Core choice for power transformer

I'm about to order few custom power transformers, some for tube amps and some for SS amps, all are for phono stages and line stages. The choice is between E-I core, C core, R core and toroid transformers.

As far as I know:
Toroid are most efficient, but don't isolate well RFI between primary and secondary;
E-I core have better isolation between primary and secondary, but they are less efficient than toroid and their regulation isn't as good;
R core have best isolation between primary and secondary and better efficiency than E-I core;
Where does C core transformers stand?


2008-09-04 1:47 pm
Probably the most important single reason is that they produce less elecromagnetic interference in surrounding components. This will be especially important with small signal devices like phono stages - the other parts in the box will actually pick up the noise through the air in between. This is why it is best to keep the transformer as far away from other components as possible, or as many manufacturers do, house it in its own enclosure.

Speaking of cores, I'd love to find a reasonable source for good audio cores and bobbins. I can't prove it, but I have an empathic intuition that solid-state push-pull amps, coupled with good transformers, emulate "Tube Sound". I built a 100 Watt PA amp about 30 years ago with PT iron OT and it sounds fine in PA use, runs directly on 12 Volts DC, idling at less than 500ma, drawing about 15 Amps at 100 W. Transformers are the levers of electricity!


diyAudio Moderator Emeritus
2003-05-29 2:57 am
Maybunga, Pasig City
I'll go for Toriods from now on. I used to use EI types that a local shop can custom make but they run so damn hot (about 65 Deg c) and vibrate. This heat generated will destroy the electro caps quickly. Granted, they were less than half the cost of a good torioid.

i can make EI's that does not run so hot.......