[BC] Comparing the audio in the Big 50's

[RichardBJohnson at comcast.net]

I believe that if it measures good, it sounds good. This was the basis of the HI-FI equipment designed  by Dan von Recklinghausen, late, of H.H.Scott. http://www.youtube.com/watch?v=uAi-uHMEk5s

To me, this means that garbage processors which create massive inter-modulation distortion and audio harmonics in the ever-reaching attempt to be loudest in the market, are the main determinants of bad sounding transmitters.

I never heard any AM transmitter (from anybody) that sounded bad when received by an AM Modulation monitor and fed to a decent monitor amplifier and speakers in the studio, unless there were distorters and clippers n the program chain. In fact, amplitude modulation has the capability of better fidelity than FM because all of its bandwidth is available for audio.

My AM transmitters, all including my two different 50 kW transmitters, had frequency response and distortion comparable to the best FM transmitters, with noise well below -60dB as well. When I designed my 50 kW transmitters, customers were concerned with square-wave response because some charlatans from the "DC-coupled" transmitter manufacturers published articles (peer-reviewed, I'm sure) claiming that all transmitters except theirs were bad. I applied simple phase-compensation and response tailoring of the kind taught to first-year college students to make the square wave response look better than anything that came out of the PWM people. It was easy because I didn't have a PWM filter that needed to have a rapid roll-off to make the FCC bandwidth requirements.

If you feed unsymmetrical audio into a transformer of any size, it will produce a DC magnetic bias. If the transformer is small, it may produce distortion, if not it won't. The older UTC "Linear Standard" input transformers worked fine with unsymmetrical audio because they were large, keeping the magnetic flux low and linear. The tiny toy transformers used later in the "more modern" (read cheaper) transmitters did not. Proper phase compensation eliminates tilt and ringing even when using transformers. There are several advantages to transformers including the fact that an arc or EMP discharge from nearby lightning is unlikely to propagate through, while OP-AMP circuits are likely to blow up.

Furthermore OP-AMPS have essentially zero common-mode rejection at RF frequencies, meaning your transmitter RF can get into the audio circuits causing gross distortion. Radio station RF doesn't get through input transformers and they can be truly balanced, not balanced each side of a noisy ground.

The two different 50 kW transmitters that I designed sounded great namely because the initial customers for the first units, realizing that 50 kW was already loud, used very minimal audio processing. The one in Annapolis, MD, used a CBS Audiomax/Volumax. It was the loudest in the whole DC area market and seemed to have "authority" during commercials and talk-programs. I expected the street lights to flicker synchronous with the announcers when listening to that station in a rented car!

Cheers,
Richard B. Johnson
Book: http://www.AbominableFirebug.com/

----- Original Message -----
From: "Dave Dunsmoor" <mrfixit at min.midco.net>

Makes me wonder, how much of the various 50kw stories regarding  
"sounds good" vs "not so much" and the 100kw power supplies, etc might  
be attributable to the "stiffness" of the utility power at the  
facility itself?