US1851905A - Vacuum tube amplifying circuits - Google Patents

Description

March 29, 1932. R. s. HAYES 1,851,905

VACUUM TUBE AMPLIFYING CIRCUITS OOO CYCLES March 29, 1932.

R. S. HAYES VACUUM TUBE AMPLIFYING CIRCUITS Filed Nov. 50 1927 2 Sheets-Sheet 2 |NvENTo R Ralph zlcgyea,

v ATTORNEY Patented Mar. 29, 1932 RALPH S. HAYES, OF ARDMORE, PENNSYLVANIA VACUUM TUBE AMPLIFYING CIRcUrs Application yfiled November 30,` 1927. Serial No'. 236,804;

'and more particularly to a new and improved method of obtaining a desired non-linear .Li gain-frequency characteristic in vacuum tube amplifiers employed in certain radio receiving and phonograph pick-up circuits.

In the present and known types of radio receiving sets, it is a well recognized fact that the very high tones and the very low tones from broadcasting stations areY reproduced poorly, and this has been found to be due to theusual flat audio amplifier characteristics of such sets disclosing a marked deficiency from perfect tone production or reproduction, which is causedby certain inherent forms of frequency distortions, wherein'the higher tones are lost by reasonof radio frequency tuning, which is called side-band cutting, and of the relatively poor efficiencies of loud speakers at the higher tones; and the lower tones by reason of the intentional softening downof the bass instruments at the broadcasting stations orkstudios, of the i comparatively attenuated transmission of such tones through the amplifiers of many stations, and of the relatively poor efficiencies of loud speakers or the pick-up attachments for phonographs for-the bass tones. Although these facts have been known in the practice, no attempt has heretofore beenr made commercially kto correct or. cure both these deficencies, principally on account of the complex apparatus thought to be necessary or required for the purpose, not only in the audio but also in the radio frequency part of the system.

It is, therefore, the principal object of the invention to provide for a simple and comparatively inexpensive method and means for effectively overcoming the serious disadvantages occasioned by the present intentional softening down of the low tones -of the bass instruments in broadcasting stations or studios as aforesaid, and the unnecessary deficiencies in receiving sets to properly and perfectlyreproduce both the high and low` tones from such stations or studios.

Another object ofthe invention is to provide for a method and means as hereinbefore generally characterized, wherein the usual fiat audio frequency characteristics of radio receiving set-s will be modified into dished gain-frequency characteristics, such as will greatly improve the quality of tone reproduction and otherwise correct or compensate for the deficiencies aforementioned throughout the range of the higher and the lower frequencies, providing, in other words, a tone control circuit.

A. further object of the invention is Vto provide for an equalizing stage of audio frequency amplification to be used in connection with radio receiving circuits as and for the purposes aforesaid and also in connection with electrical pick-ups for phonographs.

In carying out the present invention, the equalizing stage of audio amplification under contemplation is constituted in a stage of doubly tuned audio frequency amplification,

having two frequencies of maximum gain, which are obtained by the parallel resonance of two tuned parallel or tandem circuits, and one frequencyof minimum gain obtained by the resonance of the smaller' inductance with the larger capacity. This double parallel resonance circuit has the unique and appropriate qualities to accomplish the desired results, in that it gives two peaks in the amplifica-tion curve,.dependent upon the parallel resonant frequencies selected therefor and a minimum point between the two peaks dependent onr the series resonant frequency. The invention includes a higher tuned circuit and a lower tuned circuit, and at one particular frequency there is al series resonant effect of minimum gain created by the inductance of the higher tuned circuit, and the `capacity of the lower tuned circuit. In the practicalapplication of the invention, a wide range of selective constants are avail'- able to give the proper parallel resonant and series resonantfrequencies.

lith the foregoing and other equally important objects, purposes and advantages in View, the invention resides in the certain new and useful` combinations and arrangements `of instrumentalities and circuits as will be hereinafter more fully described, set forth 00 in the appended claims, and illustrated in the accompanying drawings in which:

Figure 1 is a plot of the gain frequency characteristic of the equalizing amplifier stage in accordance with the exempliied Vap,- plication of the invention,

Figure 2 is a preferred circuit arrangement, and Y i Figures 3, 4, 5 and 6 are slightly modified but equivalent net worlrs or circuit arrangements.

