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My DIY AC Line Filter Build

Posted by Maxamillion on February 6, 2011 at 11:19:56:
I decided to build a parallel AC line filter along the lines of the ones outlined here, with a couple twists. First step was to determine the values of the components needed. Since I wanted to terminate the RF transmission line formed by my dedicated AC line, I wanted to match the impedance of the filter to the characteristic impedance (Zo) of 12/2 Romex at RF frequencies, which I managed to find in a reference as 143ohms. Since I had some 1 watt 68ohm PRP resistors on hand, I decided to use two of those (68*2=136, close enough to 143) in an R-C-R configuration. I decided against using multiple R-C-R in parallel, opting for an R-(multiple C)-R configuration to keep the impedance close to Zo for as much of the frequency range as possible.

I opted for Wima caps for the filter, namely 0.47uF MP3X2, .047uF MP3X2 and .0047uF Y2, sourced from TAW Electronics. I used Mortite to damp between and around the caps.

I found a nice empty Wall Wart shell from Polycase.com which is perfect for this job, complete with Hot, Neutral and ground prongs installed. I made an inner shell of TI-Shield for the inside of the wall wart, with fully soldered joints to minimize RF leakage from the back of the unit.

Finally, in a bit of a departure from the norm, I decided to pot the guts of the unit inside the TI-Shield inner box using potting epoxy filled with carbon black. Carbon black is a strong RF absorber, so it may actually help in preventing shunted RF escaping from the unit, and may increase the RF-absorbing ability of the filter. The Mortite constrained in the rock-hard epoxy should provide good damping for the cap vibrations - the caps are already encased in epoxy for fire-proofing, so I don't think it can hurt. Even though the epoxy contains carbon black, it is not conductive (8x10^14 ohm-cm resistivity), so no shorting worries. It is certainly fireproof now, as well! The only concern might be dielectric absorption effects from the carbon-filled epoxy (dielectric constant = 3.5), but time will tell on that one.

Here are some pictures at various stages of completion:

So how does it sound, you ask? Well, I plugged it into one side of the duplex outlet into which my PS Audio PPP is plugged (everything audio runs off the PP) and took a listen. First impressions were not very good; everything sounded a bit flat, and lacking in dynamics - a bit worrying. I decided to let it break in for a few days to see if anything changed - day two was about the same. Day three was this morning, and I have to say it is starting to come around; the system has much improved dynamics and sounds very clear and clean now. I'd say it is slightly better than what it was before the filter was installed. I'm hoping the improvements will continue over the next few days - I'll report back in a couple.