Thought I would do some copy & paste from the Rob Watts BluII thread that led me to try the ferrites - apologies, but it does add some grist:
Anyway, I spent the whole of last afternoon, and much of Friday evening on it. About half-way through, I lost the will to live struggling to hear any consistent differences. But at the end it proved worth it.
So what did I do. Firstly listen to some BNC cables, and I bought these:
1M gold BNC http://cpc.farnell.com/pro-sig...o-plug-1m/dp/AV14130
2M gold BNC http://cpc.farnell.com/pro-sig...o-plug-1m/dp/AV14131
5M BNC http://cpc.farnell.com/pro-sig...o-plug-1m/dp/AV14118
After a lot of initial inconsistency, a number of rules became apparent:
1. Do not loop up unused cable - it sounds better stretched out.
2. Cables must not touch other cables (they can touch one another though - just avoid mains and other digital cables).
So which one sounded the best? Well it actually was a struggle hearing consistent differences, particularly bearing in mind comment 2 - how one placed the cables is almost as important as the actual cable.
In the end, the 2M BNC was best - it sounded a little deeper in depth than the 1M, and a tad warmer. The 5M was slightly brighter than the 2M, and had slightly worse depth.
The changes are small, level 1 - that is only audible by a careful AB listening test. So at that point, I was disappointed, as 3 hours work gave no big improvement.
But there was something else to try. Now I am convinced that the RF qualities of the cable is not too important as quality standard RG59/U sounds identical to exotic PTFE insulated cables - the only difference being length. And increasing length increases inductance, which reduces RF ground currents going into Dave - and that's what we want to do. But there is a better way of increasing the common mode impedance than simply adding length - and that's ferrites that clamp over the cable. So I had bought some ferrites to try too:
Ferrite clamp, 5mm http://cpc.farnell.com/wurth-e...m-320-ohm/dp/FT01652
Ferrite clamp, 7.5mm http://cpc.farnell.com/wurth-e...m-320-ohm/dp/FT01655
The clamp simply clip together and clamp over the cable. A plastic key is used to unlock the clamp.
Initially I tried the 7.5 mm, with one on each end of the cable. Boy what a change! This was not something small, struggling to hear a difference - it was a lot darker, with better instrument separation and focus - all classic signs of lower RF noise. And it was a lot darker (to my ears) - so much so that I suspect some people may find it too much of a change - but believe me run with it, you will just need to make your system more transparent or brighten it up with EQ, speaker placement, different headphones etc. With all of my DAC's, jitter is not an issue, so the only thing to worry about is RF noise creating more noise floor modulation. The rule is simple - if the digital source sounds brighter, it is categorically worse, even if you think it is more transparent, as it isn't. And if the digital source sounds too warm, then change the rest of your system to make it brighter. Otherwise, you will not make progress towards truly transparent sound.
The 7.5mm sounded better than the 5mm. Also it sounds better with both two 7.5 mm at Dave end. I only bought 4 of the 7.5mm, so I need to order some more. CPC have low stock, but you can buy from Farnell:
I will try more, but I suspect 4 on each cable (Dave side) will be the best. Ferrite treated cables also reduced the length effect, so the difference from 1M to 2M was smaller.
In the evening I used the cable on my main system, just for pleasure - and immediately I could tell it was a lot better - warmer and darker, with better depth. I also found that I could listen at louder levels - this is classic RF noise effects.
To conclude - don't bother with expensive BNC - just use quality RF RG59/U. But ferrite clamps on Dave side is a must.
Some additional pearls picked from different posts:
The problem RF frequency range is 50 kHz to well over 1 GHz... But my guess is, in this application, 100 MHz is the key. I did try the 5 mm ones too, these have a higher impedance, but was not as beneficial (SQ wise) as the 7.5 mm ones, and I guess these have a higher resonance frequency. Ferrites are not like inductors, in that the the series resistance increases at RF, so they provide better attenuation above resonance.
The issue here is not so much RF noise per se but when it is correlated with the wanted music signal. What the ferrites do is reduce common mode RF currents from entering Dave's ground plane. My thinking is that the correlated RF gets directly demodulated into a distorted audio signal, and this is responsible for the improvement in depth that I have heard with this treatment. The extra warmth is just straight forward noise floor modulation due to random RF noise inter-modulating with the analogue electronics within Dave. So if the common mode impedance was infinite, then we would have effectively isolated it completely from upsetting the ground plane.
Ferrites on digital cables have absolutely no down-side technically. But for analogue cables, proceed with extreme care, as ferrites, like all magnetic materials, have non-linearities, and the benefits may be be much smaller than the problems of the non-linearities. Principally we have two problems:
1. Hysteresis - this will cause straightforward high frequency distortion, and timing distortion, as signal delays depend upon previous activity. Both effects are highly audible.
2. Inductor saturation. As current flows in an inductor, the inductance value reduces with current; this in turn changes the phase angle, so the delay varies with current amplitude. This creates PIM (phase intermodulation distortion) and also creates non-linear timing errors too. Again, this effect is highly audible with large currents (loudspeaker outputs) on both inductors and in-circuit ferrites.
Experience has taught me not to use inductors or ferrite beads in-circuit with analogue, because of these problems, as they are directly measurable (with loudspeaker outputs), and certainly audible. The large ferrite cylinders are less of a problem, but nonetheless don't assume that a ferrite that is good on digital will be equally good on analogue.
As too which ferrite is best, then for sure the warmer, smoother or softer sounding is the more transparent, as the mechanism for changing the sound is RF noise creating noise floor modulation - and more noise modulation always sounds brighter. Moreover, it's very easy to confuse a bright sound with more transparency.