TK.

CEM3378 Filter: first impressions

16 posts in this topic

Today I did the first experiments with a CEM3378 filter, which some

of the germans purchased in a collection order from synthtech.com

A top view of the workbench:

mbsid_aout_lc_proto.jpg

Right side: A AOUT_LC module based on the schematics from Karl Dalen.

It mainly consists of 2 * 74HC595 and not less than 36*10k resistors to realize

a 12bit DAC, which controls the Cutoff frequency.

The final version I'm planning will consist of 48*10k resistors for an 12bit DAC

(CutOff) and 4bit DAC (Resonance) (I guess that the resolution is enough, especially

because I have to build 4 of them ;-)

In addition this module has an OP-Amp to control the gain and offset of the

output CV - for the filter it turned out that this is a very important feature

Bottom: the CEM3378 circuit (based on the datasheet) + a +/- 5V voltage regulator.

Both, the filter and the AOUT_LC circuit are powered with this low biased voltage.

In addition, the CEM3378 needs +12V

Left side: ok, my beloved MBSID

And here the first demo - MBSID in TB303 mode, controlled via MIDI (internal

sequencer not used) - filter/envelope parameters tweaked in realtime

http://www.ucapps.de/mp3/midibox_sid/mbsid_demo11.mp3

IMHO the filter sounds very good, especially on filter sweeps it can be noticed

that the volume doesn't get lower even when resonance is close to self-oscillation.

During the first tests I also noticed, that key-tracking is a must for such a filter.

Therefore I implemented it into the firmware (CC#6) - now I have to find a way

to store this value, because the SysEx data structure is completely  allocated :-/

Best Regards, Thorsten.

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sounds goood ;D

ive modded a synth that has a similar cem ic, i like them >they have a seperate VCA too and all..

low cost vcf aswell?

cheers, marcel

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nice tb303 like sound.. it obviously isnt by any means exact.. but imho.. it sounds great.. but do i hear an echo? thats something i would like to have in my mbsid.. some kind of FX processor.. (delay and chorus/flanger) and bpm synced ofcourse :P

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low cost vcf aswell?

It is - $10 + taxes + shipping is a fair price compared to the effort which is required to build a discrete filter

Yes, it's just the TB303 mode - not the typical TB303 sound. In fact I wasn't able yet to realize the same filter behaviour like on my moog-filter yet, I think that I have to optimize the OP stage at the end of the DAC, or the voltage divider at the CEM3378 filter CV input in order to fix this.

However, the filter gave me some enjoyable hours today, it already makes a lot of fun to play with this toy :)

Fx processor: there was a guy who realized a DSP project for such jobs, search for "miss parker" in the forum

On the other hand: I prefer the use of VST plugins. The usage is simple and more flexible.

Best Regards, Thorsten.

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Good Thorsten, as it sounds I can say without doubt this is the chip used in Chroma polaris ;) -------> although CEM3372

I checked out the service manual of the old fender synth. Maybe some hardware could be taken as example for a mios polaris based synth :D

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Eh.. IMHO the application of the 3372 in the polaris is sub-par.  That chip is capable of much more.  I'll have to check the sheets, but I also think there are some differences between the 72 and 78.  The 72 is also what was used for the massive multimode beast in the oberheim Martix-12 and XPander synths, the Xpander manual actually has a full app note about it too, check it out at bluesynths.com.  Quite a nice chip really.  Keep an eye out on ebay for SSM2044's too, those are nice (Mono/Poly, Wave 2, etc;)

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What additional controls are required for this filter?  Does it connect only to the master core module?

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I'll have to check the sheets, but I also think there are some differences between the 72 and 78.  The 72 is also what was used for the massive multimode beast in the oberheim Martix-12 and XPander synths...

There are some differences, but it doesn't prevent using CEM3378 in similar multimode configuration as

3372.. all the gain cells have output taps.

Keep an eye out on ebay for SSM2044's too, those are nice (Mono/Poly, Wave 2, etc;)

Keep ya hands off of SSM2044s.. I might need some for my Polysix.. :D

Bye, Moebius

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What additional controls are required for this filter? Does it connect only to the master core module?

You need a MBHP_AOUT_LC module (low-cost DAC)

A schematic can now be found here: http://www.ucapps.de/mbhp/mbhp_aout_lc.pdf

PCB Layout will also be provided (sooner or later)

Best Regards, Thorsten.

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You need a MBHP_AOUT_LC module (low-cost DAC)

A schematic can now be found here: http://www.ucapps.de/mbhp/mbhp_aout_lc.pdf

PCB Layout will also be provided (sooner or later)

Best Regards, Thorsten.

I meant to ask what knobs or switches are required.

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MBHP_AOUT_LC layout:

mbhp_aout_lc_v1.gif

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The PCB:

mbhp_aout_lc.jpg

Best Regards, Thorsten.

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can we have the .brd-file? i'm etching this weekend!

by the way; is it (going to be) possible to put the s&h module with this?

cheers, marcel

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Hi Marcel,

link to the layout: http://www.ucapps.de/mbhp/mbhp_aout_lc_v1.brd

link to orderlist: http://www.ucapps.de/mbhp/mbhp_aout_lc_orderlist.txt

Yes, the SHX8 board can be used, but there is currently not firmware which supports this combination. And I'm sure that it won't be possible with MBSID or MBFM, but it will be supported by MB64 and MBCV (sooner or later...)

Best Regards, Thorsten.

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