Doepfer A-106-1 Xtreme Filter (MS20)

I'll start by saying - this is *NOT* A STRAIGHT MS20 CLONE. To actually emulate the MS20 filter section you would need two of these patched in series - if you are purchasing for that MS20 sound bear that in mind, treat this as a completely different beast.
Now having said what this module isn't, I'll say what it is. Quirky, unusual, gnarly, crazy, raspy, openly-patchable, characterful - the list is endless. This is not a go-to bread and butter filter - take a look at the a106-5 sem filter if thats what you are after - but what it does, it does incredibly well. Balancing the input gain against the resonance and clipping levels can yield some incredible sounds - rasping brasses are a speciality for it. I also love using for drones as the deep lowpass with clipping is great when you patch out the resonance feedback path (I like using a189-1 vc bit mangler and a199 spring reverb on it). The filter responses are adjustable with trimmers on the board which is useful.
You wont find yourself using this on every patch, but it'll add so much to those weird and wonderful distorted sounds that you wont regret the investment.

It's a very characteristic filter that can scream, but can sound quite mellow when tamed. One thing to bear mind is that it can be a bit confusing getting this filter to act like a standard low pass filter. This is because the low pass filter is normalled to the high pass filter i.e the signal goes through the low pass filter to the high pass filter. In principle you can set the high pass to zero (12 o'clock on the polarizer control) and the filter will just act as a standard low pass filter. In practice it is quite hard to hit the zero mark on the high pass filter and you get some of that going into the signal, which can dramatically change the sound of the low pass filter at low frequency settings. The easy way around that is to just put an unconnected jack plug into the high pass in. This will then completely bypass the high pass filter and the filter will act as a conventional low pass filter. That was the most confusing thing I found when trying this filter for the first time. Once you've sussed that out it's a great sounding filter with lots of character.

This filter is a must for anyone with a eurorack modular.

This has become one of my favourite filters. It definitely has that MS20 character, but, as is usual with Doepfer, goes much further. Some folks complain that it doesn't sound 'xtreme' enough, but this is all down to incorrect gain staging. That's what the gain input attenuator is for! I have found that it works best when set around 1 (out of 10). Crank the resonance past 5-6 and it turns into a musical screaming beast! There's a jumper on the back to turn HP input polarizer into a regular attenuator. I prefer this mode as you can easily turn off HP input without requiring dummy cables. With 3CV inputs and a feedback path what's not to love!