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6th December 2012, 10:49 AM  #41  
diyAudio Member
Join Date: Jul 2004
Location: Scottish Borders

Quote:
Quote:
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6th December 2012, 04:42 PM  #42 
diyAudio Member
Join Date: Nov 2012
Location: Kansas

Marian would an audio amp using the above figures for power (200Vcc @ 8.5 amps) be able to produce a 30Hz audio tone at the 1200watt power rating into 4 Ohms for an extended amount of time? (more than a few minutes) to me it would seem like more current would need to be available...

6th December 2012, 05:52 PM  #43 
diyAudio Member
Join Date: Jul 2004
Location: Scottish Borders

Another Member suggesting that current is a necessary parameter for the PSU.
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6th December 2012, 06:10 PM  #44  
diyAudio Member

Quote:
It is like that with an audio power amplifier as well, as long as you supply the current needed on a given resitive load, as long as the power supply unit is capable of that current and is designed to cope with long term usage 1 minute or 100 munutes of 1,2Kw of power it is the same, the current does not drop in time, or at least it should not cus that is why you take a loong time in designing the power supply.
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6th December 2012, 06:16 PM  #45  
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Join Date: Jul 2004
Location: Scottish Borders

Quote:
An AC voltages reduces to zero twice every half cycle. It does not stay constant. The AC current through the load does not stay constant. It too varies. Don't listen to analogies that suggest anything different.
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6th December 2012, 06:35 PM  #46 
diyAudio Member

@AndrewT ofcourse the current varry on a speaker for audio signall, and that is precisely what makes things a bit easyer for the sypply cus it does not need to supply that max current constantly but in pulses. Now when designin the PSU you take in account that max current at max power and max audio signall. y do not know you so i cannot make any claim about you, but i can say that it is dissapointing to see this much missunderstanding.
@jlind54 if you trully want my help than you need to accept my imput and not debate it, this way i will be glad to help any way a can, but if i have to fight your misstrust at every turn then i am affraid i have to say no, and good luck. Best wishes to all. Marian.
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6th December 2012, 07:43 PM  #47 
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Join Date: Apr 2010

Maybe the desired sound is similar to that caused by 'sag' in a guitar amplifier...

6th December 2012, 08:30 PM  #48 
diyAudio Member
Join Date: Nov 2012
Location: Kansas

benb the desired sound is to be as accurate as possible and not exhibit a 'sag' sound similar to a guitar amp. Would having a supply with in sufficent current capacity cause that? My ultimate goal is to have something robust that can take loads of punishment. But I guess there is just something with the voltage and current to a load that I'm not understanding. Typically looking at an impedence curve of a speaker as frequency decrease the impedence decreases. I my mind a subwoofer spends most of its time in these lower octaves therefor would need a fair amount of current...

7th December 2012, 06:14 AM  #49 
diyAudio Member
Join Date: Feb 2012

Marian I'd like to start back at the basics and perhaps you could enlighten me on your thoughts for the current going into the speaker from a mathematical formula standpoint. To make things simple we can base the figures on a 100 watt continuos output into a 4 ohm speaker. This would establish a Vref of +20 with Vpk +28.2. Rounding that up a bit to account for losses one could estimate a designed rail voltage of around +35 possibly a bit more. A transformer with a 26026 winding configuration would be suitable as it would achieve near 35 volts after diode rectification. figure an amp with 50% efficiency is used would require a minimum power supply rating of 200watts. My confusion comes with rating the amp capacity... What's the best rule of thumb? Use the 200watts/70volts = 2.85 amps (power supply only), 200/40volts =5 amps (power supply/calc rms audio Vref), 200/35=5.71 amps (power supply amp draw of one winding side ), or some other combination? Apologies for so many questions to you and again thanks for the information you have provided.

7th December 2012, 06:58 AM  #50 
diyAudio Member

When dealing with a 50/60Hz linear power transformer i have always kept in mind theyr winding voltage for current calculation, and i go like this:
In your case 100W in 4 ohm resistive load a continous max stable signall voltage ( witch is the most demanding on the power supply ) the signal would be 20Vac indeed. Now you have to establish the power rails but first you need to know what power devices does the amp has, cus fets eats a few more volts that BJT's, so if i have a bjt power stage than i would reserve about 2v for them ( it would be more than enough ) so a 30Vcc power rails would be required. We know that, now we have to establish the power drawn from the power transformer, and if we are building a stereo amp than we would have 200W audio power needed, and i take in account an efficiency of about 6570% ( for class B/AB it is a good figure ) and so the power needed from the supply would be 200/0,65=307W minumum power needed. We know the dc voltage rails and power needed, now we have to establish the power transformer windings, and for 30Vcc the Vef would be 21,3Vac, at that we add the 1,4V drop on the bridge rectifier and we get 22,7Vac, we round it up to 23 or 24Vac if possible, cus in full load the voltage will drop some. Knowing the voltage for the windings is the last step needed to establish the current for the power transformer, and at 300W and 2x24Vac it is 307/48=6,39A, we round it up to 7A. Those would be the calculations i go trough when desighnin the power supply for a given amplifier, power needed and load. That is for a linear psu witch a trully recommend to most people as long as we talk about ofline PSU, they are much more easyer to bult and ofcourse much more safe for one's life. Hope that helps
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