv of It includes a time constant circuit in order that instantaneous variations of the output level will not affect `the 'supplementary amplifier. The operation of this system will be described hereinafter in connection with the description Y of Fig. 4.

Fig. 3 isa schematic diagram of an alternative arrangement of an amplifier system embodying this invention. In this figure the signal is supplied to the input amplifier II. The signal is then passed to the selective amplification control I5 and thence to the output amplier I3. A portion of the output of the amplifier I3 is supplied to the frequency filter, rectifier and time constant .circuit I4. Here the voltages of various frequeninput terminals I-2 are connected across a' voltage divided P1, the variable tap of whichserves to impress a desired portion of the input Vvoltage across the input of the amplifier tube V1. The input signal is amplified in tube V1 and is supplied through the usual network including the coupling condenser C1 to the grid of the tube V2. lThe signal is further amplified in the amplifier tube V2 and impressed through an appropriate network including the coupling condenser C2 tothe input of the amplier tube V4. Here the signal is further amplified and impressed through the transformer T1 upon the inputs of the push-pull connected vacuum tubes V5, V5. The output of the push-pull connected amplifier is connected to the primary of output transformer T2. Across one-half of the primary o f the output transformer T2 is connected a voltage divider Pz by means of the variable contact of which a desired portion of the output voltage is supplied through the coupling condenser C3 to the diode plates of the tube V2.

A circuit including resistor- Rz, condenser C4 and condenser C5 is connected between the plate of the tube V1 and the cathode thereof. A connection between the condensers C4 and Cs is made to the input grid of the supplementary amplifier tube V3 by which low-frequency-signals are impressed upon the tube Va` in a manner A diode resistor R1 is connected be-. tween ythe diode plates and the cathode of the hereinafter more fully described. The rectified output terminals z resistance of R1.

voltage developed across the resistor R1 and the normal voltage`across the cathode resistor 'Ra of the tube V2 determine thel bias voltage of the vacuum tube Vs in a manner which will be described more fully hereinafter. The low-frequency voltages are amplified in the tube Va and supplied through an appropriate network including the condenser Cs to the grid of the amplier tube V4 'in which they are amplified together with the signalvoltages supplied to the grid of tube V4 through condenser C2.

Appropriate resistors, condensers and voltage supply sources are provided as indicated. Any

of the tube V3. The gridv of the tube Va is therefore excited by the voltage drop across the condenser Cs. It is obvious that at high frequencies only a small percentage of the voltage appears across this condenser, while at very low frequencies most of the voltage is developed across it. The resistor lR2 is chosen to be of high enough resistance so that the condenser Cs does not affect the input to the tube V2 appreciably at high frequencies. Tube V3 is a high gain amplierftube, and the circuit constants are so 'chosen that with normal bias on the grid of the tube Vs, the low Vfrequencies which are amplified by V3 are amplifled-to several times the extent to which the signals are amplified by the tube V2 and its associated circuit elements. The signals which have been amplified by tubes V2 and Va are combined and applied to the grid of the tube V4 which feeds the push-pull output tubes V5 ln the usual manner. Usually a loud speaker is connected to the The signal voltage taken from the voltage divider P2 is fed through the condenser C3 to the diode detector embodied in the tube V2. The diode leak resistance R1 must be high enough so that it produces a negligible loading effect upon the output circuit. The value of the condenser C3 must be carefully chosen in relation to the If this condenser is too small, the bass notes will be accentuated too much when the output level is high, providing the music being reproduced contains few treble notes. On

the other hand, if this condenser is too large, the

desired accentuation of the bass at low` output levels cannot be achieved because the bass notes themselves will excite the diode circuit too much, on account of the relatively greater amplitude of the low frequencies in most music.

At high output levels, a large negative bias voltage is built up at the diode plates of the tube V2, and this voltage is transmitted through the filter network Re-Cv-R'z to the grid of the tube V3, suiilcient to render the tube V3 inoperative. Under these conditions the amplifier has a normal, flat frequency characteristic.

As the output voltage decreases, from any cause whatever, the negative voltage on g the diode plates decreases until, at `a certain output level,

75 the tube V3 commences to be operative. As already noted, the input circuit of me tube v1 favors the lower frequencies with reference to `the higher. The output circuit of the tube Va the plate circuit impedance is practically thatl of its plate resistor R5 in parallel with the grid leak R of the tube V4; for very high frequencies, the platel resistor R4 of the tube V1 is also effectively in parallel with the resistances R5 and Re, and since R4 is considerably lower in resistance, the gain through the tube V3 for high frequencies is considerably lower than for low frequencies. Therefore when Va is operative, it serves to accentuate the bass notes relative to those of the middle and treble resisters.

