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Old 26th February 2010, 09:30 PM   #1
dasein is offline dasein  United States
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Default Decrease gain and heat? newb question.

Hey guys,

I feel really dumb asking this question because it just shows how little I understand the circuit but I built PD's LM3875 kit and it's working beautifully (well one channel wasn't working quite well but after turning the amp off and on a few times, it started working just fine, I'm guessing burn-in I hope...)

Anyway, the amp is wayyy too loud, the gain is huge when driving my 8ohm 86db speakers. I didn't include a volume pot and I don't really need to but I was wondering if there was a way to decrease the gain permanently and decrease the heat output out of the LM3875 chips. It runs pretty hot (but no clipping or distortion) but I would be more comfortable if I could make it run cooler and lower the gain.

I've been scratching my head at the National datasheet for the LM3875 trying to understand the equations at the end and seeing if introducing a resistor between the rca input and ground would help things...
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Old 27th February 2010, 12:24 AM   #2
wwenze is offline wwenze  Singapore
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I feel most amps are designed for use with a volume control (or preamp) that's why they are so loud without one.

A voltage divider at the input, which would have almost the same shematics as a volume control, reduces the input volume.

If you don't need the extra power and are concerned about heat, reduce power supply voltage.
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Old 27th February 2010, 02:35 AM   #3
dasein is offline dasein  United States
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Yea, I have a preamp that I am using to control the volume.

It's just that the gainclone seems to run so hot that it makes me nervous and I feel like I have adequate headroom so if I can turn down the gainclone, turn up the preamp, and lower the heat that would be ideal...

I guess the only option is to drop the power supply voltage.
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Old 27th February 2010, 02:53 AM   #4
ryan750 is offline ryan750  Philippines
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don't drop the power supply, doing that, i think it would just make your amp distort sooner. just introduce some sort of a volume control at the input of your amp, like a simple pot. or you could decrease the gain by lowering the feedback resistor.
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Old 27th February 2010, 04:29 AM   #5
godfrey is offline godfrey  South Africa
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Quote:
Originally Posted by dasein View Post
... so if I can turn down the gainclone, turn up the preamp...
That's not going to change the heat.
Quote:
Originally Posted by dasein View Post
I guess the only option is to drop the power supply voltage.
That will. Heat depends on power supply voltage and how loud it's playing, not gain.

More importantly, have you got the LM3875 chips properly mounted with thermal grease on decent heatsinks? That's the way to get rid of heat.
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Old 27th February 2010, 08:03 AM   #6
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Maybe you could put some numbers to it. How hot is 'pretty hot'? If the amplifier's protection does not kick in, the temperature is probably okay.

And when does it get hot? Only when music is playing? That would be okay. Or already when no music is playing? That would not be okay.
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Old 27th February 2010, 08:50 AM   #7
AndrewT is offline AndrewT  Scotland
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show us a pic of the heatsinks and the method of mounting the chipamps to heatsinks.
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