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Old 10th April 2008, 04:15 PM   #1
fatbob is offline fatbob  Canada
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Default Identiying capacitors to use in salvage gainclone

Thanks for all the responses to my "Advice on how to start a LM3875 amp project" thread (Advice on how to start a LM3875 amp project).

I am trying to "Read the manual" before I fire off questions.....so I'm going to go with the cheapo (I'm reading a PhD and have little cash) and green (i study climate science) option and try to build a gainclone from salvaged parts and use p2p wiring.

This probably isn't the right part of the forum, so I do apologise if I am off topic.

If it fails and sound bad it means I'll have an excuse to make another. If it succeeds and sounds at least as good as my currant amp I'll be mighty impressed and will build another with better parts. To see how good I can make it sound.

So I started making a review of what I need and what I have, I seem to have everything from my two donor amps except that I don't have four identical capacitors to use as the decoupling capacitors. The kits all quote 1500 uF (50V) electrolytic Panasonic's. I've also acquired some old lab equipment that they were throwing away. Some of which were capacitance boxes (nice Bakelite boxes, heathkits I think) . There are two types of boxes, one type with standard capacitors and a wide and varied range of them, unfortunately there isn't four identical ones ( I can make up four 1000 or 1500 or 2000 uF but not identical capacitors)



The second type are 1000 to 10000uF range boxes, but have these wacky metal boxes that I can't identify, but I have five identical ones. They are about the size of a box of matches and have colour coded wires, for example a common and a 1000uF connection and a 2000uF connection.

So the question is can anyone identify the capacitors and if so can i use them as my decoupling capacitors, they are old so they might be rubbish sonically. Do the decoupling capacitors have to be electrolytic?


Also, is there a way to identify the wattage of resistors ?



This is all interesting stuff

thanks and regards,

Fat bob
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Old 10th April 2008, 05:38 PM   #2
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Well, resistors are the easy part--fairly small tan or brown resistors should do.

As for capacitors, you'll need 4 matching capacitors.

One option is to use 4 of 220uF or 330uF or 440uF--if purchased new, its about 85 cents Canadian (for the set of 4).
Then you can make your power supply out of matched pairs (same model cap on v+ as is on v-) and parallel enough to make at least 4700uF per rail.

At the thrifty shop or garage sale, you may be able to find a disfunctional old transistor stereo. Useful parts therein can include heatsinks (normally quite expensive new), and if there's especially good luck, a transformer in the range of 36vct (18+18) to 52vct(26+26). Plus, there's probably a nice bridge rectifier in there, potentiometers galore, speaker terminals, rca jacks, maybe even a working radio, and you can recycle the enclosure just by adding a nice bit of aluminum or thin wood over the front.
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