[Notch filter musing] Why can't pirates recite the entire alphabet?

[EBK]

Because they get lost at C.



I do not know what this circuit will actually sound like, but I don't care at this moment.  I'm just having fun! 

It is a pair of Fliege notch filters, with notches at 130.81Hz and 261.63 Hz, corresponding to musical notes C₃ and C₄.  Intended to be silly rather than useful.   

[duck_arse]

what's R again?

[EBK]

what's R again?

If I ever end up redrawing it, I promise I will replace all occurrences of "R" with the less ambiguous "Arrr".   

[EBK]

I do actually have this built, by the way.  I just haven't had the opportunity to fire it up yet.

[11-90-an]

That looks very neat... I wish I can draw that nicely...

Imagine if you had a randomized way of selecting frequencies to cancel so while playing there's a chance that no note will ring out...

Also, what's that heatshrink-d package over there on your breadboard? a PSU filter/VREF generator?

[EBK]

Also, what's that heatshrink-d package over there on your breadboard? a PSU filter/VREF generator?

Yes, a Vref generator.  It's a small piece of vero board with an op amp, two resistors, and a capacitor.  Apply a voltage across the red and black wires, and you get a buffered V/2 on the green wire.  I built it for another project, but thought it would be convenient to use it here.

[Rob Strand]

Yes, a Vref generator.  It's a small piece of vero board with an op amp, two resistors, and a capacitor.  Apply a voltage across the red and black wires, and you get a buffered V/2 on the green wire.  I built it for another project, but thought it would be convenient to use it here.

That's a really cool idea.   Just build one and use it for all your breadboard projects.

[EBK]

Yes, a Vref generator.  It's a small piece of vero board with an op amp, two resistors, and a capacitor.  Apply a voltage across the red and black wires, and you get a buffered V/2 on the green wire.  I built it for another project, but thought it would be convenient to use it here.

That's a really cool idea.   Just build one and use it for all your breadboard projects.

Thanks!
Here are the details, for anyone interested:

[bluebunny]

Do I get a prize for predicting pirate jokes before I'd scrolled down?   

[Rob Strand]

Do I get a prize for predicting pirate jokes before I'd scrolled down?

I could only come up with seven C's.   For wooden legs, hooks and parrots we need experts like you and EBK.

[EBK]

Do I get a prize for predicting pirate jokes before I'd scrolled down?

I could only come up with seven C's.   For wooden legs, hooks and parrots we need experts like you and EBK.

We really just need more circuit ideas inspired by dad jokes and other terrible puns. 

A bit of an update:  Despite somewhat careful wiring, the circuit didn't work at first go.  I will measure the DC voltages and then audio probe it sometime to figure out where it is going wrong.  As it stands now, I get only the slightest signal through, regardless of frequency, with a brief chirp if I hit the strings hard.  Not giving up yet though.

[bluebunny]

with a brief chirp if I hit the strings hard

That'll be the parrot.

[Rob Strand]

As it stands now, I get only the slightest signal through, regardless of frequency, with a brief chirp if I hit the strings hard.  Not giving up yet though.

It would be really funny if you could cancel one note.  You tune the notches just above the exact note so when you bend the string the sound dies.   Then take it to a guitar tech and say I've got this weird dead spot on my guitar when I bend.

[anotherjim]

Pirates can't get lost, they know exactly where they HaHarrrrgh.

But as drawn, the amps have positive DC feedback via R1, R5 so the slightest DC offset in the amps and they are off to jamb into a power rail. So as Stephen predicted, the R's have it.

[EBK]

But as drawn, the amps have positive DC feedback via R1, R5 so the slightest DC offset in the amps and they are off to jamb into a power rail. So as Stephen predicted, the R's have it.

I found this curious statement in a doc from TI regarding the pros and cons of Fliege notch filters:
"Unfortunately, this circuit uses two op amps instead of one, and it cannot be implemented with a fully differential amplifier."

What does "cannot be implemented with a fully differential amplifier" mean?  Does it explain what is going wrong?

[bushidov]

We really just need more circuit ideas inspired by dad jokes and other terrible puns.

Time to make pedals with Sziklai Pairs and call them "sick", "plagued", "disease", "scurvy", "COVID-19", the list goes on.

[Rob Strand]

What does "cannot be implemented with a fully differential amplifier" mean?  Does it explain what is going wrong?

A fully differential amplifier is one that has differential inputs *and* differential outputs.      TI brought out fully differential amplifiers and they would have fully differential low-pass filters as anti-aliasing filters before differential AtoD inputs.

It's not a limitation to get worried about  .    It won't be a *cause* of any problems as it is a limitation in the topology.   Fliege should works fine with normal opamps.

[anotherjim]

I found this...
https://www.youspice.com/spiceprojects/spice-simulation-projects/general-electronics-spice-simulation-projects/active-filters-spice-simulation-projects/twin-tee-1900hz-fliege-notch-filter/

[EBK]

I found this...
https://www.youspice.com/spiceprojects/spice-simulation-projects/general-electronics-spice-simulation-projects/active-filters-spice-simulation-projects/twin-tee-1900hz-fliege-notch-filter/

Interesting.  They added a cap in series with my R1, R5.  Maybe I'll try that.

[Rob Strand]

OK, here's a sim with a few notes.

I think the problem is the pot adjusts the notch depth but it changes the frequency.
If you want a deep notch *and* an adjustable frequency you will have to tune two resistors.