http://mrbichel.com

Almost a year ago I soldered up a pcb and ordered parts for putting together a small simplistic HIFI amplifier. Just recently I rediscovered the old project and finally got around to complete it.

The amplifier is based on the amp6-basic 2 x 25 W chip amp from 41hz connected to a standard switch mode power supply. It’s supplied only with a mini-jack input since thats what my ipod, phone and mac use anyway. A DACT Type 23 50K stepped attenuator acts as volume control. I put it all together in an anodized aluminum chassis with a thick custom frontplate.

Components laid out

I cut out the various holes in the back and frontplate using regular twist bits and flat wood bits on a column drill. I finished off the non round cutouts using a file. I ended up using a few hours with a dremmel to cut out the frontplate for the volume knob because I didn’t have a wide enough drill bit around. It turned out quite rough but I tried to use it as a design feature engraving the whole frontplate with the dremmel. I also mounted a blue LED as an on-off indicator, with a rather large resistor 13K Ohm to avoid it illuminating my whole room at nighttime.

I supply the amp6-basic with about 14 V which it can easily handle if it’s rather efficiently cooled. I mounted it to an aluminum plate in the side of the chassis furthest from the SMPS with cooling paste on the back of the chip. The power cables are wound around a large ferrite which should in theory reduce the noise.

The expenses add up to around DKK 1500 (~EUR 200). I am using the amplifier with two small Dali speakers and I am very satisfied with the overall sound quality.

Me and my friends wanted some sound in our camp at this years Roskilde Festival so I set out to build us a boombox.

I ordered an amp6-basic kit from 41hz.com because of the many recommendations I have read and because the kit comes with mostly through hole components and is relatively easy to solder.

After soldering i mounted the amp in an aluminum plate from an recycled aluminum profile. It was quite a revelation to discover how forgiving aluminum is as a material. I have added a 50k Ohm stereo log pot for volume control a stereo minijack input and a switch that i wired to the 12V input since it didn’t seem to produce any noticeably larger turn on thumbs than the sleep jumper. I somehow managed to reverse the volume pot, meaning you turn it up rotating against the clock, and the power level seems to max out when turned half way around. It is not important for this amp, but for the next one I will look into improving the volume control, perhaps using an attenuator and an active preamplifier – can anyone point me to a good DIY volume control or preamplifier guide/advice? Currently the amp is powered by a screw machine battery but we are planning to bring a car battery to power it during the upcoming festival.

The amp sounds great, and is loud enough even through 8 Ohm, and being a class T amp it is very efficient thus giving us many hours of music on a recharge.

Next up is encasing the amp and speakers providing it with weather protection and fitting it with wheels, i am hoping to find some large bicycle wheels. Right now i am considering a light design using two 8 Ohm compact speakers, or using 2 heavier 4 Ohm speakers, they are very old and rated to 15W RMS which is not quite as much as the 25W RMS the amp outputs through 4 Ohm, I am not sure if this will be a problem.

On a side-note fooling around with electronics for the past 6 months i have been wondering why we are not all running with 12V DC as mains. I suspect there are a few reasonable arguments but i haven’t been able to find them.