Rail Voltage VS Output Power ????

Zero Cool · 14th October 2004, 04:11 AM

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 DH-500 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

djk · 14th October 2004, 07:52 AM

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

AndrewT · 14th October 2004, 07:57 PM

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?

richie00boy · 14th October 2004, 08:47 PM

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.

Enzo · 15th October 2004, 12:01 AM

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.

Zero Cool · 15th October 2004, 12:30 AM

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

richie00boy · 15th October 2004, 07:30 AM

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

Zero Cool · 15th October 2004, 05:40 PM

Thanks!!!

Zero

richie00boy · 16th October 2004, 10:39 AM

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

viki · 16th October 2004, 07:33 PM

Hi, using that exel sheet amp_designer what values should I use for the transistor 2SA1302 ,since that sheet is intended for MOS-FET 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

14th October 2004, 04:11 AM	#1
Zero Cool Audio Junkie 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 DH-500 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

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Counterpoint SA20 Mosfet output rail voltage	quinnling	Tubes / Valves	0	21st November 2006 03:50 PM
supply rail voltage on 3875 output ?	Buhl	Chip Amps	10	9th November 2006 09:44 AM
A40 rail voltage on output	Eric	Pass Labs	5	24th November 2004 01:55 PM
Power transistor rail voltage question	yeshu26	Solid State	23	21st July 2004 06:46 PM
Is the output power tied to the rail voltage	Shoog	Pass Labs	2	15th August 2002 10:26 PM

New To Site?	Need Help?
Register to Participate Search Privacy Statement Contact Us	Frequently Asked Questions Did you forget your password? Mark Forums Read


	Please consider donating to help us continue to serve you. Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving

14th October 2004, 07:52 AM	#2
djk 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
AndrewT 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
richie00boy Did it Himself diyAudio Member Join Date: Nov 2003 Location: Gloucestershire, England, UK	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
Enzo 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
Zero Cool Audio Junkie 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
richie00boy Did it Himself diyAudio Member Join Date: Nov 2003 Location: Gloucestershire, England, UK	I hadn't thought of that. I will make changes and post back here when done

15th October 2004, 05:40 PM	#8
Zero Cool Audio Junkie diyAudio Member Join Date: Sep 2004 Location: MN	Thanks!!! Zero

16th October 2004, 10:39 AM	#9
richie00boy Did it Himself diyAudio Member Join Date: Nov 2003 Location: Gloucestershire, England, UK	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
viki 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 MOS-FET 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

Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)