Twin T filter as bandpass instead of notch?

[brett]

Hi
I'm after a bandpass filter and I'm wondering if I can use a Twin T as a pass rather than a cut.

As a notch, it's usually:
http://www.radio-electronics.com/info/circuits/rc_notch_filter/twin_t_notch_filter.php

Without thinking about it too much, I imagine connecting the output to ground and taking the desired band of frequencies from what is usually the earth (at the bottom of the diagram). Well, the "notch" frequencies get dumped to earth, right? So I can pick them up from there. Maybe? Or completely wrong?

An even trickier question that I'd like to know is: what is the output impedance of a Twin T used in this way?

thanks for any help.
cheers

[Bill Mountain]

If you put it into the feedback loop of an opamp it should reverse it's operation.

[duck_arse]

http://sound.westhost.com/articles/active-filters.htm#s61

that's what rod elliot told my while you was typing.

[R.G.]

See http://www.geofex.com/article_folders/wahpedl/wahped.htm#twintee for the use Bill mentions.  PAIA once offered a wah-ish circuit using this, and I believe that the Coloursound inductorless wah is this way, IIRC.

The Twin T circuit works by causing a phase shift on one leg that can be used to cancel the oppositely-shifted signal on the other leg.  I'd have to do some navel-staring to think about whether the center leg current peaks at this frequency or not. If it does, then to keep the Q high, you'd need to run the center leg current into a virtual ground to keep the Q up.

Bootstrapping the center leg from a low-impedance opamp output is used to raise the normally-low effective Q from about 0.3 or so to higher values. See
http://www.geofex.com/article_folders/eqs/paramet.htm for how you might do this.

[tubegeek]

Would it work as a bandpass if you swapped 2C for R/2? That's typical filter behavior, dunno if this one is a special case. Putting the R's all in one leg would establish a constant cut at all frequencies, the all-C's leg would do .... something ....

EDIT - I may be talking nonsense....

[R.G.]

See the Bridged T variants.

[brett]

Thanks all.
I was interested in avoiding an extra opamp (laziness, really).
Haven't heard of a Bridged T. So much fun to read about something new.
Have a great day.

[PRR]

A General Approach to Twin-T Design and Its Application to Hybrid Integrated Linear Active Networks
http://www3.alcatel-lucent.com/bstj/vol49-1970/articles/bstj49-6-1105.pdf {16MB PDF}

The General Second-Order Twin-T and Its Application to Frequency-Emphasizing Networks
http://www3.alcatel-lucent.com/bstj/vol51-1972/articles/bstj51-1-301.pdf {5MB PDF}

The papers are dense, but bibliograpies are excellent; as is the history in the beginning of the first paper.

[samhay]

I was interested in avoiding an extra opamp (laziness, really).

If you want to generate a narrow bandpass, you need to either generate quite a lot of gain at the notch frequency, or subtract a lot of gain either side of it. In either case, I suspect you are going to struggle to do this passively.

Here's something I knocked up earlier: http://www.diystompboxes.com/smfforum/index.php?topic=104233.msg933134#msg933134

[PBE6]

If I'm not mistaken, the bass boost section of the Thor pedal from www.runoffgroove.com uses a bridged-T filter in an non-inverting opamp feedback loop to emphasize a frequency of around 400 Hz, emulating the bass response of a Marshall 4x12 cabinet. Removing the switch and using a pot instead of the fixed resistor made for a really great sounding variable boost. Something like this would probably work well for you, but it does require the extra opamp.

[mac]

Colorsound Inductorless Wah in LTSpice,

http://www.diystompboxes.com/smfforum/index.php?topic=99448.0

It needs some tweaks to make it more like a real wah.

mac

[bufferz]

If I'm not mistaken, the bass boost section of the Thor pedal from www.runoffgroove.com uses a bridged-T filter in an non-inverting opamp feedback loop to emphasize a frequency of around 400 Hz, emulating the bass response of a Marshall 4x12 cabinet. Removing the switch and using a pot instead of the fixed resistor made for a really great sounding variable boost. Something like this would probably work well for you, but it does require the extra opamp.

Nice, maybe make both fixed resistors variable?