As the output voltage continues to decrease, the potential on the diode plates of tube V2 approaches that of the cathode of V2, and under this condition, the tube Vs operates with maximum gain. Therefore, for all outputs below a certain level, the bass receives the full accentuation of which the system is capable. The choice of the level at which to atten out the gaincurve l and the choice of the particular frequency'range to accentuate may be preselected by a variation of the circuit elements involved.

Although anyvappropriate amplier tubes may be used, the following American type tubes are suitable:

V1 The triode section of a type 85 tube V2 A type 85 Va A type 77 V4 andV Vs-- type 42 tubes connected as triodes.

Although the resistors and condensers shown may have any appropriate characteristics, the following values are given for the purpose of illustration;

The alternative system of Fig. 5 includes an amplifier for amplifying.the high and low frequency signals to a greater extent than the mid dleregister signals whenever the output volume is reduced. In this circuit, where circuit elements having functions corresponding to those of similar-elements in Fig. 4 are similarly designated, the input terminals I--2 are connected across the voltage divider P1, the variable contact of which serves to impress any desired portion of the input voltage across the input of the tube V1. The output of the tube V1 is connected through an appropriate network including the condenser C1 to the control grid of the amplifier tube V4. The output of the tube V4 is connected through the transformer T1 to the inputsof the push-pull connected amplifier tubes V5, Vs. The

to the primary oftransformer T2, the secondary of which is connected to the output terminals 3 4 across whicha loud speaker may be con 5 nected.

A voltage divider P2 is connected across onehalf of the primary of the output transformer T2. The variable contactof voltage divider P2 is iconnectecl through condenser C3 to the diode plates vof the tube Va. A resistor R1 is connected between the cathode of the tube V3v and the 'diode plates. 'I'he screen-grid of the tube V1 is 'connected through a networkv comprising the parallel impedances L, R, and C tothe cathode of 15 the tube Va. Appropriate biasing resistors, coupling resistors and voltage supply sources are provided as indicated. Specific details of these elements are not essential to the operation of the invention. For instance, any appropriate volt- 20 age source may be provided.

In operation, whenever the voltage of the output at the terminals 3 is high, a high voltage is impressed upon the diode plates of the tube V3 vresulting in a considerable direct-current volt- 25 age being built up across the resistor R1. 'I'his causes the grid of tube V3 to have a high negative bias so that this tube acts as ahigh resistance inthe screen circuit of the tube V1. If the resistance of the tube V3 is large compared with 30 the anti-resonant impedance of the circuit L R-C, then the amplifier will have Aa fiat characteristic. For low level signals, the resistance of the tube V3 is relativelysmall and therefore the gain-. -vs.-frequency characteristic curve of the tube V1 will have adip at theresonant frequency of L-C. The characteristics of L- RC are chosen to cause the combination to be broadly resonant at the middle register by means vof which the amplification of the amplifier at middle register frequencies may be greatly reduced at low output volumes. The circuit L--R-C may be replaced by a more complicated network to produce any desired shape of response curve.

Attention is now invited to Fig. 6, which is a circuit diagram of an arrangement related to the block diagram of Fig. 3. The signal voltage is applied to the input terminals i and 2, across which is connected a vvoltagedivider P1. The portion of the input voltage appearing between terminal 2 and the variable tap on the voltage divider is applied to the control grid of the high gain' amplifier tube V1. tion of the action of the filter circuit L-Ca-Cs for the moment, it will beseen that amplied 55 signal voltage appears across the coupling reslstor R9 and is transferredl through the coupding condenser C1 to the grid yoi the amplifier tube V4; After Abeing further amplified by tube V4, the signal voltage is applied through the push- 70 is applied through the coupling condenser C3 to the diode plates of tube V3. Resistance R1 serves as a load resistance for the diode circuit of V3. 'I'he rectified current ows throughRi resulting in a pulsating direct-current voltage appearing 75 across R1. The alternating current components Neglecting considerameans of the filter circuitR'r-e-C'z-Ra'andfthe resulting direct-current voltage ap'pliedas-"ay bias voltage to the grid'oi tube V21.-y Tube vV3 `'acts as a variable resistance rconnected in the coirifv mon ground lead of-the low-'pass filter network Direct-current plate `vvrpotential@@iisV supplied to V3 through theimpedanceZ-,Ywhich -of Vthis voltage `are subtantially removed .byl

has a high impedance to all frequencies' lvin the useful range and playsl no partir! lthe'action of the device except to prevent signal currents from passing directly from the plate of tube Va to ground through the plate voltage source, in this case indicated by a battery for the sake of simplicity.

