PA

disconnecting the loudspeaker once morę.

The morę important function of the Circuit is the protection feature and this is where Qll, Q12 and Q13 come into play. Notę that the amplifier output is monitored via a low-pass network consisting of two 22kft resistors and two 47/iF/50VW bipolar electrolytic capacitors. This renders the protection Circuit insensitive to normal AC output signals. However, if the output of the amplifier develops a negative DC potential of morę than 2 volts, Q11 will be forward biased (because its emitter will be pulled negative with respect to its base). Q11 then turns on Q12 and shorts the base of Q14 to ground. This turns Q14 and Q15 off, which turns off the relay and disconnects the speaker, protecting it from damage.

Similarly, if the amplifier output develops a positive potential of 2 volts or morę, Q13 will be forward biased, which will again short the base of Q14 to ground, with the same result as before.

Arc protection

An interesting point about the arrangement of the relay is that when the loudspeaker is disconnected, it is shorted to the 0V raił. The reason for this is that if a large DC fault occurs in the amplifier, the moving contact of the relay will morę than likely produce an

Performance of prototype

Power output

4 ohms    225 watts

8 ohms    145 watts

(measured with a 240VAC regulated power supply)

Harmonie distortion

less than .02% up to fuli power output into 4 and 8 ohm loads

Sensitivity

1,5V RMS for 145 watts into 8 ohms; input impedance 22kfi

Frequency response

-3dB down at 3Hz and 140kHz; at 1W Hum and Noise

— 116dB unweighted with respect to 145 watts into 8 ohms with input unloaded

Damping factor

at 1 kHz    > 50

at 30Hz    > 50

Stability

Unconditional

ELECTRONICS Australia. October. 1985

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