US2093248A - Frequency discriminating electric network - Google Patents

Description

Sept. 14, 1937. H. L. OURA 2,093,248

FREQUENCY DISCRIMINATING ELECTRIC NETWORK Filed Sept. 50, 1953 I WVf/VTOB 195g: 70 L. 0054 5y 00m Patented Sept. 14, 1937 PATENT FREQUENCY DISCRIMINATING ELECTRIC NETWORK Hector Leslie Oura, Hillingdon, England, assignor to Electric and Musical Industries Limited,

Middlesex, Britain England,

a company of Great Application September 30, 1933, Serial No. 691,599 In Great Britain October 3, 1932 8 Claims.

The present invention relates to frequency discriminating electrical networks, and more particularly but not exclusively to networks of the type employed in audio-frequency amplifying arrangements and the like for purposes of tone control. Networks of this type usually comprise one or more variable elements, whereby the frequency discriminating property of the network may be adjusted.

Audio-frequency tone controls have numerous applications in modern wireless receivers; they are employed, for example, for correcting for side band attenuation taking place in the radiofrequency circuits of the receiver, for compensating for imperfections in the frequency response of the loudspeaker, and for other purposes.

It is an object of the present invention-to provide frequency discriminating electrical networks of the type referred to which are simple to construct, and can readily be embodied in an audiofrequency amplifying arrangement.

. It is a further object of the invention to provide a frequency discriminating electrical network which may be adjusted to have any one, of a number of frequency characteristics within limits at which low frequencies are favoured at the expense of high frequencies, or high frequencies are favoured at the expense of low frequencies.

The invention will be described by way of example with reference to the accompanying drawing, in which Figures 1 and 2 show diagrammatically two forms of network according to the present invention and Fig. 3 shows diagrammatically a form of uni-control operating means embodying the invention Referring to Fig. 1, the primary Winding l of a transformer 2 is tapped at a point 3, and a variable resistance 4 is connected in parallel with one of the two portions so formed. A variable condenser 5 is connected in parallel with the secondary winding 6 of the transformer 2. The resistance 4 and the condenser 5 are preferably ganged together so that they may be operated by one control member as indicated by the dotted line shown, and the arrangement is made such 1 that, starting from a midposition, rotation of the control member in one direction, for example, causes variation only in thevalue of the resistance 4, while rotation thereof in the opposite direction causes variation in the capacity of the condenser 5, but does not vary the value of the resistance 4.

This may be accomplished, for example, by suitably shaping the vanes of the condenser 5,

and by arranging that when the movable contact of the resistance 1 reaches the position in which the resistance in circuit is a maximum, the movable contact then tracks over an element of good conductivity connected in series with the resistance element, so that when the movable contact passes the position of maximum resistance, substantially no further change of resistance occurs. It is further arranged that while the movable contact tracks over the highly conducting element, the capacity of the condenser 5 is varied, the capacity remaining constant at a minimum value, which may be substantially Zero, while the movable contact tracks over the resistance element itself. Such an arrangement is shown in Fig. 3 in which the movable arm H is shown at one end of resistor l, the movable plate IU of condenser 5 being secured to the same operating means, shown as a shaft l2, to which arm II is secured. l3 denotes the fixed condenser plate or plates. It will be seen that rotation of shaft I? to the left causes arm I l to slide over the conductive member it While the capacity of con denser 5 increases to its maximum value. Upon rotation of shaft l2 in the opposite direction,

however, the capacity of the condenser remains 2 substantially the same while the value of resistor 4 is decreased.

Alternatively, it may be arranged that when the value of the resistance 4 reaches a maximum, the resistance element itself is open-circuited; variation of the capacity of the condenser 5 then takes place only while the resistance element is open circuited, and this capacity is arranged to remain constant at a minimum value, such as substantially zero for example, while variation in the value of the resistance 4 is taking place. This arrangement may be had by simply omitting the track or conductive member M of Fig. 3. Methods other than those described of obtaining independent variation of the resistance 4 and the capacity of condenser 5 by means of one control member will be apparent to those skilled in the art.

The arrangement illustrated in Fig. 2 is a modification of that shown in Fig. l. The variable condenser 5 which in Fig. 1 is connected in parallel with the secondary winding 5 of the transformer 2 is replaced in Fig. 2 by a fixed condenser l in series with a variable resistance 8.

