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Euro Rack 914 FFB clone done
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author Euro Rack 914 FFB clone done
dingebre
Update 12/17/2014

My new web store is the best place to buy the PCBs. Start at:

www.analoguerealities.com

click on the Catalog and SDIY "key", navigate the menu to the Moog modules, 914 FFB. It's the only one right now, so should be easy. The checkout is through PayPal.

David

**************************************************

First, see here for the main thread:
https://www.muffwiggler.com/forum/viewtopic.php?t=116142&postdays=0&po storder=asc&start=0

This thread has photos, and other details if anyone is interested. I uploaded an audio sample comparing this 914 to Yves Usson's 914. Here's a link to the blog:
http://analoguerealities.blogspot.com/2014/12/audio-for-914-comparison .html
You can read more about what I did and how there.


If you want the inductor version, contact Carsten Tonsmann at www.Analog-Monster.de. Great guy and has an excellent site with all sorts of good stuff.

SORRY FOR YELLING smile IF YOU ARE INTERESTED, SEND ME A PM OR EMAIL. I am in the process of publishing my new website and purchasing will be done there. I purchased a bunch of PCBs so I'm pretty sure I won't run out. If I do, I'll order more.



Here is the initial post, somewhat edited:

**********************************************
NEW INFORMATION ADDED TO ORIGINAL POST

GIC - $70.00 per set (A little expensive, but they are 4 layer PCBs to minimize noise and hum)

Inductor - $50.00 per set (Will work with either the large or small core 5 Henry inductors Carsten has made)

A couple of quick comments on construction. I'll put more detail together if people buy the PCBs.

A set consists of 4 PCBs, a top, middle, and bottom PCB adn an I/O PCB which mounts to the bottom filter cell PCB. They have PCB mounted pots, Panasonic EVU-E2JFK4D54, DigiKey P3U0503-ND. You will most likely need to trim the shaft a little depending on the knob you use. the HP and LP pots are not PCB mounted and can be any smallish panel mount pot. I've calculated the values for the capacitors which are on the silkscreen. There are two choices, one for an "exact" cutoff frequency, one that pushes the cutoff either up or down about 6% to mimic what it appears Moog did (as well as Jurgen Haible on his version).

The construction is not bad. The calibration is the problem, here. Ideally, you want the overall "Q" to be about 3.7 (per Yves Usson's testing of an original 914). The trimmers will adjust that, but it's not so straight forward to tell. I've just set my trimmers to point to about 10:00 or 11:00 and the result is very pleasing. you don't want too high a "Q" or you lose some of the harmonics between the cells. Too low and you miss the nice resonance when the input attenuator is fully CW.

Be warned, both versions are kind of pricey to build. For the inductor version, you need a set of Carsten's inductors. for the GIC, you need 29 of the best, lowest noise, op amps you are willing to buy. I ended up using the OPA2227, about 4.50 each for 10-99 pieces. Probably way overkill. Jurgen used an MC4558 at about $0.40 depending on the exact one you pick.

So, enough for now. Please feel free to contact me if you are interested. I've got about 2 weeks til the PCBs are in. In the mean time, I'll work on a BOM and some construction notes.


ORIGINAL POST

I've been working on a Moog 914 fixed filter bank clone using GIC (generalized impedance converters) and inductors (Carsten Tonsmann at www.Analog-Monster.de custom wound)

I combined ideas from Yves Usson, Jurgen Haible, and my own doing to put together what I think is a faithful recreation of the original.

Good and Bad:

Good:
Euro/Frac rack compatible
PCB mounted pots (except for two)
Can run on +/-12 or +/-15 volts
Doepfer and MOTM power connectors
Two versions:
1. GIC version using op-amps to simulate the impedance of an actual inductor. All the good from a "real" inductor and none of the bad, like interaction with stray electromagnetic fields.
2. Inductor version use real inductors with the nonlinearities and other "features" you get with a real inductor.

Bad:
It uses 4 PCBs and they mount perpendicular to the panel, so you need a deep cabinet.
The inductor version is very sensitive to stray electromagnetic fields. I can't find a place in my cabinet far enough away from the power transformers to eliminate a very high level hum. But, if you use a toroidal transformer or if the transformer is a couple of feet away, you should be fine.
The GIC version uses 4 layer PCBs so they are more expensive than the standard 2 layer, double sided PCBs.