Referring to the drawings, wherein simila'r characters of reference `designate cor- Y responding elements or parts throughout the ,several views thereof, and more particularly to Figure 2, 10`and 11 indicate two threeelectrode vacuum tubes constituting preterably the first and second stages of audio frequency ampliiica'tion of a conventional form ot radio receiving circuit. ln this particular circuit the grid 12, of the tube 10, is connect ed Vby a conductor 13, and the filament 14 thereof by aconductor 15 for the application thereto cia band of audio frequencies from a suitable source (not shown). The plate 16, of the tube 10, is, in turn, connected by a conductor 17 through an interposed condenser 18 to the grid 19 of the tube 11. Connected to the con ductor 17 at a point between the condenser 18 and the 'grid 19 is one end of a resista-nce 20, from the 'opposite end of which a conductor 21 leads to and connects -with the ilainent. 22 of the tube 11. The resultant amplified frequencies from the tubes 10 and 11 are available from the plate 28, of the tube 11, through the conductor 24 leading from the plate 23.

To create the two peaks in the impedance or audio amplification curve, as aforesaid, the invention contemplates the `provision of two parallel resonant circuits, designated f1 and f2, coupled in tandem lor `in Vseries parallel, as shown in Figure 2. Une side of the circuit f1 is coupled by a conductor 25 to the conductor 17, between the Vplate 16,'of thetube 10 and the condenser 18, -and Vthis circuitjinclu'des an. inductance L1, a resistance RL and a condenser C1, all connectedin series in a local circuit.Y The conductor 25 connects the circuit `f1 at 1a point betweenithe condenser C1 and the inductance L1. The'opposite side oi the circuit f1, between the resistance Rl and the condenser C1 is connected by a conductor 26 to one side of the second circuit f2,

5 which consists of the inductance L2, `the re- `from a point between the resistance R2 and Athe condenser C2. R1 yand R2 representfthe necessary coilresistance in the coupledcin cuits f1 and f2. The conductor 27 is lead to land is connected with one terminalfof a 'resistance R3, which has its other terminal Veach end Vof this band 25 or more decibels to a maximum at -about Ll() cycles, and at 5000 cycles, two .peaksare desired or necessary in the impedance Yfrequency curve, and these a-re produced by 'the two circuits f1 and ,t2 coupled in tandem or series parallel substantially as shown. Further, the 5000 cycle tuned circuit f1 should have a ratio of L VU not too large nor too small, since the sharpness of resonance 'varies at this ratio, also, the minimum impedance of the two circuits f1 and fzshould be between 200 and 7 00 cycles, since the frequency of this minimum inipedance lis'de'termined by the resonant point of the impedance L1 ofV theOOO cycle parallel circuit f1 with the capacityCg of the L10 cycle circuit, which capacity G2 should preterably he between .2 mi". and .4l mi?. AHaving determinedfthe valueof C2, the value ot L2 /canrbedetermin'ed frointhe fact that L2 and C2' `must Vbe resonant at approximately 40 cycles. Finally,this'resistance R3, in the circuit f", which is connected in series with the two reson'antcircuits f1 and f2, serves to 'fiatten `out fthe resonant minimum gain ot the stage. The resistance in the inductances R1 fand fRz of the 4two parallel-'circuits f1 and f2, as wouldbe usedvfor such an equalizing'stage, "as hereincontemplated, have been found to have a negligible ieffect, and taking into 'account' rather large resistancesas compared to computations neglecting such resistances, the

following tendencies have been noted (a) that there is a. slight reduction in the maximum gain, and (b) a comparatively slight increase infminimuin gain.

As aneXample of the 'practical application of the invention, after havingdetermined the gain of an 'amplifier'stage to be approximately:

p, Z Voltage amplification ZH! rp ywhere u: tube amplificationv constant; Z:

impedance of the output circuit;

and r.=plate resistance oi the tube. it was evident that the gain of a stage could be de- 'signedY to` vary with :frequency :by suitably choosing the output impedance Z, since u and rplareconstants ofthe tube, and inria high mu ftype'fofftube, asl an linstanceytliese con stants are approximately 20 and 40000, respectively. Now, by trial computations, it was found that L1 should be from .25 to .50 henry for proper results, but, for present purposes, L1, in the circuit f1 (Figure 2) is indicated at a value of .25 henry, and Cl at .00404 mf., tuning to 5000 cyc1es,'wl1ile in the circuit f2, L2 has been taken at a value of 82 henries, for convenience, and C2 in three steps having values of .2; .3 and .4 mf. The

resistance R3, in series with the two circuits f1 and f2, and serving to Hatten out the resonant minimum gain of the stage, has been indicated at a value of 2100 ohms in order to show a minimum gain of zero.

