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RE: They are not necessarily mutually exclusive.

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LewinskiH01

In Reply to: RE: They are not necessarily mutually exclusive. posted by bartc on August 01, 2010 at 05:34:03

Sorry my answers came out without order. They were supposed to be answers to specific posts.

Anyway, thanks all for the great feedback. This is very helpful.

So I'm taking away I need to use a combination of approaches and I need to experiment to figure out what will work. Fair enough.
Some approaches are more expensive and more complicated, especially considering I now live in Argentina where voltage is 220V and getting stuff shipped here is possible but cumbersome.

So the first two things I want to try are RC filters and isolation/balanced power transformers. For the latter I need to speak with a local transformer manufacturer tomorrow. So that leaves me with the RC filters. Did some reading online and I gather we are talking about low pass filters. I'm fully aware the capacitor and resistor values in a filter designed for 110V will not work for 220V, but it would be very helpful if someone could point me to a schematic of a RC filter designed for AC power, and some reference info such as metal foil capacitors are to be avoided or what I should look to find as far as components. Also is the 50 vs. 60 Hz going to be an issue?

This should be fun!

Follow Ups:

Use only AC-rated noise suppression caps. - Al Sekela 18:53:32 08/01/10 (2)
Capacitors made for AC noise suppression are designed to not generate resistive leaks when exposed to power line spikes. Other capacitors may have much higher DC voltage ratings, but may not withstand repeated power line spikes without degrading and catching fire.

Noise suppression caps these days are all rated at 225 volts AC or higher, as it is too expensive to maintain separate stocks for different countries. Any of these would be safe to use with your mains.

The dielectrics in noise suppression caps do affect the audio performance, even if the filters are used on other branch circuits. The ones I've found to give the best sonic performance are Wima MP-3 metallized paper.

Use resistors in series with the caps to provide electrical damping. Placing the caps directly across the AC line will result in ringing, as the caps do not absorb noise energy. Use resistors that are flame-proof, or mount them in such a way that if the cap fails to a short-circuit, the brief flame resulting from the resistor burning out (about as much as a kitchen match) will not create a fire hazard. You are responsible to make these things safe: I'm just telling you the possible sources of hazard.

Each size of capacitor needs a different value of resistance to achieve optimum performance. I'm not ready to share the values I've found by experimentation, but you can make some experiments and find combinations that sound best to you. Start in the range of hundreds of ohms.
- "flame proof" resistors - John Swenson 16:21:59 08/04/10 (1)
  A couple years ago I was working on a friends amp that wasn't quite working right. I was bending over the circuit with a scope probe and was about to touch a wire (hadn't actually touched it yet!) when a resistor next to the probe burst into flame. It was about a 6" pure white flame that lasted quite a while, right below my face. I WAS wearing safety glasses so there was no eye damage.
  
  I checked with the manufacturer and it was a flameproof resistor. I've never seen one give off a flame like that before, even when TRYING to get that effect! (what can I say, high school electronics class)
  
  I still have that scope probe, with a black charred tip. Its hung on the wall of my lab to remind me to ALWAYS wear eye protection when working on live circuits.
  
  I don't believe in "flameproof" any more.
  
  John S.
  - RE: "flame proof" resistors - LewinskiH01 18:48:34 08/05/10 (0)
    Thanks all for the input. Very helpful. And thanks for that iso tranny schematic!!
    
    Anybody able to share a schematic of an RC filter for AC power? Been reading some more and found the Jensen 4-pole caps to be very well liked (also Blackgates), but can't get a schematic.
    
    Thanks!
I think Al already answered this before - bartc 16:01:27 08/01/10 (0)
Try the archives.

In any rate, for the R-C filters you need caps that are properly safety rated for your voltage. I have no idea what those might be, but Al might.

The Rs are matched to the cap values too. Again, I don't have the knowledge to tell you what to use in your situation.

So be safe first and get satisfactory answers from those in the know!

I don't think these are intended as low pass filters, though. These are parallel filtering schemes.

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