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 Forum index » DIY Hardware and Software » Thomas Henry designs
TH PAL Filter
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Trumpet1



Joined: Aug 02, 2007
Posts: 10
Location: Long Beach, CA

PostPosted: Thu Aug 02, 2007 3:26 pm    Post subject: TH PAL Filter Reply with quote  Mark this post and the followings unread

I have a couple of CEM chips and the schematic for this TH Polyphony mag design, and have always wanted to build it. Does anyone know of a PCB, or even just the PCB layout? As I recall, one was never provided with the article.

-David
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Dan Lavin



Joined: Nov 09, 2006
Posts: 620
Location: Spring Lake, Mi, USA
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PostPosted: Thu Aug 02, 2007 3:58 pm    Post subject: Reply with quote  Mark this post and the followings unread

Trumpet1,

Welcome to the electro-music forum! welcome

I forgot about the PAL filter! Yeah, it would be awesome to build! I still have that Polyphony issue. But to answer your question: no, I don't believe I've ever seen a pcb pattern for this. Generally speaking this group stays away from CEM and older SSM chips (with the exception of Fonik's SSM2040 LPF). Unfortunately the asking price for the chips is really high and folks are leery of making a new pcb for parts they can't easily replace.

Do you have pcb design experience? It could be possible to use ExpressPCB which would be ~$51 for 3 boards and you could maybe talk 2 others into buying the other 2. I might consider it, but no promises right now.
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Dan Lavin



Joined: Nov 09, 2006
Posts: 620
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PostPosted: Thu Aug 02, 2007 4:05 pm    Post subject: Reply with quote  Mark this post and the followings unread

Sorry. For those that don't know what a PAL filter is: Phaser, Allpass, Lowpass switchable filter
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Danno Gee Ray



Joined: Sep 25, 2005
Posts: 1343
Location: Telford, PA USA

PostPosted: Thu Aug 02, 2007 5:22 pm    Post subject:
Subject description: More info
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Fill us in!

Some of us don't have the Poliphony mags.
What CEM chip was used? Some of them still pop up on Ebay once and a while.

Don't tease...
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Dan Lavin



Joined: Nov 09, 2006
Posts: 620
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PostPosted: Thu Aug 02, 2007 6:31 pm    Post subject: Reply with quote  Mark this post and the followings unread

DGR,

The PAL uses the CEM3320 along with two 4066 and the 1980's obligatory 4136. A mechanical switch triggers 2 banks of 4066 switches to configure the filter as either low pass or all pass filter. In the all pass mode, there is a original signal blend-in pot so that it becomes a phaser.
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Etaoin



Joined: Jun 30, 2005
Posts: 760
Location: Utrecht, NL

PostPosted: Thu Aug 02, 2007 10:34 pm    Post subject: Reply with quote  Mark this post and the followings unread

I have a "spare " CEM3320 as well. Actually, it's in a Digisound module, but the PAL sounds far more interresting. Is the schematic publicly available somewhere or is the schematic even more unobtainable than the 3320 itself?
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Trumpet1



Joined: Aug 02, 2007
Posts: 10
Location: Long Beach, CA

PostPosted: Thu Aug 02, 2007 11:47 pm    Post subject: TH PAL Filter Reply with quote  Mark this post and the followings unread

Thanks for the welcome. The PAL was a 1982 TH design and I actually wire-wrapped one then, but lost track of it, if you can believe. I still have a couple of CEM VCOs, and ADSR, and the CEM 3320. I recognize it is dated, but since I've got it...

I have done simple PCB layout using ExpressPCB, and even did an EAGLE schematic of the PAL circuit to see if the Eagle autorouter could possible layout a single sided, or at least minimum jumper PCB. No such luck - the board needs two sides and about 20 vias. I wanted to etch it myself, since I have never actually used the ExpressPCB service. (I use their free software to make my own transparencies, but only for simple, single-sided boards.)

Anyway, the Eagle schematic and board files are available if anyone can use them, and if TH doesn't mind, and you let me buy a board. You would need the original magazine schematic and instructions, since I left off all the panel controls and jacks from the Eagle version - just used labeled connectors. I have that info too, again if TH (or would it be PAIA) doesn't mind. BTW, back issues are still available, or were not too long ago.

Regards,
David
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Etaoin



Joined: Jun 30, 2005
Posts: 760
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PostPosted: Fri Aug 03, 2007 12:24 am    Post subject: Reply with quote  Mark this post and the followings unread

The PAiA site says back issues should be available but they also offer to send photocopies if they are not. So I've e-mailed them to check.

