Question Regarding AS3320 Resonance Input

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Chris Willocks
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Question Regarding AS3320 Resonance Input

Post by Chris Willocks » Mon Mar 23, 2020 6:34 pm

Hi everyone,

I've designed a Eurorack AS3320 low-pass filter module and just have a query regarding the resonance input (pin 9).

As the input is current controlled and comprises a low-impedance summing node at a potential close to ground, would it be acceptable to assume that no external summing op amp would be required for inputting multiple resonance CV sources into it? This is shown below for illustration purposes:

AS3320 Resonance Inputs.png

The module has two resonance inputs; a standard normalised input from a pot on the front panel that controls the initial resonance level and another input that comes from an attenuated input socket that accepts a CV of between 0V to 5V.

Each input will provide a current of 75uA to pin 9 when fully open, which combined will provide up to 150uA maximum, as the input resistors are in parallel. I am aware that 100uA is the maximum from the datasheet, however I have read from other that up to 150uA should still be safe. The reason I chose 75uA is because this is what the Pro-One is also specified at and is enough to provide sufficient oscillation.

Hope this makes sense.

Many thanks,
Chris
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KSS
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Re: Question Regarding AS3320 Resonance Input

Post by KSS » Mon Mar 23, 2020 9:12 pm

In principle I expect it will work. But in practice I'd run it past Alfa support before using 150. They're the best situated to give or deny approval on ignoring their datasheet.

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GSE
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Re: Question Regarding AS3320 Resonance Input

Post by GSE » Mon Mar 23, 2020 10:55 pm

I've done it both ways using a summing amp, and also summing two signals direct to pin.
It's all worked for me, using the 100K min input resistor like the datasheet.
These re-issue chips are cheap now compared to NOS CEMs. Maybe get a few, test them on a breadboard.

Chris Willocks
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Re: Question Regarding AS3320 Resonance Input

Post by Chris Willocks » Tue Mar 24, 2020 5:18 pm

Hi both, thanks very much for the replies and feedback. Much appreciated.

To be honest, I tried the summing amp method i.e. running it through two inverting amps and it worked fine. However, I just want to simplify the design a little and reduce the number of components on the PCB by configuring the two signals to be summed directly. Also, I wasn't sure if summing them directly would lead to any current bleed between the two inputs, like you'd get when summing multiple signals with a non-inverting amp; due to the lack of a virtual ground?

I'll contact Alfa and see what they say regarding the maximum current.

Thanks again,
Chris

jorg
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Re: Question Regarding AS3320 Resonance Input

Post by jorg » Wed Mar 25, 2020 8:40 am

As a low-impedance ground-referenced input, it's clearly intended for summing applications. Treat it as you would any virtual-ground op amp input. The Alfa and Curtis datasheets both mention a range of 0-100uA for this input.

Make sure you go get the original full datasheet; it has a LOT more info about this input (though I believe Alfa linearized the control curve, versus a flattened curve in the original).

https://sdiy.info/wiki/File:CEM3320_datasheet_long.pdf

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