[sdiy] Seeking a very old & specific message from Dec 2006

Tim Stinchcombe tim102 at timstinchcombe.co.uk
Mon Sep 13 10:45:29 CEST 2021


Well it was not a particularly earth-shattering revelation, but interesting enough in its own way I suppose.

The notch filter concerned was this one:

https://www.ti.com/lit/an/snoa680/snoa680.pdf

The rather-too-perfect matching of component values in SPICE can make the simulated notch response really, really narrow, so it can be difficult to see that it is there at all. My suggestion was to replace the op amp follower in the simulation with a voltage-controlled voltage source where the gain can be set directly, to 0.9 say - this should make the notch much easier to see. (If the follower/buffer has gain 'k', the Q of the filter is apparently Q = 1/(4*(1-k)), so it blows up with k exactly 1.)

(And from what I could gather from quick Googling last night, the filter tends to be rather tricky to use in practice, because of its use of such big resistor values and the necessity of getting component values 'just right' in order to have the notch exactly at 60Hz.)

Tim

__________________________________________________________
Tim Stinchcombe 

Cheltenham, Glos, UK
email: tim102 at timstinchcombe.co.uk
www.timstinchcombe.co.uk


> -----Original Message-----
> From: cheater cheater <cheater00social at gmail.com>
> Sent: 12 September 2021 22:49
> To: tim102 at timstinchcombe.co.uk
> Cc: synth-diy <synth-diy at synth-diy.org>
> Subject: Re: [sdiy] Seeking a very old & specific message from Dec 2006
> 
> so what did you say? Don't keep us wondering!
> 
> On Sun, Sep 12, 2021 at 10:51 PM Tim Stinchcombe
> <tim102 at timstinchcombe.co.uk> wrote:
> >
> > Scratch that last request - I just found it, and not at all where it
> > should be!
> >
> > Tim
> > __________________________________________________________
> > Tim Stinchcombe
> >
> > Cheltenham, Glos, UK
> > email: tim102 at timstinchcombe.co.uk
> > www.timstinchcombe.co.uk
> >
> >
> >
> > > -----Original Message-----
> > > From: Tim Stinchcombe <tim102 at timstinchcombe.co.uk>
> > > Sent: 12 September 2021 21:40
> > > To: 'synth-diy at synth-diy.org' <synth-diy at synth-diy.org>
> > > Subject: Seeking a very old & specific message from Dec 2006
> > >
> > > Hi list,
> > >       I am trying to track down an old email I sent to the list on
> > > 16 Dec
> > 2006, but
> > > both archives I know of:
> > >
> > > http://search.retrosynth.com/synth-diy/
> > >
> > > and
> > >
> > > https://synth-diy.org/pipermail/synth-diy/
> > >
> > > apparently have a 'hole' around this time and don't contain it, so I
> > > am
> > hoping
> > > someone may still have the message filed away in their own personal
> > archive/PST
> > > file/whatever etc.
> > >
> > > The message was from me ('tim102 at ...'), in response to a message
> > > from
> > Aaron
> > > Lanterman on behalf of one of his students who was trying to get a
> > > 60Hz
> > notch
> > > filter running - the thread subject was:
> > >
> > > [sdiy] Notch Filter for Logon's Human-Capacitor Filter
> > >
> > > Somehow my own store of old messages doesn't contain it, though it
> > > knows I replied on 16 Dec 2006 at 16:34 - I must have not copied it
> > > across or
> > deleted it
> > > during a past re-organisation of messages or something (I keep old
> > messages of
> > > interest in at least 2 PST files...).
> > >
> > > I'm rather curious to see what I said!
> > >
> > > Many thanks,
> > >       Tim
> > > __________________________________________________________
> > > Tim Stinchcombe
> > >
> > > Cheltenham, Glos, UK
> > > email: tim102 at timstinchcombe.co.uk
> > > www.timstinchcombe.co.uk
> > >
> > >
> > >
> > >
> >
> >
> > _______________________________________________
> > Synth-diy mailing list
> > Synth-diy at synth-diy.org
> > http://synth-diy.org/mailman/listinfo/synth-diy
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