Gyrator bandpass filter

[helge-h]

Seeing videos about old test equipment (Hainbach and others), I wanted to make a bandpass filter. The passive old ones use coils, which I don't want, so I looked for schematics of newer designs. I found an article by Rod Elliott (ESP):
https://sound-au.com/articles/gyrator-filters.htm

Instead of making an EQ as he suggests, I just wanted to pass one band at a time,
such as in this video:
https://www.youtube.com/watch?v=XNQ4ULNwi7g which demonstrates an octave filter (at 7.00 onwards).

I came up with the circuit in the schematics below; I had a switch with 6 positions and two poles, hence the 6 frequency bands.

The opamp is powered by one 12 volt supply, but if you have a dual supply just connect all the "6 V" points to ground (not U1D pin 14; just disconnect this output from the circuit).

R6 determines the Q factor of the filter - experiment with other values if you want. Higher resistance increases the Q, but also lowers the output - compensate by increasing the value of R8.

The capacitors determine the center frequencies. Higher capacitor values give lower frequencies and vice versa.

One idea would be to use a couple of CD4051B multiplexers to switch the capacitors, then you could have 8 bands, but they would need to be controlled in some way, and the complexity rises quickly.

It could be made into a synth module, but I only made it in a standalone box. I find it easier to use then. My switch was a little hard to turn, but with a big enough knob it's ok.

I made some calculations and measurements on the circuit:

calculated: measured:

72 Hz 80 Hz
159 Hz 151 Hz
339 Hz 338 Hz
723 Hz 718 Hz
1592 Hz 1640 Hz
3386 Hz 3400 Hz

Q measured to ca 1.4

So it's not an octave filter as in the video; the capacitor values would have to be exactly halved for each band (200n - 100n - 50n and so on).

It's a simple circuit to experiment with and fun to use.

Edit: uploaded new version of the scematic.
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Helge

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schematic.pdf
Description:		Download (listen)
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[helge-h]

Here is an image of my unit; I used the box of an old router or something. Also a couple of sound examples with white noise. There is considerable switching noise, but that just adds to the character, I think. (I also updated the schematics; the new version is exactly the same, just removed som elements outside the page.)

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Helge

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noisetest1.mp3
Description:	Slow switching
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noisetest3.mp3
Description:	Rapid switching
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P1050237-b.JPG
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This image has been reduced to fit the page. Click on it to enlarge.

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[vladosh]

thanks for sharing this ,this is exactly what i needed ,as i was looking at some passive filters from 9th issue of electronotes and those all need that expensive coil of 500 mhy ,i built the SIAM (Siemens inspired EQ) that also has 8 gyrators and it makes nice sinewave in feedback