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Twin T Notch Filter Anomaly

Discussion in 'Electronic Design' started by Ian Bell, Jan 25, 2009.

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  1. Ian Bell

    Ian Bell Guest

    I have built a 2KHz twin T notch filter to help measure low level
    distortion harmonics in audio equipment by removing the fundamental
    (2KHz). It uses 510pF (1%)capacitors 150K (0.1%) resistors in a standard
    bridged T configuration. However a strange thing has happened. I have
    measured the notch response so I know it is 12dB down at 4KHz and 8dB
    down at 6KHz etc so I can correct the readings. However, using a low
    source resistance oscillator I was getting different distortion readings
    with and without the notch and at first I thought I had the calibration
    wrong but I rechecked it and is seems OK.

    Now I have discovered that attaching the Twin T itself alters the
    distortion products and it makes no difference to the fundamental. I
    just connected the Twin T across the oscillator output and measured at
    the same point. At first I thought this was just a property of the
    oscillator. However, I have now tried this on both the oscillator (low
    impedance) and the output of one of my 6SN7 mu followers with almost
    identical effects namely the 2nd harmonic is reduced by about 10dB and
    the third is raised by about the same amount.

    I could understand if it happened with perhaps one particular circuit
    but not with two entirely different ones.

    Any ideas what it is?


  2. Darren Holdstock

    Darren Holdstock

    Jan 20, 2009
    Could be a number of things - need more info. Supply filter circuit, cap type (I'm assuming polystyrene for the time being), load, and measurement setup.
  3. Ian Bell

    Ian Bell Guest

    Yes, that's a good idea, I'll try a 20dB attenuator and see if there is
    any observable difference.

    Yes, brain fart on my part ;-)


  4. Ian Bell

    Ian Bell Guest

    That was my first thought but I checked the response and the notch is at
    2075Hz, 4KHz is 11dB down, 6KHz is 7dB down and so one. Pretty much as

    If you are

    60dB is plenty for what I need. My HP wavemeter can reliably read
    components over 70dB down so if I can reduce the fundamental even by
    50dB I should be able to read harmonics down to -120dB.

    That is VERY interesting, especially as it is adjustable. Can you email
    me details to ianbellATukfsnDOTorg please?


  5. Ian Bell

    Ian Bell Guest

    At the moment it feeds my HP 3581A wavemeter which has an input
    impedance of 1Meg so the load would seem to be OK. Source impedance is
    another matter.
    Yes, but this will need extremely low distortion in order not to
    invalidate the results but I guess that is not too hard these days at
    unity gain ;-)

    Yes, I think I saw that in J. Linsley-Hood's book


  6. JosephKK

    JosephKK Guest

    Nothing personal, but that is a pretty sloppy notch for my typical
    uses. For me a 60 dB notch at 2 kHz; 3 kHz and 1.5 kHz are not more
    than 1 dB down.
  7. Ian Bell

    Ian Bell Guest

    NP. It is completely passive and that is exactly the sort of perfomance
    a passive twin T gives. You can, however, considerably sharpen the notch
    using feedback and obtain the sort of figures you quoted.


  8. Ian Bell

    Ian Bell Guest

    No, but then you canna change the laws of physics but if you add an op
    amp and some feedback you can improve a twin T no end.


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