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26th November 2012, 05:37 PM  #1 
diyAudio Member
Join Date: Aug 2012

is current in speaker load to be anyway closer to psu current ratings?
today i was talking to trafo manufacturing company and when I was ordering 56056v trafo the current ratings are supposed to be about 8.9amps...
now i have a doubt since the amplifier itself doesnt generate any electricity but just modulates the power in the psu I have a question regarding the psu.. lets take one practical example... when the output voltage is about 60v peak to peak and when the imp = 4 ohms then according to ohms law i=v/r so total is 60/4 which is 15 amps and when my psu itself can deliver only 8.9 amps how come this current be fed to the amplifier? or do I need double to that current rating of the psu? I just had this doubt since if its instantaneous currents then the reservoir caps will do the job but what if its continuous? My question would be what will be the voltage output of the 500w 4ohm LME49810 amp when the supply voltage at the power stage be +/78V with 5 pairs of 5200/1943 BJT combination.. do i need to consider this current factor aswell? since im ordering the Rcore trafos I need to make the decision on the current ratings 
26th November 2012, 05:57 PM  #2 
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Join Date: Apr 2008
Location: Carlisle, England

The smoothing caps supply the extra current for the transients. If you input a sine wave then the peak current needs to be multiplied by 0.707 to get the mean current.
Also bear in mind a speaker is only 4 ohms at its lowest point, it can vary wildly across the audio spectrum.
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26th November 2012, 06:07 PM  #3 
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Join Date: Jun 2007
Location: Romania

Use google translate the following article:
Surse de alimentare liniare pentru amplificatoare audio  calcul, teorie, concluzii  Surse de alimentare  Tehnium Azi  Articole (Articles)  Comunitatea Tehnium Azi The article is called: Linear power supplies for audio amplifiers  calculation, theory, conclusions In that article I wrote many things that can be answers to questions you may have. If you want any administrator of this site, I could translate the article into english and publish it here on the website diyaudio.com. I can to do that only if management diyaudio.com wants it.
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26th November 2012, 06:14 PM  #4  
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Join Date: Aug 2012

Quote:
In the mean time ill tell to moderator to give the permission to convert and put it on the site... thank you very much for the contribution... 

26th November 2012, 09:38 PM  #5 
diyAudio Member
Join Date: Apr 2010
Location: Coffs Harbour, NSW

From a practical point of view, The duty of the amplifier greatly influences the power transformer rating.
If you are building a PA amplifier that will work hard with compressed audio and limiters kicking in, The power rating may not only need to allow for high average signal level but also low efficiency with the lowest impedance of 2R which can be very inefficient even in class AB with BJT devices. If the amplifier was rated at 500W/2R, by rule of thumb you could expect to need a 1kVA rated transformer. However, before ordering transformers, check the rating of transformers in pro. gear designed for similar requirements. Alternatively, for a HiFi amplifier to impress by its rating more than requirement, 500 VA might be quite sufficient rating.
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26th November 2012, 10:00 PM  #6 
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Join Date: Jan 2011
Location: Fosser, AurskogHoland, Akershus, Norway.

This is why you never see an amp having a power output wich directly i possible to calculate from the supply rails voltage alone.
And at the same time why there is amplifiers in the maket trying to tell us that they have a power output many times greater than the power consumption from the mains line. As a few already said, there is possible for the Capasitorbank to provide peak power beyound the current rating from the transformer. Also the transformers are capable of delivering shortterm current above what is stated as the contious current output. Amplifiers very often sound a lot better when having a transformer wich is more than the double the rating of the amps given output power, opposing to similar amps that have a transformer just above the power output rating. This is due just to the fact that an amplifier needs to be supplied with more than enough power to produce the output power wanted. An old rule when checking amplifiers true rating is to look at the power spesifications sign on the back, and read out what power consumption the amp have. In my mind that consumption should be at least the double of the combined power output. Prefereably even four times the rating. Only then one can believe that the amp is capable of delivering what it promises in any sales brochure. In addition, a large filtercapbank also will help out. Let me add: Lets start with that 500VA Transformer. It's not unusual to have losses already in the transformer itself. Up to 5% in copper and iron losses can be sees. Next, the rectifier diodes. Four diodes is usually used, with a forward voltage drop up to 1V, and with 5,6A in each, we loose another 20W in those. Then the amplifier itself. A Class AB has a loss of more than 3060%, just in heat to deliver the wanted output power. This parameter is qiute dynamic, but in this case we may see losses up to 150W. There is now only about 300W left to push out to the speakers. Divided in the two channels there is just 150W each. The difference is sitting in the amp and powersupply, and is dissipated as heat. Put in an 1KVAtransformer and the picture is totally different. There would even be some power in reserve.
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Sooner or later you end up with TANDBERG Last edited by TANDBERGEREN; 26th November 2012 at 10:13 PM. 
27th November 2012, 02:20 AM  #7 
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Join Date: Aug 2012

I think thats the one reason why krell amplifiers heavy psu..
Krell Industries, Incorporated, America's premiere manufacturer of highend audio equipment: awardwinning amplifiers, preamplifiers, CD players, DVD players, surround/sound processors, loudspeakers the above amp is rated for 400watt in 8 ohms and 800w in 4 ohms the trafos are 2 x 3000va which is 6000va so its 7 .5 times the output pow... and 1,00,000uf of caps 
27th November 2012, 07:42 AM  #8 
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Join Date: Apr 2010
Location: Coffs Harbour, NSW

Krell designs are not comparable with simple class AB. Google and read up on their operating principle which was a unique method
of maintaining audio output effectively in class A. Current demand varies between class AB and class A which, if you check out the facts and figures is a lot of difference.
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27th November 2012, 01:51 PM  #9  
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Join Date: Jan 2011
Location: Fosser, AurskogHoland, Akershus, Norway.

Quote:
Pure class A demands even more to produce "enough" power. And if we look at the efficiency of those, a power with such reserves indeed is needed. Nor problems to see that. But with class ABamps, it would be sufficient with a transformer in the area of 1,5 to 2 times the combined output. It is also better with the correct selection of caps over a "dynamite" bank of 1,000,000 uFarads. If one is to examine that parameter, one could do like this: The bigges cap of around 3000uF. Next two 1500uF, 3X680uF, 4X330uF, 5X150uF, 6X68uF, 7X33uF, 8X15uF, 9X4u7F, 10X1uF, then some standard 100nF in the end. Gives a good enough capasitor bank for the most demands, AND a FAST response from the capbank. Tested this and found that my amps have excellent transient respones, despite the lack of monster filtercaps.
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27th November 2012, 02:27 PM  #10 
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Join Date: Aug 2012

but the 15amp current has to come from the supply under any cost isnt it... ( continuous power not instantantaneous )

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