Going somewhat further on the T-dot System in Japan, broadly speaking the factories were anonymous, unknown (outside of the industry insiders) manufacturers who would produce branded products for OEMs (SONY, Sansui, Kenwood, Pioneer, etc) completely ... that is the product came out of the factory as a boxed, branded unit.
Broadly speaking OEMs would manufacture their highest-priced products (Receivers, Amplifiers, etc) in owned factories but sent mid and lower-priced design criteria to the T-dot factories. In many cases the T-dot developed the product from scratch, designing the circuit and PCBs themselves based on broad criteria from the OEM.
Some brands (notably Accuphase, Luxman) did some or all manufacturing in-house, but these were rare and you could tell by the MSRP in any case, which wasn't low by any means.
In the early and mid 1970's the label on the back panel would indicate products manufactured under this system, with a logo (a "T" with a dot on top) and number, indicating which factory did the work.
By the late 1970's the T-dot mark was omitted from the labels but the practice remained. Companies like Jelco built most tonearms, Matsushita (Panasonic, Technics and at the time, JVC) built most DC direct drive motor assemblies which were incorporated into other OEMs products by the T-dot factory. Matsushita did manufacture their own turntables (with Jelco arms) but some other products went to T-dot factories. Matsushita and SONY were huge and did do a lot of their own manufacturing, but by no means all of it.
The original high quality cassette deck (Advent 201, very late 1960's, a US-based company) was contract-manufactured by Nakamichi. Nak was not a brand at the time; they got into the cassette business as a result of the contract with Advent and a decision to carry on with cassettes. Other major cassette deck manufactures were Alpine, TEAC, and a number of anonymous manufacturers.
It may seem strange today, but in the 1970's Japanese brands were considered inferior to European and North American manufactured equipment, much as it is today with offshore brands from less expensive labour regions of Asia. The "Made in California" Marantz gear was sought after once the company was sold to Japanese owners in the later part of the 1970's, again mirrored today with some "vintage" gear made prior to the 21st Century. By the late 1970's that badge had largely disappeared from Japanese made gear, but earlier it was very much evident.
A notable exception was Japanese loudspeakers, which never quite managed to overcome an inferior image until the Yamaha NS10M arrived in 1978, and found it's way, slowly, into recording studios. Some of the Technics linear phase units also saw moderate success in the late 70's.
It still is somewhat dominated by North American and UK manufacturers, although again PRC manufacturing is becoming the norm at least to a certain extent (especially raw drivers). Although there are exceptions, you could make an argument that Japanese manufactured speakers never really got a foothold in the market (even in Japan itself, where American made speakers have always dominated the high end), and today the opportunity is lost with PRC made units taking the trophy in that respect.
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Dragging myself back on topic, it's my experience that not all amplifiers sound the same. I do prefer listening followed, if possible, by measuring. Certainly you can learn some clues about the "why" but not always. It does not matter to me that one amp has this design and the other has that; I'm not interested in manufacturing amplifiers, I just want something to listen with.
I don't see how auditioning by ear is incompatible with assessing a product whose job I will put it to is playing music with other equipment I may own or am considering.
If you failed, for example, to test some of the Class-A PASS amplifiers (just an example) with transient signals versus the standard continuous sine wave power output normally measured, you may miss the prodigious short term power ability (more than 100w) of these 25watt RMS (or so) units, and be mystified as to the Sound Quality they exhibit. Even the most complex input signal, one that can be reliably removed to measure intermodulation distortions, is orders of magnitude less complex than actual music. Listening and Testing go hand-in-hand if you really want answers.
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I have an example of how amplifiers (or any component, from subtle to obvious) can sound different. If you are a laptop user, grab some music ... doesn't matter what, but less complex examples might be best, as it makes the areas to listen to a little easier to pick out ... and start playing it.
Because of a psyco-acoustic phenomena where humans are less critical of Sound Quality (SQ) when accompanied by visual cues, maybe a video isn't the best choice, when a music track is available, but it actually doesn't detract enough that this doesn't work.
Move the screen back, further than you normally view the screen. Listen to the sound (close your eyes if you want to help you concentrate). Move the screen slightly towards you, say by a half inch (12mm). Listen for changes.
Continue until the screen is getting closer to almost closed (where you can't view the screen anymore).
Now move the screen back to the original far away location. The music should be playing continuously during this time.
See how small a movement of the screen is needed before you can notice a sound difference. You may find rather subtle changes are now audible. You may even have difficulty deciding which one you prefer.
These are the kinds of differences you can expect when auditioning good, compatible gear vs another good, compatible alternative. It is not night-and-day, usually, but to say it's inaudible is really un-supportable.
