rjr@mgweed.UUCP (10/12/83)
I don't think well designed equipment should need any warm-up period, except in the case of tube-type stuff. Even that should not need more than a couple minutes warm-up time. I would say that if something needs to be warmed up, and the sound difference is noticeable, there is a problem with marginal components that are getting hot causing bias levels to change. In my many years in broadcasting I ran many proof-of-performance tests (noise, response, and distortion measurements) and I ran them both with "cold" and "hot" equipment. I never noticed any differences between equipment that was turned on, say 10 minutes before testing, compared with equipment that had been on all day before testing. The worse problems I encountered were: 1) excessive noise generated in a mixing console that used 5879 pentodes in the mic preamps. We had to hand-pick tubes for low noise. 2) Hum caused by poor wire routing in a compression/expansion amplifier. The AC power leads were laced in the same cable with the input audio leads. 3) A poor phone line between studio and transmitter. This caused bad hum. The first time the telco "fixed" it, they just put in a hi-pass filter that rolled off badly below 250 Hz. After I got the right person, they gave us a new line that solved the problem. The only equipment that stayed on all the time was the frequency monitor, the transmitter crystal oscillator and oven, and the filaments of the high power stages in the transmitter. The latter was done primarily to save the filaments from turn-on shock (when most filament type devices burn out). Some stations that do shut everything off, apply the filament power slowly to prevent this problem (use a Variac (TM)). Bob mgweed!rjr