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Old 24th June 2008, 07:24 PM   #1
ryan750 is offline ryan750  Philippines
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Default bpa200 question

i'm planning to build a bpa200 gainclone (lm3886), i'm going to use a 18-0-18v 400va sec, how many watts can it deliver to a 4 ohm load??? thnx
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Old 25th June 2008, 05:11 AM   #2
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AN-1192 tells us that you get 335 W continuous and 450 W peak using ±28 V.
With P = UČ/R you roughly get 138 W continuous and 185 W peak for ±18 V. Maybe a bit more, because your transformer is not as tightly calculated as in AN-1192.
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Old 25th June 2008, 09:41 AM   #3
AndrewT is offline AndrewT  Scotland
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forget about continuous and peak power figures.

Each chipamp will overheat if you try to get a continuous 60W from it.

The maximum power is about 50W into 4r0, but only for low duty cycle to ensure you keep the chip cold.
Using four chips in BPA configuration will allow a maximum of about 200W into 4r0, but again only if you keep the chips cold.

If the chips run hotter than the 25degC that National specify, then the Spike protection will limit earlier than the typical figures stated in the datasheet.
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Old 25th June 2008, 10:23 AM   #4
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In other words, you are fine with 185 W peak distributed over 4 ICs.

There is a lenghty essay in AN-1192 on the thermal background, and on the conditions to make 335 W cont./450 W peak available given the facts AndrewT disclosed.
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Old 25th June 2008, 03:32 PM   #5
ryan750 is offline ryan750  Philippines
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18-0-18 vac translates to about 25vdc, so in single ended lm3886, according to overture design guide, the one in microsoft excel, a +/-25vdc would have an output of 57watts in 4 ohm, so whats the total output in bpa config??
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Old 25th June 2008, 04:02 PM   #6
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Approximately double that figure. See also post #2.
The truth will be between those two figures.

It also depends on the transformer you use, the stability of your power supply and the chip temperature.

18 V translate to 25 V only under no load conditions. Under full load it should be more like 23 V. May vary according to the capacity you use, the voltage drop of your transformer under load, and of course on your mains voltage.
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Old 25th June 2008, 05:07 PM   #7
AndrewT is offline AndrewT  Scotland
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parallel operation allows nearly double the output into half the load impedance.
That translates from 50W into 4r0 to 100W into 2r0.

Bridge operation allows double the power into double the impedance.
That translates from 100W into 2r0 to 200W into 4r0 for BPA200.

Guess where the 200 in BPA200 comes from?
Yes, the maximum power into the load. The BPA200 can push 200W into 4ohm or 6ohm or 8ohm by selecting the PSU voltage to suit the load impedance.
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Old 25th June 2008, 08:35 PM   #8
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@AndrewT
Thanks for pointing out my mistake as charmingly as usual. I had not taken into consideration that the BPA-200 measurements were taken with a too small transformer for 4 Ohm. Thus I had jumped to the wrong conclusions by only using Ohms law on the measurements.

@Ryan750
Sorry for misleading you. AndrewT is right, you get 4 times what the design guide gives you for a sinlge chip.
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Old 26th June 2008, 11:25 AM   #9
AndrewT is offline AndrewT  Scotland
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Is it National that suggests using BPA topology?

I wonder if they are offering an unbiased opinion? How many extra chipamps will National sell if we were all to adopt BPA willy nilly without looking for better more affordable alternatives?

If one requires big power then go discrete or use the voltage amp stage that National sell.
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Old 27th June 2008, 09:08 AM   #10
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Of course National wants to sell. That's why they happily jump on the power supply train with that application note and AN-1849, too. There are capacitors to sell together with the LMs. And they are certainly not the only ones making money out of gainclones and capacitors. I could name a few members of this forum, if I would think you didn't know them better than I do. So what? As long as the value for money is adequate.

If 200 W are really required is a different question. How often do you use your 170 W amp at full level? You would be nearly deaf after a while, if you did very often. It's more about wanting that power than really needing it, isn't it?

Most chip-amp builders must have chosen not to go solid state, because they don't feel like accumulating the necessary amount of knowledge for that. Like me, who just wanted a small mid-fi amp for my PC, and got that in a few hours with an LM1876 without bothering about spice models, impedance matching and whatnotever.

The BPA is the way to go for more power with about the same set of skills that is sufficient for a gainclone, for anybody who likes showing off with wattage or listening to very, very loud music. So both sides win. Wouldn't you call that a good deal?
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