An audio amp that outputs 90V rms would produce over 2000W in to a 4 ohm speaker, and require + and - 140V supply rails - which is probably a little high for a telephone ring signal?.
I would expect it's probably best done with a smaller power amplifer, and with a transformer to increase the voltage?.
By some happy coincidence?, 100V line speaker transformers are common place, used in PA systems to drive long wires. They are generally specified in watts, so you could use a 10W transformer, fed from a 10W amplifer.
At a quick glance through the article I didn't notice any mention of the power requirements on the 90V signal?, do you know what they are?.
Here's a ringing circuit with a step-up transformer that produces about 90VAC to a few phones. I would use a TIP31 power transistor for the NPN and a TIP32 for the PNP.
Only 20VAC won't ring a phone. Since you don't say what it is for and don't say how much current is needed, it is difficult to suggest a change that will work.
The 1st circuit I posted uses a 120V to 9V power transformer wired backwards to boost the voltage to 90VAC, but a 9V to 20V or 20V to 9V transformer isn't available. You could power the circuit with only 5V or 6V, and use a 120V to 30V transformer wired backwards to get 20VAC output at low current. Use an LM317 variable voltage regulator and adjust its output voltage to power the circuit at the voltage that provides exactly 20VAC output from the transformer. :lol:
Canadian telcos send 90VAC/20Hz to ring a phone. Why did you want only 20VAC?
Your phone might be expecting to have the telco's 52VDC across the phone line and might not do anything without it. The 1st circuit I posted supplies 12VDC to the phone which might be enough. It shuts-off the ringing when you answer the phone. The 2nd circuit doesn't supply DC to the phone and doesn't shut-off the ringing.