People keep using the filter in various configurations, so not everyone is hitting the same issues; it’s perfectly possible to use it out of the box without any issues, especially if you’re used to attenuate your way to the right signal levels (which is something you usually need to do anyway in a full rig).
There are three level issues that people keep running into:
- The MS-20 filter has different response curves in low pass and high pass mode; in low pass, it’s two −6 dB/octave filters in a row so −12 dB/octave in total, while in highpass it’s one −6 dB/octave filter and the other stage is only used to inject resonance feedback.
- The MS-20 itself has one low-pass and one high-pass filter in sequence, so it’s not really an issue there, but here you have only half the filter, and if you flip the switch the levels will be off.
- The performance build has a much larger input resistor than the stripboard and other builds, which affects the signal level. You can swap this for lower values, but go too low and you run into the next issue sooner:
- The TL07x series isn’t a rail-to-rail opamp, and the input range (“common mode input voltage range” in the datasheet) is limited; if an input voltage goes below −8 V or so, the opamp misbehaves (this is called phase reversal). This is not a problem with inverting topologies, but the TL07x amplifiers in the filter chain are non-inverting voltage buffers, so the inputs will see the full signal voltage. Too much amplitude in or via resonance, and the filter locks up. To avoid this, you have to either manage the levels, or switch to a different opamp.
So as long as you have the right levels in and the right gain after it, the stock builds work just fine. If not, some tweaking might be needed (e.g. replacing the input 100k with 20k seems to bring up the levels to nicer but still reasonably safe levels) but you may still run into issues that are inherent in the MS-20 design and the component choices.