Stereo protector , If a d.c. coupled output amplifier breaks down during operation, the loudspeakers, particularly the bass units, are at risk. The bass particularly so is not decoupled for d.c. the because of it capacitors in the cross-oyez- network. If, for instance, the output transistor has given up the ghost, the bass units will get the full (1.e. supply voltage at their terminals.
A suitable circuit to protect the loudspeakers in such an eventuality, and at the same time obviate the annoying `plops’ on it on, is shown in the diagram. Note that it -Operates from an unregulated, unsymmetrical power supply. Normally, it may be directly from the power supply of ) the output amplifier.
The a.c. components of the signals in the output stage is bypassed by R1 and the two anti-series connected capacitors, C2 and C3. The signal at the junction R1-R2 is, therefore, the d.c. component of the loudspeaker
signal. From there it is applied to potential divider R2-R3 and then to window comparator ICS, and ICib. Since the supply voltage is fixed at 10 V by R13-D7, the window height is fixed at 2 V by R5. In other words, u2 = 6 V and u3 = 4 V. In the absence of d.c. at the output of the power amplifier, ui = 5 V. In this situation, the outputs of ‘OR gates’ Di and D2 is logic high. When the d.c. the component at the output of the power amplifier is greater than ±2 V, u1 is greater or smaller than either u2 or 113. The output of one of the opamps will then be logic low. When the power amplifier is switched on in step with the present circuit and u• lies within the window, C4 is charged via R8. After about 1.5 s, ‘Schmitt trigger’ ICid changes state and its output becomes logic high. The relay is then energized and connects the loudspeaker to the power amplifier: no ‘plop’.
If a defect occurs, or if the direct voltage at the output of the power amplifier rises, C4 is discharged via R7 within 50 ms. The output of ICid then goes low, the relay is de-energized and the loudspeaker is disconnected from the output amplifier.
Resistor R13 and the operating voltage of the relay must be suitable for the supply voltage. If that voltage is 20-40 V, a good value and rating for R13 is 4.7 k.c2, 1 W; while for 12-20 V, 1 k.(2, 1/4 W is right. If the supply voltage is, say, 36 V, the operating voltage of the relay should be 24 V. The difference of 12 V should be dropped across a suitable resistor. If, for instance, the relay draws 15 mA, R16 should have value and rating of 820 S2, 1/4 W. If there Is likely to be a requirement for switching off the circuit, S1 should be Incorporated. When that switch Is closed* the relay Is energized.
For a circuit suitable for a stereo power amplifier, only components R1—R3, C2, C3, D1, D2, D5, D6, ICia and ICib need to be duplicated. The additional circuit is connected in parallel with S1. Note that the relay should then have two working con-tacts or two relays with their contacts in series should be used.
