Saba Freiburg W10US Notch filter 9 KHz

      Dear Saba fans,
      Please, explain me, why in Saba Freiburg W10US the frequency 9 KHz is suppressed in the low frequency amplifier with Notch filter! I have met the same case in other models. And why exactly 9 KHz?

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      Sincerely: Vlado

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      Hello Vlado,

      the convention for the frequency spacing in the AM ranges is 9 KHz.
      So if you are listening to a station on a given frequency and there is another station operating on the next frequency (+ 9 or -9 KHz) there will be an audible interferences as a 9 KHz tone.
      The 9 KHz notchfilter in the amplifier suppresses it.
      Dear Vlado,

      In USA 10 kHz, in Europe 9 kHz. It is the so called whistle filter.

      Not all receivers have that. In case that two adjacent channels are received simultaneously, an interference noise can be generated. For AM (Mittelwelle) the channel separation is 9 kHz in Europe and 10 kHz in USA. That's why. For SW it is only 5 kHz. There are SW receivers that have such notch filter at 5 kHz for SW.

      Best Regards,

      Just saw that Achim typed faster than me.... same answer, though.
      Hello Achim, Reinhard,
      Thanks for the answers. I think, the problem is that in Freiburg W10 US the filter is continuously active and restricts bandwidth at VHF (UKW) and TA. In Nordmende Othello (1953/54) for example, the filter is switched-off at VHF (UKW) and TA mode.

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      Hi Vlado,

      the FB W10 was developed around 1950 with no focus on FM reception. That´s why it only has one speaker and no tweeter. The response of high frequencies wasn´t an issue at this time.
      After FM broadcast was established the transmission of the whole audible frequency range became a concern. In 1953 / 1954 tweeters were added and permanent 9KHz filters were thus abandoned.
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