Re: Twin T Notch Filter Anomaly


Ian Bell wrote in message ...
bg wrote:
Ian Bell wrote in message ...
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?

Cheers

Ian

It sounds like your notch isn't where it is supposed to be.

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
expected.

If you are
interested, I can send some data for creating a notch filter using an
inductor, two caps and a pot. It will create notches in the 60db range.


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.
the inductor is made adjustable, you can dial it in exactly where you
need
it. The inductor will typically be in the mH range at 2k . Almost
anything
you have in the junk box can be made to fit.
bg




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

Cheers

Ian

Check back here and I'll post an address for my webspace as soon as I get it
uploaded. My isp doesn't do binaries.
bg


.

Relevant Pages

  • Re: Twin T Notch Anomaly
    ... distortion harmonics by removing the fundamental. ... I have measured the notch response so I know ... None of us have a clue what your problems with your twin T filter ...
    (rec.audio.tubes)
  • Re: Twin T Notch Filter Anomaly
    ... source resistance oscillator I was getting different distortion readings ... with and without the notch and at first I thought I had the calibration ... I should be able to read harmonics down to -120dB. ...
    (sci.electronics.design)
  • Re: Twin T Notch Filter Anomaly
    ... source resistance oscillator I was getting different distortion readings ... with and without the notch and at first I thought I had the calibration ... 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. ...
    (sci.electronics.design)
  • Re: Twin T Notch Filter Anomaly
    ... distortion harmonics in audio equipment by removing the fundamental ... just connected the Twin T across the oscillator output and measured at ... It sounds like your notch isn't where it is supposed to be. ... 50dB I should be able to read harmonics down to -120dB. ...
    (sci.electronics.design)
  • Re: Twin T Notch Filter Anomaly
    ... distortion harmonics in audio equipment by removing the fundamental ... just connected the Twin T across the oscillator output and measured at ... It sounds like your notch isn't where it is supposed to be. ... 50dB I should be able to read harmonics down to -120dB. ...
    (sci.electronics.design)