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Classic gear from yesteryear; vintage audio standing the test of time.

RE: Ever auditioned

No, I haven't; although it has a terrific reputation. Sadly, I'm at the point, at 78.5, that my hearing is starting to go off in the highs. I'm a long ways from needing a hearing aid; but the upper treble is fading away. It's a lot of the reason I made the decision to get the second Adcom GTP-400 and quit there.

In one of those situations where classic measurements don't tell me anything, I've had a number of measured ruler flat frequency response preamps through here, and they all sounded and imaged differently. What I have not done, and probably should, is to run the preamps through an input/output linearity test. You put the input on the vertical axis of the oscilloscope, and the output into the horizontal axis, adjusting gain until the oscilloscope shows a 45 degree straight line. Any deviation from a single perfectly straight line shows the item under test is modifying the input. The sensitivity is such that if you can see a deviation, you generally can hear it. Phase shift changes the straight line to an ellipse, and amplitude non-linearities make the line curve. Distortion shows up as output along the X axis. Clipping shows up as vertical lines at the ends of the 45 degree line. You do need an oscilloscope that can do X-Y plotting and has a sharply defined trace, and when I bought mine, I made sure it could. That test dates back to the late 30's. It has fallen in disuse because of all the wonderful instruments we have that give numbers. The oscilloscope X-Y plot is subjective; but anecdotal evidence in both US and UK literature says it is sensitive and indicates things in combination that you can't easily measure together any other way, and does it on music. A lot of the test methods from the 30's and 40's are like that - they rely on cleverness as a substitute for measurements using precise instruments.

Jerry


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