Hi everybody,
I built a dual amplifier using 2 LM1875. Currently I am testing it with my 1A power supply that puts out +-21VDC. It isn't enough to power both speakers, so what happens is it starts popping, then buzzing, and then the speaker pulls in and out about 5 times, all in about 5 seconds, then it goes dead. Ususally I have to turn off the power supply and turn it back on. So, if I just want to listen to somthing above volume levels of wispers, I have to just dicsonnect one speaker, and only listen to one or the other. Then, either amp sounds good.
But I did notice one thing. From when I turn on the power until about 1 minute after, I notice that between that time, the speaker had slowly moved out about 1/4 inch. I don't know what would happen if I used 2 speakers, since I cant. Even at low volumes, it will only run for about 20 seconds before becoming unstable and dying. Is that becuase I am only running one speaker, yet still powering another amp?
Thanks for the help, Mike
I built a dual amplifier using 2 LM1875. Currently I am testing it with my 1A power supply that puts out +-21VDC. It isn't enough to power both speakers, so what happens is it starts popping, then buzzing, and then the speaker pulls in and out about 5 times, all in about 5 seconds, then it goes dead. Ususally I have to turn off the power supply and turn it back on. So, if I just want to listen to somthing above volume levels of wispers, I have to just dicsonnect one speaker, and only listen to one or the other. Then, either amp sounds good.
But I did notice one thing. From when I turn on the power until about 1 minute after, I notice that between that time, the speaker had slowly moved out about 1/4 inch. I don't know what would happen if I used 2 speakers, since I cant. Even at low volumes, it will only run for about 20 seconds before becoming unstable and dying. Is that becuase I am only running one speaker, yet still powering another amp?
Thanks for the help, Mike
Your amp has some serious problems, man.
If the speaker sucks in or pushes out when connected to the amp, then you have a major DC offset on the output. This could be a problem with the power supply (maybe one of the recitifiers or capacitors is bad) or with the LM1875, or a flaw in your circuit design or construction.
Here's a way to measure DC offset at the output of the amp while it is playing music. This method will allow your voltmeter to read the DC voltage while filtering out the music that could otherwise screw up the reading. Take a 3K3 resistor and put it in series with a 47 uF electrolytic capacitor (rated at least 50 V), and now connect in series another of the same capaitor, but this time, in the opposite direction. Now connect this across the output of the amp (in parallel with the speaker). Now use your voltmeter set to DV volts to measure the voltage across the capacitors while playing some music (see diagram). You will probably see the voltage constantly rising during the 20 seconds that it takes for the amp to go dead.
If you get a large DC offset then maybe:
Use a voltmeter (or ideally, two) to monitor the + and - voltage rails (that's after rectification and filter caps) while causing this problem to occur. I think that you will notice one of them drop in voltage so much that the amplifier is shutting down. If both power supply rails look fine at the time that the amp dies, then possibly the LM1875 is damaged or is overheating (you are using a heatsink right?), or maybe your circuit is flawed.
If the speaker sucks in or pushes out when connected to the amp, then you have a major DC offset on the output. This could be a problem with the power supply (maybe one of the recitifiers or capacitors is bad) or with the LM1875, or a flaw in your circuit design or construction.
Here's a way to measure DC offset at the output of the amp while it is playing music. This method will allow your voltmeter to read the DC voltage while filtering out the music that could otherwise screw up the reading. Take a 3K3 resistor and put it in series with a 47 uF electrolytic capacitor (rated at least 50 V), and now connect in series another of the same capaitor, but this time, in the opposite direction. Now connect this across the output of the amp (in parallel with the speaker). Now use your voltmeter set to DV volts to measure the voltage across the capacitors while playing some music (see diagram). You will probably see the voltage constantly rising during the 20 seconds that it takes for the amp to go dead.
If you get a large DC offset then maybe:
- the chip is bad
- you have a bad ground. Is the center tap of the transformer securely connected to the circuit's ground?
- you didn't ground reference the input, so the input is floating, which is always bad. The ground of the source must be connected to the ground of the amp. And preferably, you should have an input cap and a resistor to ground at the input (similar to the "typical application" shown on page 1 of the LM1875 datasheet) to remove DC offset at the input.
- the power supply is bad. One of the rails is falling in voltage while the other one stays high. Could be bad rectifier or capacitor.
Use a voltmeter (or ideally, two) to monitor the + and - voltage rails (that's after rectification and filter caps) while causing this problem to occur. I think that you will notice one of them drop in voltage so much that the amplifier is shutting down. If both power supply rails look fine at the time that the amp dies, then possibly the LM1875 is damaged or is overheating (you are using a heatsink right?), or maybe your circuit is flawed.
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