Original Post (September 2009)
Here are some pictures of my DIY speaker filter inspired by several threads on Audioasylum.
Very much like the Combak Enacom or the Walker High Definition Links, these are R-C networks or zobel networks that are applied in parallel across the speaker or the amplifer if you prefer (although they usually work better across speakers).
They will filter out ultrasonic garbage that happens to be in your system and their efficacy across the band depends on the value of capacitor and resistor that you use.
Why this project ? More out of necessity rather than vanity. The Alpha-Core Goertz speaker cables I use have much higher capacitance than average, enough to upset some amplifiers into oscillation. I thought I heard my AG1 speaker cables induce a hazy grain on the top end of my Hypex amplifier the other day. Maybe it was my imagination, but it was frightening enough to get this project up pronto. Zobel networks, besides their filtering effect also stabilises amplifiers.
Since this is a prototype project, I left this quite simple with a 10 nF and 1 nF cap, each in series with a 10 Ohm resistor.
It was really a surprise how much difference this simple device makes. There is a distinct clearing up of the sound, especially in the focus and articulation of the midrange. Vocals in particular sharpen in image size and the texture of the human voice becomes easily discernible. High frequency hash and the mess that happens when the recording mix becomes too heavy are cleaned up substantially by this.
Try it ! Its cheap, effective and really works.
Revisting the speaker filter - 4 years later (October 2013)
After a fair amount of experimentation using terminal blocks and spare capacitors, I made a second version of the speaker filter.
First off, the choice of capacitor is critical. The typical audiophile plastic foil capacitor does not work well here. I tried using a suitable value Multicap RTX and Vishay MKP 1837 but the results were underwhelming. Please stick to parts suitable for high frequency application like suppression caps, ceramic or silvered mica caps.
Secondly, it pays to utilise the other high quality parts and techniques recommended on Audioasylum, such as solid silver wire leads, and shielding the box - they really do make a difference. I am not sure whether the resistors make much difference, but I used 1% PRP resistors as recommended.
This filter sounds even clearer and more resolving that the initial set made. Soundstaging is particularly wide, and imaging is absolutely pinpoint. Go for this version if you can.
My audio buddy TC made a version using 3 stacks, i.e. 10 nF, 1nF and 0.1 nF but found it too dark. He removed the 0.1 nF capacitor and liked it a lot better.