The action of the control circuit is as follows:

When the signal level across the primary of T2 is high, a relatively high negative bias voltage is applied to the grid of V3, which causes V3 to have a very high resistance to signal currents. Consequently the filter circuit Cs--Ir-Cs has a `negligible effect on the transmission properties amplifier is altered so that low frequencies are amplified to a greater `extent than high frequencies.

In each of the circuits shown, the resistors R7 and Ra and the condenser C7 comprise a filter to prevent the alternating current components from reaching the grid of the control tube and Y to introduce a time constant to average the controlling voltage overv a desired, time interval.

In-the example given the time constant is 1A; second. This time constant is not critical, but should be small enough to enable the bias to follow the normal variations in loudness of a musical passage, and large enough so that no noticeable controlling action appearsduring the duration of a single normal musical note.

Fig. 7 illustrates lthe output-vs.`-frequency characteristic of an ampliiier constructed in accordance with the circuit arrangement of Fig. 4. The curves A and B represent, respectively, the output-vs ;-frequency characteristic at various audio frequencies for low and high output levels, respectively. The ordinates represent the output voltage .at-the various frequencies with reference to the output at 400 cycles. It can be seen that with low output voltage the low frequencies between 64 and 128 cycles predominateby from about 300 to 800% relative to the frequencies of more than 500 cycles. Therefore as the volume of `the output is reduced by any means whatever, the low-frequency amplification.

becomes relatively greater. Thus with this arrangement, the response at. the low frequencies is maintained atva suficiently high level to remain above the threshold value of the average ear in order that the low frequencies will not seem to fade out as the output level is'reduced.

Whereas this invention has been described for the purpose of illustration with reference to the three specific examples given above, it is to be understood that any appropriate circuit in which the shape of the outputvs.frequency characteristic curve is controlled by a voltage derived from the rectification of all or part of the audio-frequency components of the signal being amplified may be used to accomplish the objects of this invention.

What I claim:

1. In a multi-stage vacuum tube amplifier system adapted to amplify signal voltages within a wide frequency band, a path including main amplifier means adapted to amplify voltages of all of the frequencies of said band and a second. path connected in circuit with at least a portion of said first path and including supplementary "audio-frequency amplification which includes` amplifier means adapted to amplify voltages of only a portion of the frequencies within said band, means for combining the voltages so amplifled, and means for automatically varying the amplifying properties of the supplementary amplier means in accordance with the amplitude of signal voltages.

2. In a multi-stage vacuum tube amplifier System adapted to amplify signal voltages within a wide frequency band, a .path including main amplifier means adapted to amplify voltages of all of the frequencies of said band and a second path connected in circuit with at least a portion of said rst path and including supplementary amplifier means adapted to amplify voltages of only the lower frequencies within said band, means for combining the voltages so amplified, and means for automatically varying the amplifying properties of the supplementary amplifier means in accordance with the amplitude of signal voltages.

3. In a multi-stage vacuum tube amplifier system adapted to amplify signal voltages within a wide band of frequencies, a path including main amplifier means adapted to amplify voltages of all of the frequencies of said band, a second path connected in circuit with at least a portion of said first path and including supplementary amplier means adapted to amplify only voltages within a narrow frequency range in said wide band of frequencies, a restifier associated with said system to develop a direct-current potential from signal current supplied thereto, and connections for applying said direct-current potential to a control element of said supplementaryamplifler means to control automatically the amplification of said supplementary 'amplifier means.

4. In an amplifier system, the method of audiofrequency amplification which includes auto.

substantially uniformly amplifying voltages of all of the frequencies of a wide band, simultaneously supplementally amplifying the voltages of the lower frequencies of said band and automatically increasing said supplemental amplication of said low-frequency voltages as the integrated audio-frequency voltage level at a given point in the amplifier decreases.

' 6. In an amplier system for amplifying slmultaneously voltages representative of a broad band of audio-frequencies, said voltages being subject to variations in amplitude, an input circuit, an amplider coupled to said input circuit,

an output circuit, means for integrating the voltage level at a given point in the amplifier system with discrimination against a portion of the audio-frequency band, means for averaging band in accordance with the variations of said integrated and averaged voltage level.

7. In an amplifier system for amplifying simultaneously voltages representative of a broad band of audio-frequencies, said voltages being subject to variations in amplitude, an input circuit, an amplier coupled to said input circuit, an output circuit,` means for integrating the voltage level at a given point in the amplifier with discrimination against a portion of the audio-frequency band, means for averaging said integrated voltage level over a preselected interval of time, and means for automatically increasing the ratio of the amplication of lowfrequency voltages to the amplification of voltages of higher frequency as the said integrated and averaged voltage level decreases.

