Proper Connection of Safety Polypropylene Capacitors in Power Supplies for Audio Equipment

Question

I'm designing a power supply for an audio system, and I've heard that safety polypropylene capacitors are recommended for filtering high-frequency noise from switching components. I have one of these capacitors, but I'm concerned about connecting it incorrectly and damaging the entire power supply. How should I properly connect this capacitor to ensure both effective noise filtering and protection of my circuit?

This is the capacitor I'm referring to.

Here is a simple schematic to show where to connect the capacitor. The capacitors shown in the diagram are basic electrolytic capacitors; the polypropylene capacitor is not included because I'm not sure where to place it. Below is a basic circuit for the LM317T and LM337T regulators.

Answer 1

I've heard that safety polypropylene capacitors are recommended for filtering high-frequency noise from switching components

It's rather hard to have an argument with hearsay.

The capacitor you have is rated to be connected across the line voltage of up to 240VAC. So it'd be connected directly across the primary terminals of the transformer.

The diagram of the secondary you have shown will not work. The best you can get is one supply (positive or negative):

^{simulate this circuit – Schematic created using CircuitLab}

Quite a bit of switching noise comes from the rectifier diodes on the secondary side of the transformer. You need to use fast types - e.g. UF4001..4007 to minimize that. The "standard" 1N400x rectifiers perform much worse, since they take much more charge to turn off compared to the ultrafast UF400x. That means that after the secondary voltage has fallen below the capacitor voltage, the rectifier conducts too long and discharges the capacitor back into the transformer for a little bit. This builds up the secondary current. Eventually the rectifier turns off and the transformer secondary generates a voltage spike as the current collapses. That's not good, and it's oft overlooked in audio designs. The ultrafast rectifiers will turns off much quicker, and the residual turn-off current in the secondary will be much smaller.

To get symmetric supplies you need a half-bridge rectifier with a single secondary:

^{simulate this circuit}

Or you need a full bridge with a tapped secondary:

^{simulate this circuit}

You also should have a power switch and a fuse on the primary side of the transformer. Never skip those two.