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14th October 2004, 04:11 AM  #1 
diyAudio Member
Join Date: Sep 2004
Location: MN

Rail Voltage VS Output Power ????
How do you determine what rail voltage you will need for a given output power???
I have noticed most 200 WPC @ 8 ohms amplifiers use +/70V rails. But i think the 250Wpc hafler DH500 used +/ 85vdc rails... What is the ratio of output RMS to rail voltage??? if 200W @ 8 ohm = 40Vrms and 250W@8= 44.75V rms???? or if i was looking to build a 400W@8 ohm amp or 1000W@ 8 ohm amp, how would i determin what voltage rails i would need???? 400w= 56Vrms & 1000W=89.5Vrms Zero 
14th October 2004, 07:52 AM  #2 
diyAudio Member
Join Date: Feb 2001
Location: USA

For a typical supply found in audio amplifiers, and a typical output stage:
400W/8R will need ±95V DC and 1000W/8R will be a bridge running on ±80V DC 
14th October 2004, 07:57 PM  #3 
diyAudio Member
Join Date: Jul 2004
Location: Scottish Borders

You can work out the rms voltage ok.
The peak voltage formula is: Vpk= sqr root(2 * Power * resistance) Rail voltage formula is approx: Vrail = Vpk + 5 to 10 volts. I don't know how to predict the loss more closely than 5 to 10 volts. It must depend on emitter resistors, collector resistors if any, droop in the rails under load, how close to rail the VAS can drive the output stage, the number of output transistors and output resistances. Any thoughts on other losses and how to calculate? 
14th October 2004, 08:47 PM  #4 
Did it Himself
diyAudio Member

I made this spreadsheet. It's intended for MOSFETs, but will be absolutely fine for BJTs as well. Just enter the info in the unlocked cells, i.e. everything on the first worksheet in the first block, and the diode drop, Iq and regulations on the second worksheet. It even tells you what transformer you need.

15th October 2004, 12:01 AM  #5 
diyAudio Member
Join Date: Sep 2004
Location: Lansing, Michigan

The power is determined by the load. The load draws the power, the amp does not push the power out. So you have to start with the load.
If it is 8 ohms or whatever, then you can calculate what voltage must be across the load to result in the power wanted. That will also calculate how much current the amp must source at the determined voltage. Figuring a sine wave, 1.414 times the RMS voltage of the sine gives you peak. The amp must have enough voltage to hit that peak. And all the above mentioned things like how close to rail can the output be driven must be added into the mix. You can do this in reverse and determine what would be the maximum output power of an amp. The rails limit the peak output voltage. And then there is always the current to do the job. 
15th October 2004, 12:30 AM  #6 
diyAudio Member
Join Date: Sep 2004
Location: MN

Thanks richie00boy for the spreadsheet!!
Thanks exactly what i needed, i can play around with the numbers and get an idea....Any chance you could add another coloum on the 3rd sheet for US 120Vac primaries???? Thanks Zero 
15th October 2004, 07:30 AM  #7 
Did it Himself
diyAudio Member

I hadn't thought of that. I will make changes and post back here when done

15th October 2004, 05:40 PM  #8 
diyAudio Member
Join Date: Sep 2004
Location: MN

Thanks!!!
Zero 
16th October 2004, 10:39 AM  #9 
Did it Himself
diyAudio Member

I have updated the spreadsheet. You can now change the primary voltage to your country and also the wire lengths and colours.

16th October 2004, 07:33 PM  #10 
diyAudio Member
Join Date: Sep 2004
Location: portimão

Hi, using that exel sheet amp_designer what values should I use for the transistor 2SA1302 ,since that sheet is intended for MOSFET and I dont know what is the Rds(on) and Vgs(th) in this transistor
In the number of pairs of devices if I use two it mens in paralel? Tanks for help 
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