Hi Andy, all,
Let me say it straight out - a job damn well done, Andy! Very thorough indeed.
Hi Dejan,
Good suggestion, to use a sig gen into the power cords.
Tonight I have spent a few hours measuring as you suggested. Alas, I am afraid I did not find any "filtering" whatsoever ... ie. no reduction in signal voltage, up to 100KHz, but I thought people might be interested in what I found.
I should think so! Time somebody put an end to this cable filtering bull - because it is bull. If it filters, then it's because of large capacitance, and I hate to think what that would do the phase of your power sine wave, 50 or 60 Hz, no matter, not to even mention higher up.
I'd be interested in anybody's comments upon my musings:
1. I compared a few measurements between 4 different power cords. All were about 3m long:
a) a 'commercial' power cord that you get with a PC - ie., it has a mains plug on one end and an IEC socket on the other. I call this the 'comm' power cord.
b) a power cord made from Allan Wright's 'Super Cable Cook Book'. This used solid-core, 2.5 mm squ. electrician's wire for active, neutral and earth. The wires were plaited, then I enclosed the result in a copper braid which was earthed at the mains plug. I call this the 'grey' one (because I used an expandable woven grey sleeve over it).
c) basically the same as b) but I used a thicker, multi-strand earth conductor. I call this the 'black' one.
d) the TNT_AUDIO Snake. For this, I used RG213 satellite uplink coax for each conductor - active, neutral and earth. The central core is approx 2mm in diameter (AWG 13) so it is considerably thicker than b) or c).
In addition, the thickness of the insulation around the central conductor obviously separates each conductor, compared to b) and c). This will affect the cable's capacitance and inductance??
As a side comment, you took a good sample. Typical and special, about the same length, exactly the way it should be done.
As for capacitance and inductance I don't know, but I do know it will reduce any crosstalk/crossfeed between them.
2. Now the first purpose of my tests was to see whether I could see any 'filtering' of a signal fed into the power cord by the signal generator.
The way I did this was as follows:
* I connected the sig gen to the active and neutral pins of the mains plug.
* I connected the CRO to the active and neutral sockets of the IEC plug at the other end.
From 100Hz to 100KHz there was no appreciable difference to the CRO trace ... 2% at max!!
From this, I assume that the power cords were doing absolutely none of the supposed "mains grunge filtering" which I had read that the braided cords should do.
Obviously. For your information, a DeZorel LF-A1 Mk.2 filter, their cheapest, will filter by -16 dB at 6 kHz and -38 dB at 20 kHz. For those not familiar with decibels, -38 dB is 79.4:1, or, as opposed to Andy's finding of -2% max, -7,943%, just 3,972 TIMES more. I must add these are measurements I took about a year and a half ago, when I was testing their old series, in the meanwhile they have improved their results. Their current top model, Audio Reference RP1, which I just finished testing, hits -45 dB at 20 kHz, which is 178:1, more than twice that LF-A1 Mk.2.
3. So why would I hear a difference when I compared the result of plugging each cable into my pre-amp?
Maybe because of the difference in R, L & C??
Not so much individually as put together in a complex relationship.
So, as I have an RLC bridge, I decided to measure these parameters for each power cord.
4. Before I list the results, let me say that from previous listening tests, I had thought the sound of cable d) was the best! But why should this be??
Because I don't know how to insert a table into this post, I will have to provide a comma-delimited list of results. I suggest you transpose these onto a sheet of paper to understand the results properly. The headings of each column are the different cables; the row descriptions are:
* inductance in mH
* resistance in ohms
* capacitance in nF.
a) comm: 0.0031, 0.4, 0.2735.
b) grey: 0.0035, 0.270, 0.2653.
c) black: 0.0038, 0.258, 0.2862.
d) Snake: 0.0129, 0.272, 0.1216.
This shows that:
* the Snake cable, which uses braided coax, has 4 times the inductance of the others.
Q: why should this be? Any suggestions?
Q: or is the figure too damn small to worry about anyway?
It is small, but please veiw it in conjunction with the other two. Basically, to put it in plain terms, this cable will influence the voltage to a small extent, but not the current, and since its capacitance is small, it will interact with different power transformers far better than those with higher capacitance.
In other words, any phase shifts will be most likely positive, where voltage will lead over current, while others will tend to go the other way. I would always prefer a positive phase shift to a negative one, personally speaking.
* the extra thickness of the RG213 cable doesn't make any difference to the cable's resistance.
Q: why should this be? Any suggestions?
The higher value for R, for the 'commercial' cable can be expected in terms of a) thin wire used and b) 'enclosed' rather than soldered connectors.
However, I find it surprising that the extra thickness of the RG213 core doesn't deliver any reduction in resistance!
NB: most of the resistance value shown is caused by the connecting wires I had to use, to do the testing! The slightly smaller result with the 'black' cable is probably due to the fact that this cable did not have an IEC socket on the end ... just bare wires!
How about copper purity? Just as I am opposed to snake oil claims regarding materials, so I oppose total denial of the fact that the sheer purity of the material used WILL have an impact. Perhaps small, perhaps down into academic levels, but it will be there. Combine thickness with purity, then throw in some more exotic materials into the mix (e.g. carbon fibre, as done by van den Hul), and you have differences which are very real.
* the Snake cable had significantly lower capacitance than the others - which were all about the same.
Q: does this deliver any benefit?
I believe it does, and for the better.
Q: can anyone explain what the 'Snake' cable has higher inductance and lower capacitance? Could this be due to the fact that the coax insulation around each conductor separates these conductors from each other?
I would think so.
Q: a slight modification to the 'Snake' cable could be to strip the shield off. This would leave a cable constructed similarly to b) and c) except that the conductors are separated more, by the insulation ... would this be good or bad!!??
That's introducing several new variables all in one go. The answer is painfully simple - try it and see, no other way you will ever know for sure.
Now, in view of the figures you provided above, do us all a favor and try again with any one of the cables, but cut down to say one half, or 1.5 m.
These are physical values related to physical properties, so please, don't anybody tell me the length of the cable makes no difference.
Thank you Andy, that was very informative.
Cheers,
DVV
