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Old 8th July 2006, 11:49 PM   #1
beady is offline beady  United States
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Default Is one 25.2A, 2A or two 12.6V, 3A transformers enough for gainclone?

As I stated in another post, my local Radio Shack is closing, and lots of stuff is cheap. I'm wanting to build a gainclone, and was looking at what transformers they had.

The largest they have is one 25.2V, 2A http://www.radioshack.com/product/in...tId=2102703&cp

Or two 12.6V, 3A http://www.radioshack.com/product/in...tId=2102702&cp

All are only $3.15 each

Thanks,

Mike
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Old 9th July 2006, 12:46 AM   #2
jaycee is offline jaycee  United Kingdom
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The only suitable one there really is the 25.2V 2A job, but the current is too low for a good gainclone. It's only a 50VA transformer and really you want 120VA at the bare minimum.
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Old 9th July 2006, 01:46 AM   #3
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I would go for the 12.6v 3A Xformers without a doubt, because at least they have the current to drive the GC at your rail voltage. You will get about 20W per channel.

The reasoning is this:

How much current a chip like the LM3886 tries to draw is dependent on the rail voltage you supply it. 25Vrms secondaries translate to about 30Vpeak per rail. Power out, in watts is then...

Po= (Vpeak^2)/2R

where R is your speakers Impedance. So, with this rail voltage, the amp will attempt to put out 50W of power. This incurs a current draw determined by:

Ipk = sqrt[ (2 * Po) / R ]

which turns out to be around 3.5A. The 2A Xformer can't do this, although it might try, so bad things will happen

However!
If we substitute the 12.6V Xformer with its Vpk of about 18V into our Po equation...

Po = (18^2)/ (2 * 8) -- i'm assuming an 8ohm speaker here

Po = 20W give or take. Now looking at our current draw to drive 20W into an 8ohm load...

Ipk = sqrt[ (2 * 20) / 8 ] = 2.23A

And, since the 12.6V Xformer have 3A to offer, you won't starve your LM3886 or burn out you xformer.



But the real deal is you must read the datasheet for which ever chip amp you are going to use to make sure that everything fits within the parameters. If you want to use the LM3886, the math is done for you and you should be all set (but still read the ds!)
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Old 9th July 2006, 04:05 AM   #4
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You could stack some of the 12.6V trafos.

Take 2, run the primaries in parallel, and the whole secondary out to a bridge. This will get you up to near 75VA with just over 16 volt rails. For these kind of rails 3A per rail should be sufficient. Run 1 stack per channel (ie 4 transformers -- still only $12.60 for a stereo amp).

And it would be lots & lots if you built an 1875 GC.

dave
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Old 9th July 2006, 04:03 PM   #5
beady is offline beady  United States
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So basically, they aren't worth using for even a modest gainclone power supply.

For $3 each, are they still worth getting for power supplies for other projects like a preamp or electronic crossover, etc? No shipping costs.....

Thanks,

Mike
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Old 9th July 2006, 05:47 PM   #6
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Quote:
Originally posted by beady
So basically, they aren't worth using for even a modest gainclone power supply.

For $3 each, are they still worth getting for power supplies for other projects like a preamp or electronic crossover, etc? No shipping costs.....
I wouldn't agree with the 1st statement at all... it would make a decent low power GC, and would make a really good tweeter amp with an 1875.

I'd be buying as many of the 12.6s as they have. They make stellar filiment transformers -- it is easy to go in and change the secondary to 2 6.3 V windings instead of a 12.6V CT

dave
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Old 11th July 2006, 08:43 AM   #7
AndrewT is offline AndrewT  Scotland
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Hi,
Planet's idea got me thinking.

Buy 6 of the 12v6 3A transformers. Three for each channel.

Parallel the primaries. Series three secondaries. Vac=37V VA=113.

Bridge rectify the 37V giving +-26V dual polarity supply.

The big problem is finding a centre tap to reference the dual polarity supply.

You need to insert a centre tap into the middle transformer secondary. Is it a toroid? or can you buy a cheap 2 * 6Vac 3A transformer that will give you the centre tap you need.
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Old 11th July 2006, 12:47 PM   #8
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Default Series-Parallel

Beady-

You could use three of the 12.6V, 3A xfmrs, like AndrewT and alitaido both suggest. Because the middle transformer already has a center-tap, your problem of finding the center-tap reference AndrewT mentions, is already solved. There is a Radio Schmuck closing near us, and many of the same deals are to be had here, too.

Andrew, these Xfmrs are the standard E-I units, but they are shielded, which is good for audio applications.

Regards,

Steve
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Old 11th July 2006, 02:16 PM   #9
AndrewT is offline AndrewT  Scotland
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Hi,
It appears I'm too lazy, I should have looked at the links first.

The trafos are already centre tapped so three in series is just perfect for a bridge rectified +-26Vdc PSU.
They will support 75W maximum output power. 8ohm ability will be quite a bit less than this from +-26V.
If you want +-35Vdc then use eight transformers.

$18 for a pair of 113VA trafos is a bargain.
Only down side will be high regulation due to the initial small VA rating and compounded with EI being slightly worse than toroid.

Be careful to check absolute maximum voltage before buying suitable smoothing caps.
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