Lrsky: Why not set up a demo that will prove your brother to be wrong and then have him eat his own words after listening with his own ears? With all of your expertise and connections in the field of electronics and audio, finding a suitable combo of gear that will demonstrate the differences of altering just one variable within a system shouldn't be hard to do at all.
Since he is a "naysayer", i would suggest using speaker cabling as a test. If you can find some inexpensive 10 gauge zip cord and some Nordost, the differences should be highly noticeable right off the bat. While Fulton Gold would work better than the generic heavy gauge zip cord, i was trying to make this as simple and inexpensive as possible in case others wanted to try such a test themselves.
The key here is to make the listener doing the comparison extremely familiar with the sound of the system and recording being used prior to introducing the variable to be tested. This allows them to think that they will know what they will hear based on previous exposure. Given that they are already familiar with the sonic traits of the recording and the system, any differences that are easily detected will seem quite pronounced to them at this point.
If you want to take advantage of your brother's belief in AB testing, use it against him. By carefully "rigging" the results, you can achieve the results that you want. That is, IF your brother is both a reasonable listener and honest.
Play a selected song or portion of a song for him using the heavy guage zip cord as the speaker cabling. Then act like you swap speaker cabling, but don't really do it. Play that portion of the song again. Then pretend to swap speaker cables again, but don't. This will give him three times the exposure to the one set of speaker cables that he would have normally had, further aquainting him with the sonics of the system. After three identical presentations, it will also lull him into a false sense of security that there is no difference between the cables.
On the fourth try, use the Nordost cabling. Not only should the differences be markedly noticed, but he should have a hard time denying that there isn't an audible difference. As a side note, i was able to identify the differences between 16 gauge and 12 gauge Monster Cable during a similar demo at a local Best Buy. I did so using songs & equipment that i had never heard before with 100% accuracy, so this test should be even easier to achieve positive test results with. Like i said though, this would require your brother to be honest. In such a test, even an unskilled listener should be able to tell the difference.
As a side note, the use of non-locking banana plugs makes speaker cable changes the fastest, most convenient method. This requires the least amount of time as you can simply pull and insert the new cabling. The use of at least 8' - 10' of speaker cabling will also tend to further highlight the differences as shorter lengths introduce less of a sonic signature into the equation. As i've mentioned before though, longer speaker cabling lengths are only a big deal when the cables themselves are not properly designed.
The reason that i picked the cabling that i did is that zip cord tends to roll-off the top end and lends a much warmer, fuller sound to the presentation. Nordost lacks warmth and tends to accent the upper midrange and treble region, making it just the opposite of heavy gauge zip cord. Both are poorly designed cables in the fact that their nominal impedance is appr 100 ohms, but due to the differences in conductor geometry, they tend to shape the audible region in contrasting fashion. Both are "coloured" cables and we are taking advantage of those colourations to demonstrate that audible differences are discernable. Sean
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