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Old 24th June 2006, 03:48 AM   #1
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Default LM3875 Dual mono amp

Hello everyone. I'm planning my first amp, which I will be building for a friend as a graduation/going off to college present. I want to build an LM3875 amp but I have questions.

The only LM3875 amp kit avaliable is the dual mono. Can I just use one rectifier and one power supply and assemble as if it was a stereo kit? Or am I better off buying the components seperately?

Secondly, approximeatly how much power will this amp yeild into 8 ohmns nominal with a 160VA toroid with 22 volt dual secondaries?
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Old 24th June 2006, 05:16 AM   #2
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I was looking at the transformer thing a few weeks back for a lm3886 based amp.

BIG CAVEAT - I have no training in electronics

2x22VAC secondaries give you +/- ~31VDC when rectified.

If you look at the datasheet - http://www.national.com/pf/LM/LM3875.html#Datasheet & http://www.national.com/ds.cgi/LM/LM3875.pdf - on page 10 (bottom left) you can see that +/- ~31VDC would give you around 45W output power into 8ohms.

A 160VA trafo with 22V secondaries gives you something like (160/22)/2 secondaries = 3.6A on each seconadary.

On page 16 the formula (5) and (6) give us the voltage and current required for a given power output per channel (I think?)

Let say we want to find what peak voltage we need for that 40W into 8Ohms (we already know reasonably good from that table on page 6):

Vpeak = sqrt(2* 40 watts output* 8 ohms speakers) = +/- 25.29V.

But the datasheet tells us we need to add 5V for the dropout of the lm3875 so that's now +/- 30.29V and then to add %15 for regulation = +/- 34.83. Which means it looks like it will be just a tad less than 40W / channel.

And we want to know how much the peak current will be in the same situation:

Ipeak = sqrt((2* 45 watts output ) / 8 ohms speakers ) = 3.35A /channel

This might be a problem? In my figuring out of all of this I was told that this A figure was the peak and that the average is probably half this - and that the capicitors on the power rails would probably help to deliver any sudden current needs. Then again I was also told that getting a bigger trafo (like 225VA or 300VA) would mitigate the risk of running into issues further down the track.

Hopefully someone can shed some more light onto whether your trafo choice is the right one
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Old 24th June 2006, 08:11 AM   #3
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This will help get you going!
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Old 24th June 2006, 06:23 PM   #4
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Thank you guys!
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Old 26th June 2006, 02:01 AM   #5
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I've been wondering about GCs for a bit having seen so much but never built a chip amp, thanks.

What it doesn't answer is why people call them "gainclones".....??? When I first saw them mentioned I didn't realise they were just chip amps based on the national semi-conductor chips, thought they were some special diyaudio.com secret solution to amps
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Old 26th June 2006, 07:16 AM   #6
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Quote:
Gainclones, are so called because the original inspiration is a commercial product called the Gaincard amplifier, produced by 47Labs. This was the other commercial amplifier using a chip amp that had received rave reviews from the hi-fi press, and not a little controversy due to its selling price in relation to its material cost.
From the Gainclone section on Decibel Dungeon.
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