Avoiding unwanted distortion in a Sallen-Key filter

[HeavyFog]

Hey everyone, so recently I've been toying with the idea of an onboard active low pass filter/preamp for bass in the style of alembic. After looking into it and tracing some circuit boards I found that alembic stuff uses an onboard sallen-key filter with a range of roughly 300hz to 6k. Im pretty new to OP amps in general so I'm sure I've made a few mistakes here and there, but after messing around with the circuit here's what I came up with:




It's not an exact clone of the alembic filter, as I had to fill in some of the cap and resistor values I didn't know, but right now its sounding great and I'm real happy with the filter performance. The non inverting gain stage before it it meant to give a little extra volume and output and it works but is causing the filter to clip. I have the same issue when I set the gain stage up as a unity gain buffer too, just not quite as bad.

Like I said, I'm fairly new to op amps so I'm sure I've made some kind of mistake here but I cant seem to get the filter clean. I'm testing the circuit with a jazz bass and a ric 4003 and its giving me problems especially when the gain is turned up. I've tried adding coupling caps after the first stage, which just ended up killing sound altogether.

Also I am aware that alembic pickups are low impedance and have a generally low output. I'm keeping that in mind, I just want to make this work with conventional high impedance pickups too. This makes me thing an impedance mismatch could be at ply here but I'm not sure.

Usually I would add series resistance to the filter but I dont want to drop the max cutoff frequency by adding more series resistance. Maybe adding some directly to the non inverting input would work? Also I should mention this will be going in my bass to replace the tone control so I want to keep part count a minimum

[Scruffie]

Are you using a split supply? Either you're limiting your input op amp's voltage swing with that reference divider or you've got your sallen key referenced to ground instead of half supply.

[HeavyFog]

I'm using a single supply of 9v since il be using it with a battery in my bass.

[Scruffie]

Then the resistors from pin 5 & 6 of the filter should be going to a reference voltage.

[HeavyFog]

Sorry, should have mentioned that I got rid of the 0.015uf coupling cap and the resistor to ground on pin 5 because adding the cap killed the sound altogether, even with the resistor (1m) connected to a reference voltage (4.5v from a divider), ground, or 9v.

The filter still works without it, and I found absolutely no difference with or without it (other than the cap killing the signal)

[HeavyFog]

Also I should probably mention that I used a 4558D chip in the circuit. The older alembic filters used a 4558dd or n, plus it's all I had lying around.

[Scruffie]

Even still, the resistor from pin 6 should still go to a reference/shunted with a large cap or it's going to be offsetting the op amp.

And in future you'd want to put that cap/resistor straight after the proceeding op amp, although it would create a slight band pass effect.

IIRC the DD was just a slightly lower noise variant of the 4558, not the issue here.

[HeavyFog]

That's for the help! I hooked up the 10k resistor to a reference voltage supplied by 2 resistors set up as a voltage divider, with a 100uf cap on the ref voltage line to ground. The distortion is much less than is was before even with the gain up all the way. I would like to get a bit more headroom out of it if possible with a 9v supply. Any ideas?

[Scruffie]

Either a rail to rail op amp or use a charge pump for a higher supply voltage.

[PRR]

> a rail to rail op amp or use a charge pump

Or don't turn Gain up so much. As-is it should deliver 2V which ought to be enuff for almost any purpose.

[HeavyFog]

Honestly I think I've got it good now. The distortion is pretty minimal and it's only really happening with the gain on max when I really dig in on the bridge pickup on my ric 4003, which makes sense given the pickups are kinda hot with a very prominent attack to them. On my fender jazz and musicmaster bass there's no distortion whatsoever no matter how hard I dig in, so I think once I quiet the circuit up a bit everything should be good. Thanks for the help scruffie!