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13th September 2006, 11:43 AM  #1 
diyAudio Member
Join Date: Dec 2003
Location: earth

power calculation...correct?
Could someone please tell me if I have this right?
I have a schematic of a rather well know and respected japanese power amplifier with 70 volt rails, dc. equals 49.49 volts available for ac swing, rms assuming 5 volts dropped across transistor, std emitter follower bipolar output stage with 2sc3858 transistors that makes 44 volts say. which is 44v squared /8 ohms gives 242 watts into 8 ohms? now the feedback resistors are 28,890 ohms series and 560 ohms shunt gives a gain of 52.5 which makes 34.4 dbs? the sensitivity is .775 volts rms to full power, from the specifications listed, so.... a signal of .775 volts times 52.5 will give the full voltage ac out which is... 40.68 volts... so based on that, we get a power output of 206 watts into 8 ohms? so it will clip at around 242 watts? ie never? its biased at 10 mv across a 0.22 ohm emitter resistor, which makes 45.45 milliamps, and there are 4 pairs of devices, so class A output will be 45.45milliamps squared, times 4 devices, times 2 times 8 ohms? if its biased hotter, will that reduce the total power out? any way to easily calculate the db's of feedback without scoping the thing? finally, will reducing the feedback by 36 dbs increase the sensitivity and the power out? thanks in advance 
13th September 2006, 11:59 AM  #2  
diyAudio Member

Re: power calculation...correct?
Quote:
One minor point: I think you should take the 5V loss from the peak voltage, not from the RMS voltage. That will give you a bit more max output power. Quote:
If you bias it higher, nothing changes for the max Vout and thus the max Pout, although the part of the output in class A will of course increase. What exactly do you mean by the feedback dB's? The feedback is the difference between the open loop gain and the closed loop gain. The latter is (approximately) what you calculated above 34.4 dB (I say approx because it assumes infinite open loop gain which is not what you have). If, say, your open loop gain (without feedback) is 80dB, your closed loop gain 34.4, then your feedback is 45.6dB. But that ol gain will vary with frequency, so your feedback varies with frequency, and so will your closed loop gain (although much less so, if the ol gain is high enough wrt the closed loop gain). Edit: this may also help: http://wwwk.ext.ti.com/srvs/cgibin...i.exe?Company={5761bcd811f54e0884e08167176a4ed9},kb=analog,case=obj(32624),new Jan Didden
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An engineer designing a feedback amp is a mixed emotions creature  Hendrik Bode Check out Linear Audio! 

13th September 2006, 12:43 PM  #3  
diyAudio Member
Join Date: Nov 2003
Location: Brighton UK

Re: power calculation...correct?
Quote:
It will increase sensitivity and stability margins but not the power output. /sreten. 

13th September 2006, 12:45 PM  #4  
diyAudio Member

Re: Re: power calculation...correct?
Quote:
Thanks, I missed that Jan Didden
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An engineer designing a feedback amp is a mixed emotions creature  Hendrik Bode Check out Linear Audio! 

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