8. In an amplifier system for amplifying simultaneously voltages representative of a broad band of audio-frequencies, said voltages being subject to variations in amplitude, an input circuit, an amplifier coupled to said input circuit, a frequency-selective network incorporated in said system, a variable-resistance device controlling the action of said frequency-selective network, an output circuit, means for integrating the voltage level at a given point in the amplier vsystem with discrimination against a portion of the 4audio-frequency band, means for averaging said integrated voltage level over a preselected interval of time, and means for automatically varying the resistance of said variable-resistance device in accordance with the variation of said integrated and averaged voltage level.

9. In an amplier system for amplifying simultaneously voltages of varying amplitudes representative of a broad band of audio-frequencies, an input circuit, an amplier, an output circuit, a. supplementary amplier system connected in parallel with at least a portion of said amplifier, a frequency-selective network incorporated in said supplementary amplifier system, means for integrating the voltage level at a point of said amplifier with discrimination against a. portion of the audio-frequency band, means for averaging said integrated voltage level over a preselected interval of time, and means for varying the amplication of the supplementary amplifier in accordance with the variation. of said integrated and averaged voltage level.

10. In an amplifier system for amplifying simultaneously-voltages representative of a broad band of audio frequencies, said voltages being subject to variations in amplitude, an input circuit, an amplifier coupled to said input circuit, an. output circuit, means for integrating the voltage level at a point of the amplier, comprising a rectifier, a coupling between said p'oint of said amplifier and said rectiiler, said coupling including a frequency-selective network, and a load resistance for said rectiier across which a controlling voltage is developed; and means for averaging over a preselected time interval the integrated voltage developed across said rectier load resistance, comprising a coupling circuit betweeny the high-potential end of said load resistance and the point at which said contro series resistance together with shunt capacitance, said resistance and said capacitance being so proportioned that the time constant of said coupling circuit is small enough to enable the controlling voltage to vary substantially 'as the amplitude envelope of a normal musical passage and large enough so that the controlling voltage will not vary substantially during a single .musical note.

1l. .in an audio-frequency amplifier system including an input and an output, the method of audio-frequency amplication which includes substantially uniformly amplifying voltages representative of the entire audio-frequency band in one path, in a separate path amplifying voltages representative of the .low-frequency portion of the band, and increasing said last mentioned amplincation as the-level of the voltage in said output is decreased.

12. In an amplier system for amplifying voltages of varying amplitudes within a broad band of frequencies, an input amplifier, an output amplier. a path coupling said input amplier `to said output amplifier, a separate path coupling said input amplifier to said output amplifier and including a supplementary amplifier, and a nlter whereby only a predetermined part of the frequencies of said band are impressed upon said supplementary amplifier, and means for automatically controlling the amplification in said supplementary amplifier in accordance with' the amplitude of the amplified voltage in the output of said output amplifier.

13. In an amplifier system for amplifying voltages covering a broad band `of frequencies and subject to variation in voltage level, an input, an amplifier tube connected to said input, a supplementary amplifier, a coupling separately connecting said input andsaid supplementary amplifier, said coupling including means for discriminating against voltages of certainfrequencies in said band of frequencies, individual output circuits coupled to said amplifiers, means for controlling the amplification of said .supplementary ampliiier in accordance with the amplitude of the signal voltages, and means introducing a time constant into the action of said amplification controlling means.

14. 'In an amplier system for amplifying Voltaojaavo's voltage is applied, said coupling circuit including agesv covering a broad band` of frequencies and subject to variation in voltage level, an input circuit, an output circuit, two parallel amplifier circuitscoupled betweensaid input and output circuitsa network included in one of saidv amplifier circuits for discriminating against voltages of certain frequencies in said band of frequencies, and a coupling between the output circuit and said last-mentioned amplifier circuit for'accent- -uating therein the amplification of voltages of 16. An amplifier system for amplifying signal voltages within a Wide frequency band, a main path designed substantially uniformly to pass the frequencies of said band, a separate path electrically connected in. circuit with at least a portion of the main path and designed to pass only a portion of the frequencies of said band, and means for automatically controlling the passage of frequencies through said separate path according to the amplitude of said signal voltages.

17. In a multi-stage vacuum tube amplifier system, a path including amplier means designed substantially uniformly to amplify signal voltagesof all frequencies within a wide band, a separate path electrically connected in circuit with at least a part of the iirstsaid path, including ampliner means adapted to amplify voltages of frequencies within a narrow range included substantially in said wide -band, and means for automatically varying the amplification of said second amplifier means according to the amplitude of said signal voltages.

18. In an amplifier system, the method of amplifying voltages of frequencies within' a wide band. which includes simultaneously and automatically augmenting the amplification of lowfrequency voltages as the amplitude of the signal decreases and maintaining the amplification of higher frequency voltages substantially constant.

NELSON PERRY CASE.

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