[sdiy] Transistor Sallen-Key lowpass filter question

Tom Wiltshire tom at electricdruid.net
Mon Nov 19 00:59:37 CET 2018

Can anyone tell me why a transistor-based Sallen-Key filter like this one would have such a heavy roll-off at the bass end?


According to the sim (LTSpice), the response looks like this:


There’s a significant drop with a -3dB cutoff of about 200Hz. What’s causing that?

Using the same passive components, but with a op-amp in place of the tranny, with the tools at..


..doesn’t give the same effect. Am I seeing a simulation effect or a real phenomena?


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