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 Forum index » DIY Hardware and Software » Jürgen Haible designs
914 FFB and Moog incomplete schematic
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dingebre



Joined: Aug 10, 2008
Posts: 257
Location: Salt Lake City, UT USA

PostPosted: Sat Jul 06, 2013 3:12 am    Post subject: 914 FFB and Moog incomplete schematic Reply with quote  Mark this post and the followings unread

In the course of my own FFB design, I discovered a couple of things which may be helpful to those building or who have built Jurgen Haible's 914 clone. This is just a for what it's worth post. It is in no way meant to be critical of Jurgen's design. It's because I purchased 2 sets of his PCBs that I even started to seriously look at how this filter worked. Then recently I found www.analog-monster.de, and found that Carsten is hand winding inductors for these clones. I started simulating and analyzing the 914 circuit and decided to lay out some PCBs to easily fit behind a eurorack panel, and it kind of went from there...

First, The Moog schematic for the 914 is incomplete, I had the occasion to look at some photos of two different 914s (some of them are here http://electro-music.com/forum/viewtopic.php?highlight=moog+914&t=14857), and in both modules, there is a resistor in parallel with the capacitor in the second section of each bandpass filter cell. After a lot of simulating in MultiSim, and staring at the enlarged photos, and a little theory, it looks like a value between 4.7k and 8.2k, with 6.8k working best when comparing the AC response from Multisim to the specs published by Moog. This resistor is critical in establishing the proper Q for the filters and if left out, the response is not the same. The 907 may also benefit from this resistor, but I haven't really looked at it.

I'm attaching some images to show the Moog specification (downloaded from http://moogarchives.com/), Jurgen's response with the actual inductance of his GICs, and then one using the ideal inductance and corrected capacitors.

Second, should you build the 914 with real inductors, you will need to adjust the bandpass filter capacitors to match your inductors. Jurgen appears to have chosen his values to match the actual inductance of his GIC cells some of which are off of the Moog specified values. Some are very close. for example, the 3 henry GIC is really 3.001 henries with the exact capacitor values Jurgen specified for the GICs. But, the 1.2 GIC is really 1.06 henries with the GIC capacitors Jurgen chose.

I've played around with some ways to "correct" the --filter-- (not GIC) capacitors given any specific value for a real inductor or GIC. A bit long to explain here, but if you're interested, send me a P.M. or an email and I'll explain more.

Third, the initial series resistor into the bandpass and HP cells should be about 10 to 20 k. I think the photos show a 10k resistor in series with an unknown value trimmer. I suspect the trimmer will be about the same value as the series resistor.

Again, for what it's worth...

David


Moog specification.jpg
 Description:
This is the original Moog "typical" response specification. You can see in this one and in the simulations with the added parallel 6.8k resistor, the shape of the peaks and troughs, and the slope of the response is different with and without the
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Moog specification.jpg



JH no resistor.jpg
 Description:
The AC response of JH's design without the additional parallel resistor and using the actual GIC inductance, not the Moog specified inductance. Note that the peaks and troughs are rounded, whereas in the Moog response, the peaks are a little flatter and b
 Filesize:  283.51 KB
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This image has been reduced to fit the page. Click on it to enlarge.

JH no resistor.jpg



JH with resistor.jpg
 Description:
This is Jurgen's circuit with all his chosen capacitors for the GICs and the filter cells. I added the 6.8k parallel resistor and changed the series input resistor to 15k.
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This image has been reduced to fit the page. Click on it to enlarge.

JH with resistor.jpg



Moog with resistor and corrected capacitors.jpg
 Description:
This is the Moog circuit with added parallel resistor, 15k series resistor, and capacitors corrected to match the "ideal" inductances. I'm still looking at the HP and LP responses.
 Filesize:  274.92 KB
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This image has been reduced to fit the page. Click on it to enlarge.

Moog with resistor and corrected capacitors.jpg



_________________
David M. Ingebretsen, M.S., M.E.
Collision Forensics & Enginering, Inc.

dingebre@3dphysics.net
http://www.xmission.com/~dingebre/Synthasystem.html
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