Capacitive voltage divider question

Question

I see a few discussions on this but cannot really find the "definitive" answer.

I am building an audio amp and have two modules, one requires 18-0-18V AC and the other 12-0V AC, the 12V device requires 500mA approx. I have an 18-0-18 120W 50Hz transformer so have no issue with the 18V supply, but want to avoid using another transformer for 12-0V AC.

I have looked at resistive dividers but at 3:2 ratio at 500mA, these will be more like a heater, estimated need 10W resistors.

Now capacitive appears an option with 80uF (40Ω) and 160uF (20Ω). I will breadboard this but two questions:

My calculations show this will deliver around 450mA, what type and rating of cap should I use? I presume polyester 150V, but need guidance.

Other question, the heat dissipation appears to be around 8-9W, like the pure resistive solution, its significant, what is the effect on the caps?

Never tried this before so any pointers will be helpful. If the idea is a non-starter and my limited electronics knowledge has led me to a dead end, please be kind!

Answer 1

I am going to answer something you did not ask. You state that you have "two modules". The description of the power requirements make it fairly likely that you are talking about a power amp with symmetric voltage rails and a preamp with internal DC offsets (typical for a discrete preamp). The signal grounds of those are not independent. In addition, power amp rails are fairly tolerant of ripple and may exhibit significant fluctuations under load that also affect their DC behavior.

There are transformers built explicitly for that kind of strong/weak load typical for amps. Short of such a transformer, an extra small transformer is not the worst idea. Otherwise it's an option to just pick off the rectified voltage from the positive supply rail of the power amp (which is about +24V) and stabilise your preamp voltage off that. With a linear regulator, you'll be cranking out 12W of heat. It seems saner to just get an extra 12V transformer here and be independent.

Capacitive dividers, in contrast, will cause you trouble without end. Particularly because you will be combining them with a voltage source with an inductive inner resistance and possibly high intermittent load.