By the way, double-sided with vias does not necessarily rule out DIY'ing the board.
I did the double sided board below myself, with 35 vias. It does depend on whether the rest of the PCB is DIY friendly or needs holes to be metalized through.


clockboard.jpg
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DIY double sided board
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Dan Lavin



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PostPosted: Fri Aug 03, 2007 5:18 am    Post subject: Reply with quote  Mark this post and the followings unread

Trumpet1,

Thomas also designed a 12dB state variable (LP,HP, BP,notch) VCF around a CEM3320 in one issue of Polyphony. Actually that's what I was thinking about doing with my spare CEM3320. I'm a big Oberheim SEM fan and thought it would be cool to make a not-quite identical clone. There is artwork for the pcb and it is single-sided.
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Trumpet1



Joined: Aug 02, 2007
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Location: Long Beach, CA

PostPosted: Fri Aug 03, 2007 9:35 pm    Post subject: Reply with quote  Mark this post and the followings unread

I thought you all might be interested: here's a photo of a TH VCO Deluxe from the Polyphony articles, but in my own funky 4U form factor. Really great performance. If this shows OK, I'll post a couple more.


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Trumpet1



Joined: Aug 02, 2007
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PostPosted: Fri Aug 03, 2007 9:37 pm    Post subject: Reply with quote  Mark this post and the followings unread

Here's a TH EG from the Polyphony series.


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Trumpet1



Joined: Aug 02, 2007
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PostPosted: Fri Aug 03, 2007 9:38 pm    Post subject: Reply with quote  Mark this post and the followings unread

And here's a TH VCA.


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Trumpet1



Joined: Aug 02, 2007
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Location: Long Beach, CA

PostPosted: Fri Aug 03, 2007 9:41 pm    Post subject: Reply with quote  Mark this post and the followings unread

antman49443 wrote:
Trumpet1,
...There is artwork for the pcb and it is single-sided.


Hey, this sounds right up my alley. Got a URL?

Thanks,
David
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Randaleem



Joined: May 17, 2007
Posts: 456
Location: Northern CA, USA

PostPosted: Sat Aug 04, 2007 1:33 am    Post subject: Reply with quote  Mark this post and the followings unread

antman49443 wrote:
Trumpet1,
Thomas also designed a 12dB state variable (LP,HP, BP,notch) VCF around a CEM3320 in one issue of Polyphony. Actually that's what I was thinking about doing with my spare CEM3320. I'm a big Oberheim SEM fan and thought it would be cool to make a not-quite identical clone. There is artwork for the pcb and it is single-sided.


Hi,

Here's one that sounds like it may be similar to the Thomas Henry PAL.

Craig Anderton's MIF (Multiple Identity Filter), using the 3320 for a filter with switch selectable LP,BP,HP,AP and Notch response. Most modes have 12, 18, or 24dB slopes available. It uses a LOT of Panel toggles. I was going to try some analog switches and a PIC in an updated version.

The complete Contemporary Keyboard magazine article (From Nov '79, running three issues) is available somewhere online. No PCB layout, but the circuit is pretty simple with only 3 IC's. Looks more complicated due to the switches in the schematic. So far, it's the most comprehensive circuit I've seen for a 3320.

Randal
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v-un-v
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Janitor


Joined: May 16, 2005
Posts: 8932
Location: Birmingham, England, UK
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PostPosted: Sat Aug 04, 2007 1:55 am    Post subject: Reply with quote  Mark this post and the followings unread

fwiw, the Elektor Formant 24db/oct filter had similar specifications but used cascaded CA3080's (like the Oberheim design), however it also used a notoriously difficult to find ua726 matched transistor pair.

I've got the plans still for this filter

But I do like the sound of this filter. Very buttery.

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Dan Lavin



Joined: Nov 09, 2006
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Location: Spring Lake, Mi, USA
Audio files: 21

PostPosted: Sat Aug 04, 2007 5:40 am    Post subject: Reply with quote  Mark this post and the followings unread

I agree with Randal, the MIF is a much more versatile filter and not that hard to build. Your panels have enough real estate to handle all the required switches. I just downloaded that article a couple days ago after googling "CEM3320 circuits" The file was MIF.pdf, but I don't remember the exact website.

Your panels and pcbs look pretty good! You obviously know what you're doing.
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Trumpet1



Joined: Aug 02, 2007
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Location: Long Beach, CA

PostPosted: Sat Aug 04, 2007 8:02 pm    Post subject: Reply with quote  Mark this post and the followings unread

Thanks, all. I found the MIF article at http://www.crowncity.net/ratcave/Electro/MIF/MIF.pdf
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