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lm 3876 impressions
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Old 5th August 2002, 06:39 PM   #11
mrfeedback is offline mrfeedback  Australia
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Default Unobtainium

Yeah me too - I just need to get a round tuit.
If you see one, please let me know.

Eric.
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Old 5th August 2002, 06:46 PM   #12
jackinnj is offline jackinnj  United States
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lm 3876 impressions
Default I use the inverting flavor

In the inverting version you don't have to worry about an electrolytic (C5) in the ground return path. In the non-inverting example which you show, this cap should be bypassed anyway with a mylar (although you won't see it in the Nat Semi site).

You can be a little clever if you burn some printed ciruit boards for this by placing R2 and R3 on the bottom of the board. A 1/4 watt resistor will fit perfectly. This keeps the leads very short. I also cut off the "NC" leads since they just get in the way.

note that the pins for this chip are on 0.067" centers. I created a component in Ultiboard for this and one of Nat Semi's motor control chips (LMD18200/18201) which uses the same layout.

I found that the distortion with these chips was actually a little better than Nat Semi claimed. They state 0.06%, 20Hz-20kHz, I found 0.04% (at least in the inverted version).
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Old 5th August 2002, 06:54 PM   #13
planet10 is offline planet10  Canada
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Default Re: I use the inverting flavor

Quote:
Originally posted by jackinnj
[B]In the inverting version you don't have to worry about an electrolytic (C5) in the ground return path.
This is the one that people seem to like the best and the way i'll build mine.

dave
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Old 5th August 2002, 11:15 PM   #14
Jean is offline Jean  United States
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What else is involved in the inverting version of the circuit ?
whats the main benefit other than less parts (no c5) ?

thanks
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Old 5th August 2002, 11:56 PM   #15
jackinnj is offline jackinnj  United States
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lm 3876 impressions
Default Here's a rough schematic

The 1.0 uf capacitor bridges the V+ and V-. This particular design is for the '3875 which lacks the mute feature. I have used 330uF caps and 1000uF caps and can detect no difference.
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File Type: gif inverted.gif (3.7 KB, 1827 views)
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Old 6th August 2002, 12:11 AM   #16
planet10 is offline planet10  Canada
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Quote:
Originally posted by Jean
What else is involved in the inverting version of the circuit ?
whats the main benefit other than less parts (no c5) ?
There is a lot of discussion & experience over the Amp Chips DIY Forum. For the inverting circuit, look for Thorsten Loesch.

Note: There are also 2 other active related threads:
http://www.diyaudio.com/forums/showt...&threadid=5018
http://www.diyaudio.com/forums/showt...=&postid=48964

dave
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Old 6th August 2002, 02:38 AM   #17
fender4 is offline fender4  United States
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Default Fun, easy, cheap...and it even sounds good!

I've built a stereo pair of LM3875 based amps similar to the Thorsten circuit posted on the DIY Amp Chips board, except I didn't include the 1.0uf cap or the resistor on the output....7 parts, not including the pot or the chip itself. No hum, extremely quiet, and sonically pleasing. Actually, it's the best sounding amp I have, but I only have mid-fi Circuit City stuff.

Keep the layout tight so noise problems don't bite.

-f4
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Old 6th August 2002, 03:34 AM   #18
jackinnj is offline jackinnj  United States
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Default Nope, you need the res, zoebel or inductor

this amp will go into hf oscillation with disastrous effects if you don't take precautions.
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Old 7th August 2002, 11:33 AM   #19
fender4 is offline fender4  United States
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Jackinnj,
Without a 'scope, is there a way or me to tell if the amp is going into HF oscillation? I've have listened to this amp or 100's of hours now, and I have never heard anything but clean, "sparkling" ampliication of source material, with ZERO noise at full volume with no signal (if I put my ear withing 1cm of the tweeter, I can hear a VERY slight hiss). I have used it with three different sets of speakers, and I haven't noticed any damage to them. The chips get warm, but not hot, on the heatsink.

BTW. the amps are built from Rat Shack components, and I am using a 18-0-18 Piltron transformer with a single bridge for both amps (I know.... ). I am going to uilt a new pair soon with niver components, so I would like to know if this I am creating an amp that is flawed, even if the flaw is not noticable to my ear. I know that adding the additional components would add "stability", but is it absolutely necessary if my layout if the very tight and I experience no problems with it.

I am no expert with electronics by any means! I just really like a minimalist approach and would have a hard time adding components to a design that seems to work fine. I am open to suggestions, though!

Thanks!
-f4
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Old 7th August 2002, 12:18 PM   #20
jackinnj is offline jackinnj  United States
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Default The added components add nothing to the noise

the inductor in the Nat Semi (non-inverted) design is about 7 turns of wire on a 2 watt resistor, and the 0.22R resistor in the Thorsten (inverted) design won't contribute any meaningful noise either (I use a pair of 0.47R's as a I have hundreds of these bought for a buck). At any rate, a pic is enclosed showing the resistors attached to the bottom of the board.
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