One solution for increasing the power of an amplifier running on a low-voltage supply, like a car radio powered from at best 14 V, is to use a ‘bridge’ configuration, i.e. to connect the loudspeakers between the outputs of two identical amplifiers whose inputs receive the same signals, but in opposite phases. This doubles the apparent voltage applied to the loudspeaker, which in theory quadruples the maximum power available. In practice, because of the various losses in the power transistors, we can only triple it. The peak-to-peak voltage applied to the loudspeakers in the car radio example is 28 V, less the losses in the power transistors, i.e. around 24 V. So we have an RMS voltage of around 8.5 V (24 V / 2√2), which gives an RMS power — the only one we hear — of 18 watts (8.5 V² / 4 Ω).
The booster described here does noticeably better, as it can deliver up to 55 watts rms into 4 Ω with distortion of less than 0.5 % — and it’s capable of 70 watts rms if you can put up with 10 % distortion. To achieve this, it does not break the laws of physics, but it does use a very original system for boosting the supply voltage, using integrated power switches and high-value electrolytic capacitors.
It uses jus t a singl e I C p er channel, a TDA1562Q from NXP which handles both the power amplification in class H and the voltage boosting. Since our circuit is intended to be fitted ‘behind’ a car radio, it has no volume control and its high-impedance input allows it to be connected to either the radio’s loudspeaker output or, preferably, to the line output that some car radios have these days.
Capacitors C3 and C6 are used for the voltage boosting mentioned above. Via the TDA1562Q’s integrated electronic power switches, these are alternately charged up to the circuit’s supply voltage, then put in series with the supply, thereby doubling it to supply the power output stages. Given the very high currents drawn by such a process when C3 and C6 are being suddenly charged, the supply voltage needs to be very heavily decoupled so as to ensure that it doesn’t collapse momentarily when C3 and C6 are connected across it. This is the role of C2.
Transistor T1 drives a ‘diagnostic’ LED from information provided on IC1 pin 8. This LED, off in normal operation, flashes when the IC detects output distortion (in fact clipping, i.e. distortion of 10% or more) and lights steadily when the IC detects a short-circuited output, in the absence of an output load, or when its thermal protection comes into operation need it. This is a mute control that puts the circuit into stand-by when grounded. No output signal is produced and the consumption is reduced to a minimum.
The PCB  carries all of the components and two of them will need to be built for a stereo application. Given the heavy currents involved, the wiring for the supply and the connections to the loudspeakers will need conductors with a minimum cross-sectional area (c.s.a.) of 2.5 mm2. Obviously, the TDA1562Q must be bolted to a heatsink, the efficiency of which will govern the maximum possible time it can work at full power.