The values of the several condensers C, impedances Z and the resistance R; the frequencies, both resonant and anti-resonant, obtaining in the circuits fland f2,

in circuits f2 are to be computed as follows, by way of example 1. Coupling resistance (Ra) for zero gain R3 Ohms 2. Resonant .frequencies for 40 cycle circuit (a) Assuming 02:,.2

' (b) Assuming 02:.3

fummo# 62808203 (c) Assuming 02:.4

3. Resonant frequencies of L1 with C2 38 cycles 32 cycles 28 cycles (a) Assuming 02:.2

fuman# v(S28/.250.2

(b) Assuming U2=.3

=710 cycles (c) Assuming 02:.4

1000 y f f3 500 cycles 4. The impedance Z1 of a'parallel resonant and of thel gains with the condensers of different rating` 35' circuit is computed .by the usual formula for the impedance of anr inductanc'e in parallel with a capacity (neglecting coil resistance), 1. er v Suchcalculations result in the following impedance values for the various tuned circuits at various frequencies, where- Z1=impedance of 5000 cycle tuned circuit, ZM: impedance of 40 cycle tuned circuit with 5. The voltage amplification and the equivalent gain in decibels are computed from the relations Voltage arnplification=gZ y f Decibels 2O log Zrp.

Using the previously mentioned value of R3, 2100V ohms, and mu equals 20, there results the following decibels gain for` the entire amplifying stage for the three values of()2 In the use yof this equalizing stage, the following agreeable impressions obtain, (a) an effectivcsharpening of the radio frequency tuning at'tlie cut-off frequencies, which gives Y the effect, of band-pass radio frequency tun-v .-i.

ing in a simpler manner; (b) the bass fundamentals are reproduced with an appreciable increase of the naturalness in reception; (c) a natural sparkle and brilliancy tothe program due to the improved reproduction of the higher harmonics; (d) in speech reception, thesounds Z, S, F, TH and V are reproduced with clearness and better understanding; (e) a feeling is experienced of volume without loudness,-and (f) an apparent inc crease in the broadcasting station or studio reverberation, which `gives somewhat of a binaural eHect to the reproduction. ,c

It is to be noted that the equalizing stage, as designed, assumes a fiat characteristic from the broadcasting station, which, however, is not always the case, but, if the low tones are only partially transmitted, the using ot' the lower Values oi C2 is of considerable benelit. Again, if there is an vover emphasis of the higher tones from the broadcasting station, as sometimes may be the case, then the 5000 cycle circuit f1 of the equalizingstage should be'partially shunted by a resistance.

Aln, fact, bya proper selection of the constants of the circuits, by partially shunting the circuits with resistances to dampen their effect, any desired Variation of the dished gain frequency characteristicv may be obtained.

lt is to be further noted that full beneiit from this equalizer stage cannot be obtained without sutlicient power handling capacity in the final power amplifier and in the loud speaker. y

In Figures 4, and 6, are shown net works or circuits coupled in series parallel or in tandem, each olf which net works have` been found to be substantially equivalent to the two parallel resonant circuits f1 and f2, as illustrated .in Figures 2 and 3.

In Figure 4,'the lead 256 connects a conductor extending between one terminal of an impedance coil L5, of a by-pass condenser C5, in the circuit f6, and an impedance coil L6, in the circuit f7, with its point of connection lbeing preferably between the terminal of the condenser C5 and that of the impedance coil The other terminals of the impedance coil 'L5 and the condenser C5 being connected one to the other by a conductor, which, in turn, is, at an intermediate point, connected to one terminal of ai condenser C6, inthe circuit f7 denser C6 being, in turn, connected by a conductor tothe other` terminal of the impedance i conductor vforming one side ot the circuit f, the latter conductor being extended between and connected to'one terminal of a condenser` CS and tot an impedance coil LS. The other terminal of the impedance coil L7 is connected to one terminal of a condenser Cain the circuit 74S, while the remaining` terminal of the latter condenser@7 is, in turn,

i connected tofan intermediate pointon a conduction extendingbetween the remaining ter- Tlie remaining` terminal of the latter con-V pedance coil L10, of the circuit 7m. The other terminal of the condenser C9, orp lthe circuit f1", is connected by a conductor to one ter# minal ot an impedance coil L9, which has its other terminal connected to an intermediate point on a conductor extending between the remaining terminals of the condenser C10 and `the impedance coil Lm, of the circuit fn.

