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Old 2nd April 2007, 06:16 PM   #1
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Default What are your RFI precautions?

I will be starting a preamp project in the next few weeks, with another amplifier in the next year. I would like to hear some opinions on filtering RF from the AC line. Not really addressing speaker lead pickup, signal lines, or rectifier switching noise for this thread. I see a few options, or combination thereof:

1. Installing Corcom type filters (custom or OTS) immediately after input fuse. Most Off-The-Shelf modules use both X and Y caps, though Morgan Jones seems to favor a design addressing differential RFI only (I believe he says the Y caps for common mode add to the ground noise). Others seem to disapprove of these devices entirely, with the claim they degrade sonics.

2. Using electrostatically shielded transformers for CM noise rejection. I like this idea, along with using a split bobbin design for filament transformers. Unfortunately, many of the power trannies I would like to use do not have the shield, and I don't want to order customs.

3. Installing ferrite beads on the power supply, and adding ceramic caps to the heater pins. I also think common mode chokes for the heater supply is fairly easy and cheap.

4. Here and there I see someone using CM chokes in their B+ supply.

So my question is what do you typically build into your designs? All or some of these, and are there any others? I would always shoot for the shielded transformer if available. My preamp won't have one, so what would be the best options?
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Old 2nd April 2007, 06:25 PM   #2
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Many COTS EMI filters wont handle amplifier psu duties due to the very high peak currents. The common mode choke in them is made to a price.

Some people dont like large X caps, but find smaller ones to be ok (ei 4.7nF v 22nF.) Y caps put leakage currents on the ground.

Ive ripped the ferrite toroid out of disused computer switching power supply and wound a common mode choke on it.

A drastic measure Ive seen taken is using balanced power. A large (preferably shielded) transformer provides 60V-0-60V power instead of the usual 120V-0. This is not to code outside the amplifier in most residential settings. The transformer must be large to avoid degrading attached PSU performance.
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Old 2nd April 2007, 07:22 PM   #3
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Quote:
Originally posted by Tweeker
Many COTS EMI filters wont handle amplifier psu duties due to the very high peak currents.

Agreed, but I will be running all choke input filters (including on DC filament PS), and PSUD shows me a reasonable crest factor on current.

I would also suggest running about 10X rated current i.e. if amp draws 2A current, I would pick a 20A rated Corcom module.

So your preference is to run more choke than cap, and eliminate the Y caps?
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Old 2nd April 2007, 07:43 PM   #4
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Some people dont like large (inductance wise, more current rating is better, asides stray capacitance) CMCs either. I cant say I have a preference on choke H v cap F, though I do favor eliminating Y caps (ideally with balanced power). I was reporting prior discussion at this site.

The thing that really needs eliminating is all the triacs in the house.
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Old 2nd April 2007, 10:14 PM   #5
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I live in a fairly large city with a lot of rf sources nearby, I have never found it necessary to do anything other than pay attention to internal signal grounding paths, and to use power transformers with electro-static shielding.

Those corcom modules are more effective for keeping conducted emi out of the wall socket from high speed digital noise sources like pcs than for anything else imho.

Generally I would otherwise echo Tweeker's comments about x and y caps as well as cm chokes.

You are far more likely to have problems with line and low level (phono) inputs than power or even speaker lines (as long as global feedback is not employed)

Fully enclosed metal chassis will work wonders if you use a little aforethought to keep most of the rf out of the box.

RCA jacks should never be directly grounded to chassis, either star grounding or bussed grounds back to a single star ground is preferred, however in RFI rich environments small ceramic caps from the jack gnd shell right to chassis will usually help. Grid stoppers on input tubes, and in some cases resistors right at the jack will help a lot as well - I have never had to use a bypass cap on a signal lead, but if needed use the smallest value that does the job and insert some series resistance ahead of it so that the driving source does not become unstable. Silver mica, and some low inductance foil types may do the job and not degrade sound quality too much - ceramic types are to be avoided due to the distortion they can generate when audio is impressed across them.

IMO for whatever reason in tube gear I have never found that a corcom entrance module results in better sound when employed - in fact much the reverse. I won't use them except in test equipment and digital projects.
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Old 3rd April 2007, 12:30 AM   #6
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That's good advice.

One note, though, is that I am unfortunate enough to not have a readily available transformer with a shield, and could not locate any off the shelf's that fit my req's.

If I could, I would be less concerned about RFI. But now I face the H-L capacitance of my power tranny, and that can be significant. I am also vain enough to insist on using exotic wood chassis with top nickel plate, so I lose some chassis shielding. Nickel does provide a little magnetic shielding, tho.
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Old 3rd April 2007, 12:54 AM   #7
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You could line said fancy chassis with aluminium if it really needs to be. No amount of PSU filtering will help if the RFI is coming into an antennea on the amp.

On line filter C and L, it really doesnt take much to achieve a <160khz corner.
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Old 3rd April 2007, 03:19 AM   #8
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Copper tape works wonders on the inside of wood chassis, and you can get it pretty cheap at most garden centers where it is sold as a slug repellent barrier. All that is required in addition is a metal bottom plate that is effectively connected to the shield and top chassis plate.

I should mention the tape usually has adhesive on the backside and adheres well to wood. Score the area where the tape overlaps to assure a good electrical connection and/or solder.
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Old 3rd April 2007, 11:48 AM   #9
EC8010 is offline EC8010  United Kingdom
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Thanks for the copper tape purchasing tip. How does it repel slugs? Is it some sort of chemical reaction, or are high voltages involved? I imagine with their good contact, a few volts would be enough.
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Old 3rd April 2007, 11:54 AM   #10
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Copper is toxic to many things. Copper sulfate is used as an algicide.
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