[sdiy] Question about AS3350 filters in sequence

[synth at schmitzbits.de]

Hi Tom and all,

I have used the chip but not in this exact configuration.
As a first step I would use a unity gain buffer.
What ever the input bias current, would be provided by that.

The OTA outputs would not like to be loaded anyway.
So it's basically the circuit as depicted, leaving out the divider at the LP input and open the 220R at the top opamp.
After that you know if it's worth directly coupling the LP to the HP, which probably also works.

Cheers,
 René

________________________________
Von: Synth-diy <synth-diy-bounces at synth-diy.org> im Auftrag von Tom Wiltshire <tom at electricdruid.net>
Gesendet: Montag, 19. Januar 2026 13:59
An: synth-diy at synth-diy.org <synth-diy at synth-diy.org>
Betreff: [sdiy] Question about AS3350 filters in sequence

The Alfa AS3350 dual SVF shows two different filters in the datasheet:

https://alfatriode.lv/eng/sc/AS3350.pdf

One is HP/BP and the other is LP/BP. All outputs are via a non-inverting op-amp with 22K/220R feedback resistors, so set up for a x101 gain. The LP/BP design has a matching 22K/220R divider on the input to reduce the signal level going in.

Ok, so now my question:

If I wanted to have a HP filter followed by a LP filter, would it be possible to drop the HP's output gain and then also drop the LP's input divider. It seems a bit crazy to boost the signal and then reduce it again straight afterwards. However, perhaps the lowpass input needs the 220R to ground for biasing? Would that need to stay? In which case, can the direct highpass output cope with feeding such a low impedance?

Has anyone tried anything like this? Any experience to offer before I give it a go?

Many thanks,
Tom

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