I built my M2x with an Antek AS-4218 . A 400VA transformer.
Why did I select that particular transformer?
Because Antek was out of stock on the 300VA transformer I really wanted, the AS-3218. I was in a hurry, I had ants in my pants. So I compromised and bought the next larger transformer which WAS in stock. Luckily I had not taped-out my self-designed, custom PSU circuit board to fab, so I slimmed the PSU board in order to accommodate the fatter donut.
Why did I select that particular transformer?
Because Antek was out of stock on the 300VA transformer I really wanted, the AS-3218. I was in a hurry, I had ants in my pants. So I compromised and bought the next larger transformer which WAS in stock. Luckily I had not taped-out my self-designed, custom PSU circuit board to fab, so I slimmed the PSU board in order to accommodate the fatter donut.
This is also the case now - the 300 is out of stock but they have some 400s.
My idea was to go dual mono with a 200va per channel. Is there any reason for / against this? The difference in cost with this is an extra bit of money and space consumed?
You want to oversize a class A transformer primary for thermal reasons. A transformer running at full rated capability will invariably get hot (easily 30 to 40C above ambient). Ok for an industrial application but not ideal for an audio amp where the amp itself is also running hot.
I think you are on the money with a 200VA component for each 25W class A channel (400VA total for both channels).
I think you are on the money with a 200VA component for each 25W class A channel (400VA total for both channels).
Ok I tweaked the spreadsheet a little bit. It now calculates against the DC for current draw. I added a section at the bottom that shows the AC current draw so I know the size ish of the fuse and the min size of the transformer - 300 😉 I'm sure someone could further extend this and calculate their electric bill since we know the current draw would be fairly constant!
The sizing did not change much it still came out to a 300VA minimum. 400 actually may be a better size just in terms of voltage drop. I don't know if that factors in much if at all but going with a larger transformer I think keeps closer to that 24v out - per the spec and what I observed personally with a smaller transformer it went from 24vdc -> 18 when drawing the max of the transformer. Going with a 300VA you might have this situation.
I'm moving on from this unless someone points out something wrong with the calculation ( please speak up if its wrong, I want this to be accurate! ).
Thanks to all who have provided responses on my question!
The sizing did not change much it still came out to a 300VA minimum. 400 actually may be a better size just in terms of voltage drop. I don't know if that factors in much if at all but going with a larger transformer I think keeps closer to that 24v out - per the spec and what I observed personally with a smaller transformer it went from 24vdc -> 18 when drawing the max of the transformer. Going with a 300VA you might have this situation.
I'm moving on from this unless someone points out something wrong with the calculation ( please speak up if its wrong, I want this to be accurate! ).
Thanks to all who have provided responses on my question!