QSC Audio ABX Comparator

Introduction

From http://www.bostonaudiosociety.org/explanation.htm

As Wilden A. Munson came up with the idea of a simultaneous acoustic signal comparison to figure out the well-known equal-loudness phenomenon in 1949, two members of the Southeastern Michigan Woofer and Tweeter Marching Society (SMWTMS), Arny Krueger and Bern Muller, did the very first hi-fidelity audio comparison in 1977. And in 1982, David Clark officially debuts this ABX paradigm to the Journal of Audio Engineering Society.

In 2012, we have a Foobar2000-based software ABX comparator at out disposal, but has this hardware-based ABX comparator ever been widely available to the general public? Why can't we just purchase these anymore? Today's Hi-fi audio industry does not make this kind of product anymore, due to the cold fact that these comparators reveal too much transparency among audio devices; In other words, upon a direct 1:1 comparison, it always turns out most of audio devices sound very similar, if not the same, to one another. Consequently, audio companies lose their spots when the comparators become widely available to the public. There is absolutely no point for them in investing wealth when there is nothing to be gained.

The ABX company, founded by six members of the SMWTMS, produced various comparator models from 1980 to 1987, and QSC Audio rolled out the hi-fidelity version in 1998 like a blitzkrieg, with only 300 units produced. It is also worth to note that a few DIY attempts, such as Rod Elliot's and Matrix HiFi, have been made as well over the years.

Since QSC Audio's comparator was said to essentially meet the strict criteria set by the recommendation ITU-R BS.1116-1, I've been trying to get hands on it for quite a while. And after 3 years of anticipation, I finally had a chance to acquire the legendary hi-fidelity product, made by QSC Audio in 1998. What a break-through!

Then how does the unit fare in regards to technical accuracy? What's the spec? Two main factors are of a concern:

1. To execute a hi-fidelity audio comparison, the switching delay must be near-instantaneous.
2. To execute a portable audio comparison, the comparator must not change the impedance of DUT. In other words, a closed-circuit impedance of the comparator should be close to zero.


1. Switching delay

On 'Note 1' of '4.2 Grading Phase' of the recommendation ITU-R BS.1116-1, it is stated that the switching delay must not exceed 40 ms for the subjective assessment of small impairments in audio systems. In order to figure out the exact amount of delay, or the quality of the relay used, I quickly ran a sine wave sample through the unit, and recorded the switching delay:

The delay is around 24 ~ 25 ms, which is extremely close to near-instantaneous switching.


2. Closed-circuit impedance

Its rated specifications are as following:

Small-signal section (line-level): mechanically balanced DPDT relay switching; Input impedance: 200 kilohms balanced; 100 kilohms unbalanced.
Heavy-duty section (speaker-level or line-level): mechanically balanced DPDT relay switching
Frequency response: 20 Hz–20 kHz 
THD+N (20 Hz–20 kHz): +0, -0.1 dB

And according to Thanh Nguyen, the service development engineer at QSC Audio,

" All relay contacts are ideally 0 ohm so it would not change or alter DUT (device under test)’s impedance.  Note that no electronic device was used in between INs and OUTs."

So I ran an impedance check, and voila, Mr. Nguyen was correct. Even if I connect a portable device, I could only obtain the source device & cable impedance within 20 - 20,000 Hz range.


The end and a new beginning

Although the comparator was successfully obtained, I realized the fact that I live in the country where a snake oil hoax becomes something equivalent to that of a Viagra, and a tax evader becomes a GOP presidential candidate.  On the other hand, my comrade, Salsal, is living in South Korea where subjectivism and objectivism still clash to each other. I believe he is a visionary who possesses a drive to make a difference in the audiophile community, of which industry-biased commercial reviews are prevalent. So the unit has been sent to Salsal once and for all. Have fun, Salsal! (It seems he already dissected the whole thing)