[sdiy] Question about AS3350 filters in sequence

[Mike Bryant]

Looks like it should work.  To be safe you could just put a unity gain op-amp between output and input as that would also bias the internal input op-amp.
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From: Synth-diy <synth-diy-bounces at synth-diy.org> on behalf of Tom Wiltshire <tom at electricdruid.net>
Sent: 19 January 2026 12:59
To: synth-diy at synth-diy.org <synth-diy at synth-diy.org>
Subject: [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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