Amplifier input circuit having a figure eight conductive pattern

Disclosed is a circuit arrangement having an amplifier with first and second input terminals which are adapted to be coupled to a playback head, wherein the first input terminal of the amplifier is coupled through a first circuit section of a conductive pattern to a first terminal of the playback head and the second input terminal of the amplifier is coupled through a second circuit section of the conductive pattern to a second terminal of the playback head. The first and second circuit sections (3, 4) are arranged to insulatively cross each other at an intersection (6) to form a figure eight pattern having two loop sections of substantially equal areas to equalize the amounts of currents which are oppositely respectively induced in the first and second loop sections by an externally generated magnetic flux.

BACKGROUND OF THE INVENTION 
The present invention relates to a noise immune printed circuit 
particularly for use in tape recorders and record players. 
Playback amplifiers used in tape recorders and record players are usually 
provided with an equalizer to emphasize lower frequency components to 
compensate for particular record-playback characteristics. The input 
circuit of the playback amplifier comprises a first circuit section which 
couples the base of a transistor to a first terminal of the playback head, 
and a second circuit section which couples the emitter of the transistor 
to the second terminal of the head. The collector of the transistor is 
coupled by way of capacitors to the equalizer. A loop is thus formed from 
the first and second circuit sections of the amplifier input circuit. In 
the presence of an external magnetic flux generated by a power supply unit 
or the like, a current will be induced by the magnetic flux in the loop 
circuit and fed as an induced noise to the amplifier. Since the frequency 
of the noise is low, the induced noise is considerably amplified by the 
equalizer. To suppress the noise the following points have usually been 
taken into consideration: 
(1) To place the input circuit and the noise generating source as far apart 
from each other; 
(2) To design a circuit pattern that minimizes the area of the loop; and 
(3) To provide a magnetic shield over the loop. 
However, difficulties have been encountered to meet these requirements 
simultaneously. 
SUMMARY OF THE INVENTION 
An object of the invention is therefore to provide a circuit arrangement 
which overcomes the disadvantages inherent in the prior art circuit 
arrangement. 
This circuit arrangement of the invention includes an amplifier with first 
and second input terminals which are adapted to be coupled to a playback 
head, wherein the first input terminal of the amplifier is coupled through 
a first circuit section of a conductive pattern to a first terminal of the 
playback head and the second input terminal of the amplifier is coupled 
through a second circuit section of the conductive pattern to a second 
terminal of the playback head. 
The invention is characterized in that the first and second circuit 
sections are arranged to insulatively cross each other at an intersection 
to form a figure eight pattern having two loop sections of substantially 
equal areas to equalize the amounts of currents oppositely respectively 
induced in the first and second loop sections by an externally generated 
magnetic flux. The induced currents are cancelled out by each other and 
the noise is suppressed significantly without employing means for 
shielding the input circuit of the amplifier. 
The invention is particularly advantageous in situations where the power 
supply unit and transformer are to be located in specified positions and 
there is no freedom with which the circuit is designed.

