Welcome to the retro electronics world!

Transformerless Quasi-complementary 50 watt power amp

Also a classic is the circuit of this 50-watt power amplifier. It is suitable for universal use, e.g. for power mixers, active speakers, instrumental amplifiers or more powerful hi-fi devices. I used such circuits earlier for playing party music. It is, as the 15W power amplifier, a quasi-complementary power amplifier with two equal power transistors. The main difference is in addition to stronger transistors and higher supply voltage, that here an additional preamp stage is inserted. As a result, high idle respectively loop gain is achieved and more efficient negative feedback can be performed. It is set by the ratio of 2.2kΩ to 100Ω resistance at the emitter of the first pre-amp transistor (BC307A) and determines the voltage gain of the entire circuit. With the specified dimensions, it is good 20, so that the power amplifier with just over one volt input voltage can already be fully drived or rather overridden.

The stronger negative feedback leads to a smaller harmonic distortion. It reaches about 1% only at full drive and remains significantly smaller at lower levels. The transfer range covers the entire audible frequency range (about 16 ... 20000 Hz). Often disregarded in power amplifiers, the power frequency response. While the transfer range refers only to the gain or rather its drop at its upper and lower limits, the power frequency response indicates how far the achievable power drops at the limits. Based on a drop of 3 dB (= half power) it is about 30 ... 15000 Hz in this circuit. In music with powerful lowbasses, e.g. at electronic dance music (techno, etc.), the amplifier may thus be quieter compared to a 25W amplifier, if the power frequency response of that reaches until under 10 Hz. In the bass range, in particular, a lot of power is needed, and with a drop of 3dB at 10 Hz, it can be assumed that the drop in power at the lower hearing limit is already negligible.

The stabilization of the output stage bias current takes place in this circuit via a transistor (BC237B). It must be thermally coupled to the output stage heat sink and thus mounted on this. This more efficient type of circuit is advantageous when the amplifier is operated at the power limit for a long time. As the 15W amplifier, this amplifier has no electronic protection circuit, so that short circuits in the speaker line during operation must be avoided. A certain protection offers the micro-fuse in the speaker line. It should therefore not be bypassed or replaced by a higher value. Unexpected failures are possible in such circuits, especially when using cheaper end transistors, also by the so-called secondary breakdown. To avoid this, the 100Ω resistors at the bases of the final transistors must by no means have larger values. More safety is achieved by smaller values ​​(for example 39Ω). Thus, the driver transistors are not overloaded, then they should be cooled.

This is my private web presence on the topics of amateur radio, music electronics, self-assembly of devices and the history of technology. Any use of the content beyond personal information, in particular the texts, drawings, circuit diagrams, photos, videos and music, requires my written approval!