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Old 5th January 2005, 05:11 PM   #1
ejn327 is offline ejn327  United States
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Default Parallel gainclone questions

Iíve been checking out the forum for a while now and I think I am going to build a gainclone. I will probably get the LM3875 kit when they are available again from chipamp.com.
I have a few questions though. I will be building a 2 channel amp with room for a third channel later.
The speakers that this will be powering are 4 ohm MTMís. I have been looking around and understand that with 4 ohm speakers a lower voltage (~25V rails) power supply should be used. I think I have the terminology correct, the rail voltage would be 1.41 * the transformer voltage
I have also read that you can run 2 amps in parallel (with a .22 ohm resister) to drive lower ohm speakers. Do I put a resistor on each output and then combine?
In the manual for the kits it talks a little about running a parallel configuration and says that the amp will see double the load. says this will give higher power but not double. Does anyone know what kind of power I could expect?
If I do this, can I run higher voltage (31V rails) to the amps?
I plan on getting a transformer with around 400-500 VA or should I do this or run with 1 transformer for each channel?
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Old 5th January 2005, 06:28 PM   #2
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I think lm3886 would be better 4ohm speaker,if you use a single chip

Yes lower voltage would make it easier on the chip

Yes the voltage is around 1.414*transformer secondary

Yes one resistor goto each chip output then combined

Goto I believe www.the12volt.com there are programs for getting output power
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Old 5th January 2005, 08:22 PM   #3
ejn327 is offline ejn327  United States
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are their kits with the LM3886? I don't have any clue about designing or building a PCB, and I don't want to do a PTP

A lower voltage would be easier on the chip, but if I use adequate heat sinking could I run with higher voltage and be safe?
I am also curious about regulated power supplies. What advantages does this give?

I looked around at 12volt.com but it looked mainly geared for car audio, I didn't see anything about amplifier power. - will have to look harder
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Old 5th January 2005, 08:35 PM   #4
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Yes It should not be much of a search to find a lm3886 kit,so do a net search

Yes with a big enough heatsink you can use higher voltage but nothing over 40volts(lm3886) between 30 and 35volts should do

goto www.the12volt.com then look for "ohm law" you will see a chart and calculators
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Old 5th January 2005, 08:52 PM   #5
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Ejn327 Get the application note for whatever chip, the kits uses. You find information about how much power,you will get for a certain voltage supply
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Old 5th January 2005, 09:11 PM   #6
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National Semi has the following tool that you can use -- it also has the equations for parallel and bridged amplifiers:

http://www.national.com/appinfo/audi...gn_Guide15.xls
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Old 6th January 2005, 05:28 PM   #7
ejn327 is offline ejn327  United States
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thanks for the links guys. I ran some numbers using the spreadsheet from national for 18, 22 and 25V transformers. It looks like if I go with the parallel 3875 and use 18V I would get 93W. Anything above this (i.e. 22V or 25V) is current limited and the output stays at 95W.
Seems like a no brainer to go with the 18V supply.
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