LM3886 rail voltage difference, problem? - diyAudio
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Old 20th October 2012, 07:17 PM   #1
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Default LM3886 rail voltage difference, problem?

Hi everyone,

I'm building a subwoofer amplifier with dual lm3886 chips in bridged configuration.
I bought two toroid transformers from a factory surplus store. At first glance, they looked like twins, with 20v secondaries. I was thinking to use each for a rail, with dual bridge power supply design.

But today, when i was about to test the circuit, i noticed that one of the transformers had a 220V primary and the other one had 240V primary windings. As they are using the very same wall socket voltage, 220V, the outputs of them differs about 3-3.5 volts after the rectifier bridges, one outputs about 29V and the other 25.5-26V.

So i didn't connect them to the amplifier circuit, as i don't know if it could harm the chips and decidet to ask you.

Is this "different rail voltage" issue a serious problem? or should i ignore it? if it's a problem, what can i do to reduce the voltage? Would it be useful to put 4 or 5 diodes in series to drop the voltage on the higher voltage rail? Or can you please suggest me any other solutions?

Thanks in advance,
Özgün Dönmez
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Old 20th October 2012, 07:35 PM   #2
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That is not really an issue.
The amp will just not clip exaxtly symetrical.
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Old 21st October 2012, 09:26 AM   #3
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Thanks for the answer, so it won't cause dc offset also?
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Old 21st October 2012, 09:34 AM   #4
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As long as input and output has a common reference (ground), this will not create any offset.
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Old 21st October 2012, 10:09 AM   #5
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No prob. with 10% diff in voltages. If you are still worried, then put 3 or 4 diodes of high current rating in series between bridge rectifier & filter capacitors. And watch out for DC offsets(Few tens of mV is OK. Nothing in Volts range )
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Old 21st October 2012, 11:56 AM   #6
AndrewT is offline AndrewT  Scotland
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you can add some primary windings to the 220Vac transformer.
This will reduce the output voltage and much more importantly reduce the idle current in the primary due to the extra inductance afforded by the extra turns.
There is a further advantage to adding extra turns to the 220Vac transformer. It increases the ampere turns of the primary. This reduces the flux in the core and makes for a lower temperature transformer as well as a transformer that is less susceptable to saturation. The only disadvantage is a slight reduction in the VA. But since it will run cooler it will tolerate overloading better and thus perform as well as or better than the 240Vac transformer.

Try adding 20T. Connect that in series with the primary.
Use you bulb tester to power up the transformer.
Measure the output voltage.
Unplug.
swap the phase of the extra winding.
Power up via the bulb tester and re-measure the output voltage.
the "correct" phase is the version that gives the lower output voltage.

Now compare the output voltage to the 240Vac version. Are they similar yet?
Decide if you want/need to add another 10T or 20T.
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Last edited by AndrewT; 21st October 2012 at 11:59 AM.
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Old 21st October 2012, 01:24 PM   #7
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Quote:
Originally Posted by AndrewT View Post
you can add some primary windings to the 220Vac transformer.
This will reduce the output voltage and much more importantly reduce the idle current in the primary due to the extra inductance afforded by the extra turns.
My advise is NOT do anything when it comes to changing the transformers. It won't add anything positive.
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Old 21st October 2012, 01:29 PM   #8
yd1cvj is offline yd1cvj  Indonesia
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Originally Posted by peranders View Post
My advise is NOT do anything when it comes to changing the transformers. It won't add anything positive.

Ya agree with you. As the function for subwoofer, I think should be no problem as long as 0 DC output.
Unless you have experience to modification toroid.

Last edited by yd1cvj; 21st October 2012 at 01:37 PM.
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Old 21st October 2012, 01:37 PM   #9
AndrewT is offline AndrewT  Scotland
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Originally Posted by peranders View Post
........ It won't add anything positive.
i disagree.
And to be unbiased I pointed out the one negative if am aware of, which when examined in more detail may turn out to be not -ve.
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Old 21st October 2012, 04:22 PM   #10
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Thanks a lot for the suggestions.

I think i'll try to add extra windings to the primary, as i have some coiling wire in the attic, if i fail, i'll connect 5 high ampere diodes to the higher-voltage rail then. I'll share the result here.

Again, thanks everyone for the responses and suggestions.
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