[sdiy] Transistor Sallen-Key lowpass filter question

rburnett at richieburnett.co.uk rburnett at richieburnett.co.uk
Mon Nov 19 11:38:56 CET 2018

You need to take the bottom of R4 to a negative supply rail, or bias the 
input to about half of the positive supply voltage if you want it to 
work with a single rail supply.  (There examples of this in Roland's 
Juno series chorus circuits.)

I love that website, BTW.  It's great for designing anti-aliasing 
filters for audio and data acquisition type applications.  I'm 
particularly fond of the MFB low-pass filter because of it's superior 
stop-band performance compared to the more well-known Sallen-Key 
arrangement.  It's also easy to dial in a bit of gain, which can be very 
useful in some occasions.

Less fond of the cryptic error messages in Kanji & Hiragana when you 
enter unreasonable design parameters though! (>.<)


On 2018-11-18 23:59, Tom Wiltshire wrote:
> 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?
> 	https://electricdruid.net/wp-content/uploads/2018/11/DelayFilterSchematic.png
> According to the sim (LTSpice), the response looks like this:
> 	https://electricdruid.net/wp-content/uploads/2018/11/DelayFilter.png
> 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..
> 	http://sim.okawa-denshi.jp/en/Sallen3tool.php
> ..doesn’t give the same effect. Am I seeing a simulation effect or a
> real phenomena?
> Thanks,
> Tom
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