Idea:
I wanted my own version of the 914. I built Yves Usson's excellent version, but it is totally an active filter circuit. It also uses a PCB much more suited for a 5U module, not euro or frac. I bought circuit boards for Jurgen Haible's version, but again they are more suited for a 5U format. With all due respect, Jurgen also missed an error on the Norlin schematic where a resistor in parallel with each second band pass cell was omitted. Without this resistor, the "Q" in each cell is too high and you get too much resonance in the filter.

So, I borrowed Yves' (and Kazike at COTK although he brings every cell out) idea to also bring out every other cell as an "even" or "odd" cell. I also decided that a back to back GIC low pass cell was not ideal. So, I designed a salen-key low pass filter cell similar to Yves' and Jurgen's design but with standard component values so no need to put resistors in series or capacitors in parallel.

I use the original Moog input and output amplifiers for the combined filter outputs, but do a simpler op-amp (either a single stage or dual stage if you want) output for the even and odd outputs.

There is an interesting feature in the 914 (Jurgen discusses this, too) output amplifier, a high pass filter with a cutoff at about 70 Hz at the input to the output amplifier. I included this filter as did Jurgen at the input to my even and odd op-amp output stage. It's already in the Moog stage. I think Moog included this to help filter out stray 60 cycle and to eliminate the muddy sounding very low frequencies. This filter is intended to help form certain instrument sounds by emphasizing certain resonances, so I think they wanted to get rid of "boomy" bass when using the filter. But what do I know. I'm looking at a 30-40 year old circuit trying to second guess what the design engineer's intent was. But this is my story and I'm sticking to it.

Update:
OK, after many, many hours and lots of prototype PCBs, I think I have a good handle on the filter. I just ordered what I hope is the final revision for PCBs for the GIC version. I went to some very extreme measures to minimize outside noise and hum pickup, like a 4 layer board, ulta low noise op-amps (OP2227PA), etc. I think I can safely say any noise is due to the original filter design. I think the inter PCB connections carrying the summed output signals may derive benefit from shielded wires due to the low levels from the resulting summed filter cell outputs.

For comparison, I did some quick "ear" checks against my Yves Usson 914 filter clone and I was very happy. I'll work on some more analytical comparisons over the next week or so.

I need to think about the Inductor based version a bit more, but I think I have the final version of those PCBs ready to go, too. I will buy a set of last prototypes soon and will post here as soon as I have them built.
**********************************************************

Happy knobbing...

David
sempervirent


Euro panel interest check thread:
https://www.muffwiggler.com/forum/viewtopic.php?p=1738117
Synthsense
Awesome project! Count me in please Guinness ftw!
dingebre
My new web store is the best place to buy the PCBs. Start at:

www.analoguerealities.com

click on the Catalog and SDIY "key", navigate the menu to the Moog modules, 914 FFB. It's the only one right now, so should be easy. The checkout is through PayPal.

David
bananeurysm
Cool project!

I wonder if you could post audio demos of the differences between your builds and the yusynth version?

I'm thinking about building one in 5u.. I was interested in an inductor version - but it sounds like you've worked out a potentially even better approach (GIC) smile
sempervirent
See the first post here for an audio file:
http://analoguerealities.blogspot.com/
dingebre
bananeurysm wrote:
Cool project!

I wonder if you could post audio demos of the differences between your builds and the yusynth version?

I'm thinking about building one in 5u.. I was interested in an inductor version - but it sounds like you've worked out a potentially even better approach (GIC) smile


Thank you. These PCBs will certainly fit behind a 5U panel. I put some mounting holes on the front of each PCB in case someone didn't want to use the PCB mounted pots.

sempervirent posted the link to the audio sample I made on my blog comparing the inductor version to the YUSynth version. I think they are very similar except for a stronger bass response in Yves' version. I attribute this to the high pass filter (see the blog page for more information) that Moog added before the output amplifier stage of the 914 (which is curiously not in the 907 design if my memory is right).

It's a very interesting filter. I quite like it personally.

David
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