Moog Music Forum

Hi,
I've been a big fan of Moog sounds for a long time, and recently I've started to dig deeper into the different types of filters Moog uses. I know the legendary Ladder Filter on the Minimoog and many other classic lines, but I've also seen some more modern lines like the Sub 37 or Subsequent 37 use OTA filters.

I'm curious about the distinct tonal differences between these two types of filters. Ladder Filters are often described as "fat", "creamy" and have a very distinctive resonance. So what kind of sound color does the OTA filter on the Sub 37 bring? What are its advantages and disadvantages compared to the Ladder Filter in creating different sounds (e.g. leads, bass, pads)?

I also wonder if this difference affects how these synths interact with external modules?

Does anyone have experience with both of these filters and can share some comparisons or sound examples? I would love to get a better understanding of the sonic "soul" of each Moog line.

"OTA" filters is a generic term. You can cascade a series of fixed mode OTA filters (IE lowpass) to realize a 12dB or 24dB lowpass filter.

The other "OTA" filter is the more common multimode filter (aka SVF=state variable filter), made famous in legacy Oberheim synths. These filters produce lowpass, highpass, and bandpass responses simultaneously.

While I don't know which OTA filter is implemented in the Moog products, the Oberheim SVF is 12dB and the typical Moog Ladder filter is 24dB. They both have their different sound, and history has established that they both co-exist nicely without much redundancy.

The SVF is a little buzzier, the resonance is unique, and the low end doesn't attenuate as you increase the resonance. The Moog filter has that "Moog sound". Both can sound fat. The Moog filter will self oscillate but not the SVF.

I prefer the SVF for brass sounds, I prefer the ladder filter for bass sounds, I can get good strings from either of them. While you may not need brass/strings, it demonstrates that having both filters is good for experimenting with patches.

It is worth noting that a ladder filter in 12dB mode will not sound like an OTA filter in 12dB mode.

Hambsood wrote: Mon May 05, 2025 3:52 am ...I know the legendary Ladder Filter on the Minimoog and many other classic lines, but I've also seen some more modern lines like the Sub 37 or Subsequent 37 use OTA filters.

The OTA based filters are easier to build. No transistor matching process necessary. Probably cheaper in production.

Hambsood wrote: Mon May 05, 2025 3:52 am ...Does anyone have experience with both of these filters and can share some comparisons or sound examples? I would love to get a better understanding of the sonic "soul" of each Moog line.

I cloned both. The sound differences are as MC already described.

I was flashed when I cloned the Moog 904-B and 904-C modules. I noticed that for me a Moog Ladder low pass (904-A) is not enough, not sufficient, and my favorite sound is the NOTCH patching of the filter module trio. Unfortunately the MINI does not provide that, what is a pity.

But despite of being 24 db the Moog filter modules sound more smooth, so I prefer the Oberheim State Variable Filter, what is a matter of taste obviously.

Examples:

Moog Ladder in System 55 and Prodigy

Oberheim SVF