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Class D Switching Power Amplifiers and Power D/A conversion 

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15th January 2021, 02:36 PM  #1 
diyAudio Member
Join Date: Oct 2019

Noob question about DC resistance
I have an Amplifier board rated 40W 8Ohm and 80W4Ohm if my speaker Znom is 8 ohm but Re is 5 Ohm. How many watts will draw for the amplifier? Something between 40W and 80W?
Tank you and sorry if is a stupid question but i couldnt find any information about this. 
15th January 2021, 02:40 PM  #2 
diyAudio Member
Join Date: Jan 2008
Location: Carp

Speaker Znom varies with frequency, so anything from a couple of watts to 80 or more. Its not unusual for a woofer to be 30 ohms or more at resonance. look at some impedance charts of speakers vs frequency. At normal listening levels most people will use a watt or 2.

15th January 2021, 03:16 PM  #3 
diyAudio Member
Join Date: Oct 2019

I'm designing a Boombox and the goal is maximum SPL so I want to know the peak of watts the speaker will draw from the amplifier to design the box correctly.

15th January 2021, 04:29 PM  #4 
diyAudio Member
Join Date: Mar 2015
Location: Austin, TX

If you're designing a boom box, then the peak watts the speaker will draw is: all of them. As multisync mentioned, speaker Z varies by as much as 10 times the Re.
You can plot yours if you have a signal generator that you know the output impedance of. If you can set the output impedance to 600 ohms or more, do that. If you cannot, then put 550 ohms in series with the signal generator's impedance, which is probably 50 ohms. Drive the loudspeaker directly from the signal generator, through the resistor. With 1 V sig gen output, use a multimeter on it's AC Voltage setting, lowest range, and measure the voltage on the loudspeaker. Vary the frequency by starting at 20 Hz and multiplying by 1.1 to get the next frequency: 20 22 24.2 26.6 29.3 etc. Write down all the measurements. Zspeaker = 600 ohms * (Vspeaker / (1Vspeaker) ) ~= 600 ohms * Vspeaker Your multimeter will probably not be accurate above 1 kHz, so you can stop there. For a boom box, you can stop well below 1 kHz.
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Best Regards, Russell Wherever you go, there you are. Buckaroo Banzai 
15th January 2021, 04:36 PM  #5 
diyAudio Member
Join Date: Mar 2015
Location: Austin, TX

Also, a subtle point: An amp makes much less power when the impedance is high.
A 40 W amp can deliver about 18 Vrms to a speaker. If the speaker impedance is 30 ohms at resonance, then the power the amp can deliver at that frequency is 10.6 W
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Best Regards, Russell Wherever you go, there you are. Buckaroo Banzai 
15th January 2021, 05:38 PM  #6  
diyAudio Member
Join Date: Aug 2008
Location: Dolceacqua

Quote:
Re is the DC resistance, in reality Impedance is about 6 ohm at around 200300Hz You guessed right, around 53W. This is the maximum 

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