Gainclone: Clone of 47labs Gaincard Minimalist Power Amplifier

Slowly and steadily, over the years, I’ve been working on building a Gainclone — that is, a DIY version of the popular (in some circles) 47Labs Gaincard using a torroidal transformer, bridge rectifier, and one NatSemi LM3875 monolithic chipamp per channel (plus a handful of other parts per channel).  A built a Squelette amp from the MAKE zine vol. 23 (using LM1875 and a Radioshack transformer) which was neat and sounded okay when driven with a DIY Pass Labs B-1 buffer in front of it.

A few months ago, I picked up the torroid and an amp kit from Audiosector.com (Peter Daniels on diyaudio.com).  The kit was simple (minimal part count, after all) and the power amp has no volume pot or other controls, so the parts could be soldered up easily in one afternoon.

However, building an enclosure was something that I’ve failed at many, many times.  Add to that, these opamps need to dissipate a great deal of heat — not Class A amounts, but still a bit of heat.  I found a scrap metal reseller down in SODO, picked up some thick aluminum bar and sheet, and went to town.  Using some salvaged hardwood that I’ve had on hand (from an old entertainment center I picked up off the side of the road and broke down about a year ago), I had the enclosure put together in a few months, here and there doing 2-3 hours of work.

The folks down at the West Seattle Tool Library have a pretty good table saw, crosscut sled, and router + router table these days.  That made short work of the side rails (the hardwood).  The top plate was the trickiest since I had to cut it freehand with a jigsaw + metal cutting bits.  After a lot of drilling and tapping, I had a box.  And the tolerances were within the margins that I had in mind (about 0.5mm +/- for all joints).  I still have some fit and finish work to do, but all the major functionality is there (except for the LED indicator on the front panel).

Now I just hope the wooden rails don’t expand/contract enough to crack under pressure.  And how does it sound?  Quite great for a $100 amp!  My bass reflex Mark Audio CHR-70 1st gen full range speakers are sounding lively, quick and with a surprising amount of bass.

Next up is the Salas shunt regulated B-1 buffer from the diyaudio.com store.  I might forego the enclosure construction next time, however.

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