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Old 17th September 2014, 08:27 AM   #1
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Default PSU voltage for 3886 amp in Beh. B2031A

I have a pair of Behringer 2031A speakers and the mains transformers have blown (gone open circuit primary) and I need to replace them. The amp uses LM3886 which is why I've posted about it here. I've managed to source a schematic of the B2031A, but the power supply rail voltage is not shown, so I have no idea what voltage transformer to buy.

LM3886 data sheets suggest the ideal voltage for a 3886 is +-32V so I'm thinking a 24-0-24V transformer, at perhaps 120VA. Does this sound about right, or does anyone know the exact voltage/size of the transformer in a Behringer B3021A.

Thanks for any advice.

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Old 17th September 2014, 08:37 AM   #2
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Hi Pete, the most important thing will be the impedance of the speakers. If they are 8 ohms you should be ok. If they are closer to 4 ohms then a 24-0-24 transformer will be too much. I can't remember off the top of my head but I think around 20-0-20 is max recommended for 4 ohm loads. It is in the datasheet too.

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Old 17th September 2014, 08:42 AM   #3
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22-0-22 is high enough. 24 will give you +-34Volts!
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Old 18th September 2014, 08:06 AM   #4
AndrewT is offline AndrewT  Scotland
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LM3886 data sheets suggest the ideal voltage for a 3886 is +-32V
National's 3886 datasheet does not state an "ideal" supply voltage.
The 3886 works with 20Vdc to 84Vdc, i.e. +-10Vdc to +-42Vdc. (see datasheet for corroboration).

But for 4ohms use I'd suggest much less, maybe <=+-28Vdc

Transformers from 10Vac+10Vac to 26Vac+26Vac will all power a 3886 well.
Aim for 160VA, or more, if that will fit inside the case.
100VA and 120VA will work, just not as well.

What size of heatsink is fitted.
that too will limit the maximum supply voltage that the chipamp/s can use. see p14.
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Last edited by AndrewT; 18th September 2014 at 08:11 AM.
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Old 18th September 2014, 09:49 AM   #5
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TRANSFORMER TRA-0034 XFMR PWR TOROID 115/230V ED14270R2 B2031 115/230V $42.00
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Old 18th September 2014, 05:41 PM   #6
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Hi All,

Thanks very much for your replies. All hugely helpful. I have no idea of the impedance of the drivers, so I'll play it safe and go for 22V. I've managed to find toroidals from China for about 20 ($32) plus shipping, that should fit. They're 120VA.

Thanks Dan for the link to the actual part as a spare. I would go for those, but the shipping cost and duty would be high from NYC I think.

Next thing is to figure out the transformer that was in my B2092A subwoofer, which also blew up at the same time! All caused by using a solid state relay to switch the power to the monitoring system :-(

All the best,

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