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Rotel Bias ...

Discussion in 'Electronic Repair' started by Arfa Daily, Feb 22, 2008.

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  1. Arfa Daily

    Arfa Daily Guest

    Anyone know off the top of their head, or have the manual to check what the
    correct bias figure for a Rotel RB970BX Mk II is ? Working along the lines
    of the 'standard' 20mA figure (about 5mV across the TPs which are across a
    0.22 ohm emitter resistor) the output stages idle quite cool, and I see no
    signs of x-over distortion on the 'scope, but would be nice to know the
    correct recommended figure.

    Arfa
     
  2. Eeyore

    Eeyore Guest

    20mA sounds a bit low to me. I'd go for at least 30mA.

    Graham
     
  3. **8mV across TP1 & TP3 & Across TP2 & TP4. Those are the figures for the
    RB970BX. (Not MkII). I'll try to locate the data pertaining to the model you
    have.

    Trevor Wilson
     
  4. If you have a distortion meter -- especially one that can show the levels of
    each harmonic (eg, an audio spectrum analyzer) -- you might try adjusting
    for the lowest distortion or the least odd-order distortion.
     
  5. Arfa Daily

    Arfa Daily Guest

    Thanks very much Trevor. That equates to not far off Graham's figure of
    30mA, so that's where it's now set, and going back to the shop this
    morning - it's one of those jobs where the owner is panicking the shop for
    it on a daily basis. Customers, don't you just love 'em !! d;~}

    Arfa
     
  6. Arfa Daily

    Arfa Daily Guest

    I can live with that, and 30 is pretty close to Trevor's figure (see his
    post).

    Thanks
    Arfa
     
  7. Arfa Daily

    Arfa Daily Guest

    I do have one, but it only shows THD. I was expecting the figure to be
    somewhere between 20 and 50mA, and was erring on the side of caution and
    cool running with 20mA. It's now set for about 30 per Trevor's useful info,
    and heatsinks are just barely off room temp at idle. I doubt that there
    would be too much to be measured as a difference in THD between 20 and 30mA,
    and the owner probably wouldn't be able to tell the difference ... :)

    Arfa
     
  8. I'd be foolish to suggest there even _might_ be an audible difference.
    However, the harmonic-distortion spectrum of transistor amplifiers can vary
    quite weirdly with bias and signal level.
     
  9. **Some. I have a few clients who are like gold. Some, OTOH, I just never
    want to see again.

    Trevor Wilson
     
  10. Arfa Daily

    Arfa Daily Guest

    Amen to that, Trevor !!

    Arfa
     
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