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Old 4th December 2012, 07:53 PM   #31
MarianB is offline MarianB  Romania
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@jlind54 you make the same mistake i've seen many people do... ok let us look at a verry simplistic schematic of audio power amplifier:
Click the image to open in full size.

What you should look at are Q9 and Q10, they are the output devices, they take an voltage amplified signall from the other stages and delivers it to the load with current amplification ( it's sort of simplistic sayd but enough to get you started ), so how do they deliver the signal? well imagine a sine wave, as i hope you know it is a wave that goes up and down a center line, now immagine that center line to be the ground of the load/speaker and the upper part of the wave as Q9 and the lower part of the wave Q10, now as the wave goes up above the center line ( that is the ground here as we have established ) it develops positive voltage and it powers up Q9 witch is an N channel one and it conducts current to the load, as the wave is up Q10 witch is a P channel, cannot conduct any current cus it has no negative gate voltage to get it started, so only Q9 conducts the current to the load, this is the first part of the cicle, now as the wave is a sinus one it goes down too so that means it generatesc negative voltage this time, what that means is that Q9 cannot conduct cus it has no positive voltage any more at it's gate so it cuts of, Q10 on the other hand likes negative voltage on it's gate so it starts conducting current to the load, so when the wave goes down only Q10 conducts current to the load, that is the full cicle.

What did we learn from all of this?:
-The sine nature of audio signall means that only one rail at a time can power the load/speaker, so the voltage signall on the speaker can only be provided from one single rail Vcc at a time. The speaker has one side connected at the ground, the other side of the speaker is connected by the power devices alternatively at possitive and negative side, up and down, and so on again and again, at the signall frequency.

Hope you have better understanding now of what happenes in an audio power amplifier, and also understand why i have sayd that i do not recommend any tinkerring with ofline SMPS for now, you lack too much basic knowledge and ofline SMPS is extreemely dangerous for one's life when you do not know it well enough!
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Old 4th December 2012, 08:31 PM   #32
jlind54 is offline jlind54  United States
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Thanks for the above explination as it helps visualize and step through the workings of the amp. However, is it not possible to have two circuits like the above creating a situation were the speaker would be tied from the +35volt side to the -35volt side and not tie the speaker to a ground leg?
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Old 5th December 2012, 06:11 AM   #33
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Yes. Bridged configuration.
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Old 5th December 2012, 06:36 AM   #34
MarianB is offline MarianB  Romania
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@jlind54 Bridging 2 amps get's you about double theyr individual power, in this case about 250W on 4 Ohms loads, nowhere near 1,2kW of power that you want, it seams you still don't get it...
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Old 5th December 2012, 01:56 PM   #35
jlind54 is offline jlind54  United States
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When I mentioned the +-35 I was speaking of Vref not the actual supply rails shown above in the example. As you mentioned previously they would need to be higer (near 50). So in theory a +-55 volt power supply with a bridged configuration would be capable of supplying 70 volts across the speaker and be capable of supplying 1.2kw of power correct?
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Old 5th December 2012, 02:16 PM   #36
MarianB is offline MarianB  Romania
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For 1,2kW of power on 4 Ohm load you need about +/-75Vcc supply rails, but do you have any ideea how much more stress this config at this voltage rails puts on the output devices? i think you do not, in fact i think you do not have a grasp of most basics on the power amplifier, you really need to learn more, to read more, there are way too much stuff you do not understand, you cannot espect us to teach you all the basics, trust me there is much you need to know.
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Old 5th December 2012, 02:23 PM   #37
jlind54 is offline jlind54  United States
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MarianB thanks for all your input. I'll take your advice and contiune reading. Do you have any suggestions on good literature?
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Old 5th December 2012, 06:51 PM   #38
jlind54 is offline jlind54  United States
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MarianB in the above example stating the need +-75 Vcc to achieve 1.2kw at 4 ohms in a bridge configuration would mean an amp would have to have near 93% efficiency correct?
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Old 6th December 2012, 08:44 AM   #39
AndrewT is online now AndrewT  Scotland
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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.
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Old 6th December 2012, 10:48 AM   #40
MarianB is offline MarianB  Romania
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@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.
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