Having issues with LM1875

Thread Starter

Mohamad Tarabah

Joined Jul 4, 2016
54
I did try to power a 4 ohm woofer from a LM1875 circuit (it's the second circuit in the datasheet?) And it worked but not giving more than 3watts. Now I went into trying the first circuit in the datasheet but because it's a little bit complicated I did re-draw it and soldered the parts together. You can see in the pics below the schematic and compare it with the original one in the datasheet. First when I touch the pin which should be Vin, no noise is coming out. And secondly I'm a bit confused about where should I input the negative side of the power supply (12v lead acide) is it -Vee or just ground or both ? And where should I also connect the input signal
I appreciate your help
 

crutschow

Joined Mar 14, 2008
34,669
You are confused because the first circuit requires two separate supply voltages.
It won't give you more power unless you add a second supply.

For a single supply you need to stay with the second circuit.
According to this:
upload_2017-3-13_10-56-49.png
you should be getting about 5W output into an 8Ω speaker with a single 12V supply.
You should be getting more than that into a 4Ω speaker but perhaps it is current limited at that supply voltage.

If you want more than that from your 12V supply then you need to go to a bridge output type of amplifier.
That will double the peak speaker voltage and give 4 times the power output (since power is proportional to V²).
There are a number of 12V audio power chips available with a bridge output, which are common for automotive applications.
Here's an example.
 
Last edited:

DickCappels

Joined Aug 21, 2008
10,224
If you are going to run from a single power supply you should consider putting a large capacitor in series with the speaker, as shown in the second schematic in the datasheet. That will save you a lot of wasted power.
 

crutschow

Joined Mar 14, 2008
34,669
Now I've already built the first circuit. What should I do please ?
You could build another and use them in a bridge configuration (speaker connected between the two outputs).
For that you would also need a gain of -1 inverting op amp to invert the signal to one of the amp's inputs, so that one amp is pushing while the other is pulling.
For a bridge configuration you don't need the large output capacitor in series with the speaker.
 
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