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Old 16th June 2010, 03:50 PM   #1
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Default Questions on First Chipamp

After a 20+ year hiatus as an electronic tech I've pulled out my old scope, variac and DMMs. They all still work, but I'm pretty rusty myself. and most of my books are gone.

I decided to build a chipamp and may have rushed things a bit in my parts selection. I picked up two transformers and four boards on ebay

http://cgi.ebay.com/ws/eBayISAPI.dll...d=482789703007

28Vx2 200VA Toroid Transformer DIY HiFi Power Amp Kit - eBay (item 250649543698 end time Jun-18-10 12:38:43 PDT)

After reading through the diy forums I'm starting to think the LM3875 isn't that great a chip anymore and the supply voltage will be too high for reliable operation. I plan to drive 2 different pairs of 8 ohm speakers as surround sound on my patio. I also was thinking of finally finishing that Phoenix Systems Parametric Equalizer project from Popular Electronics that I bought pcbs for long ago, and try to use it as some sort of preamp. It's supposed to go in the tape loop. Can I just feed it the headphone out from an mp3 player or Squeezebox Radio? Where's the best place for a volume control?
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Old 16th June 2010, 05:56 PM   #2
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Location: Cape Town
your link for the kits you selected doesn't work (or is it just me?)

transformer voltages are too high
For 8 ohm loads they should be 25Vx2

You might want to build regulator circuits ,but that might prove to be far to much effort and a waste of power.

here is an example of such a circuit:

PA100 DIY 2x LM3886 in parallel gainclone audio amplifier
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Old 16th June 2010, 07:11 PM   #3
AndrewT is offline AndrewT  Scotland
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Location: Scottish Borders
Hi,
that is a 115/230:28+28Vac transformer.
It will give too high a voltage for a 3886 from 240Vac.
It will probably run the 3886 at just outside it's specification maximum rating when mains voltage is high.

It should just about be OK when run from 220Vac mains supply.
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regards Andrew T.
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Old 23rd June 2010, 07:01 AM   #4
Arx is offline Arx  Canada
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Location: North Vancouver, B.C.
Nothing wrong with the 3875.

It's one of the easiest to build with on minimal Point to point wiring. Some of the newer amps make a bit more power (especially with 4 ohm loads), but the differences aren't exactly huge.
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