An electronics oxymoron...

[ExpAnonColin]

Easily tunable active bandpass filter.



-Colin

[Peter Snowberg]

What's wrong with a classic state variable style? One quad opamp, a few resistors, and a few caps and there you go.

Take care,
-Peter

[R.G.]

You take the schematic of the Mutron 3 (or Neutron...) and change out the envelope detector for a knob setting the LED current. Bango, easily tuned bandpass (and highpass, and lowpass) filter of the state variable kind.

You can also do the same with a dual OTA chip and some jellybean opamps to let the OTAs be the varying input resistors and a capacitor on the outputs of the two OTAs be the integrators and get a two chip current controlled state variable bandpass.

Hmmm... if you read some of the stuff at GEO, you could pick up on gyrator-based bandpasses and resistor-to-ground wah bandpasses.

There's even a diode-based Sallen-Key bandpass filter somewhere in my files.

And your pleasure is...?

[ExpAnonColin]

You take the schematic of the Mutron 3 (or Neutron...) and change out the envelope detector for a knob setting the LED current. Bango, easily tuned bandpass (and highpass, and lowpass) filter of the state variable kind.

You can also do the same with a dual OTA chip and some jellybean opamps to let the OTAs be the varying input resistors and a capacitor on the outputs of the two OTAs be the integrators and get a two chip current controlled state variable bandpass.

Hmmm... if you read some of the stuff at GEO, you could pick up on gyrator-based bandpasses and resistor-to-ground wah bandpasses.

There's even a diode-based Sallen-Key bandpass filter somewhere in my files.

And your pleasure is...?

Good points, very good points.  My original post should have been more specific-it's near impossible to create a single-ganged-pot-tuned single op amp bandpass filter.  There are, of course, countless ways to do the same sort of an idea-my personal favorite being the wah-antiwah-but I was just getting tired, hence the post.

-Colin

[ExpAnonColin]

What's wrong with a classic state variable style? One quad opamp, a few resistors, and a few caps and there you go.

Take care,
-Peter

I was unclear with a state variable-does varying the resistance between the Vref and the BP out change the frequency, or do you have to change all of the resistor values?  It seems the latter to me.

-Colin

[R.G.]

it's near impossible to create a single-ganged-pot-tuned single op amp bandpass filter.

Depends on how much performance you're looking for. There is a single-opamp/single pot schemo at GEO for a wah bandpass using an inductor, and also a single pot schemo using a Twin T filter. You can also do a bridge-T and get single pot tuning. They're limited in the sweep range and Q, but they usually work OK for audio.

I was unclear with a state variable-does varying the resistance between the Vref and the BP out change the frequency, or do you have to change all of the resistor values? It seems the latter to me.

You only change the two series resistors in front of the two integrator stages. A dual pot works. If you don't have a dual pot, use an LED/twoLDR setup.

[spongebob]

TI has a chip called UAF42, Universal Active Filter:

http://focus.ti.com/docs/prod/folders/print/uaf42.html

Description:
---
The UAF42 is a universal active filter which can be configured for a wide range of low-pass, high-pass, and band-pass filters. It uses a classical state-variable analog architecture with an inverting amplifier and two integrators. The integrators include on-chip 1000pF capacitors trimmed to 0.5%. This solves one of the most difficult problems of active filter designâ€"obtaining tight tolerance, low-loss capacitors.

A DOS-compatible filter design program allows easy implementation of many filter types such as Butterworth, Bessel, and Chebyshev. A fourth, uncommitted FET-input op amp (identical to the other three) can be used to form additional stages, or for special filters such as band-reject and Inverse Chebyshev.

The classical topology of the UAF42 forms a time-continuous filter, free from the anomalies and switching noise associated with switched-capacitor filter types.

The UAF42 is available in 14-pin plastic DIP and ceramic packages, and SOL-16 surface-mount packages, specified for the -25°C to +85°C temperature range.
---

Looks like an interesting chip. It's a bit expensive though, but you could try to get on TI's sample program :wink:

[Peter Snowberg]

Cool chip Spongebob!

One resistor, one dual pot, and you're ready to go!

For musical apps, it's going to take two more resistors, or possibly four more to get the range you want out of the pot.

http://focus.ti.com/lit/an/sbfa003/sbfa003.pdf

Take care,
-Peter

[Tim Escobedo]

A single LM13600/13700 can be configured as a voltage controlled BPF tunable over a wide range with a single pot.

For simplicity, though, the Twin T/Bridged T does pretty well.

[ExpAnonColin]

That's quite a cool chip.  Thanks for the suggestions, guys.

-Colin