audio amp Power Supply

Published:2013/7/15 20:03:00 Author:muriel | Keyword: audio amp , Power Supply | From:SeekIC

The power supply is simply a straight forward symmetrical plus/minus supply constructed from a transformer with a balanced secondary and a bridge rectifier module. Even though the transformer was a self-wound unit, any transformer with a 25.2VCT secondary at 2.0A will be a good substitute. Such transformers oughht to be available as VT heater transformers for high power ham RF amplifiers. It is essential that the primary side of the transformer be connected exactly as shown. This PS uses a MOV for over voltage protection. These things have a nasty habit of not turning off once they turn on. The fuse needs to be connected between this MOV and the mains. Don't attempt to go cheap (or lazy) and fail to include it. A 2.0A, slow blow type works well here. It will stand up to the initial surge as the filter capacitors charge up, and will prevent damage or worse should the MOV be triggered. This supply is somewhat over rated for the application, and is heavily bled for good voltage regulation. There is still some 120Hz hum that could be eliminated with a more involved PS circuit. However, as this wasn't intended as a premium amp, that wasn't necessary. The hum is most noticeable when running with no input signal. While actually playing sound, it's no longer noticeable. If you want to substitute a better supply, then that's OK, too.ImprovementsThese units worked quite well, and do sound good, despite the inherent limitations of BJTs as analog amplifiers. There are a couple of improvements possible. The first would be the use of a full-complimentary final. If this is done, then any complimentary BJTs with a PC of 5.0W or more will work. Again, the most important part of this will remain the input Darlingtons. Even better would be the substitution of complimentary (N-Channel/P-Channel) VMOSFET power transistors. These, of course, don't require Darlington pairs in order to operate. If VMOS power transistors are used, it's important to include a 100 ohm resistor in series with the gate terminal, mounted close to the transistor body with the shortest lead length. This will help prevent the possibility of RF oscillation. (Unlike the BJT, the VFET is a high frequency device, and the inadvertant construction of VHF oscillators with these is quite easy.) Parts List: AmplifierCR1 -- 6: 1N914Q1: 2N3819NPN: 2N3904 (or other small signal NPN)PNP: 2N3906 (or other small signal PNP)Q8: 2N3904 (Critical)Q10: 2N3906 (Critical)Q9, 11: 2N3053R< sub>: 1.0K/¼W miniature pot. Parts List: Power SupplyC1 -- 2: 20000uF/35WVDCCR1 -- 4: 50VPRV/5.0A Bridge ModuleCR5: Green LED Panel MountF1: 2.0A, Slow Blow FuseT1: PRI: 120VRMSSEC: 25.2VCT/2.0AZ1: 120VRMS MOV



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