Amplifier Drive Indicator Circuit Diagram

Introduction

An amplifier drive indicator circuit is a useful tool to measure the strength of an audio signal passing through an amplifier. It shows the amount of power that a speaker is receiving from the amp, so you can make sure your audio setup is working properly and isn’t an overloading-a common cause of the distorted sound. With these handy gauges, you can easily adjust your volume levels to ensure optimal performance with any given setup. They’re especially ideal for live performances, both onstage and in the studio, where one small adjustment can make or break a performance.

Circuit Diagram

of Amplifier Drive Indicator Circuit

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Working Explanation

of Amplifier Drive Indicator Circuit

The indicator shows the four important drive states of an output amplifier: Class A (low power), Class AB (normal operation), half power and full power. The indication is provided by a dual (red/green) LED, which also shows the colours orange and yellow. The circuit may be used with most types of the output amplifier.

The input signal is derived from the output of the amplifier and is applied to a full-wave rectifier, IC1, via attenuator network R1-R2-R5. The symmetry of the signal is set with P₁. (This is done simply by first applying a positive direct voltage of, say, 15 V to the input and then a negative one at the same level. Turn P1‚ until the output of IC1b is the same in both cases: +5 V).

The rectified signal is compared in a three-fold comparator IC20-C2c with reference voltages provided by potential divider P2-R7-R8-R9-R10. They assume a normal load and represent powers of 2.5 W (limit of Class A operation); 25 W (half power) and 50 W (full power). The divider may be calibrated with the aid of P2, which should be set to a position for which the potential at junction R7-R8 is 9.3 V. If this proves impossible, the value of R7 should be altered as appropriate.

The outputs of the comparators are applied to MMVs (monostable multivibrators) IC 3a, IC4a, and IC4b. These retain the relevant levels for a short while so that the appropriate LED lights clearly even with short power peaks. In all three cases, the time constant, R14-C1, R15-C2, and R16-C3, is 1 second.

When not one of the comparators is enabled (Class A), the Q output of IC3a is high, whereupon the green section in D3 is actuated via R19.

When the first comparator has changed state (Class AB), the red section of D3 is actuated via IC4b. The value of R17 is such that the diode then emits a yellow(ish) colour light (since the green section has stayed on).

When the second comparator level is exceeded (half power), the red section of the LED gets extra drive, whereupon the colour becomes orange.

When IC2c is enabled, the Q output of IC3a changes from high to low, whereupon the green section goes out and the LED light is red to show that the amplifier has reached its drive level.

The supply is derived from that of the output amplifier via network R20-R21-D4-D5. The indicator draws a current of 25 mA from the +ve line and 50 mA from the -ve line. Note that the -ve line has to provide the current for the LED.

Since the Zener diodes require some time to warm up, it is best to leave the circuit switched on for, say, 15 minutes before adjusting P1‚ and P2.