75TL thrown in for size
By adding a second strategicly places winding on the secondary I am able to get three different turns ratios which all have essentially the same frequency characteristics. When you work the speaker impedance back to the load you get a number of options to play with. Below is a screen grab of the spreadsheet showing how the numbers work out.
The three possible connections are:
Secondary 1 alone (Sec 2 remains floating)
Sec 1 in series with Sec 2 adding
Sec 1 in series with Sec 2 subtracting
since secondary two has a small number of turns and is never referenced to ground, it doesn't introduce the capacitive issues that plague other transformers causing varied response as you adjust the impedance.
To make things even worse (and I assure you i did not draft this on 1, April.... take a gander at this.