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help with lm3875? Answered

hi im trying to make a simple amplifier using a lm3875
i am trying to power a 30w 8 Ohm speaker 
i tried googled for this chip and got a lot of info but all of them had really powerful power supplies for my project 
what kind of power supply do i need for this purpose can anyone please help 
i have 12v 2A, 32v 1A, 22v 1A supplies can i use one of them?
also this ic requires v- how can i achieve that with one of my power supplies 
im a newbie to all this stuff so please dont call me dumb if i asked some weird questions 


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Best Answer 8 years ago

While Steve's answer is partially correct, he didn't do the full math.
(His answer with the 30V supply is correct, but only for a 30Ohm speaker)

So, you want 30W at 8Ohm. Let's find the voltage and the current.

P = I * U,  I = U / R, 

P = U² / R,  or:  P * R = U²,  or U = Sqrt(P * R)

P = 30W, R = 8 Ohm

U = Sqrt(30 * 8) = Sqrt (240) = 15,5V 

Let's just say, U has to be 16V. Now do the check:
A voltage of 16V will drive a current of 2A through 8Ohm and 16V * 2A will yield 32W. QED.

So, with the given speaker, you need a 16V power supply with at least 2A. (At least. This calculation doesn't care for losses, switching limits, saturation etc.) You can try with two 12V 2A supplies by combining them into a +/- 12V supply.


Answer 8 years ago

"You can try with two 12V 2A supplies by combining them into a +/- 12V supply." as in by reversing the polarity?

can you also assist me with the resistance capacitance circuit in order to make a low pass filter as i want to use this as a sub woofer driver
Thanks a ton


Answer 8 years ago

This works only with independent power supplies (no common ground)!

Connect the + output of power supply A with the - output of supply B. This connection point is your ground (0V). The minus output of supply A is -12, the + output of supply B is the +12V output.

For the filter network? Sorry, I'm more a digital guy. The internet should have some resources. Just make sure, you filter a) the input to the amplifier or the frequency response of the amplifier (i.e. make it amplify lower frequencies more). Filtering the output is possible, but you'll need bigger components (current wise) and convert some of the amplifiers power to heat not sound.


8 years ago

You need at least a 30W power supply ! Your 32V 1 A supply sounds about right. on the face of it, the chips specced from 20 to 80 odd volts, but personally I'd want to run it on +/- 15 V rails (AT LEAST 1 amp), or the circuit design becomes quite nasty.

Big power amps need big inflexible power supplies. You want one that gives +/- 15 V under all load conditions, and not just when its piddling around with 100mA of output.