Resonant Lowpass Filter Problem

[rackham]

Wondering if anyone can help?

I've been breadboarding a resonant lowpass filter, using the (sort of) schematic here:

http://thesquarewaveparade.com/images/TSP-rlpf.jpg

Only I can't get it to work.

I'm using a LM741 opamp and rather than a dual (500k) pot, 2 x 500k single pots just to test it.

The Res control is a 50k pot and the resistors are 100k to ground and 330k between the res pot and the cap.

I've been juggling cap values and the best I can get is a really tiny signal.

I'm thinking that either the 741 isn't the right opamp for the job, or I've just been miles out with the cap values?

Can anyone offer any suggestions?

[Mark Hammer]

A few observations. 

1) Usually, resonance/gain in such a filter is set via the resistor between the inverting input and ground.  It actually doesn't take all that much gain to produce audible resonance.  So, a feedback resistor of, say, 22k and a ground-leg resistor of 18k-22k with a 25k variable resistor in series, would actually give  a wider range of resonance than you'd think.  Really.

2) There absolutely needs to be fixed resistors in series with the dual ganged pot shown, so as to set maximum cutoff frequency.  I realize the schematic does not show them, but it is a valueless conceptual schematic not a "real" one.  Ideally, you would probably want to use a value for the fixed resistor that is 1/10 the maximum value of the pot.  So, if each pot section is 50k (and I think 500k is way too big), then make the series fixed resistor 4k7 or 5k1.  The reasoning behind this is that you want to strike a balance between so wide a range that it gets hard to dial in anything with precision, and a range so narrow that you feel the need to switch cap values.

3) The formula for each pole is F = 1 /[2*pi*R*C].  Assuming you used the suggested 50k+4k7, a cap value of 2200pf (.0022uf) would score you a filter cutoff range from 1.3khz at the low end to 15.4khz at the top end.  As another example, using .0033uf (3300pf) caps would get you a cutoff range of 880hz to 10.26khz.  You can feel free to play with the pot value, series resistor and cap value to achieve different ranges.

[RickL]

Maybe a little obvious, but that circuit uses a bipolar supply. I've had circuits not work because I assumed a single supply.

[rackham]

Thanks very much for the help Mark and Rick.

I'd definitely missed biasing the Opamp  so that was a start.

I'm still not 100% there but it's really handy to know the theory behind what I'm trying to achieve.

Mark, just to recap, would you suggest all the pot values be 47k then?

Cheers

Jon.

[Mark Hammer]

Anything between 10k and 100k duals will make use of easily available cap values feasible.

[rackham]

Right, so I've been messing about with this for the last couple of days and I'm still getting nowhere

I've used the same biasing as a GGG dist+ to power it and have found an alternative (better) source for designing a Sallen-Key filter:

http://www.t-linespeakers.org/tech/filters/Sallen-Key.html

Still not getting much success. Should I be able to plug a guitar into the circuit and out into an amplifier or does it need the gain raising before or after?

Also is a dual-gang pot essential for testing rather than 2 x single pots?

Cheers

Jon.

[Paul Perry (Frostwave)]

Two single pots are perfectly OK, if you are able to get them adjusted to the same point easily enough. Within 10% would be fine.
I don't think you would need any extra boost to the signal level.

[rackham]

Thanks Paul.

Finally got this thing working yesterday and I'm planning to make it switchable between lo-pass and bandpass, which I've managed so far on the breadboard.

I think I'll put a basic fuzz option in front of it too for a bit more harmonic content.