Freiburg 100 Automatic Stereo - Question about stereo

      Freiburg 100 Automatic Stereo - Question about stereo

      I am looking at the schematic for the Freiburg 100 Automatic Stereo and have a few questions about some oddities in the design..

      Firstly, am I right to conclude that there is no way of adding a stereo decoder, and that in FM it can only be operated in mono?

      Secondly - there is an extra tube in the Right channel audio path using the EBC91 as a low gain inverting amplifier - why is this done? it seems oddly extravagant to use a triode +2 diodes to exchange the phase of the channel - why not just reverse the wires to the speaker? Can anyone explain why this is done? There does seem to be extra switching of the output windings of the audio transformers, so I'm wondering if its some sort of push pull arrangement in mono...

      Finally, surely closing S13 puts a short across the transformer TR3 - what is this switch for (I can't work out its function from the schematic).

      Many thanks for any wisdom,

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      Hi Kevin,

      yes, it´s a push-pull design in regular (radio) operation and two single ended power stages when switched to the Tape or Phono input.

      The EB"C"91 triode does the phase inversion in push-pull (mono) operation but since the output winding of one output transformer is connected in reverse polarity everything´s fine. The two output signals would show a 180° phase shift due to this triode stage in Stereo operation which would make a very poor sound. That´s why the inversion of the output transformer is cancelled by "St3" in stereo operation only. So everything´s o.k. again.
      Tricky, isn´t it?

      S13 is for muting when the set is switched off.

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      Thanks for the explanation. It makes sense now, though it seems over complicated. I have a Dorchester radiogram which has 2x ELL800 push pull - I guess the complexity is still great but different.

      The muting switch - one of the nice SABA touches then.. that makes sense - better hope it doesn't fail short :)

      I was looking at one of these sets that is for sale in the UK, but as it cannot decode stereo FM, I may give it a miss. Interesting anyhow to review the schematic - the Automatic tuning engine looks impressive. It seems to have its own IF stage and detectors.

      Thanks for your clarifications on the audio.

      Kind Regards,