lm 3876 impressions

Just for fun today I put together very simple amplifier based on the lm 3876 circuit. I used Avel 25-0-25 transformer, 25a bridge, and two 10,000 uF Mallory for the power supply. I didn't really know what to expect when it was complete. I am using onkyo 2ch receiver connected to my computer and diy 2way speakers. I went ahead and replaced one channel with the lm3876 ic amplifier and fired it up. I was surprised that it sounded better than my receiver. The biggest difference in sound is the bass response, compared to the receiver it made my right speaker sound much better. There is no hum or noise when there is no signal, don't hear anything with my ear close the speaker. I used my maudio 24/96 as the source w/o a passive volume control in line.

I am going to finish it up and use it as a computer speaker amplifier :)

I am really impressed with this little IC.

thanks for reading.
I do not have any experience with these except for servicing consumer gear using them.
In the typical Jap home amp they work quite decently imo.

In my experience on other amps I have found that the quality of the feedback AC shunt (R5 in above diagram) is important. Decoupling caps are important too ime.
Ime, SMPS grade caps work a lttle better and can give a more pleasant sound.

Dave, any experience with this on these type amps ?


Ps - I have recovered an SMD chip in the past by grinding the epoxy back with a dremel tip and exposing the broken leg so that I could attach a wire. Not difficult at all.
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).
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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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. :eek:

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

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.... :rolleyes: ). 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! :)

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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