DETAILED DESCRIPTION 
In a schematic illustration of FIG. 1, a preamplifier 1 is formed on a 
printed circuit board, or substrate 2. For purposes of illustration, the 
preamplifier 1 is shown in an operational amplifier configuration. The 
input circuit of the preamplifier comprises a first circuit section 3 and 
a second circuit section 4. The first circuit section 3 couples the 
noninverting input of the operational amplifier 1 through a terminal A to 
the playback head 5, and the second circuit section 4 couples the ground 
circuit of the amplifier 1 through a terminal B to the head 5. The first 
and second circuit sections 3 and 4 cross each other at a point 6 to form 
a figure eight pattern so that the two sections S1 and S2 of the figure 
eight pattern have substantially equal size and are configured such that 
their centers are in close proximity to each other. Since the magnetic 
fluxes generated by a power supply, not shown, pass through the loop 
sections S1 and S2 in the same direction with substantially equal 
densities and since the loops are cross-coupled at the intersection 6, 
currents induced in the first and second circuit sections 3 and 4 are of 
opposite polarities and of substantially equal amplitude. In contrast, the 
prior art input circuit can be illustrated as shown in FIG. 2 in which the 
portion marked S is subject to the magnetic flux of the power unit which 
contributes to noise. 
In one embodiment of the invention in which the loop sections of the 
pattern are formed on the same side of the substrate, the intersecting 
conductors 3 and 4 are isolated from each other such that the first 
conductor 3 is interrupted by terminations through which the second 
conductor 4 extends and a circuit component of the input circuit, such as 
a capacitor, is coupled between the terminations. Whereas, if the loop 
sections are formed on opposite sides of the substrate there is no need to 
provide such circuit component at the intersection 6 and a greater degree 
of design freedom is allowed. 
FIG. 3 is an illustration of the detail of the preamplifier 1 of FIG. 1. 
The preamplifier 1 includes a transistor X1 having a base coupled by a 
capacitor C1 to the terminal A and an emitter coupled by a resistor R1 to 
the terminal B with a junction between the resistor R1 and terminal B 
being coupled to ground. The collector of transistor X1 is coupled by a 
resistor R2 to a DC voltage supply +B, the junction between the collector 
and resistor R2 being coupled to the base of a second transistor X2 whose 
collector is connected to a junction between capacitors C4 and C5 and 
through a resistor R4 to the voltage supply +B. The capacitors C4 and C5 
are connected in series from the collector of transistor X1 to an 
equalizer 7 having the characteristic of low frequency emphasis. The 
emitter of transistor X2 is coupled to ground by a resistor R5 which is in 
shunt with a capacitor C6. The base of transistor X1 is further coupled by 
a capacitor C3 to its emitter and by a resistor R3 to the emitter of 
transistor X2. The terminals A and B are coupled together by a capacitor 
C2 as illustrated. The input circuit of the preamplifier 1 is indicated by 
a broken-line arrow 8. 
A practical embodiment of the preamplifier 1 having an input circuit of the 
figure eight pattern is shown in FIG. 4 which will be described with 
reference to FIG. 3. Input terminals A and B are coupled via a shielded 
wire to the playback head 5 and therefore no or little noise is introduced 
into the connecting circuit therebetween. The first and second circuit 
sections 3 and 4 of the preamplifier and its associated circuits are 
formed by a printed conductive pattern on one surface of an insulative 
substrate. The first circuit section 3 extends from point A1 corresponding 
to the terminal A and advances through a portion 9 of the conductive 
pattern to one terminal of the capacitor C1 which is mounted on the other 
side of the substrate. The other terminal of capacitor C1 is connected to 
an island 10 to which the base of transistor X1 is connected. The first 
circuit section 3 terminates at the base of the transistor X1. The emitter 
of transistor X1 is provided on an island 11 which is coupled by resistor 
R1, which is also provided on the other side of the substrate, to a 
peninsula 12. The second circuit section 4 commences at the emitter of 
transistor X1 on island 11 through resistor R1 and passes through a 
portion 13 which extends over the capacitor C1 and over the capacitor C2, 
which is also provided on the other side of the substrate, and terminates 
at point B1 corresponding to terminal B. 
It is seen that capacitor C1 serves as an insulating element between the 
first and second circuit sections 3 and 4. As discussed above, the loop 
sections S1 and S2 of the figure eight pattern have substantially equal 
areas equally exposed to magnetic flux from the power supply unit. The 
currents induced by the magnetic flux in the first and second circuit 
sections 3 and 4 therefore cancelled each other. 
The foregoing description shows only a preferred embodiment of the present 
invention. Various modifications are apparent to those skilled in the art 
without departing from the scope of the present invention which is only 
limited by the appended claims. Therefore, the embodiments shown and 
described are only illustrative, not restrictive.