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CEM3320 / AS3320 for 12V: design problems
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heriks



Joined: Jan 14, 2018
Posts: 2
Location: Malmö

PostPosted: Fri May 31, 2019 5:37 am    Post subject: CEM3320 / AS3320 for 12V: design problems
Subject description: I would appreciate some help understanding the data sheet, basically.
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Hi!

I am in the process of building a Eurorack VCF based on the AS3320, pretty much using the reference design from the data sheet. One difference is that it is based on +/-15V supply, where I have +/-12V. The one part where I have big problems understanding the data sheet is calculating the resistor values in the signal path: Rf, Rc, and Rb.

http://www.bustedgear.com/images/datasheets/CEM3320.pdf

This is what the data sheet sounds like to me (the section "Operation of each filter stage):
Iref = a voltage depending on Vcc divided by 100k
Rf = the same voltage depending on Vcc divided by Iref
To me, that sounds like Rf will always be 100k, regardless of Vcc, but that feels strange, because why would the data sheet then spend time talking about how to calculate Rf?
The only thing I can think of that explains this, is that the calculation for Rf is for the "simple case of Figure 1", i.e. one might imagine other circuits where it's not that straightforward.

I tried finding existing 12V designs to learn from, but I only found two:
- EricaSynths multimode filter: the schematic is really hard to understand for me, so I cannot say anything for certain. Grasping for straws (and guessing), I did spot at least one combo of the 100k/91k/220k used in the reference circuit.
- The Elka Synthex mentioned in Electric Druid's walkthrough of different 3320 designs. I checked the original schematic, where applicable, they seem to use 81k/81k/220k for Rf/Rc/Rb.

It's probably easy to tell that I am rather lost; I'm happy for pretty much any advice at this point.
I would especially like to get a real understanding of what the data sheet means, but I would also appreciate links to other 12V implementations out there.

Thank you![/url]
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brock



Joined: May 26, 2011
Posts: 112
Location: Canada

PostPosted: Sat Jun 01, 2019 10:34 pm    Post subject: Reply with quote  Mark this post and the followings unread

The only resistor you probably want to change is Ree from 1.5k to 1.2k.
Everything else can stay the same and should work fine.

Rf is chosen for unity gain in the example circuits in the datasheet. You might want to change this and the bias resistors Rc and Rb to meet the 6.9V bias voltage for the outputs and current conditions described in the Sample Filter Circuits section but the values are probably close enough even at +/-12V.

I will be building a V3320 filter in a few weeks and will try it at +/-12V to see how it works.

Brock
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bubblechamber



Joined: Nov 04, 2006
Posts: 279
Location: NYC
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PostPosted: Fri Jul 26, 2019 6:50 am    Post subject: Reply with quote  Mark this post and the followings unread

i found this to be really helpful when i built my 3340 VCOs

https://electricdruid.net/cem3340-vco-voltage-controlled-oscillator-designs/

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