Quote:
Originally Posted by daveobuchanan
... In my investigations and researches I concluded that to get Dolby B to work properly the higher frequency performance is absolutely critical...
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Yes for Dolby B the overall response, theoretically up to 20kHz, needs to be close to flat. The tape recording process will be the weak link, as it always was, especially with cassettes. Transistors and valves shouldn't be the issue although of course the insertion of transformers with valves might be a cause of non linearity. Interestingly, the later Dolby C while more touchy in other respects is less sensitive to high frequency errors as it is deliberately only responsive below 10 kHz, a feature they probably should have included in the original Dolby B.
Again I suspect that if you do it well, the valves will make little difference. The weak link will be the tape recording and playback process itself, especially its alignment when Dolby is involved. Type II or even Type IV tapes make a difference.
I tend to go the opposite way with tape machines, replacing older valve circuits or inferior transistor components such as noisy Germanium trannies with later lower noise designs.
I suspect a well designed older valve reel machine ensured the tape saturated before the valve circuitry did, so the latter was effectively transparent. That was also the aim with transistor circuits. It was always something of a compromise especially with 2 head cassette machines where you needed a relatively high impedance head for low noise playback but relatively low impedance for recording to avoid the record amp clipping.
Tim