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Old 8th December 2002, 10:42 PM   #1
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Default 1/2 DIY LM3886 amp...

I (my dad) bought a second hand amp from cash converters, but it had bad distortion or something (can't remember) I wanted to remove the existing poweramps (not that good anyway) and replace them with a few LM3886 based ones. The problem is that I don't know what size heatsink I need!!! (ok, I'm stupid) and whether or not I would be able to fit 4 amps in teh case or only 2... If I was to use 4, I would use the existing preamp/input selector, etc for 2 of the channels, and havew the other 2 channels volume/tone controlled by the audio sorce (my computer) to drive my subwoofer, while the other amps would drive my other speakers..... Or, in teh future I may modify the amps front pannel to have 2 volume controls, but then the knobs wouldn't match the rest of the case.. lololololol so, what rating heatsink do I need for each amp? I intend to use about a +-30-35volt rails, as my main speaker are 8ohms... any problems with this idea??
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Old 9th December 2002, 01:04 AM   #2
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Hi Skinny

I use one of these-
WANTED! P3A Amp heatsinks!
cut in half, one part per LM3785, for a stereo amp running at 30V lines.

Even under full load, they only get warm to the touch, so you might be able to get away with two channels on each half.

If you want the tech. calcs, I can probably work them out, but I thought a practical example might be easier to get your head around

BTW, the LM3875 sounds much better than the LM3886, and the power difference is negligable at normal listening volumes.

For a circuit diagram, do a search for Gainclones, or check out this website
http://pub4.ezboard.com/ffakeidsfrm1
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Old 9th December 2002, 01:13 AM   #3
halojoy is offline halojoy  Sweden
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Default LDO amplifier and LM3875

Quote:
Originally posted by pinkmouse
Hi Skinny
If you want the tech. calcs, I can probably work them out, but I thought a practical example might be easier to get your head around.
BTW, the LM3875 sounds much better than the LM3886, and the power difference is negligable at normal listening volumes.
Interesting about LM3875
I maybe will change my POWER-IC favo to that from LM3886

You haven't seen
the circuit I drawn for you.
Might give you a new hint what can be done
when it comes to LDO-regs
The practical components values, you probably can work out by yourself.
It is not as hard as it might seem
Otherwise I can try and help you out.

for you others: see Thread: Electronic Parts, "LDOs"
/halo
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Old 9th December 2002, 10:04 AM   #4
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Quote:
Originally posted by pinkmouse
Hi Skinny

I use one of these-
WANTED! P3A Amp heatsinks!
cut in half, one part per LM3785, for a stereo amp running at 30V lines.

Even under full load, they only get warm to the touch, so you might be able to get away with two channels on each half.

If you want the tech. calcs, I can probably work them out, but I thought a practical example might be easier to get your head around

BTW, the LM3875 sounds much better than the LM3886, and the power difference is negligable at normal listening volumes.

For a circuit diagram, do a search for Gainclones, or check out this website
http://pub4.ezboard.com/ffakeidsfrm1
Ok thanx for that.. but I kinda, sorta wanted to use LM3886s as I already have a few of them.. :P

Calculations are not required.. lol

I sorta wanted to make the highest power amp possible for as cheap as possible, but I need a transformer, which will be EXPENSIVE!!!!!!! hmm, anyway.. thanx for the help..
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Old 9th December 2002, 10:07 AM   #5
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Umm. I looked at the data sheets for both ICs.. are they pin-pin compatable?? they aren't, are they.. which means I have to make my own boards.. hmm...

Just looked at the data sheet again, and I think I'll go with the LM3886s...

The problem is I want to drive 8ohm with 1 pair of amps, and 4 with another, both using the same supply voltages.. If I use LM3875, with a +-28volt supply, I get ~50watts into 4 ohms, but only ~38watts into 8ohms...

If I increase the voltage to say.. +-30volts I get over 40watts @ 8ohms, but the 4ohms drops...

this is not the case with the LM3886, is it? not as much, right??
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Old 9th December 2002, 10:17 AM   #6
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Quote:
Originally posted by pinkmouse

BTW, the LM3875 sounds much better than the LM3886, and the power difference is negligable at normal listening volumes.
Hi Pinkmouse,

have you used both chips with exactly the same support circuitry. If so, what kind of a difference did you perceive?

I have heard similar claims which all seem to go back to some post by Thorsten in another forum, and knowing him and the kind of crusades he can embark on, I have eyed them suspicioully. Also, just going by the data sheets, the 3886 would seem to offer superior THD and IMD.

So, any kind of listening experience or measurements would be very helpful. And I promise I won't riducule any reports.

I have used the 3886 with very low source and feedback impedance (on the order of 500 R) and without any coupling/DC blocking caps, in the noninverting configuration, and I was quite impressed what the little devil could do.


Regards,

Eric
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Old 9th December 2002, 10:21 AM   #7
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How do I attach more than one picture?
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File Type: jpg 1.jpg (46.7 KB, 1705 views)
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Old 9th December 2002, 11:17 AM   #8
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LOOK AT IT ALL!!!!
didn't know how to upload multiple pics, so I made it 1 pic...

You know what.. I give up... you can be happy with only 1 picture!!!!!
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Old 9th December 2002, 11:20 AM   #9
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Hi Halo, thanx for the LDO post, I have seen it, I just need to get the breadboard out and have a play, as I find that easier than doing it in my head!!

Eric, yes, Thorsten can be quite evangelical at times, but I think his heart is in the right place

My listening tests come purely from breadboard versions of the gainclone, I never actually built a proper dead bug or pcb variant. I used the same basic inverting circuit for both, using standard quality components.

I found the LM3886 slightly more dynamic on bass, but with a very closed in sound on mids, almost as if some sort of compression was going on, but with good treble. The LM3875 had a little bass roll off, ( which I sorted in my "production" variant by adding two more 1000uf caps near the diodes), but a very clean mid and top.

Perhaps "Much better" is perhaps a slight exaggeration, but the differences are there, and the LM3875 just seems more musical and open. I wonder if the LM3886 was perhaps designed for boom box style hi-fi, where perceived power is a better gimick than real power, who knows...
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Old 9th December 2002, 11:25 AM   #10
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This is the insides, with the power amp removed, the brown is a piece of cardboard, which will be replaced with aluminium to mount the transformer on top. This also means that a large section of the preamps is fully shielded.....
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