Living with Dolby B (continued)
It seems that in order to reduce noise levels, typical Dolby unit input impedances are reasonably low at around 10 to 30 KOhm. My Sony's DIN output impedance is much higher, at 80 KOhm, while the Hitachi's output impedance is all of 1 KOhm. The only loser is the poor old signal. In fact, to record anything at all meant twisting the PD2B's record level controls to maximum and taking the Hitachi's controls up to around 90%. This had two results — it barely made the Hitachi's VU meters twitch (except during the Rite of Spring) and it invalidated the calibration that had cost me so many bitter tears.
At this stage I seriously considered going off to Tibet. Instead, though, I wrote to Phoenix-Videosonic-Uher and got a nice letter back from their Technical Manager, RL Campbell. A kindly colleague1 also showed me a copy of Hi-Fi Answers containing an article called "A user's guide to Dolby B". From the article, I learned that a way round my impedance troubles might be to use the amplifier's phono tape sockets and from Mr Campbell's letter I learned the following two, vital points:
1) The oscillator's tone may have to be recorded at well above or well below 0 dB; it depends on the recorder. (The meters of which, incidentally, have no relevance during
calibration — what about that "half-scale deflection"? Oh, that's only a starting point, forget it.)
2) Once calibrated, the Hitachi's record level controls must not be touched. All level settings must be done via the PD2B's level controls. "I trust" wrote
Mr Campbell, "this clarifies the procedure".
Well, I nipped off to my local hi-fi dealer and bought a 5-pin DIN to 4 phono plugs converter. I then spent a few frantic minutes trying to track down which plug was which channel etc.
Anyway, I finally got it all together, and tried again.
Bliss! Perfection squared!
I now have the Hitachi playback level controls at about 70%, the record level controls at about 30% (out of deference a) to the "Which?" article and b) to Angus Mackenzie's comments about the Hitachi's Dolbyised brother) and I experience no trouble whatsoever. Well, that's if you don't count the crackle when you tap the PD2B; the thump needed to cure the occasionally dead left-hand Hitachi channel; the 50Hz buzz that from time to time appears, drowning my cartridge's left-hand channel; the police breakthrough on Radio 3; the...
