Sunday, January 14, 2007

Chip Amp - Project Notes

A Chip Amp in a Humidor - The Huma-Champ
Another experiment in Audio Addiction...

The Huma-Champ
So this story begins at and the LM4780 Kit. "Kit" is more like it (emphasis on how kitty the kit is). It is an excellent printed circuit board and parts to put on it. No wire, No enclosure, No Heat sinks, No Mounting Parts, and dam near no Instructions. So the challenge was on. Could I make this diminutive bag of parts come alive.

I don't like to bring my table saw out of the basement. I don't have anywhere to store it, sawdust goes everywhere and I am concerned my neighbours will get pissed. So my first concern was what to stick this amp project in.
One day while walking aimlessly through the mall I saw out of the corner of my eye a Cigar humidor. And the idea was born. I would buy a humidor and use that as the enclosure. I did some surfing on e-bay and found a winner - new glossy black for just over $40.

Next was the heat sink. I don't' do thermal calcs. I studied the LM4780 design guide for days trying to make sense of the math and finally gave up. The calculation said I needed a heat sink with a 1.5 C/W heat transfer (what ever that meant). So then I started cruising the web for heat sinks. I found a vendor that seemed to have a wealth of heat sink material that fell into that category of heat transfer and gave them a call. HA! they asked how many feet I wanted. I only want 2 3" pieces. The guy actually laughed. Then I hear the rapid strokes on his 10 key and I get a blunt - that will be $89 not including shipping. This project was beginning to piss me off.

Back to e-bay - typing in heat sink I land my prey. A fine gentleman is selling heat sink that appears to have turned up on his kitchen table after working second shift at a aluminum extruding shop. He was happy to saw it to size for me and ship for under $30. Now we were talking.

My final stop was on the transformer. Since the "kit" did not include any power transformer, switches, wire etc. I went to Parts Express and picked up one of their torrodial's. As far as sizing goes - the LM4780 wants somewhere in the neighbour hood of 20-28 Volts - pre-rectification. I ended up selecting a 22V, 22V model at 160VA. I think this is working out fine although I'm sure some people will say that I'm underpowered and should have gotten the 22V, 22V at 320VA'ish. Bla Bla. I doubt I'd hear the difference. Besides - I did a little more upgrading.

Enter Mouser - I decided to implement the snubber configuration on the boards from audio sector and I think it worked out well. I purchased some 15,000uF capacitors from Mouser along with the resistors and a couple of orange drops to complete the setup. Although the capacitors don't fit in the top of the board they plug nicely into the underside. This will give me a "nearly dual mono" configuration and the extra juice from the caps will probably make up for the partly anemic transformer.

As you read the web on implementing the LM4780 you will learn about the backside of the chip carrying -35V. It does - insulate against it. I ended up with the sil-pad. That was a lesson learned.
So - here's some pictures of the final Huma-Champ. My first un-kit DIY project. I'm thrilled with it. It sounds wonderful, is dead quite, and packs a punch. Enjoy!
Note the stand off and break in the RCA cable is for a Line level L-Pad to absorb a hiss from my preamp. If you are going to give this project a try. Do so without this correction unless you have the Bottlehead FPIII with the C4S upgrade.
Lessons Learned
Coming from the rear - Figure out how to integrate RCA jacks in the rear. I wired directly to the board but it would be better if I figured out how to put in room for the RCA jacks. Problem is none of them come long enough to simply drill a hole in the rear of the unit. The speaker posts work because I found some that are threaded and are long enough to fit.
Blue Tape - I covered the box in Blue Tape the moment I got it. I believe it helped keep the finish throughout the construction - I'd do it again.
Grounding - This was my first attempt. Seems as though I needed to maybe take some electronics classes. You can see in the photos above that I have a "star" ground fabricated out of a terminal strip. All ground leads go here. It was how I eliminated the buzz.
Fiddle, Spark, Boom - Don't try and diagnose a grounding problem while the unit is on. I bought a new kit from audio sector by trying to see if I could eliminate a ground loop problem before I made the star ground above. Lucky I'm alive. Ruined a speaker driver the in the process.
Tapping the Heat Sink - When tapping very small holes like the #4-40 screws I used to secure the chip to the heat sink I wasted two taps and countless holes. Go SLOW and back out often and clean the tap. I tried to force the tap through the metal and ended up snapping the tap off in the handle.


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