Passlabs Amplifier Projects |
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The Aleph5 - Construction The following discussion is aimed at non-EE type people (like myself), who may want to contemplate this sort of project but lack some of the basic electrical theory. The "learned few" will find it almost insulting, so you've been warned ;-) Note: many of the recommendations are specific to a class A, constant current draw amplifier - class AB may vary.
So, I settled for a 650VA, 28-0-28VAC toroid from Tortech, who make good quality toroids. This is just adequate on the total power dissipation front, but this company is conservative in their construction and in designing the case the additional diameter of the 800VA unit (which I would class as "ideal" for this amp) didn't fit quite as nicely .... yes, even DIYer's have to make design compromises! You will note from the picture above that I included a high current (10A) line filter to remove any "trash" from the AC mains. I think if you are building one of these amplifiers this is a justifiable expense.
The next step is filter capacitors. Here you can spend quite a lot of money and at some point you get into the land of diminishing returns in terms of extra $$$ versus power rail ripple/hum. The original Aleph5 used a total of 100,000uF, which is probably adequate. I happened to have a supply of larger caps, so used 4x 80,000uF for a total of 320,000uF. This is actually overkill, and it makes current draw at start-up even more of a problem (uncharged a large cap acts as a short-circuit across the bridge), so the in-rush current limiting thermistor was an essential. The final rail voltage under load is 36VDC. I considered using Pi-filters, as I had done in my Aleph4, however the design layout of the 5 really didn't suit this, so I thought I would try the amp without filter chokes first and add them if I found the rails contained too much ripple. I stole the idea of using brass plate for the supply busses from pictures of the X-series amps. The brass off-cut cost little and I simply brushed it with some 180 "wet-n-dry". Heat dissipation is another major factor in designing/building any Aleph series amplifier. To quote Nelson, "there is no such thing as too much heatsink", or the other memorable quote, "we dont do calculations, we just make 'em really big!". Back to reality, this can be one of the most expensive parts of the amplifier, both in terms of dictating final size and cost. For we mere mortals calculations really are helpful. If we start with the power dissipation, we need to account for 150W/channel of heat. As a rough rule we do not want heat sink temperatures greater then 60C, so we need heatsink with a thermal resistance as follows:
The original Aleph5 sinks ran at about 55C and were temperature protected at 70C. We can afford to go slightly higher than this, with the understanding that the higher the FET temperatures, the shorter the life of the devices. I was fortunate to find a heatsink manufacturer in Oz who is happy to sell direct to the public. The unit I used (diagram) was 300mm x 150mm with 40mm fins and dual flanges for device mounting. The heatsink Rt = 0.25 C/W, which was right on the limit, however I would have the rest of the case for additional heat dissipation.
Again I decided to make the PCBs myself (link), and despite 1 foul-up the end result turn-out well. I found a good source (cheaper than Farnell) for positive resist PCB and then used the developer/etching solution left-over from the Farnell kit that I used to make my Aleph4 ..... very bad plan, do not try this :-( Using the complete kit (boards/developer/etching solution) from Kinsten worked just fine! If you would like a copy of the artwork, you can download a TIF file here.
I was really quite pleased with this .... eventually! During the first run an insect landed in the 2nd spray enamel coat and I had to strip the whole thing and start again :-( You can download a PDF file outlining the chassis construction, PCB layout + stuffing, plus parts list here.
The top is well ventilated having multiple slots cut prior to powder-coating. You will note that the heatsinks are 25mm above the amplifier base. There were several reasons for doing this, partly to improve ventilation and heat dissipation (note the side of the base beneath the heatsink has ventilation slots also) but mainly because the height of the 80,000uF filter caps mandated additional internal height! The sheet-metal dimensions for the base, rear and top are also in the above PDF file. There's more to come ..... please visit again soon ;-). |
Prior to brushing, I mounted the LEDs either side of the logo, recessing a 5mm LED from the back so the front of the LED was just above the surface of the panel, then used a 45deg. countersink bit to make a bevel. Once the surface was brushed I coated it with clear satin enamel spray-pack to protect the aluminium, which would have tarnished with fingerprints etc. if it were left raw.
