Powering a speaker via. wal-wart dc and it is humming?

[TheLyst]

I have a circuit with a speaker that is being powered from an AC/DC adapter and the speaker is humming and I need to know how to get rid of the hum. When I power it with a 9v battery it sounds great. I suppose it has something to do with the noise from the AC transformer? How do I filter out the hum?

 

[Hero999]

The filter capacitor inside the adaptor is probably nit big enough.

You could use a regulated adaptor or connect a large capacitor (1000µF to 10,000µF) across the adaptor's output.

 

[MikeMl]

Most of the plug-in power supplies that say "DC" have just a half-wave or full-wave rectifier with NO filter capacitor. First try a 500uF 15V electrolytic capacitor wired across the Wall-Wart output (observe polarity). Or start with your existing DC plug-in, add the filter capacitor, and then add an LM317 IC chip regulator circuit between the filter cap and your amp.

Finally, there are 9V or 12V DC REGULATED power supplies out there that you can purchase ready made. Check the surplus electronics stores.

 

[Hero999]

I'd say unless the adaptor is a battery charger it will more than likely have a filter capacitor. The trouble is the filter capacitor is invariably woefully undersized.

I've never seen a 500µF 15V, try 470µF 16V.

 

[TheLyst]

Thanks for the response guys.

So I tried several capacitors, from about 240uf up to 6800uf. It seemed promising because the hum was gone (or greatly diminished depending on the cap.) but, when I turned on the transmit function the audio had dropped to a whisper. Any thoughts? As I said before, with a battery it works properly. I have tried about 5 different AC/DC adapters.

My project is as follows. I have a wireless phone handset and base station that I hacked apart. I removed the little electret mic and fitted a small super high gain amplifier I made. So I have the handset transmitting a very amplified signal to the phone base. Where the phone jack was I took the leads and wired them to a little mono earphone jack. Normally you know the active phone line gets about 6v so I figured why not just leach some power from the phone station's charging terminals into my mono jack wiring. . . so it hums. . . I wired it to a battery and it worked . . . I wired it to a wal-wart. . . it hums. That gentlemen is my conundrum.

 

[MikeMl]

Probably RF getting into the mic preamp. When you connect the wire from the plug-in wall-wart, you create an antenna (or at least a ground for one), and rf currents go where they do not go if running on battery.

 

[Hero999]

Adding a ferrite bead and 1nF to where the power supply enters will attenuate any RF.