Audio circuits and wall wart

[Jester]

I'm working on an audio design that is/was battery powered, now I have been asked to add a jack to allow external power from a 9Vdc "Wall wart". I recall reading somewhere that powering audio circuits from a Wall Wart is problematic in that these supplies are usually quite noisy. Can anyone make some suggestions on appropriate filtering when using a Wall Wart?

Thank you.

 

[barry]

There is nothing intrinsically noisier in a wall wart than any similar circuit. A linear one would be less noisy than a switcher. There's no one answer to your question about filtering, it depends on what's coming out of the wall wart and what your requirements are.

 

[BradtheRad]

I tested several inexpensive DC adapters from my collection, looking for one to power my computer speakers.

Finally I found one where the sound was loud enough, without much distortion. Evidently the voltage held sufficiently high, and with sufficient filtering.

There were one or two other wall warts, whose transformer put out a high enough voltage to sound good at soft volume, but was unable to provide sufficient Amperes at louder volume. Distortion was the result.

Another cause of distortion might be an undersize filter capacitor. At louder volumes, it tends to increase ripple, while volt level drops.
Therefore it increases mains hum, which is a bass frequency. It comes out as distortion through cheap speakers.

 

[schmitt trigger]

As Brad mention, not all wall-warts are created equally.
The one that worked for my the best was a wall wart from a discarded printer. Printers can draw very significant current surges when the motors get running.

 

[jiripolivka]

Like with any power supply, you must follow your demands and "wall-wart" specifications. Using a tiny battery will cause you the same problems.