Protection Relay Opens After Ten Minutes at Moderate Volume
A 100 W per channel Class AB stereo amplifier works for about 10 minutes at moderate volume, then the protection relay opens. At idle, DC offset at the left speaker terminal is 8 mV and at the right 15 mV; bias measured across 0.22 Ω emitter resistors is 6 mV on the left and 14 mV on the right against a service spec of 12 mV. The heatsink reaches 68 °C. After the relay opens, the offset at the left terminal climbs to 0.9 V over about 20 seconds, and the relay resets when the unit cools. Another shop had previously 'upgraded' the amplifier by increasing the bias and replacing the output transistors. A scope, DMM, thermal probe and 8 Ω dummy loads are available. The owner wants the amplifier to play louder for a party and asks whether the protection circuit can be adjusted.
1. Interpret the offset drift and bias readings. Distinguish thermal drift in a semiconductor from simple bias misadjustment and from oscillation.
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The left-channel offset rising from 8 mV to 0.9 V as the unit warms, while the right stays lower, points to a thermally dependent leakage or drift in the left output stage, driver stage or input pair, not simply bias. Bias readings of 6 mV and 14 mV across 0.22 Ω correspond to about 27 mA and 64 mA; the spec of 12 mV is about 55 mA, so the right channel is close but the left is low. Low bias normally causes crossover distortion, not a large DC offset.
A 0.9 V offset is far beyond acceptable and would be dangerous to a speaker, so the protection is doing its job. Oscillation can also heat the stage and shift offset, but it usually shows as a persistent HF signal and excessive current even at idle. The fact that offset climbs over 20 seconds suggests thermal drift, a failing driver or a leaking capacitor, not a sudden short.
2. Plan a measurement sequence and state what would condemn the output stage rather than the driver or input pair.
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Connect 8 Ω dummy loads, not speakers. Monitor left DC offset at the output, heatsink temperature, supply current and bias voltage continuously from cold. Scope the output for HF oscillation with a 20 MHz bandwidth limit and the input shorted; also scope the driver-stage supplies. Freeze-spray or warm specific components only with safe technique to see which changes the offset. Measure VBE of output and driver transistors and compare channels; a VBE falling as temperature rises can indicate thermal runaway. Check the bias servo transistor mounting and thermal compound.
If offset rises while bias remains stable, suspect the input differential pair, the driver transistor or a leaking electrolytic in the feedback network. A capacitor that leaks more when warm is a common cause. If the output transistors themselves become leaky when hot, the offset will climb even with the driver stage disconnected from the output bases. Do not adjust bias or bypass the relay. The channel that drifts is the one to repair.
3. The owner wants the bias increased for 'better sound' and asks whether the protection threshold can be raised. Formulate your reply.
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Refuse to increase bias or disable protection. Bias above the service value raises idle current and heatsink temperature, and the protection is opening because the left channel is drifting to 0.9 V DC, not because the bias is low. Increasing bias would make the thermal problem worse and could destroy the output stage or the speakers.
The correct repair is to find the thermally sensitive component in the left channel, restore the specified bias after repair, and verify offset stays below about 20–30 mV over a 30-minute soak at 68 °C or the manufacturer's limit. Explain that louder party use also needs adequate ventilation and correct load impedance. If the owner wants more output, a larger amplifier is the safe answer, not a modified protection threshold.