US2612558A - Crossover filter network - Google Patents

Description

Sept. 30,-1952 p. w. KLIPSCH 2,612,558

' CROSSOVER FILTER NETWORK Filed Aug. 13, 1946 Y I )2 L, 2c a PA UL M KI-IPSCH INVENTOR a, KM

ATTORNEY Patented Sept. 30, 1 952 "fflzmlasss a v Paul W., Klipsch, Hope, Ark.

A pneatiomn eusma, is egs ri i; 1.teases l sprains. (c1. ns-+44) This invention ;-relates -to electric 1 filters and particularly to crossover filter networks used to allocate ,power in different frequency ranges to differentiloads,especially to two or more loud speakers.

.An "object is. to provide a "simplified crossover network of 'lowpcost but with frequency response performance fcomparable with one using costly high-fidelity components.

Another "object'is to provide a high fidelity crossover network using fewer and less expensive parts than :generally required for ---the"--same quality.

A'fui'ther object is to provide a crossovermetwork-operable at a high impedance level where capacitor sizes aresmall and hence inexpensive.

A still further object is-to limit -overall---distortied-(both harmonic and intermodu'lation) -of-the combined network and transformers to -"even lower levels than have-been possible with conventional designs. a W

Another object isto provide a network in which the; transformers .becQmea-part, twherehy each -of saidtransfo'rmersican be designed for minimal distortion both from the. standpoint of smoothness of frequency response as well as harmonic and intermodulation of [the transmitted wave forms. I

The foregoing together with other objects. and advantages will become Lapparent, vfrom the following description considered in connection with the accompanying drawings-inwhich: I

, Fig. ;1 isa schematic circuit diagram. showing one form 'of the invention which has proved highly satisfactory; 1 y

, IFig. 2 3 shows .another ,form, .,of .,the .inventioil adaptable-to .sing' leended ampl Fi .3 is a f qu nc response icu'r i on .a network,designedjjbr.A00}:

' pass portion is not necessary as most, gwggg quency loudspeakers will cutpffsomewhere near the crossover frequenqy. ,The high-frequency speakers, when fed power'at frequencies below their. acoustic :cutofi, 1. are zvery :apt uto radiate harmonics of-sthe. received-power as indicated -by applicants zpapen f'A note onacoustic hornsfl Brocal. RsE -ivol. 33; No;-;7;pp;;447, 448, Julyiiafi;

Sin arm nic constitute one formlof ii'stor ti0I ,Qh 'i b avo ed- .Thusasteep off is provide'd in the highepass filtfi'rby the ho e of 1 sections;- Ideally, the /zesection low-'p fs portion wouldjprdduce 12 decibels loss peri octave bo u f; an t il' /zesectionhigh-jpa s per-i tionwould give 27 decibels"l'oss per octave be'idw cutoff; practically, the loss is somewhat .lessi'due to'the fact that the reactive elements .v's'zilllnot exhibit zero power factor. h

Fig. his ,a circuit, based on-zthejdesign'991 313.}. but adapted forpush-pull output, thatfis to say; a balanced line. Inductor L1 is .split sofhailf .of'jit' is on each sideflofthe line. This'is the, circuit as actually used in applicant's 'develqpmental sound reproducing system.

D s n equations The numericalifiesign follows conventional practice; see for example, Radio Engineers Handbook? Te'rm an (McGraw-Hill, 1943); The impedance "level-"was chosen as 5000'ohms; the nominalloadfor a ,pair of type12 A3 tubes operated self -bias. The class A triode was chosen because of its inherently 10w distortion; *with pentode-orbeam tubes it :is'imperative :that feedback be employed to minimize distortion within the amplifier --as well .as to produce a low equivalent generator impedance to preventdistortion from being produced by thespeakers- The cutoff frequency f was'chosen1as"500 cycles The half sec tiqn loWrDaSs portion irequires :eleinents 5000 arf The isolated half-section causes a nominal loss of 12 decibels per octave-with the interce-ptnf the zero loss line andthe asymptote of .12 decibels per octave slope occurring at v1,.4lqtimes the nominal cutoff frequency. This intercept maybe moved down to the cutoff frequency by doubling L, C, or the LC product. g

In the conventional half-section, the capacitance would be Ck'. Thiswas doubled, to make 01:0 so as to give 3 decibels lossatthe nominal cutoff frequency and the desired slOD8Tbeyonfd' cutoff. Henceyin the figuresciistwicethe coniputed-value, so that in 2, Li'" 'L;{= .61 henrys, C1=Ck=0:13 micro'farad, and"in1 fig 1, L1'=" /2Lk=1;6 henrys reckoned with 'thej2 j' ings connected series aiding'and 201" 2 51 which "i Sw s *gi es the e valenti .l This arbitrary change does not appear we have 3 caused any undue variation in the input impedonce.

