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Old 11th May 2004, 06:27 PM   #1
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Default gain calculations

i was wondering how the gain factor of an amp is calculated...i was reading the ESP website, and rod said that for a 60W amp, the gain is about 27..how do you get this figure?
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Old 11th May 2004, 07:04 PM   #2
sreten is offline sreten  United Kingdom
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Nominal max output voltage / Nominal input voltage for max output.

sreten.
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Old 11th May 2004, 07:07 PM   #3
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You would use normal op-amp gain calculations, Rf/Ri for inverting, 1+Rf/Ri

to get the gain in dB , 20Log(gain)

In the case of Rods amp ( I assume you were reffering to his P3A?)

It looks like Rf=22k and Ri = 1k which gives the voltage gain of 27dB
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Old 11th May 2004, 08:00 PM   #4
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i understand the formula...its the same for inverting and non inverting op amp circuits...so am i to assume that it would be

gain = Vout(rms)/Vin(rms)

where Vin(rms) is approximately 1V (depending on desired sensitivity)?
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Old 11th May 2004, 10:27 PM   #5
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The AVERAGE power into a load of resistance R for a sinewave is (Vpeak)^2/(2R).
So you work out the Vpeak you need for the average power you want (sometimes average power is incorrectly refered to as "RMS power" by the way - "RMS power" doesn't exist), then divide this by the Vpeak you have available at the input and this is the voltage gain you need.

Example: You want 100W average into 8-ohms at full volume and your CD player gives 2V peak as an input. The output peak voltage needs to be at least SQRT(100*16) = 40. So the voltage gain must be at least 40/2 = 20.

A voltage gain of between 20 and 50 is usual for domestic power amps.
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Old 12th May 2004, 12:33 AM   #6
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cheers mate...i really appreciate everyones help ..makes learning so much easier when you can read books and ask questions too
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