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Old 25th July 2009, 04:45 PM   #1
tom91 is offline tom91  United Kingdom
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Default Preamp for LM1875

I've built an LM1875 power amp, using the circuit diagram from a kit (http://www.quasarelectronics.com/305...ule-lm1875.htm). I've not included a volume control yet.

It sounds great when I plug it into the headphone jack on a small radio, and I can get a decent amount of volume without distortion or the chip getting too hot. However, when I use it with my MP3 player, I don't seem to get enough volume out the amp even with I have the MP3 volume on full. It works fine with other amps.

Could I build some sort of preamp to boost the input audio level? Any ideas on circuits? 741 or LM386 might be suitable? (I know the LM386 is technically a power amp, but the radio is probably putting a similar level out it's headphone jack and that seems to be fine.
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Old 25th July 2009, 06:29 PM   #2
AndrewT is offline AndrewT  Scotland
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Location: Scottish Borders
what is the sensitivity of your speakers and what is the gain of your power amp?

Have you measured the output voltages of your sources?
Have you checked that all your source outputs are free from DC offset?
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Old 26th July 2009, 07:53 AM   #3
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If you need more gain, you could increase R5. E. g. replace 180k with 1M8. That could lead to higher noise however. Considering the really low cost it should be worth a try.

Another alternative would be to decrease R4 and increase C3 by the same factor. E. g. replace 10k with 1k and 22 with 220. In that case you will need enough space for the bigger capacitor.

In both cases your amplifier will be louder, so be careful, when you connect other sources without volume control. If it still is not enough or if it is too much, experiment with different values. The gain for your amplifier is (R4+R5)/R4. C3 and R4 are a filter, and C3xR4 should remain the same, before and after any modifications for a start.

Follow the link within your link and download the datasheet. Everything you need to know is explained in it.
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Old 26th July 2009, 03:08 PM   #4
tom91 is offline tom91  United Kingdom
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I tried a 1M8 resistor at R5, and the gain is much better. I can now get a decent sound even with the volume on my MP3 only halfway up. Problem solved.

Thanks for your help!
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