bridge rectifier

There are basically two versions of the P3a: a 60W into 8 ohms (with ± 35V supply rails) and a 100W into 8 ohms (with ± 42V supply rails). I decided to build the version ± 35V, for two reasons. A 60W, which is more than enough. The other is that the version is ± 42V can not drive 4 ohm speakers. I wanted the option of driving 4 ohm speakers, so the choice was easy. A ± 35V, it can be 60W at 8 ohms and 100W into 4 ohms drive.
2.2. Transistors

The transistors recommended are not available here, so I went for one of the previous recommendations. For the output transistors, I used MJ15003 and MJ15004. Used for the three drivers that I have is two BD140 (a class-A driver and an output driver) and BD139 (the other output driver).

The power supply consists of a 625 VA 25-0-25 transformer, a bridge rectifier 35A and 4 x 10,000 uF capacitors. This seems excessive, and may for all practical purposes, but which is theoretically necessary to four 100W (4 ohms) amplifier lead to an efficiency of about 70%.

The power supply requires heavy use of a starter, for which I used P39 Rod Elliott. I chose the ballast resistors so that their maximum current is used up to 200% of the transformer. In this way, the main fuse will blow, even if there is anything wrong with the transformer. Also, I attached a thermal fuse, resistors on a piece of aluminum and epoxy resin heat for added security.

As always, an amplifier is not complete without a DC protection, and I have included Rod Elliott P33 for that. See my article DC protector subwoofer for more information on this particular circuit.

I am very satisfied with the performance. But I think to do, amplifier does not affect the sound quality through a lot (as they are a decent design), and therefore I must not expect much difference with my previous (what receiver Kenwood 1978 ). Aspects that are the most visible things such as noise and the level of snoring, which are very good. There is a very very very slight hum in the speakers, but you have to press your ear against the speaker to hear it. There, I found this part of the inherent hum P3a is design, so there’s not much I can do about it, both in regard to the wiring or the power supply configuration. But because it is so marginal, it does not matter. At the end of the amplifier is exactly what I needed to, and performs accordingly.