56W LM3886 / LM3876 Gainclone


56W LM3886 / LM3876 Gainclone


It is a 2x56W power amp based on National Semiconductor's LM3876T chip (they come in 2 versions T and TF the latter having an insulated case), this type of amp is also known as a gainclone because it is an improved copy of Gaincard amplifier. seriously this amp can outperform most commercial amplifiers/receivers (minus the video upscaling/switching) when built properly ie it has very low THD you will not be disappointed by how good it sounds . When connected to AV equipment such as dvd players this thing gets insanely loud (despite having only 56W per channel), i've have never required more than 25% volume because it hurts my ears NOTE: a even more powerful version using the same circuit with a LM3886 chip can be made giving 68W per channel.

56W LM3886 / LM3876 Gainclone

Power Supply
See that you'll need both +28V and -28V hence two rectifiers(you can use normal bridge rectifiers) Transformer to be used for 2 channels: 160VA-250VA toroidal (the donut thing) with 2 x 25-28V secondaries. If you are powering more than 2 channels a 300VA or 500VA transformer is recommended. The capacitors used after (40000uf-60000uf 50V) will bump this up to around 30V. NOTE: the two grounds shown are to be joined together to form the 0V line which is also connected to mains earth.

The chip in the centre can be interchanged between the LM3876 (56W) and LM3886 (68W) because they have the same pin configs except that the LM3886 requires that V+ is connected to pins 1 and 5 while V+ for the LM3876 is only pin 1. Resistors are preferably 1% tolerance otherwise 5% is fine, all electrolytic capacitors are 50V/63V (including those on amp and psu). You will require one of these circuits for each speaker.

Things to take note when assembling:
1. All the grounds (including input negative and speaker negative) get joined together on the PCB and to the 0V point which goes to the mains earth.
2. Never run the amplifier without a heatsink attached to the LM chip(s)
3. Test the voltage produced by the power supply before connecting to the amplifier circuit and ensure that it is correct.
4. If you are using the T versions (metal tab/casing) of the LM chips you must insulate the chip from the heatsink using mica sheets OR insulate the heatsink from the case if using a metal case. Do not ever allow the heatsink to short to ground. TF versions can be directly screwed to the heatsink with thermal grease applied.
5. Do not short the V+ or V- wires of the power supply to ground (0V) because it causes big scary sparks. Also do not short the speaker output (+) to ground because even though the LM chip has SPIKE protection (protects it from over-current, overheating, etc) it will most likely result in a dead chip.
6. Always double check your soldering on the amplifier and power supply before connect to power. Any mistakes will probably blow caps, the chip and/or other components.
7. Use shielded microphone cable from the RCA connectors to the volume pot and to the amp pcb and keep these wires as far away from the AC line otherwise you'll get funny humming noises...
8. And finally after you’ve done all this and nothing has exploded and nothing smells burnt, connect an audio signal and test it on a crappy speaker first (you wouldn’t want to blow those AWESOME Klipsch speakers would you now Empty Quarter? :Bleh)
FINAL NOTE: this amp only runs speakers between 4ohm to 8ohm, DO NOT connect 2ohm loads (blown chip again)
If you hear some music, connect up to a better speaker, turn up and enjoy!

56W LM3886 / LM3876 Gainclone

56W LM3886 / LM3876 Gainclone

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