"rule of thumb" used to determine xxVA for a power supply's transformer? - Page 2

AndrewT · 22nd July 2008, 07:48 AM

Hi,
yes, add the channel Pd together to get total Pdtot.
Yes, the dissipation drops slightly when delivering the last 3dB of output power, when driving a resistive load.
Unfortunately driving real speakers, that are reactive, increases the dissipation of the output stage and is much more onerous on the amp reliability.

The big saving with sensible unclipped replay is that average levels are, or should be, about 20dB below maximum output.

This ensures that the majority of the transient signals pass through unclipped and overall the sound is so much nicer.
The downside of this is that the 8W amplifier is outputing just 0.08W on average to your 90dB/2.8V/m speakers and they will sound quite quiet. About the same or slightly lower than a normal conversation between two without raising their voices. OK for background music.

If you really want to use single digit output powers then look for affordable speakers that have at least 5dB and preferably 10dB more sensitivity AND 8ohm if driven by a chipamp.

CopyofAudiófilo · 22nd July 2008, 12:01 PM

Ok. I think is time to start building and testing results...I have a 13V@10A transformer/rectifier circuit, a pair of Woofers/Midrange/Tweeters and Xover Network. The problem Is they have 4 ohms but I just want to test and analyze everything it was said here. I also have a pair of already made (comercial)8ohms enclosures, so I think that all the necessary stuff to test results and compare is available. Thanks everyone for the replies and as soon as I finish (and during...) testing I will be posting my results here. Greetings.

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22nd July 2008, 07:48 AM	#11
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	Hi, yes, add the channel Pd together to get total Pdtot. Yes, the dissipation drops slightly when delivering the last 3dB of output power, when driving a resistive load. Unfortunately driving real speakers, that are reactive, increases the dissipation of the output stage and is much more onerous on the amp reliability. The big saving with sensible unclipped replay is that average levels are, or should be, about 20dB below maximum output. This ensures that the majority of the transient signals pass through unclipped and overall the sound is so much nicer. The downside of this is that the 8W amplifier is outputing just 0.08W on average to your 90dB/2.8V/m speakers and they will sound quite quiet. About the same or slightly lower than a normal conversation between two without raising their voices. OK for background music. If you really want to use single digit output powers then look for affordable speakers that have at least 5dB and preferably 10dB more sensitivity AND 8ohm if driven by a chipamp.

22nd July 2008, 12:01 PM	#12
CopyofAudiófilo diyAudio Member Join Date: Jul 2008	Ok. I think is time to start building and testing results...I have a 13V@10A transformer/rectifier circuit, a pair of Woofers/Midrange/Tweeters and Xover Network. The problem Is they have 4 ohms but I just want to test and analyze everything it was said here. I also have a pair of already made (comercial)8ohms enclosures, so I think that all the necessary stuff to test results and compare is available. Thanks everyone for the replies and as soon as I finish (and during...) testing I will be posting my results here. Greetings.