Using 2 transformers for 1 amp

Hi

I want to use 2 separate 225VA 40-0-40V toroid transformers for my mono amp and was wondering how you can do this - ie: connect both transformers to the same pair of supply rails. Would you use 2 bridge rectifiers, thus preventing the transformers interfering with each other? The amp runs off +-56V rails.

:confused:
 
Amp power supplies

You can certainly use one transformer to power both channels.

The next best thing to using separate transformers, bridges and rectifiers is using two bridges with two capacitors for each channel. Next best is two bridges and two capacitors, where you still get a cleaner ground.

Go here for an excellent article series on power supplies:

http://www.zero-distortion.com/techno/powersupply/powersi.htm


Carlos
 
Yes, a picture can't hurt but generally it's very rare with center taps on toroids, almost allways separate windings. My general advice is to avoid parallelling windings directly if possible, only beacuse of possibility to get increased power losses, but this is not very harmful. The transformer gets heated that's the only thing, may also sound a little bit more.
 
If it's a Class A amp, where a little bit of series resistance doesn't hurt, you can build up two separate power supplies and then bridge the outputs together using resistors.
I was also once considering a scheme using two capacitance multiplier circuits, each with an emitter resistor and the outputs tied together. I'll sketch it up if anyone is interested but don't quite know what I'm talking about. This should work okay for Class AB amps.
 
OK, nobody asked but I'm interested in what people think. You could adjust the two emitter resistors to share the load unequally, if you had two different size transformers.
p.s. I've kept this simple to show the concept. Yes, you'd have to do this twice for +ve and -ve rails, and obviously use PNPs for the -ve rail.
 

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I was thinking about doing this. It makes sense that if one of the transformers has a slightly lower output voltage than the other, then the bridges will prevent the one transformer feeding into the other. But would each transformer be used 'equally' to charge the capacitors - ie: is the one with the lower output voltage just going to sit there while the other does all the work?

:confused:
 

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Using 2 transformers

Yes, if the two xformers are somewhat different in their output volts, there is a chnace that only one does the work. In your case, I would follow the advise of Peter Daniel I think it was to make two seperate supplies, one for the pos rail, one for the neg rail. If you cannot separate out the two center taps, use only two diodes in each bridge, that is connect the two winding ends to the ac terminals and use either the + terminal or the - terminal to feed the caps, depending on the rail polarity. The not used terminals of the bridges are left unconnected.

Jan Didden
 
Two transformers

Make separate transformer bridge circuits for positive and negative voltages. Use only one of the secondary windings on each transformer and full wave rectification on each transformer.
The two fiiter caps should be connected at the central supply ground and be the only commom connection on the secondary side of the transformers. This will work great and the increased winding resitance of one verses two in parallel is not important and actually can have benifits that are little beyond the scope of this discussion.


Chance
 
Re: Two transformers

Make separate transformer bridge circuits for positive and negative voltages. Use only one of the secondary windings on each transformer

But the point of me using two transformers is so that i can double the VA rating of one. i have two 225VA's and need 450VA. Using only one secondary means i have two 112.5VA windings which means 225VA again - what is the point then?
 
Paralleling transformers

>But the point of me using two transformers is so that i can double the VA rating of one. i have two 225VA's and need 450VA. Using only one secondary means i have two 112.5VA windings which means 225VA again - what is the point then?>

The point is that paralleling transformers is not as easy as doing that with resistors or capacitors. Particularly for power amps.


Carlos
 
Re: Two transformers

cgardener said:
Make separate transformer bridge circuits for positive and negative voltages. Use only one of the secondary windings on each transformer and full wave rectification on each transformer.
The two fiiter caps should be connected at the central supply ground and be the only commom connection on the secondary side of the transformers. This will work great and the increased winding resitance of one verses two in parallel is not important and actually can have benifits that are little beyond the scope of this discussion.


Chance

cgardener,

Your approach will give lower output voltage because of the extra IR losses and the extra diode drop. It more or less wasts half of the transformer capacity. If the unmentioned benefit you noted is concerned with the smoothing effect of the extra resistance, if that will happen (which you really can't say without more info on the particular transformer), that will again lead to a lower output voltage. If you want that, the advice could be to get a smaller transformer. If you want the maximum out of the existing transformer, the advice should be to use both winding halves.

Cheers, Jan Didden
 
"Yes. I've seen that done" - Chance Gardener

"it more or less wasts half of the transformer capacity" - Suck What ?.

Harry Gardeners advice is correct.
First thing is that a 225VA transformer is a 225 VA transformer, and using one or both secondaries will not change the total throughput capability of this transformer.
Because you can never be perfectly sure that each secondary has EXACTLY the same number of turns, connecting them in parallel is entirely unwise.
If one winding has at least one partial more turns than the other, you will have a shorted turns condition, which will thermally destroy a transformer.
The only way to ensure safety using four secondaries would be to use FOUR bridge rectifiers.
As Harry said, the reason for totally seperate B+ and B- supplies is that the supply common point is at the supply stage center star earth point and not in the transformers.
As he also said, use only one secondary per rail because adding an additional secondary in parallel introduces extra parasitic capacitances, and supply resonances.
Any droop in supply voltage at full power because of winding resistance is insignificant.

Eric.
 
so are you saying...

Okay. Another point now needs to be explained:

How is the VA rating obtained? I believe it is a rating based on how hot the transformer gets. So if you used only one secondary of a 225VA Xfmr, does this mean that you could draw 225VA from that one winding (5.6A at 40V)? This would then be great, since i could use one secondary off each 225VA Xfrmr to get the needed 450VA.