The high-pass portion is calculated from Lk=5000/41rf=0.8 henry Ck=1/(5000X41rf) =0.032 M.

In Fig. l, the inductance L1 should be wound with the two windings closely coupled to enforce balance; it has been found satisfactoryto put one winding on top of the other. -Both L1 and L2 should have adequate iron and copper to permit opening up a large air gap to hold the inductancesconstant at all levels of A.-C. voltage under which they willj be, required to operate. The matching transformer Tz'can be .any readily available transformer of proper ratio which will give the required 2 henrysfiriductance when the air ap "is opened up sufficiently to hold the inductance nearly constant over all levels of excl-- tation.

frran f 'rmeii'ri' should h ve a high assay inductance, of the order of. at least 100 henrysto keep the exciting current low at the lowest frequencies; to be transmitted, but its leakage inductance need not be held as low as demanded. byexpensive high-fidelity equipment; the leakag'einducta nce as measuredsat the primary can.

be as high as one henry. e Transformer T2 must have a low'leakage, preferable less than 0.05henry to hold the loss at kilocycles to less than 3 decibels. It may be a.

lowcost unit, however, since its primary induct-- ance. need be only 1.6.henry after opening-the:

airgap, say 100 henrys before adding said air gap.

Formulation of transformer requirements Let a.

Lp1=primary inductance of former T1 L 2=primary inductance of high frequency transformer T2 r Li=leakage inductance of T1 Lzzileakage inductance-of. T2 v v l R =plate impedance of the driving amplifier Rz.=load impedance at the plate side of the transformers w=21rf where f1=lowest frequency to be transmitted; fz highest frequency to be transmitted; fs=crossover frequency D=the per-unit distortion;

it an be: shown that:

low frequency trans- If we put distortion at /2% or D=0.005, F1=30 or wig- 187; and f2=14,000, w2=38,000, the values would be Lb=710 and L=0.04, a ratio of over 17.,000." tQ one, That would be difficult if not impossible of achievement in the present state of the art. .By using 2 transformers, and :a crossoveriof say 500 cycles, tum=3140, then the ratios of primary to leakage inductance become of the order oflqomli which is readily attainable even in lb il e i n e l l y he 2 transformers will not only result in less distortion, and at least as good a frequency response, but also cost less than a single transformer of the high fidelity class.

Transformers T1 and T2 should have the proper turns ratios to match the 5000-ohm filters to the respective loads.

It should be understood that all the inductive elements must be designed .to carry their exciting currents without distortion, in the same way that any output transformers should be so designed. The air gaps in the inductors should be large so that harmonics of the exciting voltage contained in the exciting currents are small; the transformers should have high primary inductances to minimize the exciting current. If these precautions are not observed, intermodulationdistortion may be so high as to render the 2 speaker system less than desirable than a single speaker.

Collecting-and tabulating the constants for Fig. l glves 1 Impedance level 5000 ohms. Crossover frequency 500 cycles Inductance L1 1.6 henrysf(series I aiding)? f i Inductance T1 'l00 henrysfjmini- Inductance L2 Q 0.8 henry -'(with air i g L 'ga'p). v Inductance T Q 1.6 henrys.(with air sap Capacitance -Q1 0.12s microfara'd or 201"; 0.256 microfarad Capacitance; C:; .0.064 microjfaradl or ;4Ca 0 128micro'farad Leakagednd. of T1 ,Not over 0.8 henry Leakage ind. of T2 Notover 0.05 henry Tolerances of 15% 'or'even 10%" may'be'p'ermitted. i a Conversionpf the constants given for other impedance levelsand/or other crossover frequencies is well'known and willnotb e discussed.

