connecting primaries in series
 

Hi,
I have two toroidial 115V transformers while we have 230V. I want to connect the primaries in series and use for a mosfet class ab power amplifier. I wonder if there should be any problem doing that. I assume that the load is balanced on the two rails and since we have capacitors, there won't be load variations on the two transformers. What do you guys think?

Hi elic
Do you intend to have separate rectifiers on each secondary for the + & - rails? If so, there shouldn't be a problem.
However, if you plan to connect them in series or parallel, you have to consider their phasing. More details please.

Hi Bibliophile,
thanks for your response. I usually use a single rectifier but I have no problem using two of them. Now for some details on my project:
I am going to build this amp 300 Watt MOSFET Real HI-FI Power Amplifier | EEWeb

I am going to have 2 x 27VAC transformer and would welcome a suggestion for a quality power supply.

Hi elic
Each of those 27 Volt windings would give you about 38 Volts DC if used individually.
If you want to combine them into a single supply, you would have to series the secondaries and use the junction as the centre tap.
To make this work, the windings have to be correctly phased or you will get nothing out. To do this, connect your primaries in series, the actual wiring is not important at this stage. Power it up and measure the voltage accross the two secondaries in series. If you get double the voltage of each secondary, all is well. If not, reversing one secondary or primary winding should fix the problem. There is no need for a series light bulb in the primary but by all means use one if it makes you feel more comfortable. If you were paralleling windings, then a light bulb is a must.

Thanks. I would like to convince myself that connecting in series the primaries of the two transformers will not do any problem. Can tell what's the difference between using a single or two rectifiers?

Running two 115 volt primaries in series is only safe if the secondary loads are balanced, If they are not the transformer with the greater load will attempt to take more current and therefore the voltage across it will reduce and the other transformer will see an overvoltage. Since I would assume that you would run the 27 volt secondaries in series to achieve the + and - supply rails the load cannot be assumed to be equal on each transformer if the amplifier current is not exactly symetrical.

important question : each xformer have single or dual secondaries ?

But the question is can't I assume that in an audio amplifier the two rails are balanced since it's an AC signal. Maybe not exactly but close enough to not show any noticeable difference.

Each one has a single secondary.

The two halves should be balanced but they may not be. More importantly you are assuming no fault condition. If one half of the amplifier fails in some way or even a supply wire falls off you are likely to end up with quite a lot of smoke. It is bad practice to operate transformers in this way even if it will mostly work.