Best Way to Suppress TX AC Secondary Noise??

[Ron AKA]

I'm refurbishing a Hafler DH-200 power amp. Hafler has put a single 0.01 uF ceramic cap across the AC input terminals on the bridge rectifier. Based on the recommendations in this article, I was thinking of replacing that with two 0.1 uF ceramic caps to ground. They show as Cs9 & Cs10 on the schematic. However, this article seems to be based on a design using a three prong grounded 120 Volt AC supply. The Hafler design uses an ungrounded non polarized AC power cord. I was going to ground the caps to the center tap of the transformer secondary.

However, now I'm thinking that is not such a good idea. If the cap was to short I would be putting full AC on the floating ground of the amp. Now, I'm thinking I will just stay with the Hafler design and go with a 0.1 uF ceramic across the AC input. If it fails shorted the main AC supply 3 amp fuse should protect from too much damage.

Your thoughts and suggestions?

[AndrewT]

You must use X rated or Y rated capacitors across any of the mains power leads.
Only after the isolation transformer can ordinary capacitors be used.

[Ron AKA]

This would be on the secondary of the transformer.

[Robert F]

Originally Posted by Ron AKA

This would be on the secondary of the transformer.

I think it was John Swenson who found very good compromise values which work for damping most transformer secondaries. From memory the values were 330ohm R in series with .022 uf capacitor. These are wired in series across the secondary of your transformer: the input to your rectifier bridge would be a good point to solder them. Please search to verify the values, this is from memory. I have tried this on my equipment and found that it works very well.

[Eva]

X rated capacitors are intended for line-to-line differential-mode EMI filtering applications, these are intended to whitstand transients (like moderate inductive kick-backs) without shorting (self healing) and are usually films available from 100nf upwards.

Y rated capacitors are intended for line-to-ground common-mode EMI filtering applications, these are usually ceramics and available in smaller values (typ 220pf to 4.7nf) and are intended to whitstand even higher voltage transients like lighting without shorting.

Note that there are sub-ratings too, like X or Y followed by a number.

Standard 500V/630V ceramics/films are not recommended for anything involving mains lines, they tend to end up shorted or exploding after some time.

[Ron AKA]

Thanks for the tips on X & Y rated caps. That classification is not used in Canada, but I will check out the safety issues.

I guess my basic question is whether or not a line to line X7R ceramic cap would work as well as a line to ground in suppressing high frequency noise?

[kevinkr]

Originally Posted by Ron AKA

Thanks for the tips on X & Y rated caps. That classification is not used in Canada, but I will check out the safety issues.

<snip>

The classification is used in Canada. Both CSA and UL require the use of X across the line and Y caps from line to chassis/safety ground as appropriate in line powered applications. CSA standard 22.2 applies in Canada and UL1414 in the USA. (Amongst others) There is also an IEC standard from which these and the TUV (German/CE mark) standards derive. See the first five paragraphs here: EMI/RFI Filtering

Here is some more information on noise filtering, X, Y caps, and safety standards: http://www.capakor.com/product/pdf/%...nformation.pdf This provides quite a lot of background on the standards, and practices involved..

In your application I would use X1 or X2 with Y1 caps..

[DF96]

I would not take a lot of notice of that NatSemi article. The authors admit that they have not calculated or measured anything, but merely copied what they see others doing. This is fashion-following, not engineering. Well below NS usual standard for application notes.

What are you aiming at with your proposed change in caps? Incoming mains noise, or rectifier switching noise causing transformer transients? A mains filter on the primary will solve the first problem. The existing secondary cap or a snubber, as suggested above, will solve the second one.

[Ron AKA]

Originally Posted by DF96

What are you aiming at with your proposed change in caps? Incoming mains noise, or rectifier switching noise causing transformer transients? A mains filter on the primary will solve the first problem. The existing secondary cap or a snubber, as suggested above, will solve the second one.

Will put 0.1 uF ceramic snubber caps on each of the bridge diodes to attenuate diode switching noise. The purpose of the caps I am asking the question about is to shunt any noise on the AC feed to the bridge. Hafler has a 0.01 uF there now, and am thinking now just to upgrade it to a 0.1 uF as I will have extra of those. The bridge will be upgraded as well to 50 amps to improve reliability and reduce forward voltage drop.