LM3875 Gainclone Amplifier

Indroduction:
The LM3875 chip from National Semiconductor is a 56W continuous power amplifier into an 8-ohm load. This chip delivers unbelievable quality, especially for it's low power levels. The only donwnside is it is only capable of driving an 8-ohm speaker. If ran at a low voltage, it will be able to drive a 4-ohm load, though not well. There is a way around this, though, involving using two chips per speaker. Using a 0.1ohm 5W 1% tolerance resistor on the outputs allow you to hook each amp module in parallel mode, allowing it to drive a 4-ohm load.

Schematic:
This schematic is for half of the board only. The board also has a spot for a 22uf non polarized capacitor between R4 and ground. This part may or may not be needed depending on your application of the circuit. If your input source is always connected, or you use a preamp, they are unneccesary, and I reccomend bypassing them with a jumper to have as little unneccesary parts as possible in the signal path. However, if you may ever have the amplifier powered on but disconnected from the input source, it is reccomended that you use this capacitor. This capacitor prevents large DC currents from being sent to the speaker when the input connection is disconnected. To bypass the capacitors, just run a jumper for each of them.



PCB:
This PCB is a stereo / parallel board. If you are planning on driving 4-ohm speakers, use one board per speaker, with the positive outputs wired together. This will provide 110W into each speaker. It is reccomended to use a .01 ohm 5W wirewound resistor for stabilization. See below for how to wire this circuit into parallel mode. If your speakers are 8-ohm, you only need one pcb for both speakers, with a power output of 50W per speaker. Using parallel mode on an 8-ohm speaker does not increase power output considerably. Increasing power output into an 8-ohm speaker is trickier. When the amplifier is bridged to increase power output to 220W into 8-ohm speaker, the load each amplifier "sees" is 4-ohm, which the LM3875 is not capable of powering. As a result, you will need to use four amplifiers (two boards) per speaker and a circuit capable of inverting the signal to one of the amplifier pairs. Both boards will be wired in parallel mode, and then the input to one of them will be inverted for more power. An example of a circuit that could do this would be using a Texas Instruments DRV134 chip to invert one of the two non-inverted amplifiers. This is all explained further down.

Click here to download the eagle PCB file


To print the PCB you MUST use the Eagle file, not this image or the scale will be incorrect. This image is to be used as a guide while assembling. The brown lines are jumpers that need to be wired into the circuit.




Parallel Mode:
Parallel mode allows the amp module to drive a 4-ohm load at 110W. Rather than using one board for stereo, you will need 2 boards, each one running in a parallel mode. Here is how to wire the board for 4-ohm speakers:




Bridge/Parallel Mode:
You can also quadruple the power into an 8-ohm speaker. This requires 2 boards (4 amplifiers) per speaker, plus a DRV134 board. The DRV134 inverts and balances the signal so that one of two non-inverted amplifiers amplifiy an inverted signal, thus allowing bridge mode to work. Currently, I'm not done with the circuit or board for that yet. It will be done soon though. You wire the two boards using the parallel mode schematic. Rather than connecting the ground to a speaker, you take the positive output of the resistors from one amp and connect them to each side of the speaker. When the DRV134 board is finished, you simply connect the input to it, and one of it's two outputs to both amp inputs.

The wiring is done like this:


Results:
The sound is excellent, by far the best I've heard from a chip amp. For only 56W, the sound is, as I've said, fantastic, and sounds like much more than 56W. I compared a few different amps. First was a 100W x 2 Kenwood amp. I noticed much more detail and the sound was much more controlled with my amp. Undistorted volume levels were also much better. I also compared it with a 100W x 6 home theater amp, and the home theater amp (in stereo mode), was able to reach higher volume without distortion, but they were very close in sound quality. I can't comment on sound quality in parallel mode, because I only wanted to use that mode to try and run a 4 ohm subwoofer speaker. I've never tried bridge / parallel mode either.

Photos of my amp:

Picture of amplifier PCB


Original Internal Layout, before adding power switch and Neutrik RCA connectors



Finished amplifier. The case is not the nicest (I just found it sitting around) but it works fine and it looks a lot better in real life.

The PCB file is avaliable in Eagle format, ready to print. Please download the freeware version of Eagle Layout editor from here to view any pcb file on this website.

- PCB - Eagle Layout Editor File

- Schematic - Eagle Layout Editor file