choosing right toroid (for a 1272)

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Jonkan

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Quick newbie question. What would be good values for a toroid that would power 2ch of 1272 preamp?

how many VA is needed and what Vac should the secondaries give?

/Jonas
 
would 20va work too? And would 2x24v secondaries be ok? What would be the minimum ac voltage out from the secondaries? Would a lower value on the secondaries work or not?

Im sorry, but i know nothing about power transformers... yet atleast :green:
/j
 
would 20va work too? And would 2x24v secondaries be ok? What would be the minimum ac voltage out from the secondaries? Would a lower value on the secondaries work or not?

20VA 2x24v would work OK. The voltage rating is an RMS value. The peak value which is what you get when you rectify the a.c is 1.414 times the RMS value. as long as you have more voltage than your regulator chips need to regulate then you will be OK.

For example if you arte using a 7824 24v regulator they need 4 odd volts for it to regulate so you need to supply it with at the very least about 28v, or else it won`t regulate. 30v would be better. So anything less than about 20v is not brilliant (20 x 1.414 = 28v)
 
thanks for the explanation! Seems like i can actually learn something from this! thanks!

If i run the secondaries in series on an 2x15v i would get 30v ac right? which would mean 30x~1.4= apr. 42v rectified. Have i understood this correct?

would that be ok?

(im asking because i have a toroid with those values)

btw, How do i know how much VA the toroid needs to be?
/j
 
If i run the secondaries in series on an 2x15v i would get 30v ac right? which would mean 30x~1.4= apr. 42v rectified. Have i understood this correct?

Yes. You will need to check that the regulator that you use is capable of dumping 42 - 24 = 18v. If it is a 7824 then from memory I think this is OK.

btw, How do i know how much VA the toroid needs to be?

The VA rating is effectively a power rating so for example if you have a transformer with a 24VA rating and it has a single secondary of 24v it will be able to supply 1Amp at 24v. If its VA rating was 48VA then it would supply 2Amps at 24v.

For 2 channels of 1272 24v at 1A is plenty.
 
ok, i see. This is fun, im learning stuff. :green:

So if i have a toroid with a 22v secondary, that is 15VA. That would make
the toroid able to supply 15/22= 0.68A @ 22v AC (right?)....which would give me 22*~1.4=30v aproximately rectified.

so that would come out as= 0.68a @ 30v DC, which would be 6 more than 24vdc.. Should be a piece of cake for the regulator, right?

would the amperage supplied be ok for two ch of 1272? I found a cheap toroid used, with those values.

/J
 
A 1272 amp will draw about 150ma of current at +24V. So figure that, plus 10mA at 48V for phantom power. If you have LEDs, figure 10ma per LED.

Add all this up, and give a factor of safety (for inefficientcies, heat deratings, etc), and double it and treat that as your minimum.

So, for a 2X1272 figure 150mA X 2 channels X 2 factor of safety = 600 mA. Phantom for each channel will add 10 ma X 2 channels X 2 factor of safety X 2 (accounting for phantom at 48V rather than 24) = 80mA.

Total = 680 mA.

Cheers,

Kris
 
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