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It all began in the early days of my childhood and I remember how I turned, with efforts, the handle of an old trophy gramophone in anticipation to hear the seemingly live sounds this box extracted from the magic black disk.
Then came the transistor era of audio reproduction, vinyl long-play disks appeared and amateur building of audio components became easy and widespread. This enthusiasm was fed by the desire to get most natural, high-fidelity sound, it led sometimes to success, sometimes to disappointment.
My first attempt to build an audio amplifier was a failure - the sound this amplifier produced wasn't of music nature, it was caused by deep internal high-frequency oscillation. I understood how helpless our ears can be when encountering such sort of audio devices' misoperation. In the end, the problem was fixed after connecting an oscilloscope to the amplifier output and the clearly observed oscillation was then removed by a proper choice of correction capacitances.
This was just the case when it's better once to see than many times to hear, since then the oscilloscope has become the main instrument in my laboratory.
There are many nuances in the behavior of audio circuits which can be investigated by monitoring their output on the screen while reproducing a musical program. For example, increasing gain until the output clipping, you can see is this clipping symmetrical or not, soft or sharp, clear or accompanied with a small local hf-oscillation. Most important, however, is that this monitoring demonstrates all the complexity and dynamics of the real audio signal which almost never likes a pure sinusoid.
It's ridiculous to talk about creating a measurement instrument for evaluation of the real audio signal, because the criterions of such evaluation are too difficult even to formulate, for further hardware realization. But why do we need to invent the instrument already existing? This instrument, our ears, has unique capabilities, particularly when making listening comparisons. This allows to use the only right and simple criterion for listening evaluation of the real audio signal (music, human voice) produced by the device under test (amplifier, loudspeakers) - similarity of this signal or, correctly, sound to the original live sound, the latter being a part of our environment and therefore remaining always fresh in our memory.
There are, however, two weak points inherent to this evaluation. Firstly, it is subjective, so every individual has his or her own, depending on various factors, listening comparison scale and own live sounds memory. If you are an experienced unbiased listener, always sober and in good mood, and also frequently attending live music performances, your conclusion about the evaluated sound may be considered as objective and trustworthy.
Secondly, the evaluation is integral and we analyze the sound as a product of the whole audio system - from a source recording (LP, CD etc) to the output loudspeakers, each part and even connections between them contributing its own details to the final sound. If the sound seems not satisfactory, the problem to find where its degradation occurs and why arises, but our ears hardly can help us promptly in its solving. They can only indicate the disappearance of this degradation.
Therefore, repairing of existing and development of new audio components require detailed analysis of their circuits which are considered as usual electronic structures and tested with the help of standard, not musical signals (sine-wave, square-wave etc). The typical procedures here are visual monitoring, various measurements, the choice of electronic components, necessary adjustments. Again, the final stage of the above work must be a strict listening expertise, just this sound evaluation answers the main question - can the produced sound bring pleasure and satisfaction to a listener.
This is the ultimate goal of audio and we try to reach it every time putting together LP or CD, a corresponding player, an amplifier and loudspeakers. The importance of each of these audio components for making the output sound isn't equal - it's decreasing from the first component to the last, because if something, approaching the sound to its live original is lost within the preceding components, this something cannot be compensated by the subsequent ones.
This rule can be applied and to the audio signal traveling within one audio device, for example, an amplifier. Just the amplifier first stages (phono-stage particularly) are most responsible for the signal's integrity and originality while it undergoes amplification and correction.
I have built very sensitive instruments for measuring the audio signal degradation, and these instruments, along of course with my ears, are indispensable assistants in creating the whole range of my audio components. Their circuitry is specific, most suitable for handling audio signals and it's why this audio successfully combines excellent objective characteristics with the sound strikingly close to its live original.