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Old 6th February 2007, 09:12 AM   #1
impsick is offline impsick  United States
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Default can someone please check this out

can somebody please explain what im seeing here.
so i get the whole green blue puple is 24vct.
so is the orange and yellow, and grey and white dual secondaries?

can this push a 3 channel LM3886 amp?


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Old 6th February 2007, 09:51 AM   #2
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Hi,

red and brown are the primary (not shown in the picture).

Orange and yellow form a secondary of 26.8V/4.8A.
Gray and white form a scondary of 26.8V/4.8A, too.

Green and purple form a secondary of 48.8V/4.8A with a blue center tap in the middle, giving you two 24.4V/4.8A rails with a common ground.

You can retrieve a second ground-referenced pair of rails by independently rectifying ORG-YEL and GREY-WHT (using one bridge rectifier per secondary wire pair), then connecting them in series. The connection would then become their ground reference.

Either 24V and 26V AC on the secondaries seem a little high, as it would be about 34V and 37.5V rectified and filtered DC. The specification of the 3886's assume 28V rails.

It will work, but you shouldn't connect a load lower than 8Ohm, as the SPiKE-Protection of the chip would ruin the sound. The datasheet shows around 35V as an optimal rail voltage for Ohm speakers (in order to achieve the highest dynamc range), but with a lower impedance power gets limited internally.

For connecting three chips, use two at one pair of rails and the third at the other pair - e.g. two channels main speakers and one channel subwoofer... The current capability is fiar for this purpose (with 8Ohm speakers only!), although not high. I'd say domestic hifi, but no sound reinforcement.

See LM3886.PDF and AN-1192.PDF at http://www.national.com

Hope this helps,
Sebastian.
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Old 6th February 2007, 12:56 PM   #3
impsick is offline impsick  United States
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it helps allot! thanks Sabastian for your time.
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Old 6th February 2007, 02:23 PM   #4
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You're very welcome.

Just to make sure, check all voltages between all possible pairs of secondary wires using a multimeter. Connect a power resistor in the order of 1kOhm at 5 Watts between the leads while you measure (in order to load the winding and give meaningful readouts at the meter).
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Old 6th February 2007, 11:56 PM   #5
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cool. will do thanks again
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Old 7th February 2007, 06:58 PM   #6
AndrewT is offline AndrewT  Scotland
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Hi,
each secondary winding has around 120VA rating.

Each pair is thus 250VA or 230VA.

You can run upto 150W of output power from each pair. (VA~=1.5* max Pout)

This indicates you could run 4 amplifiers, each into 8ohm, from this one transformer. If you are prepared to accept slightly lower power ability you could run five or six channels from it.

I would recommend a separate rectifier and pair of smoothing caps for EACH amplifier.

Caution,
at these DC voltages, most chipamps will not like 6ohm or 4 to 8ohm speakers that are now very common. Stick to real 8ohm speakers Re>=6r0).
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Old 8th February 2007, 02:53 AM   #7
impsick is offline impsick  United States
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Andrew, so are you saying orange & yellow, and white and grey should go to seperate rectifier boards?
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Old 8th February 2007, 01:56 PM   #8
AndrewT is offline AndrewT  Scotland
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Hi,
I did not recommend using separate rectifiers for one of your dual polarity supplies. That is a choice you can make if you want.

Here is something to think about before you make that decision.

The lower voltage centre tapped winding can only use a single rectifier to a series pair of smoothing capacitors. Each supply rail will lose 0.7V through the rectifier.

The twin windings can be centre tapped and single rectifier again to a series pair of smoothing capacitors. This will give a higher voltage on the DC supply rails.

Alternatively,
you can put a rectifier on each winding and then each rectifier feeds a single capacitor. This loses 1.4V instead of 0.7V on each supply and will bring your two sets of PSU voltages closer together at the cost of one extra rectifier.

What I was saying is, if you want to put more load on the transformer you can add extra rectifier and pair of smoothing caps for one extra amplifier, and repeat this as often as you need up to a limit on each pair of windings of about 150W to 180W. This will be 4, 5or 6 channels if you require that many. It is a big transformer to supply just two chipamps outputing a total power somewhere about 100W to 120W.
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Old 8th February 2007, 05:13 PM   #9
impsick is offline impsick  United States
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What I was saying is, if you want to put more load on the transformer you can add extra rectifier and pair of smoothing caps for one extra amplifier, and repeat this as often as you need up to a limit on each pair of windings of about 150W to 180W. This will be 4, 5or 6 channels if you require that many. It is a big transformer to supply just two chipamps outputing a total power somewhere about 100W to 120W. [/B][/QUOTE]



Andrew i appreciate your time. thanks.
i understand what your saying but im a visual person so to attempt this would be insane. here is a drawing of what i understand.

[IMG]http://i59.photobucket.com/albums/g307/impsick/****.jpg[/IMG]
its probably totally not what you meant. if your bored and feel like drawing please correct me.
dont worry dont panic im not gonna try anything stupid.
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Old 8th February 2007, 05:26 PM   #10
AndrewT is offline AndrewT  Scotland
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H,
I can't see your sketch.
you have 4* in the search string.
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