Characteristics of d.c. offset protection circuits (abstract)
NOTE: These characteristics do not apply to amplifiers with only capacitive coupling to the load terminals.
Characteristics to be specified
The following characteristics should be specified:
a) rated maximum offset voltage at the load terminals, stated by the manufacturer in the specification, that does not operate the protection circuit;
b) rated response time of the protection circuit, optionally stated by the manufacturer in the specification, to an offset voltage of 30 % of the highest d.c. supply voltage of the amplifier output stage;
Methods of measurement
Methods of measurement for amplifiers which continuously sense the d.c. offset condition
The procedure is as follows:
a) The amplifier is brought under standard measuring conditions, and the input signal frequency is then changed to 20 Hz. A d.c. oscilloscope and a d.c. voltmeter are connected to the load terminals.
b) The d.c. balance of the amplifier is then slowly changed, by a suitable method which depends on the detailed design of the circuit and cannot be standardized, and the maximum reading on the d.c. voltmeter, before the protection circuit operates, is noted. This is the maximum offset voltage that does not operate the protection circuit.
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Measurement of DC offset protection of Hypex NC252MP
This amplifier is DC coupled and it transfers amplified DC voltage from input to output.
Input test signal - 20Hz sine + DC offset and amplitude jump after 1s
Input signal spectrum before the amplitude jump
NC252MP measured response to the input signal
The input signal level in the 1st second is at "standard measuring conditions" level of the output. Then comes the 20dB jump. One can see that NC252MP transfers DC average level of 10.69V to the output, without any action of the module DC protection. This is dangerous and may lead to speaker voice coil damage in case of DC error signal at the amplifier input. After 1s, when the input signal DC jumps of 20dB, the protection circuit finally takes action. It shuts down the amplifier output in 7.8ms after heavy DC clipping.