Trying to test LM1875 circuit, don't know where to place voltage probes

Thread Starter

askDIY

Joined Oct 18, 2023
17
Hi all,

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

1711588749053.png

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.
1711588871164.png
 

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MrChips

Joined Oct 2, 2009
30,986
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?
 

Thread Starter

askDIY

Joined Oct 18, 2023
17
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.
 

MrChips

Joined Oct 2, 2009
30,986
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.
 

BobTPH

Joined Jun 5, 2013
9,182
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.
 
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