What makes a good piece of audio gear ?

Good design, good parts, good build. Features that prove pertinent to you. Ending with a feel and most important a sound that moves you. The right gear will cause you to listen more and have a gleam in your eye even when turned off. And if you see smoke...something may be wrong.

In order of importance:

1) Good circuit/crossover design.

2) Parts choices (and spending your parts budget on the parts of the circuit where quality parts will make the most difference).

I will also add that in my experience a good power supply in a component makes a big difference. Particularly in amps and preamps. And quality transformers matter (for power supplies, tube output transformers, and SUTs for LOMCs).

A good piece of audio gear is something that accurately and consistantely reproduce the musical signal, does not load down negatively the next in line component, does not affect the sound/signal, is designed such that cables make little or no difference to the signal and can drive the next in line component accurately within its design specifications, and most importantly, does what the customer wants it to in his/her system well. If, all things being equal, your system is almost "there" and you add this piece of audio gear in, and then you are "there", then that makes it a good piece of audio gear.

enjoy

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question: What makes a good piece of audio gear ?

answer: The Audio Research factory.
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There are no secrets or magic involved. It's a mature industry, and has been for decades. A great piece will be one that meets your particular requirements and will be very reliable. Build quality is real, high quality components is not. All else is marketing and one-upmanship. Did I mention reliability? The more exotic or complex, the greater the failure rate.
Cheers