I made a little box with 2 headphone cables coming out of it with standard 1/4" phone plugs on them, a DPDT switch, and a single female headphone jack.
This allows me to directly compare 2 different amps with the flip of the switch.
The headphone cables I used are pedestrian, just some OFC extension cables I got from Parts Express, from which I cut off the female ends to allow me to wire the cables to my DPDT switch. OK, so they are a compromise here but the SAME type of cable is used for each amp so the differences in the amps, if any, should still show up/
I put the thing in a little Hammond plastic project case.
NOTE: THIS IS NOT A SCIENTIFIC TEST. Not by any means. But I found it interesting just the same.
I just finished this switch box today and only had a few hours to do some A / B listening, so I only have 2 amps compared so far.
I used a Cambridge Audio DAC Magic as the source, fed by SPDIF from a Squeezebox III. I used the RCA outputs of the DAC Magic, with RCA splitter cables. (Again, a compromise, but the same cable type is used for each amp under test.)
I listened with Audeze LCD-2 and then Sennheiser HD800 phones.
Today I compared a 3-channel Beta 22 and an M³ amp. The Beta 22 has the Sigma 22 power supply; the M³ uses a 24 volt Elpac wall wart. The M³ uses AD843 op amps.
I carefully matched the volume levels of the two amps using a 1000 Hz tone.
I listened to a variety of music.
I was very hard pressed to hear ANY difference between the Beta 22 and the M³ on the LCD-2's. MAYBE a touch better bass control on the Beta 22 but then again maybe not. I would NOT bet money on me being able to tell these two amps apart in a blind test with the LCD-2.
With the somewhat exaggerated highs of the HD800's I began to hear a subtle difference between the two amps. The highs on the M³ sounded a tad bit harsh. Also, the "separateness" of various instruments on a track was better with the Beta22. But these differences were quite subtle.
1. I was surprised that I didn't immediately hear incontrovertible differences between these amps. I was expecting a fairly obvious difference in their sounds.
2. These are both good amps- they both sounded pretty wonderful.
The Beta 22 and M³ amps have things in common- MOSFET output stages, 3-channel "active ground" design- maybe this is why any differences I may have heard were so subtle.
This makes me want to A / B others amps. I also urge anyone reading this post who owns more than one headphone amp to build their own A / B box and report on your tests.
I expect I will hear more noticeable differences with other amps - such as Cavilli-Kumisa vs. Beta 22, and also Beta 22 vs various tube amps in my collection.