4PM Filter oscillation stops below 1440 Hz

I’m tuning my 4PM filter tracking, but the resonance doesn’t seem to go below about 1440 Hz. Anything below that just makes a short sound and doesn’t start oscillating.

Is that how it should be? I’ve checked all the resistor values but I can’t find anything suspicious.

One of 4PM filters I’ve built exhibits the same behavior, however to a lesser extent: filter self oscillation dies at about 180Hz. The other 2 4PM I worked with oscillate all the way down to 20Hz. I could never find the reason for this… stopped trying since it turned out that i actually like it this way since this seems to result in beefier basses.

Strange. But 180 is still 3 octaves below mine :slight_smile: I think I’d prefer it to go lower than it does now.

What kind of capacitors do you use?

It’s build from the 4PM kit you released a few months ago, so I guess the caps are mine. The 220p are polystyrene the same ones as used in the Anushri kit. The rest is ceramic.

O except for the 4.7u, 100u, and 220u, those are the same as specified in the BOM

My 560p 100p and 10p look different from those in the picture. They are brown and flat, but I guess that doesn’t matter its still ceramic and they’re all not accurate. Looking at the BOM I think all the caps are the same as listed. Anything else you can think of that might influence the resonance frequency? What resistors or ICs should I focus on?

I’m using matched pair of this polystyrene caps from Mouser.

Could this really be the reason why the frequency is so far off? That my already accurate caps are not matched properly?

How do you match caps anyway? Do you need special tools for that?

Not really. The tolerance of the original part is good enough for the resonance to work correctly. It’s just my habit of matching the parts that are supposed to have equal values in critical components. To match the capacitors, I use LCR meter to select a pair with the closest values.

Multimeter with capacitance setting :slight_smile:

BTW, one observation: in my case the resonance level degradation at lower frequencies is not caused by any active component: once I swapped all IC’s from my 4PM and another 4PM with great resonance at low frequencies and behavior did not change.

maybe just a bad joint around those relevant caps?

No bad joints as far as I can tell. This really sucks. It’s the first time I took the risk of soldering the ICs directly to the board, because I didn’t have all of the sockets. I mixed up 100k resistors with 100 and I had to replace those. Maybe I damaged one of the ICs by de-soldering the resistors. But there is no easy way to to tell since I can’t pull them out.

Could you post a photo of the board? I’ll have a look…

Thanks I’ll take a picture tomorrow in daylight.

I think I’m going crazy. Today I checked all the resistors again, and when I couldn’t find anything I decided to plug it in one more time… and now it just freaking works! I managed to tune the v/oct tracking down to the cent precision over 4 octaves and the resonance goes as low as my speakers do :smiley: Yay.

I really have no idea what went wrong last time. I think my mind is playing tricks on me, or eating those mushrooms from the forest wasn’t such a good idea after all. Anyway life is looking bright again, just 5 minutes before my birthday :smiley:

hey that’s the best if they repair themselves! happy bday!

Check the solder joints on the TL072 non-inverting inputs (grounded pads).

Nothing visibly wrong with the solder joints, but I’ve given those grounded pads a little extra just to make sure. Thanks all for the help.