Hi all,

I'm using Multisim and trying to simulate the LM1875 single supply config from the datasheet.



Where exactly do I place the voltage probe so that I can see the output voltage? I tried to place one at the "output" and got a negative voltage. I am a bit of a beginner at this so any help would be appreciated.

 

Where exactly do I place the voltage probe so that I can see the output voltage?

Across the speaker.

 

Across the speaker.

I have an 8 ohm resistor there to simulate the speaker, when I place probe after it, I'm getting no output.

I'm also getting these warnings:

 

Your first test point at the output is correct.
Is there anything wrong with a negative voltage? Think about this more carefully. What does the output capacitor do?

 

Your first test point at the output is correct.
Is there anything wrong with a negative voltage? Think about this more carefully. What does the output capacitor do?

Filters out DC and provides a stable voltage level and reduces noise. I do want to say that the voltage is fluctuating heavily, it goes from -6 to 12 to -5 to 11, etc... all in a span on a second. I cannot seem to figure out what's wrong.

 

Filters out DC and provides a stable voltage level and reduces noise. I do want to say that the voltage is fluctuating heavily, it goes from -6 to 12 to -5 to 11, etc... all in a span on a second. I cannot seem to figure out what's wrong.

No. The purpose of the output capacitor is to remove DC. In other words, the average voltage is zero otherwise the loudspeaker cone would be displaced from its neutral position. The loudspeaker would also be destroyed.

Hence the output voltage can swing both negative and positive. Taking a voltage reading with a voltmeter at the output is not particularly useful.

 

I do want to say that the voltage is fluctuating heavily, it goes from -6 to 12 to -5 to 11, etc... all in a span on a second. I cannot seem to figure out what's wrong.

If the voltage was steady, what would it sound like? Of course it fluctuates, that is how audio is produced. You need to view the waveform as a graph, not the instantaneous voltage.

 

It looks like the SPICE program has no model of an LM1875.
The DC voltages are simple to measure with a voltmeter, they should not fluctuate.

I reduced the large spaces between your parts on the schematic.