The conductor 27d leads from plus B battery, and connects anr inte `mediate point on the conductor extending vbetween the condenser C9 and the impedance coil LD of the circuit flo.

In employing any one of these modified equalizing circuits with a receiving circuit hook-up, as described in conjunction with the first or preferred term of equalizing stage or circuit, the conductors 25a, 25?), 250 or 25d will be connected to the bus-wire or conductor 17 after the manner of the conductor 25, and the conductors 27a, 27?), V27e and 27d to the plus B battery connections of the said liookup aforesaid, after the manner ci the conductor or lead 27.

It is to be noted of these modified circuits that, after determining the proper values of the several capacities, impedances and resistances comprising the same, in accordance with the usual computations ot such Values and in the manner et the examples given in connection with the iirstinstance of the invention, the same are capable of being substituted, with equal eliiciency in use, one for the other or for the first of the similar circuits described herein. g I

Vlith'out Yfurther description, it is thought that the features and advantages of the invention will be readily apparent to those skilled in the art, and it will oi course be understood that changes in the form, proportion and minor details ot construction may be iesorted'to, without departing from the spirit of the invention or its scope as claimed.

-Iaving thus fully described'the invention,

what is claimed is:

l. In' a radio receiving system, means for equalizing the audio ainplilication of the system, comprising an auxiliary stage of audio frequency amplification formed ot circuits electrically connected together and coupled in on the output'side of the usual audio frequency amplifying means of the said system, said circuits being tuned in suchV amanner vthat peaks spaced relatively tarV apart and isa corresponding in number to that of said circuits are produced in the impedance curve of the system, which act to modify the-usual flat audio frequency characteristic of the system into a dished gain-frequency characteristie.

2. In a radio receiving system, means for equalizing the audio amplification of the system, comprising an auxiliary stage of audio frequency amplification formed of two resonant circuits electrically connected together and coupled in on the output side of the usual audio frequency amplifying means of the said system, said circuits being tuned in such a manner that two peaks spaced relatively far apart are produced in the impedance curve of the system, which act to modify the usual fiat audio frequency characteristic of the system into a dished gain-frequency characteristic.

3. In a radio receiving system, means for equalizing the audio amplification of the system, comprising an auxiliary stage of audio frequency amplification formed of two resonant circuits electricallyl connected together in series-parallel and coupled in on the output side of the usual audio frequency amplifying means of the system, said circuits being tuned in such a manner that two peaks spaced relatively far apart are produced in the impedance curve of the system, which act to modify the usual fiat audio frequency characteristic of the system into a dished gain-frequency characteristic.

4. In a radio receiving system, means for equalizing the audio frequency amplification of the system, comprising an auxiliary stage of audio frequency amplification formed of a plurality of resonant circuits electrically connected together in series-parallel and coupled in on the output side of the usual audio frequency amplifying means of the said systern. said circuits being tuned in such a manner that the interaction thereof will produce peaks at different ends of the resonance Vband in the impedance curve of the system, which modify the usual flat audio frequency characteristic of the system into a dished gainfrequency characteristic.

5. In a radio receiving system, means for equalizin the audio frequency amplification of the system` comprising an auxiliary stage of audio frequency amplification formed of a resonant circuit tuned to a high frequency and a second resonant circuit tuned to a comparatively low frequency, said circuits being electrically connected together in seriesparallel and coupled in on the output side of the usual audio frequency amplifying circuit of the said system, whereby two peaks are produced in the impedance curve of the system, which act to modify the usual flat audio frequency characteristic of the system into a dished gain-frequency characteristic.

6. In a radio receiving system, means for equalizing the audio frequency amplification of the system, comprising an auxiliary stage of audio frequency amplification formed of resonant circuits having different impedance andcapacity values, said circuits being electrically connected together in series-parallel and coupled in on the output side of the usual audio frequency amplifying circuit of the said system, and tuned in such a manner that two peaks spaced relatively far apart are produced in the impedance curve of the system which act to modify the usual flat audio frequency characteristic of the system into a. dished gain-frequency characteristic.

Signed at Philadelphia in the county vof Philadelphia and State of Pennsylvania this 21st day of November, A. D. 1927..

RALPH S.y HAYES.

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