LM3875 for 4 ohm speakers

sutton1

Member
2013-02-27 1:28 am
Hey all,

I'm building Peter Daniel's LM3875 gainclone kit, and having concerns about the gain of the amp. Clearly this design has worked for a ton of people, so I know I am misunderstanding something. I am a huge noob here (degree's in mechanical engineering, not this magical electrical stuff), so correct me when I'm wrong. The way it's set up gives a closed-loop gain of 100. If I'm playing from my iPod, which I intend to, that's an input voltage of 1.5V.

1.5V*100 is 150V coming out of the chip? Then 150V into a 4 ohm load is going to give 5625W and draw 37.5A. Things are going to go up in smoke! So, if I've got this right, I'm going to need some more input resistance?

Even if I use 8ohm speakers, that's still 2812W and 18.7A.
 

sutton1

Member
2013-02-27 1:28 am
But those numbers seem ridiculous. Since posting, I read in the datasheet that the chip has overvoltage protection which limits the output current. Is that what I've been missing?

Also, Peter says that R1 (the input resistor) is optional and that he just puts a wire usually. That would make for a very high gain using the A = Rf/R1 +1 equation.
 
You can't get output voltage from nowhere. It has to come from the power supply and cannot exceed this. In fact, the maximum output voltage swing will be somewhat less than the power supply voltage.
If you attempt to exceed this the amp will clip.
I suspect that this is the fact you are missing.

Frank
 
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