Quality Control differences = variations in sound quality?
I ordered 4 Sonic Impact amps recently and I've noticed a very strange phenomenon between the first three I've listened to. They all sound different!
The first amp I hooked up sounded warm and soft - musical but just a little on the dull side. Treble was nicely balanced but pace and rhythm was slightly subdued.
The second amp, which I intend to send to my brother, sounds much more etched and analytical. For the first seven hours the treble was really prominent and unpleasant to listen to. The treble did mellow out a bit, but the sound still favors "detail" over warmth, much more so than the first amp.
I just hooked up a third amp twenty minutes ago, and this one fall somewhere in between. It has a much better sense of pace and liveliness than the other two amps, is less analytical than the second amp, and more involving and rhythmic than the first amp.
I consider myself a fairly experience listener, so I don't think I'm imagining things. The system is exactly the same save for the amps.
Could it be that the sound of all of these amps will converge with sufficient burn-in and eventually sound the same? Or are there differences in quality control/parts lot variations that may account for this?
As it stands, the third amp is so much more enjoyable than the first two that the listening experience really is of a different caliber.
Incidentally, I experience a similar thing with my Gaincard. When the stereo shop I used to work for started carrying the Gaincard, I had a chance to bring the demo unit home and use it in my system. I already owned a Gaincard for about two years before that, so the store amp went into my secondary system.
Just for a yuk, I swapped Gaincards in the main system and, lo and behold, the two sounded different. One had a warmer, lusher, more musical midrange, but the bass was a bit mushy and lacking in pace. The other Gaincard was tighter and more tuneful, but it was drier and stiffer sounding. I wouldn't say that one sounded better than the other, but you would definitely pick one over the other depending on your musical priorities.
I know that the store demo unit had been suffiicienly burned in as it had made its rounds as a review model for the press.
So I'm listening to the 3rd Sonic Impact amp right now and it's moving me in a way that the first two didn't. I'm really enjoying so much more than the first two. Crazy? I want to know what's going on, too.
Anybody have similar experiences, or so sort of explanation?
Incidentally, I'm powering the amps with a 12v/12ah SLA.
My explanation would be that it is all in between the ears. Whenever you change something in a setup, you are bound to hear differences.
It is rather unlikely that two otherwise identical modern pieces of electronics perform different enough for the ear to pick up differences, unless at least one of them is broken.
Therefore, the question is, did you take any measures objectifying the listening tests? For example, did you repeat these observations without knowing on forehand which amp you were listening to?
If these are class (A)B amps, the bias setting may be different on each which could cause an audible difference.
But, as Vacuphile notes, the first thing in order is to make certain the effect is real.
Even plugging/unplugging cables can make a difference due to contact cleaning effects.
If you want to know if they're actually different, take a frequency response measurement of a loudspeaker driven by each amp (in your possession, at least) in succession.
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