A few popular circuits supplied by Maplin (now discontinued).
This design suits transformers of 22-0-22 to 28-0-28V and inspired a matching 12V switched mode power supply. A high quality (E-I) mains transformer with separate secondaries (LW34M) was available.
An impressive performer, J. Ellis' observations on bias stability can easily be applied to these arrangements with good effect. Recommended Iq was 20mA (if this could not be set then reduce 4k7 in bias chain to 3k9).
A less sophisticated, but meatier design intended for 32-0-32V transformers. Fit 3A fuses, all output devices mounted on 2.1°C/W heat-sink. Uprating the output stage with devices with a higher Vce, or reducing the supply voltage, will increase reliability. Set Iq for 70mA, then 75mA when warm.
A mosfet design intended as a guitar amp that has seen upgrading, which has ranked amongst Maplins' best selling products, and with good reason. This was available with a bridging module for high power demand. Note use of low noise, high voltage, small signal transistors. This design certainly ranks as the simplest mosfet design employed and, importantly, none have failed.
This was derived from an Hitachi application which spawned a number of copies and derivatives.
Output devices could be easily paralleled (with their attendant gate resistors) for higher power handling and although the PCB was small, compact and proved to be stable, a redesign would have put the differential pairs in thermal contact, re-orientated the TO3 cans to give shorter gate connections and extended the earth plane. A decent bias pot (not open skeleton carbon), preferably sealed with multi-finger wiper, will help prevent possible future (corrosion) issues. Iq was set at 50mA for the first version and 100mA (80-120mA) for the latter. The later version recommended making direct power connections to the o/p device drain connections using single-core 30A runs (<300mm). The (original) transformer for one amplifier into 4 or 8R would be a 35-0-35 @ 2.28A, with fast supply line fuses (post-smoothers) of 2A for 8R and 3A for 4R for each amplifier rail. The later type specified 3.15AS fuses for the supply lines.
This design was used for experiments with sub-bass units using the modifications set out below. Power supply smoothing was increased substantially, as was heat-sinking, and where a LF roll-off was required, an input coupling cap was re-fitted. A power output of 30Wrms max suited most applications involving 12-18" speakers. Later designs used class D.
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