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Old 6th December 2012, 10:49 AM   #41
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by AndrewT View Post
1200W into 4r0 is equivalent to 98Vpk & 24.5Vpk from a "normal" single ended amplifier.
If you go bridged you need two amplifiers each capable of 600W into 2ohms.
Each of the pair delivering 600W into 2r0 is equivalent to 49Vpk and 24.5Apk.

The total "load" on the PSU becomes 49Apk since the two amplifiers draw current at the same time, but from opposite rails.
Allow an overhead through the amplifier and cabling of ~5V. Allow an unregulated PSU droop of 5V to 10V, depending on how you have designed it. If you use a regulated PSU, you will need to work out the droop on load.
Quote:
Originally Posted by MarianB View Post
@AndrewT with all do respect, that is ridicoulus, you really should research more.

How do you calculate the power output for a given load and voltage signal? it is:
Click the image to open in full size.

Now if the reverse is the case and you need to calculate the signal voltage on a given load and power you do the reverse math:
Click the image to open in full size.

And in this case we have P witch is 1200 and R witch is 4, now 1200*4=4800, the square root of this is 69, so U=69Vac, that is the signall voltage needed to generate 1,2Kw of power on 4 Ohm load, now ther power rails must be 69*1,41=97,2Vcc minimum, not accounting for power device losses, and let say for ease of argument a PSU of +/-100Vcc. Now how much power does the PSU needs? simple, foe 1200W of audio power on a B class audio power amplifier for example, accepting an efficiency of about 70% that means that 30% of the power from the supply will be lost as heat on the cooling system, and we need to provide enough power from the supply to cover both the audio power and losses, so at 70% eff we need 1200W/0,7=1700W, that is the power needed from the PSU, now we have established earlier that the power rails voltage is +/-100Vcc so that means from tail to rail 200Vcc, the total power of the PSU is related to that entire voltage so 1700W/200Vcc=8,5Amps, that is the minimum current the PSu must supply the amp, but that is on a resistive load with a continous voltage on the load, but the musick is not like that and the speaker coil is reactive, so in reality the current is drawn more like in pulses and most of it from the lower spectrum of audio witch is anything but continous.

So again quit with that Apk current and transients, and step down to earth, audio is not SMPS, do more research ( much more ) test more amplifiers for current consuption and you will realise how wrong you are.

best of wishes.
Marian.
which bit or bits is rediculous?
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Old 6th December 2012, 04:42 PM   #42
jlind54 is offline jlind54  United States
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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...
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Old 6th December 2012, 05:52 PM   #43
AndrewT is offline AndrewT  Scotland
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Another Member suggesting that current is a necessary parameter for the PSU.
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Old 6th December 2012, 06:10 PM   #44
MarianB is offline MarianB  Romania
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Quote:
Originally Posted by jlind54 View Post
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...
I know i promised to ignore this thread but i see you need help and are asking for it in a verry polite manner, it would be foolish for me to refuse it, so what you need to understand is that power from a PSU is the product of voltage and current, not the current alone, and that as the voltage increases so does the power. Now let us immagine a constant current source givin a constant 1A, if for example at 12V you supply a resistive load of 12 Ohms that load will need exactly 1A, thus we say we supply the load with 12W of power, now if the load is powered 1 minute or 1 hour it makes no difference, as long as the PSU is capable of that 1A needed by the load it does not matters one bit how long the load is powered, the only thing that would be influenced is the temperature of the power devices on the constant current source.

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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Old 6th December 2012, 06:16 PM   #45
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by MarianB View Post
...............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.
An AC voltage supplied to a speaker MUST result in a varying current through that load. You can substitute a resistor to make the circuit analysis simpler, but you will still find that the current in the resistor load VARIES with time and in time to the varying voltage fed to the load.
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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Old 6th December 2012, 06:35 PM   #46
MarianB is offline MarianB  Romania
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@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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Old 6th December 2012, 07:43 PM   #47
benb is offline benb  United States
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Maybe the desired sound is similar to that caused by 'sag' in a guitar amplifier...
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Old 6th December 2012, 08:30 PM   #48
jlind54 is offline jlind54  United States
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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...
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Old 7th December 2012, 06:14 AM   #49
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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 26-0-26 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.
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Old 7th December 2012, 06:58 AM   #50
MarianB is offline MarianB  Romania
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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 65-70% ( 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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