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Old 20th January 2009, 10:17 PM   #1
rtill is offline rtill  United States
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Default wattage question

just curious after reading this thread...

http://www.diyaudio.com/forums/showthread/t-112471.html

I am unclear of the how many watts this amp puts out. I am calculating about 60watts. I have been known to be bad at this amp calculation stuff, so I figured I'd ask.
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Old 20th January 2009, 10:25 PM   #2
lineup is offline lineup  Sweden
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40 Volt - 5 V (mosfet gs) = 35 Volt total
35/2 = +- 17.5 Vpeak
17.5/1.414 = 12.4 Volt rms max ouput

Max 8 Ohm:
12.4 x 12.4 / 8 = 19.2 Watt RMS
Max 4 Ohm:
12.4 x 12.4 / 4 = 38.4 Watt RMS
Something like that for a 40 Volt DC supply.

The output, between MOSFETS, should be ajusted to something like 15 - 17 Volt.
Either for symmetrical clipping or minimum distortion at higher levels.
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Old 20th January 2009, 10:26 PM   #3
rtill is offline rtill  United States
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wow, I was way off!
glad I asked.
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Old 20th January 2009, 10:57 PM   #4
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The irf540 being a 88w device in a to220 case may not be
up to the task of handeling all that heat!
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Old 20th January 2009, 11:10 PM   #5
Beto is offline Beto  Brazil
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lineup excuse my ignorance but
This calculation is made in each MOSFET = 2.5 v total of the two 5v.
you would use bipolar transistor 0.6 +0.6 v I correct?
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Old 20th January 2009, 11:42 PM   #6
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Even without output losses this voltage gives a max power of 24W at 8 ohm. Switching to bipolar won't buy you much additional power.
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Old 21st January 2009, 12:11 AM   #7
lineup is offline lineup  Sweden
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Hello Beto.

See attachment.
Actually I forgot that 2V is used by lower MOS. Like 2 A ClassA bias.
While like 5V, VGS, is lost in upper.
So totally we have 40 -2 -5 = 33 Volt (at best for output swing)
Same as +- 16.5 swing.

At idle, output voltage is at 18.5 VDC.
Output can not go lower than 2V .. because of MOS Current source.
Output can not go higher than 35V, because there Gate of MOSFET is at 40 VDC (supply max).

Result output +-16.5Vpeak = 11.66Vrms
Power 8 ohm : 11.66x11.66 /8 = 17 Wrms
Power 4 ohm : 11.66x11.66 /4 = 34 Wrms
These are teoretical max. In reality will be some losses and less power out.
====================

This amplifier maybr benefits from BOOTSTRAP.
Split up R3 (6k8) into two 3k3.
Bootstrap with 47-100uF, from output to BETWEEN 3k3 resistor.

Regards
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Old 21st January 2009, 12:33 AM   #8
Beto is offline Beto  Brazil
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Ok thank you for explanation lineup.
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Old 21st January 2009, 08:29 AM   #9
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Default wattage concuest may be ????

in my company i own about a 50 pro amps for PA rentals ....the smalest is like 350w per channel .......

still all day i listen to music coming from a nicelly made P3 from esp .....something like 40+40 watts ....

can't change that .... now i am an edict .....
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Old 21st January 2009, 06:10 PM   #10
Limhes is offline Limhes  Orkney Islands
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It seems to me that the bias is all wrong:

BJT base voltage is around 5V
BJT DC current is around 5mA
=> BJT collector voltage 10V

Now, how can you expect a decent voltage swing? Or am I missing something? I would decrease R3 to, say, 3k3 or something...
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