Spoiler: Source: Grok KI

Hey, fellow K-2 MKII owner here (or at least, I've dug deep into this one). Yeah, I totally get the frustration—Rev1 sounding like it's auditioning for a white noise generator while Rev2 is all pristine and polite. It's a common gripe, and it's not just your unit being quirky; it's tied to the synth's architecture and a bit of a known gremlin in the design. Let me break it down based on what owners are reporting across forums like Mod Wiggler and Reddit.

Quick Recap on the Filters
The K-2 MKII (a faithful-ish clone of the Korg MS-20) has two filter modes selectable via that REV switch:

• Rev1 (OLD): Mimics the original MS-20's early revision filter. It's got that raw, aggressive 12dB/octave low-pass/high-pass vibe with a more feedback-heavy circuit. This makes it punchier and more "vintage" but also inherently noisier—think higher self-oscillation hiss, especially at max resonance. The extra feedback loop in the transistor ladder (Sallen-Key style) amplifies thermal noise and any minor imperfections in the components, turning it into audible white noise/hiss. It's "by design" for that gritty character, but on the MKII, it can feel exaggerated if your unit's tolerances are on the looser side.
• Rev2 (NEW): Based on the later MS-20 revision, it's smoother and more stable with refined feedback paths. Less aggressive resonance, cleaner highs, and way less inherent hiss. Owners often call it "too clean" because it tames the wildness that makes Rev1 fun for weird, screechy patches.

In short: Rev1 is supposed to be hissy compared to Rev2—that's the architectural difference from the original Korg. But if it's way more excessive (like constant white noise even at low resonance or without signal), it might be amplified by a unit-specific issue.

The Bigger Issue: Resonance Drop + Noise
A ton of early MKII units (especially first batches) have a widespread bug where Rev1's resonance fades out after 1-2 hours of warmup, dropping from full scream to barely there. This doesn't hit Rev2 at all. It's likely a leaky cap or buffer issue in the Rev1 feedback path (possibly those 10uF electrolytics degrading under heat/load), which can also contribute to extra hiss/noise as the circuit destabilizes. Not every unit does it—some folks report zero problems after hours of pinging the filter—but it's common enough that people are testing it right out of the box.

There's even a YouTube demo from early 2025 showing the resonance bug in action on the low-pass side, with audio clips that might match your hiss (worth a quick watch to compare).

Is It "OK" or a Defect?

• By architecture? Mostly yes for the baseline hiss—Rev1's meant to be the dirty one. The MS-20 original had similar character (check old Korg threads; owners loved/hated the noise floor). If you're into that lo-fi edge, crank it and embrace the chaos.
• But... If the noise is overpowering (e.g., audible at zero resonance or drowning out oscillators), or if you're seeing resonance drop-off, it's probably the bug. Not a recall yet, but Behringer's aware from forum noise—hit up their support with a video demo (cold start vs. 1-hour warmup on Rev1 at max peak/resonance, no input signal).

Tips to Tame It

• Warmup test: Power on, set to Rev1, cutoff ~6, peak 10, mix to oscs off (pure noise gen). Record resonance sweep at 0min, 30min, 1hr, 2hr. If it quiets down, bug confirmed.
• Workarounds:
  • Stick to Rev2 for cleaner stuff, or blend filters via patching (HP from Rev2 into LP Rev1 for hybrid grit).
  • External noise gate/compressor post-output can squash the hiss without killing dynamics.
  • Modders on Mod Wiggler suggest swapping those feedback caps if you're handy (cheap fix, ~$5 parts).
• If it's driving you nuts: Return it if under warranty, or grab a used original MS-20 (Rev1 units are hissier but battle-tested). Some folks just avoid Rev1 entirely and never look back.

Hit me with more details on your setup (e.g., does the resonance fade? Patch examples?) if you want deeper troubleshooting. What's your go-to patch on this thing anyway? The ring mod + external input is killer.