PA 100 voltage input if its 4 ohm driver / 8 ohm driver - diyAudio
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Old 15th August 2012, 07:25 PM   #1
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Default PA 100 voltage input if its 4 ohm driver / 8 ohm driver

I would like to use 2 lm3886 chips to drive a 4 ohm speaker and sometimes 8 ohms speaker is it suggested to use 35 volts after rectification? Is that ok? but when I use the Overture design guide.xls its showing error. Is that fine when there are two chips in parallel?
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Old 15th August 2012, 07:41 PM   #2
AndrewT is offline AndrewT  Scotland
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You should try find what in your data input is causing the apparent "error".

It could be that one of your inputs is going to kill your real life assembly.
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Old 15th August 2012, 08:10 PM   #3
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there is voltage if i reduce it to 25v then its stable at 4ohms
but doubt is that If i want 60watts into 8 ohms and 100 into 4 ohms what must be the supply voltage or can we increase this to 100watts to 8 ohms and like 140 watts into 4 ohms if we parallel the chips. I am a bit confused on this load imp and volt supply...
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Old 16th August 2012, 12:41 PM   #4
AndrewT is offline AndrewT  Scotland
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The maximum specified by National is 68W into 4r0.
But we know that we can also get 69W into 8r0 if we use the correct supply voltage. National give us all the details for choosing the correct supply voltage to get that 68W into 8r0.

You cannot get 100W into 8r0. Not even if you double up or even triple parallel the 3886 chips. It's down to supply voltage and output voltage. There is a relationship between the two and National spell out the details.

If you can't be bothered reading through and learning to interpret the datasheet then a fall back position is to use the spreadsheet that you found.

But it too is telling you that your target is a NO-GO area.


BTW, to get that 68W into 8r0 reliably and with good quality reproduction, you must keep the chipamp cool. The table showing how to choose the heatsink to suit the proposed supply voltages and resistor load assumes the chip will run hot. Very hot by my interpretation.
I recommend that you double the dissipation capacity that the table predicts to get a good quality output, that minimises the clipping and limiting episodes in your music playing.
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Last edited by AndrewT; 16th August 2012 at 12:44 PM.
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