There was a lot of deliberation on the type of transformer to get, particularly the
secondary voltages. The concern here is the target load as, generally speaking, the higher
the voltage, the less suited it is to drive low impedance loads (4ohms). I settled on
20 volt secondaries which I think is a good compromise, feeling that 18v is perhaps a little too
low and 22v a little too high to get the max with a 4 ohm load whilst minimising output loss
if using 8 ohms. The ability to run stable at 4 ohms is paramount to me to keep it open to
different applications, and output wattage is relatively unimportant for this design.
The VA rating was a much simpler decision. Go for as much as possible within a reasonable budget (but a minimum
of 100VA per channel). I wanted to use a single transformer (with dual secondaries). It turns out
that 300VA was the best value.
Transformers with 20v secondaries aren't usually something you find off-the-shelf, but I found that having them custom wound
was not expensive. I also
opted for the 'audio' variety which are potted with flex leads and finished in black. The finish is actually
quite important as I may decide for an open/transparent case where it can be seen. Much to my amusement, the cost
of this custom wound toroidal transformer was cheaper than standard off-the-shelf products of lesser quality! The fact that
the toroids are 'audio' graded, and potted (supposedly to reduce noise) is even more of a bonus.
The toroids arrive and are weighed out of curiosity at 2.4Kg each. They are actually rated at 230v on the primaries, so with
my higher mains rating (which I measured at 248v), the secondaries will be a little higher than anticipated.
The first toroid measures ~22.78v on each secondary, and the second toroid measures ~22.73v on each secondary.
The winding colours for these Airlink transformers are as follows:
- Black start, Red finish
- Orange start, Yellow finish