In another of my project pages (see 14 – Power Amplifier Bridging Adapter), there is a design for a simple add-on bridging adapter for stereo power amplifiers. There is, however, an even simpler way, provided you have (or can trace out) the appropriate section of the amplifier circuit.
Nearly all modern amplifiers use a long-tailed pair as the input and error amplifier (the error amp is the LTP, which detects any variation between its inputs – an error voltage – and corrects it). The input is connected to the base of one of the LTP transistors, and the feedback to the other. The feedback signal is attenuated by the network, by an amount equal to the gain of the amplifier.
By connecting the output of one amplifier to the feedback point in the other, using a resistance equal to that for the feedback resistor, the second amp will have a signal gain of unity, and will be inverted, since the feedback is always applied to the inverting input.
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