While I agree that using a iron-core, 50/60 Hz transformer makes a lot of sense for audio amplifier design in the range of 10 -100W of output, I think that customers wish for smaller box, along with new requirements for standby power efficiency will push the use of switching power supplies more and more over time. These switching power supplies don't suffer the poor power factor of a bridge-rectified iron transformer with big filtering capacitor, plus they allow designer to use tricks like variable power rails to save power when the volume control position is low. These can also suppply the control voltage along with the power rail.
The initial investment in designing those may pay off for big manufacturers. This is obviously more difficult for a small shop to design and certify a line-level switching power supply for a power amp so there is a definite opportunity here for companies to design specialised off-the shelf unit for sale to the smaller audio components designers that can't afford to design them.
Although much is made of the "low" external magnetic field of toroids, they must be rigorously constructed to live up to that reputation. First, each winding must COMPLETELY cover the entire core. In most commercial units, the magnetic field emanates from the egress of wire leads, where core coverage has a gap. Second, virtually all power transformers are designed to be as cheap as possible. Less core and less copper are used in designs that operate very near magnetic saturation of the core. But the downside is that, as saturation is approached, radiated magnetic field drastically increases. It's also worth noting that secondary (load) current does NOT increase flux density. Only the primary voltage affects flux density for a given design. Therefore, a design on the verge of saturation at 115 VAC will likely be a problem at 130 VAC. To make any power transformer magnetically-friendly, simply operate it at reduced primary voltage (with reduced secondary voltages, of course). Another problem with toroids, because they don't have even the smallest air gaps in the magnetic path, is inrush current when they're first turned on. In large transformers, this can cause nuisance breaker tripping. I think the advantages of toroids are usually over-stated. -- Bill Whitlock, president & chief engineer, Jensen Transformers, Inc. www.jensen-transformers.com
Regarding your comment:
"It's also worth noting that secondary (load) current does NOT increase flux density. Only the primary voltage affects flux density for a given design."
It's very refreshing to hear someone with strong links to the audio community expressing this simple but so frequently misunderstood truth. Time and again I wearily see reviewers talking with great approval of (needlessly) huge power transformers with high current ratings, used in order "to avoid saturation".
The problem doesn't seem to be confined to the "hi-fi" community either: one close friend who's a seasoned amplifier designer of some repute was surprised when I put him right on this point at a recent AES convention. The ignorance of transformer basics seems so widespread that I'd actually thought of submitting an AES tutorial paper called "The Misunderstood Transformer" (or something like that). I've held back to-date not least because although I've designed plenty of power transformers (for low-frequency and HF switched-mode use) I've less experience of detailed design for high quality audio, and there are other luminaries such as your good self (and dare I mention Brian Sowter, Per Lundahl...) who have greater knowledge here. Still seems like a good idea though.
Perhaps we could get some T-shirts made bearing the slogan "The Volts Determine The Flux" !! :-)
This is initially only going to be of benefit for the big boys again!! While the smaller organisations will have to wait for an out the box equivalent.
The only difference is it has a multi-volt flux capacitor, apologies for the poor attempt to a bad joke!!