In SX Microcontrollers, SX/B Compiler and SX-Key Tool, dpaton wrote:

Andrew-

There's nothing wrong with 6550s and EL34s. I use them for my bass guitar rig. For cheap power density with reasonable quality and true low end in a small package (tube output transformers that can do 20Hz are HUGE) the LM3886, it's chip-amp brethren, and the solid state world make me smile. Did I mention they generally run much much cooler? ;-)

The L-pad/transformer volume controls don't actually clobber the phase response as much as you'd think. The L-pads present a constant impedence to the amp, which is part of their design. The transformers are more problematic, but mainly because they saturate much too easily. Getting bass thru them is hard, as it tends to get 'flabby' and 'soft' (amplitude limiting by the saturated core and horiffic phase shift as a result).

Regarding the amps:

Once Chris and I finish nailing down the module design, I'm going to build as many as I can fit on 3x 60sqin panels (15-25 or so per panel, so ~50-75 boards) and offer them at cost (probably a few bucks plus shipping) to anyone interested. The LM3886s can be bought or sampled from National Semi (if you only need 2 or 3..if you need more, I'd suggest shelling out the $5 each for them). The resistors and onboard caps are all stock items and can be had reasonably. I figure about $7-$10 in parts per module depending on how much you buy and how much you scrounge. The case, heatsinks, main caps and power transformer are always the hard parts to get cheap, but scouring the surplus markets can net you some really good deals on transformers and caps. Heatsinks can be the most troublesome parts, but ham radio swap meets and the internet are your friends. A lot of the fun is in the thrill of the hunt for parts, at least for me ;-) If you go with all new parts, things can get expensive fast ($450 easy), but scroungers and bargin hunters always do better (my $200 number).

Günther-

Vielen dank. The image your description brought into my mind was priceless. A nice quiet geek about to turn on his new stereo for the first time, and diving for cover as a dozen Dallas digipots turned into shrapnel with a pop and the acrid smell of the mythical blue smoke.. ;-)

-dave

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