Power amplifier 60W Class A          

There is one amplifier configuration that is universally accepted as the ideal for audio use: Class A operation . Many early amplifiers operated in Class A, but as output powers rose above 10W the problems of heat dissipation and power supply design caused most manufacturers to turn to the simpler, more efficient Class B arrangements and to put up with the resulting drop in perceived output quality. Why Class A ? Because , when biased to class A, the transistors are always turned on, always ready to respond instantaneously to an input signal. Class B and Class AB output stages require a microsecond or more to turn on. The Class A operation permits cleaner operation under the high-current slewing conditions that occur when transient audio signal are fed difficult loads. His amplifier is basically simple, as can be seen from the block diagram [Fig.1].

The output stage is quite substantial, using a total of six 250W power transistors. Fairly 'old- fashioned' power transistors have been used [MJ4502/802 family] in preference to some of the higher performance devices now available. They have been chosen because the die used to mount the semiconductor junction is of a large area ; the device is quite rugged and can handle high currents. The short term current capability of the output stage is, in fact, of the order of 90A, somewhat in excess of the current capability of the wiring. The rest of construction is equally massive with a steel chassis supporting six very large heatsinks. However , construction is straightforward provided that the builder has strong arm muscles and circuit alignment simple- there are but two adjustments- quiescent current and DC offset voltage nulling.The coil L1 is wound onto the body of R41. This is not a critical procedure- about 17 to 20 turnoff enameled copper wire should do nicely. Particular care should be taken in mounting the power transistors. Good quality insulating washers and bushes should be used and a generous smearing of thermal paste is essential. These transistors should be bolted to the heatsinks very tightly to ensure good thermal contact at all temperatures. One final point regarding construction. Once the amplifier has been completed and tested, it should be switched on and allowed to reach its normal operating temperature [about 20 minutes]. The amplifier should then be switched off and all the screws tightened up. Differences in thermal coefficients of expansion can result in some of the screws becoming slightly loose, particularly those holding the heatsinks to the top and bottom covers.

My own proposal, for whoever it decides makes the amplifier, is him it manufactures in two separate boxes, one for each channel, [monoblock manufacture ], separating thus and the power supply's. This will facilitate too much the mechanic and electrical manufacture. For all the manufacture, is required certain relative experience, in these sectors. The sound result is sure that it will vindicate, that tries, one and proportional performance amplifiers in the trade, costs exceptionally expensively. ETI 8/81

• DC Protection and Delay time

• Bridge (Stereo to Mono), Power amplifier converter with electonic Balance input

Here is the circuit of power supply, for Power amplifier 60W Class A. He is one relatively simple and classic power supply. In the line of input, exist, the classic circuit of protection and soft start. In the circuit appear two transformers. This can change and all the coils they become on one transformer. What it should we are careful, they are the cables that transport power voltage � V1 [ � 40V ], are also big cross-section with investment of silicone, so that, they bear in the currents that they will go through from in their, but also the temperatures in which they will be found. The connections in the various points should become with screws. For other two coils of transformer, does not need something particular. If you did not need the coil of � 15V, we can suppress, also the supply of circuit of protection, it become from the other coil of transformer. Attention should be given in the region, round relay RL1, in primary of the transformer, which should also header well. The bridge of rectifier BR1, should bolt in some point of chassis, so that is frozen. Obvious is that will be supposed exist a transformer, for each power amplifier.