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11th May 2004, 05:27 PM  #1 
diyAudio Member
Join Date: Apr 2004
Location: BC

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?

11th May 2004, 06:04 PM  #2 
diyAudio Member RIP
Join Date: Nov 2003
Location: Brighton UK

Nominal max output voltage / Nominal input voltage for max output.
sreten. 
11th May 2004, 06:07 PM  #3 
diyAudio Member
Join Date: Mar 2004
Location: Prince George, BC, Canada

You would use normal opamp 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 
11th May 2004, 07:00 PM  #4 
diyAudio Member
Join Date: Apr 2004
Location: BC

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)? 
11th May 2004, 09:27 PM  #5 
diyAudio Member
Join Date: Jan 2002
Location: Earth

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 8ohms 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. 
11th May 2004, 11:33 PM  #6 
diyAudio Member
Join Date: Apr 2004
Location: BC

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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