The amp described on this page, is a very simple poweramp based on the National Semiconductor chip LM3875. According to National it’s a chip meant for TVs, compact stereos etc. But many people claim that these chips are great high-end amps…
So I decided to try building one. The “design” work was quickly done, as I just used the guidelines and sample circuit of the datasheet. I designed a small PCB for the amp (I’m lazy), and I made it double sided to make it easier to keep all the ground lines separate, as recommended by National.
The prototype board can be seen below with a 100VA toroid I used for testing.
I made some measurements on the prototype board. The transformer is a 2*15V type, but the output voltage is actually 2*18V. The DC supply voltage is approx. +/-24V at idle.
The maximum output voltage without clipping was 19V peak. The DC voltage dropped to +/-22.3V at that point. My load resistor is 8.33R, so the max. output power is 22W. The output looks like a perfect sine wave until the clipping point. Above that you get a strange looking waveform caused by the protection circuit I think (the SPiKe stuff – see the data sheet). The frequency response is good. The output started dropping slightly above 100kHz, but that should be enough I guess…
The amp can run on higher voltages, so the output power can be increased to about 50W with a larger transformer (and heatsink).
LM3875 amp PCB
A close-up photo of the PCB
I repeated the test above with a 2*22V transformer. The DC voltage was +/-32.2V at idle, and it dropped to +/-30V at full power. The output just below clipping across the audio band (20Hz-20kHz) was about 37W.
The frequency response (at about 7W) was 17Hz-110kHz at -1dB and 7Hz-220kHz at -3dB.
So is it the best amp I have ever heard? No, but it still sounds great. If you need a simple trouble-free power amp for something, this could be it.