The 'fact that a pair of 2A3 or 6 A5G tubes exhibit a plate impedance ofonly 2000 ohmswhen the load impedance and the surge impedance of the filters are 5000 ohms raises the question-of reflections; The mismatch I is not great enough, to cause seriousfreflection from a theoretical standpoint A measured performance' curve, Fig; 3, shows each'output to'be-fiat in its transmission range and tofall oif smoothly in its attentuation range;

This curve was run on a network, usingthe mutual speakers as connectedllqadsrath'er than using fixed resistors; forf such' loads. Variations invoice coil impedance oi"the' speakers, account for the minorvariations in output, in no case exceeding 4-decibelsand this extreme only at the extreme frequency limits. This curve was 101' a 400 cycle crossover frequency. I a

The aforementioned mismatch is necessary for properloading of the output tubesand if this is a; necessary evil from thefilter standpoint it is .ofsufiicientunimportance in. the present application to be disregarded. It should be mememberecl' that speaker. .voice-coil impedance is itself highly variable; l I

Rerjormance iThis'crossover network hasbeen in experimen; tal .useywith a speakercombination. comprising a, low-frequency horn;as described in applicant s papenjf-Alow frequency horn of small: .dimen;

sions, Jour. Acous. Soc. Am., vol. 13, No. 2, pp. 137-144, October 1941. See also U. S. Patents 2,310,243 and 2,373,692. This speaker was originally designed to operate between 40' and 400 cycles and a high-frequency horn operating from 400 cycles up. The high frequency horn was described in applicants paper A high quality loudspeaker of small dimensions, Jour. Acous. Soc. Am., vol. 17, No. 3, pp. 254-258 January 1946.

Two types of low-frequency horns and three types of high-frequency horns, all of different acoustic length, have been tried in various combinations. The output appears smooth throughout the transmission range including that in the immediate vicinity of crossover. Phase relations at this point seem to have little or no bearing on performance. Even complete 180 degree reversal of one speaker (in any of the several combinations) makes no difference in performance. Thus phase shifts in the described crossover network or in the speakers themselves may be neglected. Of course, if two identical speakers are used on one channel, they should be properly phased to prevent peculiar radiation patterns.

An obvious advantage of this crossover network is the fact that low-cost components may be used to give the same high-fidelity performance as more expensive equipment.

A less obvious but even more important advantage is that distortion produced by the necessary coupling transformer may be drastically reduced by the use of two transformers and the permissive design of each transformer to cover less than the entire audio spectrum.

The experimental model was constructed at less cost that that of a single high-fidelity matching transformer, and the performance is such that the combined power output is fiat within 2 decibels from less than cycles to cover 10,000

cycles.

The distortion is extremely low; intermodulation between 60 and 6,000 cycle waves simultaneously applied are within tolerable listening limits up to the load limit of the amplifier.

Alternately, in some cases, a high pass portion of one section instead of 1 /2 sections, may be employed. In this case, the capacitor values are altered; in Fig. 1, the capacitor marked C2 becomes 2C2, and in- Fig. 2 the third of the three capacitors marked 2C2 is omitted by bridging the diagram at that point; also the inductance of T2 is not reduced to the indicated value in the table by opening up an air gap but is left as high as obtainable, preferably of the order of 50 henries or more to minimize the exciting current.

A network such asdescribed herein, but designed for 400 cycles crossover was described in applicant's article Woofer-Tweeter crossover network, Electronics, November 1945, and the performance curve for that same network was shown in Design of two-horn loudspeakers, Electronics, February 1946.

The invention claimed is:

1. A crossover network comprising low-pass and high-pass portions each associated with a matching transformer, said low-pass portion comprising a balanced two-winding inductor with one of said windings in each side of a balanced line, a one-half section filter with said inductor as input element and with equal series capacitors as terminating element, said one-half pillar inductor and terminated by the other of said matching transformers with low leakage inductance.

2. In a crossover network for allocating power in different frequency ranges from a power source to different loads, the combination comprising, separate loW-pass and high-pass circuits connected to the source of power to be allocated, said low-pass circuit including a pair of closely coupled windings and an output low frequency transformer connected thereto and having a high primary inductance, and said high-pass circuit comprising a high-pass filter and an output high frequency transformer connected thereto and having a low leakage inductance, said last mentioned transformer having a leakage inductance less than the ratio of impedance to the maximum angular velocity to be transmitted, and each of said transformers having a primary inductance equal to or greater than R/3wD, where R is the source impedance, (.0 is the lowest angular velocity to be transmitted by the transformer and D is the permissible per-unit fraction of harmonic distortion.

3. A crossover network to be operated between a power source and a load and comprising, lowpass and high-pass filters each terminated by a matching transformer, the two transformers exhibiting the parameter limits REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,010,131 Beers Aug. 6, 1935 2,084,160 Minton June 15, 1937 2,113,393 Bierwirth Apr. 5, 1938 2,227,384 Wiessner Dec. 31, 1940 2,235,018 Hagen Mar. 18, 1941 OTHER REFERENCES

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