What capacitors should I use to filter the noise coming from the power line in a amplifier?
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M$2 Answers
-Supply voltage.
-Supply type (AC/DC?)
-Current draw of amplifier.
-Freq. of noise (just high or low is ok).
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M$
1. ground loop
2. bad ground / bad shielding or bad soldering on sleeve of coaxial cables.
3. faulty filter capacitors and / or rectifier diodes in power supply
You'll need to open the amplifier. Warning, there is source of deadly voltage inside! Be sure not to touch anything inside while power amplifier is plugged in to the wall.
Fixing ground loop is usually luck / eyesight / soldering iron skill based repair. The usual suspects are corroded contacts on the connectors, corroded traces,
leakage on the PCB, loose / unintended contacts.
Fixing bad ground or bad soldering will require trained eye capable to recognize faulty soldering points, some flux and soldering iron.
Faulty filter capacitors and rectifier diodes are common fault on used audio equipment. Methods for testing electrolytic filter capacitors are kinda murky, but you can always replace suspected capacitors with the similar capacitance, same or higher voltage caring on polarity. If there is no more hum, your problem is solved.
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M$
You have a ground loop.
http://www.epanorama.net/documents/groundloop/
Rangerx52 is right, you do have a ground loop....somewhere.This fault can be caused by the amplifier in question, another device connected to the same circuit or (theoretically but unlikely) as far back as the supply (your electric company). If you don't want to, or don't know how to chase down the source of this problem; you can use a capacitor to create a high pass filter.
Given that you said the supply voltage is 220V, I assume you're somewhere in Europe which means that your supply freq. should be ~50-60Hz. You'll want to build a filter that blocks anything > ~60Hz. The math to determine the proper capacitance is pretty simple but it can't be done w/o knowing the load value of the power supply for the amplifier. To determine the load you'll need to either open it up to determine the inductance of the transformer or use an ammeter (mains voltage, be careful!!) to determine its current draw.
If you want to use loose numbers (manufacturer specs) then you can take the "max power consumption = 25W", divide it by 220Vac to determine current draw which comes out to 113.6mA. Then divide 220Vac by 113.6mA to come up with a total impedance of 1.936K ohms. You now know that you need a capacitor that will provide a capacitive reactance (Xc) of 1.936K ohms @ 60Hz. The formula to determine capacitance for Xc @ a given frequency is 1/(2pi*f*Xc). So 1/(2pi*60*1936) = 1.37 *10^ -6 or 1.37uf.
Remember that this is all based on a bunch of rated values and assumptions so it would be much better to take some measurements and re-calculate.
Edit: in previous post; ">" should be "<"
Thank you for you interest, here are more info:
Supply: 220v AC
Current draw: I don't know if this helps, but the max. power consumition is rated as 25w.
Noise frequency: low
If there are some other info needed, just ask me. I'm ready to open the thing (power supply + amp + speaker, all inside a box for 2.90€ at the local second hand store)