Questions about LM4780 amplifier

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Questions about LM4780 amplifier

Postby Bruno12 » 09 Feb 2017, 08:41

What I have:

• A transformer rated at 150VA / 30V (center tapped) / 5A
• Speaker rated at 30W / 8 ohms (measured 6.3 ohms with my multimeter)
Will use class AB amplifier:
• LM4780 (http://www.kynix.com/Detail/1143170/LM4780TA%2FNOPB.html )

• Question:
If I send +30V & -30V to this chip, I will get 45W at 1%THD+N:
Image
Is it possible to keep +30V & -30V but limit the maximum power to 30W? Is it by playing with the gain (feedback resistor value)?
I read somewhere "...power amps at lower gains requires more input voltage to get the same power output, with the same speaker impedance".
How to adjust/calculate the gain so that I get a maximum of 30W output with +30V / -30V power supply? I own the transformer already so I plan to use it as the power supply (+30V / -30V). It is center tapped. It is 30V-0V-30V.
Image
I know that sending +40V/-40V is almost at the maximum (recommended) value of 84V. Is it possible to use more diode to drop the voltage or any other method (I didn't found any regulator that fit my needs)? Maybe I can send 40-0-40 but then my output power will be around 82W and my speaker is rated 30W. In this case, I can lower the gain to 20db to help (minimum value) and add a resistor before the speaker (ie: 4 ohms, but very high watt)? I guess I could add a potentiometer at the input (A-B) to limit the power sent to my speaker right?

From the title of the chip, I think I cannot go higher than 60W in stereo: "Overture Audio Power Amplifier Series Stereo 60W". What will happen if sending 40-0-40?
If sending 40-0-40 to the amp then the output power per channel will be around 82W. What will happen (an implosion or a black hole)?

I am connecting a line out having 16ohms impedance and 1V RMS signal level. What if I add a potentiometer / volume control at the input of the power amp, thus limiting the amount of signal level going in the amp. Would the power amp consume the 80V even if the signal level is only 500mV RMS (ie)?
Any advice would be much appreciated.
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Re: Questions about LM4780 amplifier

Postby phatt » 09 Feb 2017, 12:28

Just a 10k~20k pot in front and problem solved. Often called a master volume on 100's of brand name schematics. :thumbsup

Oh and if you are not chasing max wattage then stay with 30-0-30VDC supply rails. These chipamps might look clever but they do have limits if one cares to read the fine print. :secret:
Phil.
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Re: Questions about LM4780 amplifier

Postby Bruno12 » 10 Feb 2017, 03:00

Thanks for your suggestion, Phil :D
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