904C Filter Coupler for System 55

[@amsynths]

Fantastic video and Mark is a great guy! Its a great module kit and sounds fab. The AM904C assembled module will be shipping in February, a small amount of soldering (6 wires) is needed to connect the daughter boards. Or you can purchase modified Behringer 904A/Bs with the AM904C, which are plug and play using one internal cable. PPG and Projekt Elektronik made single modules that do the same job, using two minimoog filters in the PPG 317 and two ARP 4012 filters in the PE 290. Both of these will appear as AMSynths modules in 2025. Klaus Schulze happily used band reject on his Moog 904C in his live shows but Chris & Edgar went down the PPG route, whilst Paul used the big PE system.

[@ozhalljr]

Rob, thanks for that added detail!

[@svetkolisica1635]

Hello Gentlemen. I can't seem to find the modified 904B from the AMSYNTH Site?

[@ozhalljr]

@@svetkolisica1635 I can't speak for Rob ( @Amsynths ), but I suspect he will add the modified 904A & B once the 904C is available.

[@svetkolisica1635]

@ozhalljr Thanks for the video OZ. Like the 904C combo with the 904A/B🙂

[@ozhalljr]

Thanks for saying so!

[@moogfooger]

Mark is very personable and a terrific source of info. Another high quality content video OZ. I notice you have an Abacus in place of your 914 Fixed Filter Bank. I have one too and look forward to your investigation of that. Cheers

[@ozhalljr]

Cheers Tony!

[@fitzgreyve]

Many thanks for your kind comments O.Z. (and Rob!) General observations: the 904C is a bit of an "odd" module, it has some sweet spots but also a lot of "dead" zones - you need to read (and I find re-read!) the original Moog instructions. It's not as "obvious" to use as, say, a modern state variable filter. There can be some variance between the output levels of the Behringer 904A and 904B filters which can lead to an unbalanced 904C output, specifically in the "band-reject" mode. This can be compensated in the 904C by modifying couple of resistor values.

[@ozhalljr]

Mark, thanks for the observations. Where can we find out more about the resistors to modify. At $29 per module (904 A/B), replacing the resistors seems not a (potentially) expensive risk. Are the original moog instructions in your build document for the 904C? Are they in the service manual (starting p. 39): www.synthxl.com/wp-content/uploads/2019/01/Moog-Modular.pdf Is there a different source to which you're referring?

[@fitzgreyve]

@@ozhalljr The resistors in question are in the 904C: R20 and R27. They are both specified as 10K. The issue that was flagged to me was that the B55 LPF output was lower level than the 904B HPF output. Reducing the value of R27 slightly (I think, I havent tried it) should compensate. Ideally you need to check the 904A and B output levels against a fixed input level to gauge the size of the R27 reduction. Yes, the moog instructs (plus other comments) are in the 904C build doc.

[@ozhalljr]

Thanks Mark!

[@Leerf-driftingpitches]

Thank you -great as ever

[@ozhalljr]

Thanks!

[@impurfekt]

Very interesting. I'd love a proper multi-mode 24dB ladder filter. Not sure why they are so rare. Excellent presentation! P.S. How do you like the Victor? I was considering getting one. I did watch a couple of your tutorials.

[@ozhalljr]

Thx. I like the Victor. Would like to spend more time with it.