Adjusting means associated with resistance 4 are l preferably ganged to similar means associated with resistance 8, one control member being provided as indicated by the dotted line shown whereby simultaneous variation of both resistances may be obtained. The arrangement is I by-passing action of the condenser I.

preferably such that as the resistance 4 is increased, the resistance 8 is decreased, and vice versa. V

The network illustrated in Fig. 2 is provided with two input terminals 9 and 9. Connected in series between the terminal 9' and the upper end of the winding I is a resonant filter circuit comprising an inductance coil ill in parallel with a condenser II, this circuit being tuned to a frequency against whichitis desired that the net- 7 work should discriminate. When the network is associated with a low'frequency amplifier, the

circuit In, H may serve to provide a sharp high frequency cut-off, the amplifier and other apparatus with which it is associated being arranged to have little response above the required cut-off the secondary winding 6 of the transformer 2 tend'to be reduced in intensity on account of the As the valueof resistance 8 is increased, this action is reduced, and the value of the resistance I is also reduced. The latterresistance and a portion of the winding I constitute an inductive circuit.

As the resistance 4 is decreased, the impedance'of this inductive circuit is decreased, and the effective inductance of the primary winding 1 is also decreased. The transformer accordingly becomes less efficient in the transmission of the lower frequencies, which thus tend to be suppressed. I

It will be seen that by varying simultaneously the resistances d and 8, the frequency response of the whole network may be varied progressively from a response which falls from a maximum at the lower frequencies, to one which rises to a maximum atthe higher frequencies.

It is to be noted that the frequency discriminating networks described above are given by way of illustration only, and many variations within the scope of this invention, as defined in the appended claims, will readily occur to those skilled in the art; for example, in place of the transformer employed in the arrangements described above, an auto-transformer or like device may be employed. Furthermore, the networks according to this invention are not limited in their application to providing tone-control in broadcast receivers, but may find many other uses in the electroacoustic field.

I claim:

1. An electrical network for transmitting a wide band of audio frequency currents comprising the combination of a transformer having a primary winding and a secondary winding, a

variable resistor connected between an intermediate point and one end of one of said windings, a circuit having a capacitative reactance connected in shunt across the other of said windings, a direct connection between the low potential ends of said windings whereby said ends are maintained at the same potential, and a single adjusting means arranged to varythe impedance of said circuit and the value of said resistor.

2. Apparatus constituting a frequency discriminating electrical networkjcomprising an audio single adjusting-means for varying both the value of said resistor, and the capacity of said condenser. V

3. Apparatus constituting a frequency discriminating. electrical network, comprising a transformer having a primary winding and a second ary winding, one of said windings being divided by means of a tapping. into two portions, a vari able resistance element connected in parallel with one of said portions and constituting therewith a circuit having an inductive reactance, a circuit having acapacitative reactance and comprising a condenser in series with a variable resistance element connected in parallelwith the other-of said windings, and a uni-control adjusting means for varying the resistance of both said elements.

4. Apparatus constituting an audio frequency discriminating electrical network,zcomprising an 5. Apparatus constituting an 'audiof frequency.

discriminating electricalgnetwork, comprising an audio frequency transformer having a primary winding and a secondary winding, a circuit having an inductivereactance and comprisingavariable resistance element associated withoneg'of said windings, a circuitincluding aJVariabIewrevided into two portions, a variable resistance element connected in parallel with one of said portions and constituting therewith a circuit having an inductive reactance, a variable condenser connected in parallel with the other of said-windings, adjusting means for varying the resistance of said element and the capacity of said condenser, and coupled to said adjusting means an operating member for varying,over a part'oi its range of movement,the resistance of said element and, over another part of said range, the capacity of said condenser.

7. Apparatus constituting a frequency discriminating electrical network, comprising an input terminal and an output terminal, a transwith one of said portions and constituting there- 7:5,

former having a, primary wi d a ailysecdnm ;,7 0

with a circuit having an inductive reactance, a circuit having a capacitative reactance connected in parallel with the other of said windings, an inductance coil in parallel with a condenser connected effectively in series between said input terminal and said output terminal, and a single adjusting means for varying the impedance of both said reactive circuits consecutively.

8. Apparatus constituting a frequency discriminating electrical network, comprising an audio frequency transformer having a primary winding and a secondary winding, one of said windings being divided into two portions, a variable resistance element having its ends conductively connected in parallel with one of said portions and constituting therewith a circuit having an inductive reactance, a variable condenser connected in parallel with the other of said windings, and a uni-control adjusting means for varying both the value of said resistance and the capacity of said condenser.

HECTOR LESLIE OURA.

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