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Old 7th March 2004, 05:19 AM   #1
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Smile New Cigar Box Gainclone!

I just completed my cigar box gainclone, inspired by someone else on this forum who did the same thing (I really like it). I've been messing around with the gainclone idea for several months, and I have a HORRIBLE habit of starting about thirty projects at once and never finishing any of them, so I decided to make a real effort to complete this one. It's split into two cigar boxes, one that that houses the 450VA toroid transformer and two bridges, and the other one has the caps, heatsink, and LM3875s (it's a dual channel amp). It happily runs four speakers if I hook two speakers up to each channel, and it goes way louder than I'd ever need <grin>. No volume control at the present, but I have one of those ebay attenuators on order, which hopefully I'll be able to add in a few weeks. I used four pin microphone connectors to create a power supply cable that goes between the two boxes, and the schematic is based around the 'Ultimat' gainclone design thread.

Oooh, and the point of it all - pictures!

I'm uploading them now to http://hvcircuits.com/firebug/amp - sorry for the file size, but at least they're good quality

--
Jay
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Old 7th March 2004, 10:35 PM   #2
Ropie is offline Ropie  United Kingdom
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Very nice - congratulations on a nice couple of cases, but what about the inside?
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Old 8th March 2004, 02:11 AM   #3
moe29 is offline moe29  United States
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I resized one of Firbug24k's pictues so that slow connections could
see the great work! Hope you don't mind.
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File Type: jpg cigar amp.jpg (78.8 KB, 718 views)
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Old 8th March 2004, 04:02 PM   #4
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No, I don't mind at all - thanks for attaching the image.

The insides aren't all that pretty - that's why I spent time making the case nice! It's all done with point to point wiring, basically. I used some thermal epoxy to glue the two LM3875s to the heatsink. This thermal epoxy is nice stuff - the way the heatsink is shaped, I couldn't bolt the chips on, so the epoxy turned out to work pretty well.

I'll post some pictures of the guts sometime later if anyone wants, though.

P.S. The thermal epoxy can be obtained from http://www.melcor.com
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Old 8th March 2004, 11:25 PM   #5
Nuuk is offline Nuuk  United Kingdom
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Super neato Firebug24k! Another one up for the non-metallic case brigade.

I would really like to put that one in the DD Gainclone gallery . Let me know if that is OK.
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Old 9th March 2004, 12:00 AM   #6
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I'd like to see pictures of the insides. My linestage is in a cigar box, so putting my amp in a cigar box would be pretty neat. I'd worry about heat though... how hot do the heatsinks get? Did you do anything specific for ventilation? Drill holes or something else?
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Old 9th March 2004, 12:05 AM   #7
GregGC is offline GregGC  Canada
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That looks pretty nice.
I hope you used some kind of shielding inside the box to keep the RF out.

/Greg
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Old 9th March 2004, 06:28 AM   #8
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At the moment, there is no RF shielding in the box. You do get a bit of a hum if you set the power supply directly on top of the amp. If you reverse it and put the amp on top of the power supply, no hum . I'll fix it when I get some free time - I'm a EE student in the middle of midterms at the moment.

As to the heat, at the volumes I listen to it at, I don't have any problem with heat at all. I did nothing special for ventilation, and even after driving four speakers for a couple hours, the heatsink doesn't get hot enough that you can't comfortably touch it with your hand. Now, if you drive speakers bigger than what I have, I'm sure that the story would be different.

<story about speakers>
I have a set of logitech Z-560 computer speakers (the amp that is in this set of speakers is a LM3876, I believe, someone posted more info in a different thread about this somewhere on the forum). The subwoofer on the speaker set died a few months ago, so they had me cut the power cord off the sub and mail it and the control pod back and they shipped me a brand new set of speakers. So I had eight speakers, working subwoofer/amp and a dead subwoofer/amp. I trashed the dead parts, but I still had 8 speakers. So I built this amp to run four of them. I haven't measured the impedence of the speakers, but I bet it's fairly high, which is why I get away with running two in parallel.
</story about speakers>
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