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 Forum index » DIY Hardware and Software
Roland 100m 121 'HOT ROD' VCF Layout
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frequencycentral



Joined: May 25, 2008
Posts: 186
Location: UK

PostPosted: Mon Apr 09, 2012 7:27 am    Post subject: Roland 100m 121 'HOT ROD' VCF Layout
Subject description: Verified PCBs available
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Another in my 100M series of layout. This one is 'hot rodded' with an additional 12dB/Oct output and bandpass capabilities, as well as full stereo should you wish it.

I elected to use CA3080 as I have a bunch of them, they make for a neater layout than using LM13700, and it's pretty damn close to the original.

Verifed PCBs available from myself. £15 GBP per PCB including shipping. Paypal GIFT to:

rickholt22 (AT) hotmail (DOT) com

Please state your forum name, full address etc.


Full build documentation here:

http://dl.dropbox.com/u/967492/121%20VCF/Roland%20121%20VCF%20Build%20Doc.pdf

Posted Image, might have been reduced in size. Click Image to view fullscreen.

Posted Image, might have been reduced in size. Click Image to view fullscreen.

Posted Image, might have been reduced in size. Click Image to view fullscreen.


What’s changed:
- CA3080 used instead of BA662A
- 2 x LF351 used instead of 4558
- 2 x back to back 22uF caps used instead of 10uF BP caps
- Addition of 12dB/Oct output
- Fixed HPF not included

12dB/Oct Output:
This output has a shallower cutoff than the 24dB/Oct output, and the frequency range is slightly offset. The 12dB/Oct output is taken from the output of the second stage. It is fed through a transistor inverting amplifier (basically a modified EH LPB1). The 12dB/Oct output is 180° out of phase with the 24dB/Oct output, therefore mixing them together results in bandpass filtering:

Posted Image, might have been reduced in size. Click Image to view fullscreen.

In practice, you’ll probably be able to use two inputs of a VCA rather than having to use a dedicated mixer. The two outputs should be of equal volume so that 100% phase cancellation occurs, but you may need to slightly tweak the volume of one or the other, allowing for resistor tolerances.

Also worth feeding the two outputs to two different VCAs for stereo phase cancellation bandpass uber coolness:

Posted Image, might have been reduced in size. Click Image to view fullscreen.

…and I’m assuming you guys know how to patch a highpass filter out of a lowpass filter right? Here’s how:

Posted Image, might have been reduced in size. Click Image to view fullscreen.

Then it’s just a case of a little mixing, you’ll find that the LP filtered signal is a little quieter than the direct signal, so set it to maximum and lower the volume of the direct signal until 100% phase cancellation occurs. In the above diagram you can use either the 12dB/Oct or the 24dB/Oct. So…….with this little filter you can do:

- 24dB/Oct lowpass
- 12dB/Oct lowpass
- 12dB/Oct bandpass
- 24dB/Oct highpass
- 12dBOct highpass

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vladosh



Joined: Aug 02, 2010
Posts: 402
Location: macedonia
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PostPosted: Tue Apr 10, 2012 3:45 pm    Post subject: Reply with quote  Mark this post and the followings unread

Thanks for sharing the layout !
cheers
Vlad
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LektroiD



Joined: Aug 23, 2008
Posts: 975
Location: Scottish Borders
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PostPosted: Mon May 14, 2012 9:57 am    Post subject: Reply with quote  Mark this post and the followings unread

Does anyone have a copy of the build docs? The host site is dead
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frequencycentral



Joined: May 25, 2008
Posts: 186
Location: UK

PostPosted: Mon May 14, 2012 10:24 am    Post subject: Reply with quote  Mark this post and the followings unread

Dropbox say:

Code:
Error (509)
This account's public links are generating too much traffic and have been temporarily disabled!


I'm now hosting directly from my website:

http://www.frequencycentral.co.uk/wp-content/uploads/2012/05/Roland-121-VCF-Build-Doc.pdf

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