Ranting in the dark

There. I finally decided that it is finished. I can play with it an tweak it for eternity, but it's done. My 2x50ish watt LM3886-based amplifier.



This baby is a chip-amp, or a "Gainclone", which means it's a single-IC, minimalistic design. It's also a dual-mono design, which means the channels are completely separated, with their own power supplies. it's a power amplifier, so no volume controls or input selections, just pure muscule. The theoretical limit to output power of this IC is 68W, so I've provided more than enough headroom for that. It's powered by two 120VA toroidal core transformers. That's quite a bit more than the 80VA I was planning on, but it did cost the same amount of money, so why not. Now even the craziest bass peak will not trouble my system. With 20 000uF of capacitance per channel, smoothing the output of two rectifier bridges per channel, there's nothing to worry about.

The assembly takes extreme care of making sure that everything is bolted down, glued together and double, even triple insulated. Heatshrink covers every pair of wires. The primary is fused on both live and neutral wires, so are all the four power supply rails inside. To ensure nothing unwanted goes out to the speakers, a DC-protection circuit is installed, that automatically turns off the output in the case of anything wrong with the output signal. It also ensures, along with the LM3886's mute circuit, that there's no output while the amp powers up, as well as the immediate disconnection of the speakers at turn-off.

The chance of ground loops occurring should be taken care of by the ground loop disconnecting circuit, that combines a power resistor and a heavy-duty bridge rectifier to ensure that no unwanted current would flow during operation, but that the immense fault currents will safely flow to ground and break all fuses and circuit breakers, should anything bad happen.

I'm testing it as I am writing this, listening to the amazing Ugress and I can tell you, it was worth any minute I've put into building it. It's small, slick, all-black, with a single white diode mounted at the front of the case. It looks wicked and it delivers a punch to justify its looks.

Simple - possibly quite simple. Awesome - quite a bit indeed. It's so small I can put it in my backpack and carry it anywhere. Yet it sounds fantastic and delivers enough power so that my Cerwin Vega speakers threaten to tear down the house. And despite its miniature size, it doesn't even remotely heat up, even after hours of driving my speakers at painful volumes.

If you've got some spare parts you can build those babies extremely cheap. I did spend some money on transformers and chasis, the major expences related to this project, but you can work around that quite easily without compromising on quality. It still ended up being cheaper than a decent commercial amp, for all it's worth.


Load testing

Well, I've tried and tested the system a lot, during construction and it behaved really well. No one tells you how it would behave after 6 hours of extensive use, wired to two badass speakers and the volume cranked up in party mode. That is to say, stupidly loud, all the time. Apparenly, in small cases, things are expected to heat up.

Heat up, they certainly do. I've had bigger amplifiers than this radiate heat like crazy in such situations. The question was whether the tiny case I'm using can handle all this heat. It heated up, heated up a lot. The cooling flanges on the sides, which are usually only hot in the rear side where the chips are mounted, got heated up all the way. A good thing abount the tiny chasis is that it is well connected to the flanges on the sides, wich means that heat gets transferred across the entire chasis, which seems to help quite a bit. A lot more surface, I guess, even if it doesn't have the best thermal conductivity properties.

There was also one factor that probably influenced temperature in a very bad way - the case has no feet yet, so it was placed on top of a magazine. Otherwise the bottom of the chasis could scratch the surface. The downside of this is that all the air openings on the bottom were shut and there was essentially no convection flow through the amp. I cant bet on it but I'd expect that this airlfow would have reduced the temperature significantly.

The final word on this - it can handle it. No thermal protection kicking in (oh yes, it has that. It's disgusting, but at least it doesn't MELT), no ugly distortions, nor other noticeable effects of a significant increase in temperature. So, hurray, it's passed the final test, what I'd call the "stupidly drunk overload" test. Yey me!