I need help with an amp using an lm1875. I am using a 15 volt wall wart as a power supply. But i am getting 6 volts of dc through my speaker. Below is the schematic I am using. I forgot to order a 220k resistor for the one near the ic so I had to use a 22k. Could that be the reason?

 

You are using single supply at 15VDC, correct? Then you cannot directly couple the speaker to the amplifier. Put a 2200-4700uF capacitor in series with the loudspeaker.

 

MrChips pretty much nailed the reason you are getting 6 volts across your speaker.

The 220K resistor you replaced with a 22K will lower the gain of the amplifier by a factor of ten.

 

Like this? Or do I need to switch the polarity?

 

Thank you for explaining the resistor les. I was wondering why it was a little quiet.

 

You need to switch polarity. Negative lead of the capacitor goes towards the loudspeaker.

 

You need to switch polarity. Negative lead of the capacitor goes towards the loudspeaker.

That's what I meant. And does it matter which cap I use? I have both a 4700 uf and a 2200uf.

 

Hello,

When I look at your last picture, it looks like you are connecting the speaker to pin 1.
Pin 1 is one of the inputs of the amplifier, pin 4 is the output.
See this example schematic from the datasheet:

Bertus

 

Hello,

When I look at your last picture, it looks like you are connecting the speaker to pin 1.
Pin 1 is one of the inputs of the amplifier, pin 4 is the output.
See this example schematic from the datasheet:



Bertus

No I have pin one as input and pin 4 is output.

 

That's what I meant. And does it matter which cap I use? I have both a 4700 uf and a 2200uf.

Try both of them and listen to the bass response. If the 2200μF cap sounds good to you then go with that. You can even compare it with a 1000μF cap.

 

I'm wanting to make a 3 band eq for high mid and low. How should I go about putting that in there? Also I am putting an lm386n pre amp on it.

 

Hello,

A LM386 is not a preamplifier.
It is an 1 - 3 Watt power amplifier, depending on the model.

If you are looking for a 3 band tone regulation, have a look at the following schematic:

Bertus

 

Thanks and what would be a good ic for a pre amp?

 

Hello,

Do we need to present everything on a golden platter?
A quick search on this forum will give you this:
Op Amps

Bertus