I want to make a very simple high pass filter at around 70HZ. anyone know what value of capacitor I would need for this.
thanks
Rod

Hmmm, do you want to block the low freqs and pass-thru the high freqs? Or, block the highs and pass the lows?
70 Hz is pretty low to go.

pete

I want to block the low freq. from a 10" full range speaker. After re caping I still have a 60HZ hum on an old radio console. This seems like the easiest quick fix for now.
Thanks
Rod

Hotrod,

I think a phase cancellation technique would be much more effective than a simple HP filter in your application. If the radio has a power transformer take a sample of the filament voltage and insert it in the right anti phase and level to the speaker voice coil to exactly cancel the hum. Hook one side of the filament winding to one side of the speaker voicecoil and run the other side of the filament winding through a resistor and pot wired in series and then to the other speaker terminal. You might try a 100 ohm resistor at about 2 watts rating and a pot of about 500 ohms. The 100 ohm resistor is there to prevent applying too much power to the speaker at the minimum pot setting. You would adjust the pot until the hum was minimized. It may take some resitor value experimentation as I just pulled these numbers out of my head. If the hum just gets louder reverse the connections from your cancellation circuit to the speaker. Obviously you still leave the original speaker hookup to the audio output transformer undisturbed.

Let us know how this works if you try it.

Of course another way to stop it from humming is to teach it the words. :lmao:

Rob

I just had another thought. Just in case the hum is leading the phase of the power line you might have better cancellation effect if you also add a capacitor in series with the pot and resistor. A 0.5 uF at 100 volts or 1.0 uF at 100 volts (voltage rating can be higher) might help. Use a non-polarized cap, not an electrolytic.

A drawback of this circuit is that although intheory it could completely eliminate the hum, better than anything but a complicated multi-pole filter, it will only work best at one specified volume level. If yo usually run the radio at the same volume then set the anti-phase pot to have best effect at this volume level. Phase cancellation is a delicate balancing act. Of course you could always inject anti-phase sample hum into an audio stage before the volume control but that gets harder to explain and more tricky without my seeing a schematic.