This amplifier was designed as a booster to enable output powers of around 4 to 5 watts rms to be obtained from a radio/cassette unit. Of course, the amplifier is also suitable for other applications; it has an output sensitivity of approximately 350mV rms into ,10k for maximum output, and is intended to feed an 8 ohm load.
Circuit in lines (Click to enlarge)
The circuit uses a well known configuration which has common emitter input stage (Q1) direct coupled to common emitter driver stage (Q2), which is in turn direct coupled to the complementary emitter follower output stage (Q3 — Q4). R7 provides virtually 100% negative feedback at DC, giving the circuit approximately unity voltage gain at DC.
R1 R2 and R4 form a potential divider which bias the input of the amplifier to about half the supply potential and the output is also biased to about this level due to the DC unity gain. This bias level gives the optimum unclipped output voltage swing. R1, C1 and C2 filter out any hum or noise which might otherwise be coupled from the supply lines to the input via the bias circuit. R6 and C5 are used to decouple some of the feedback at audio frequencies, and thus give the unit a useful voltage gain at these frequencies.
D1 is used to give a small standing bias to the output transistors and, together with the fairly substantial amount of negative feedback, reduces cross-over distortion to an unnoticeable level. The emitter follower output stage gives the circuit a low output impedance so that the load can be efficiently driven with high output currents. Q3 drives the speaker during positive going output excursions while Q4 drives the speaker during negative output excursions.
C6 provides DC blocking at the output, and C3 provides the same function at the input and C4 aids the stability of the circuit. RV1 is a volume control,: and results will probably be best if the volume control on the cassette radio is set for a fairly high output (but not so high as to cause clipping), and the volume is adjusted using RV1.
The circuit requires a stabilized supply of 18 to 22 volts, and capable of providing up to 400mA. Q2 should be fitted with a clip-on TO-5 size heatsink. Q3 and Q4 are both fitted with commercially made, finned, bolt-on heatsinks.