A question about Moog filter design.

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Celluloide
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A question about Moog filter design.

Post by Celluloide » Wed Jan 14, 2009 5:34 am

I’m curious about so called ‘Moog filter’. As far as I know, the filter is a patent ‘ladder filter’ designed by Dr. Bob Moog. It is actually 24db low-pass filter.

What I’m most curious about is if all moog synthesizers have same filter, at least filter with same design. But each model of Moog monophonic synthesizers sounds different. For example, Little Phatty sounds different with Rogue and Rogue sounds different with MiniMoog model-D… and so on.

Then are there many variations of ladder filter design? Otherwise, is it caused by different components of filter, though they all have same filter design?
Moog Little Phatty Stage II, Clavia Nordlead 2X, http://moogsynth.tistory.com/

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Post by Johnny_Cradle » Wed Jan 14, 2009 11:06 am

Looks like you got a very cool website... from what i can see (language barrier)

but looks very interesting non the less_

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till
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Post by till » Wed Jan 14, 2009 11:25 am

Moog used this filter for nearly all if not all synths.

The variation in sound does not come only friom the filter alone. different oszillator sound different. Different envelope shapes. And then it gives you different results at what level you send the oscillators to the filter.
And all this is interacting. So it is a complex thing.
keep on turning these Moog knobs

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Post by EricK » Wed Jan 14, 2009 12:36 pm

Then like on the Taurus, it was straight from the 904a filter module. The exception was that it was the only SYnth made by Moog with a specially overdriven filter. It wasn't duplicated until the Phatty and will be exactly replicated on the Taurus III.

Eric
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Celluloide
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Post by Celluloide » Wed Jan 14, 2009 8:31 pm

Thank you Till and Eric K for those answers.

So to sum it up, though most of them have filter with same design, but they sound different because of different components or other designs for certain parts like envelope. Is that correct?
Moog Little Phatty Stage II, Clavia Nordlead 2X, http://moogsynth.tistory.com/

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Post by Voltor07 » Wed Jan 14, 2009 8:32 pm

Celluloide wrote:Thank you Till and Eric K for those answers.

So to sum it up, though most of them have filter with same design, but they sound different because of different components or other designs for certain parts like envelope. Is that correct?
That's it in a nutshell! :mrgreen:
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MC
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Post by MC » Wed Jan 14, 2009 9:00 pm

Many neglect the feedback path in the filter, which has a lot more to do with its resonant quality than oscillator timbre or coupling of stages. It is also the reason why a minimoog sounds different from a micromoog from a taurus.

Tube amp designers have long acknowledged the impact of the feedback path on the tone of their tube power amps, and the same holds true for the moog ladder filter. The feedback path consists of the current recovery amplifier off the top of the ladder and any circuitry providing feedback back to the ladder filter which causes it to resonate.

I had flight time with the model C minimoog with Audities when I was working with Dave Kean. It sounded radically different, its resonance was not as lively as the model D. Dave Kean of Audities had a schematic drawn up of the hand built model C and when I studied the filter I noticed that the recovery amplifier was centered around JFET transistors as opposed to the BJT used in model Ds. JFETs are opposite devices from BJTs in that the former is voltage based while the latter is current based, both constructed with completely different substrate components and architecture. Transistors are highly non-linear devices and as a rule JFETs are going to sound different from BJTs.

I did a study of the filters in various models and found that the major difference was the feedback topology - minimoog used discrete opamp built from BJTs, taurus used OTA (CA3080), micromoog used a convoluted discrete setup that doesn't look like the textbook differential amplifier. Every moog model had a different feedback architecture.

If you study digital implementations of the moog ladder filter, you will find they are not complete in that they omit the components in the feedback path. They do not consider the sonority of a discrete opamp built from BJTs from an OTA. This is a major reason why I have yet to hear a digital filter rival the real thing.

And the Satellite and Minitmoog (not mini) have 12dB ladder filters, not 24dB.

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Post by Anna's Digit » Fri Jan 16, 2009 4:44 am

ohhh nice to now ! :P
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Post by EricK » Fri Jan 16, 2009 12:36 pm

MC,
PLease correct me if Im wrong, but everything that Ive read about the Micro says that it has the exact same filter as the Mini.

I didn't know you worked for Audities. Man here in Arkansas the most Moog stuff Ive ever seen....is in my house. Ive seen a LP one time that the Guitar center. Im probably one of the handfull of people in Arkansas with Moog stuff.
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Post by MC » Sat Jan 17, 2009 6:56 pm

EricK wrote:MC,
PLease correct me if Im wrong, but everything that Ive read about the Micro says that it has the exact same filter as the Mini.
They have the same ladder transistor configuration but the feedback loop is worlds apart. Crank any amount of resonance in the filter and they will sound worlds apart.

Minimoog is a discrete differential amp in the feedback loop.

Micromoog has a dual matched FET pair followed by an OTA followed by a darlington transistor configuration in the feedback loop.

The trend I have seen is the simpler the filter feedback loop, the better it sounds. Why did Moog modify the filter circuit for the micromoog? The Minimoog has a "fault" in that the resonance (and self-oscillation) disappears at low cutoff frequencies. The micromoog design attempted to correct that but the sonority of the filter resonance suffered. You can't have both, there is going to be some trade-off. Just because it LOOKS good on a spec sheet doesn't mean it SOUNDS good.
I didn't know you worked for Audities. Man here in Arkansas the most Moog stuff Ive ever seen....is in my house. Ive seen a LP one time that the Guitar center. Im probably one of the handfull of people in Arkansas with Moog stuff.
My tenure with Audities was short due to opposite coasts but it was definitely fun. That was my first exposure and flight time with a moog modular, and it was Roger Luther's huge modular at that!!!

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Post by EricK » Sat Jan 17, 2009 8:16 pm

THats good information.

Now how about this:

Ive heard that the FIlter design of Bob's had a design flaw that was pleasing to the ear. Now this "Fault" that you were refering to, is that it? Or is this so called design flaw in the filter essentially the Moog ladder filter design? I wrote it just like I remember reading it somewehere.

Great info!

Eric
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MC
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Post by MC » Sat Jan 17, 2009 8:49 pm

EricK wrote:Ive heard that the FIlter design of Bob's had a design flaw that was pleasing to the ear. Now this "Fault" that you were refering to, is that it? Or is this so called design flaw in the filter essentially the Moog ladder filter design? I wrote it just like I remember reading it somewehere.
Yes the "flaw" is the resonance dropping off at low cutoff frequencies.

The minimoog had it, the modular had it. The succeeding models "fixed" this flaw, although I am not sure about the Sonic Six as that is one of the few moog synths I have never played.

bosonob
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voyager filter

Post by bosonob » Thu Jan 22, 2009 6:21 pm

MC,

I have heard that the voyager filter had the "fault" fixed.Meaning the dropping off of resonance as a function of frequency. Is there a way to undo the fix from a setting aspect? I heard that near oscillation the filter becomes non linear in resonance or is that only the model D?

Also, I have tried to find the filter section on the voyager but to no avail. There are many transistor arrays in there. I just did not see the ladder caps you would normally see, even between the trans. arrays you don't see a pattern. Do you know what component numbers make up the ladder?

Thanks,
Stefan

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Post by MC » Thu Jan 22, 2009 10:28 pm

The non-linear resonance is on the minimoog and modular 904A filter.

As far as duplicating the effect on a Voyager, if you're using an EG to spike the filter you could also route that EG to filter resonance. Haven't tried this trick though but I did pull it off on the Andromeda.

Regarding the location of the filter components, I know it is built around a CA3086 transistor array but I don't know where on the analog board they are.

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