904a and Minimoog filter.

System 55 • System 35 • Model 15
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Rarecomponentadsr
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Location: Derbyshire England

904a and Minimoog filter.

Post by Rarecomponentadsr » Sat Oct 31, 2020 10:19 pm

Despite painstaking attention to detail and creating as much authenticity ( Moog copper boards, vintage components, perfectly matched transistors, vintage main capacitors) as possible on my 904a clone ...when compared to the Minimoog filter, it seems pretty lifeless and diluted. The Minimoog filter on the other hand is biting, crisp and ...well, say no more .. classic. !

Im perfectly willing to accept I’ve done something wrong on my 904a but cannot find the slightest discrepancy anywhere. The 904a is set up as per instruction on the schematic.

Can anyone tell me their experience of 904a and Minimoog filter in comparison with each other ? It seems the Control and Resonance of my 904a goes to 7 while the Minimoog goes to 11. Maybe my 904a is indeed correct ...that’s how it should be ?

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MRNUTTY
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Re: 904a and Minimoog filter.

Post by MRNUTTY » Sun Nov 01, 2020 6:03 pm

Rarecomponentadsr wrote: Sat Oct 31, 2020 10:19 pm Despite painstaking attention to detail and creating as much authenticity ( Moog copper boards, vintage components, perfectly matched transistors, vintage main capacitors) as possible on my 904a clone ...when compared to the Minimoog filter, it seems pretty lifeless and diluted. The Minimoog filter on the other hand is biting, crisp and ...well, say no more .. classic. !

Im perfectly willing to accept I’ve done something wrong on my 904a but cannot find the slightest discrepancy anywhere. The 904a is set up as per instruction on the schematic.

Can anyone tell me their experience of 904a and Minimoog filter in comparison with each other ? It seems the Control and Resonance of my 904a goes to 7 while the Minimoog goes to 11. Maybe my 904a is indeed correct ...that’s how it should be ?
What're we starting from? Norlin schematics? Bread board? TH? SMT?

I have this on my todo list too. Have yet to go through the dry design details yet, but I never read a data driver analysis
VoyagerEB, Minitaur, LittlePhattyII, 4xSlimPhatty, Sub37, MF[2x101,2x102,103,104M,105,105B,105M,2x107,108M], 3xCP251, XV351, MP201;
2xMother 32, DFAM, Subharmonicon;
System 55 and Minimoog clones with lots of mods.

Rarecomponentadsr
Posts: 111
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Location: Derbyshire England

Re: 904a and Minimoog filter.

Post by Rarecomponentadsr » Sun Nov 01, 2020 7:13 pm

Norlin schematics. Exact circuit board copies. Exact components , even vintage ones. Exact wiring ..as perfect to an original as I can possibly get it. The only slight deviation is the piggy back resistor R11.
In all I’m 100% certain no mistakes on the build and that all components are working fine.

Yet as stated the AJH MiniMod LP filter trounces this. But as mentioned it’s possible that this is simply as it is.

The 904 self oscillates ..I’ve tried many adjustments but still it feels weak
and diluted.

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AlakaLazlo
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Re: 904a and Minimoog filter.

Post by AlakaLazlo » Sun Nov 01, 2020 7:53 pm

The 904a and Minimoog filters are different and sound different. (I have both.) The "Mini sound" is partially due to oscillators overloading the mixer and then the filter input. The CP3 mixer in the modulars can overload, but it still doesn't sound the same. There are going to be subtle differences even between 2 Minis...

Rarecomponentadsr
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Location: Derbyshire England

Re: 904a and Minimoog filter.

Post by Rarecomponentadsr » Sun Nov 01, 2020 9:08 pm

Thanks for reply .

Can you tell me ...is there a big difference between the two or would you say a subtle difference ?

So far I’m finding theres a huge difference. The 904 a clone I have is no where near that of the Minimoog filter. I wish it had the response of the Minimoog ...but it simply doesn’t.

Rarecomponentadsr
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Re: 904a and Minimoog filter.

Post by Rarecomponentadsr » Mon Nov 02, 2020 12:13 pm

Issue is more or less resolved.

For those interested and looking for comparisons I’ve done some basic tests with AJH MiniMod filter/Osc and my cloned 904a.

Quite simply, I was surprised to find that when I fed an AJH MiniMod oscillator directly into the 904a using saw wave, I obtained, more or less the same results as I hear from the MiniMod oscillator into the MiniMod Filter, or to put it another way, the classic Minimoog sound more or less. So in other words both the filters, when fed by a MiniMod (or Minimoog)
oscillator are more or less the same. It seems it’s all to do with oscillator signal and the Mini oscillators and as AlakaLazlo said, overloading the filter input.
In comparison, the standard 921b to 904a seems more refined and rich buts lacks that strength and “kick” the Minimoog osciltors to 904a have.

So I now believe my 904 a filters are correct and working properly. It really wasn’t about the filters at all, more about the signal they receive.

The tests I did were quick and a little haphazard so I’m open for correction
here ..but the jist of the results are as I’ve described.

noober
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Re: 904a and Minimoog filter.

Post by noober » Mon Nov 02, 2020 9:32 pm

Interesting! I wonder what if you patch some external drive/distortion like the Culture Vulture between the CP3 and 904.

Rarecomponentadsr
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Re: 904a and Minimoog filter.

Post by Rarecomponentadsr » Tue Nov 03, 2020 6:56 am

I have a COTK 921 and this module has a far far greater output then the original Moog 921.
I’ll do a test regarding saturating the filter as compared to the original 921bs
I also need to flip the test and feed a 921b into the MiniMod filter for comparison.

I get the feeling that lots of timbres Ive long tried to replicate and that “classic” Moog sound has a lot to do with filters being overdriven or saturated rather than the standard 921b -Cp3 -904a- 902 set up.
It seems to make the filter hyper responsive.

One quick rough test I did was to feed a 921b into one Cp3 then into a second Cp3 to increase saturation which partially works...adding a third Cp3 and the audio signal seems to phase out and disappear (from both + or - outputs.)

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