Ferrite beads and analogue DAC output

[axle_69]

OK, I know this is going to be a bit controversial...
Yesterday I was cleaning up the bookshelf and found two large ferrite beads, for fun put them around the balanced cables that go from the DAC to the Stax amplifier. The sound seemed to change a bit, not in frequency, but everything seemed more focused, less harsh. Didn't make much sense so I asked my wife to be the guinea pig, she gave me that look but agreed, with closed eyes she told me there was a big difference, voices were more clear and instruments more separated, that anyone could hear the difference (obviously she hears better than I).
I'm using Vovox Sonorus balanced cables that are relatively thick, so they just pass once through the ferrite beads (I put a thick cloth as spacer to keep the same distance around the cable), so not many turns and not a lot of filtering. Cables have no shielding but are balanced, so any RFI noise coming to the cable should cancel out.
Ferrite beads filter in the 10s to 100s of MHz region, so very, very far from the audible range.
Audible frequency seems the same. Played some monotonic test tones at different levels, with and without the ferrite beads, and the result is the same. I wish I had an oscilloscope but I haven't.
As hypothesis I would say some MHz noise is being generated inside the DAC (an AQVOX) and goes with the phase and inverted as well. But, even being true, it would be very far from the audible range. Anyone knowledgeable has an explanation, apart that we are crazy?
The ferrite beads are close to the Stax amplifier but when close to the DAC output the result is the same.
The connection between the computer and the DAC is Toslink so completely isolated.
The DAC and the amplifier are powered with Wireworld Stratus 5 power cords.
Cheers,

 

[Mython]

This might be of interest to you:
 

 

At the risk of being flamed, I don't see how the composition of the USB cable wire can add warmth to digital data from whatever device is being used as a transport

I understand those concerns to - after all the data is the same. But there are solid scientific reasons why they can make a difference.

 
In the 1980's, people started talking about mains cables making a difference to the sound quality - and I didn't believe it either - particularly as my pre-amp had 300 dB of PSU rejection in the power supply. But I did a listening test, and yes I could hear a difference. Frankly I still could not believe the evidence of my own ears, so did a blind listening test with my girl friend. She reported exactly the same observation - mains cables did make a difference to SQ.
 
To cut a long story short, I proved the problem was down to RF noise. RF noise inter-modulates with the wanted audio signal within the analogue electronics, and if the RF noise is random, then the distortion is random too and you get a increase in noise floor with signal. This increase in noise floor is noise floor modulation, and the brain is very sensitive to it; you can perceive tiny amounts of noise floor modulation as a brightening or hardening of the sound. By tiny I mean the noise floor modulation needs to be well below -200 dB, so the brain is very sensitive to it. With the right test equipment, you (APX5555 is only test equipment that has no innate noise floor modulation) can easily measure the effect.
 

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So what are the best USB cables? Firstly, be careful. A lot of audiophile USB cables actually increase RF noise and make it sound brighter, and superficially impressive - but this is just distortion brightening things up. Go for USB cables that have ferrites in the cable is a good idea - it may also solve any RF issues from the mobile that you may have too.

 

Rob