CorOS 4.01 — Fizzy, Harsh, Oversaturated Tone on Captures: Firmware Bug or Hardware Fault?

Quad Cortex
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Hi everyone,

I’ve been dealing with a frustrating tone issue on my Quad Cortex running CorOS 4.01 and wanted to see if anyone else is experiencing the same thing — particularly whether this might be firmware-related rather than a hardware fault.

The short version: my QC sounds fizzy, harsh, and oversaturated compared to identical captures and patches on a Nano Cortex. The tone feels like the signal is being over-driven somewhere in the chain, even on captures that should sound clean and natural. This is noticeable on high-gain captures I knw well on Nano. I don’t hear it as much on clean tones.

What I’ve already tried (extensively)

  • Verified input gain staging in I/O settings — not clipping

- Built patches from scratch using only an amp block + cab block with factory cabss

- Applied HPF (~75 Hz) and LPF (~75 kHz) inside the cab block

- Checked output routing — no cab sim stacking

- Tried both XLR and TRS outputs, some cracking in the monitors when using XLR (using as interface)

- 2 full factory resets

- Compared the same chain side-by-side on the Nano Cortex — the Nano sounds correct, the QC does not (Nano no longer with me)

- Different guitars direct into either input

The unit is brand new, so a hardware fault is possible. However, my concern is that this is a CorOS 4.01 firmware issue with how captures are being processed or rendered — in which case a warranty replacement would simply repeat the problem.

I’ve seen older threads about oversaturation and fizz on the QC (the “QC sounds oversaturated” bug report thread, the “sounds very different from demos” thread from early 2026, etc.) but I haven’t found anyone specifically comparing QC behavior to the Nano Cortex on the same captures under CorOS 4.x.

Has anyone else seen this on CorOS 4.01 specifically? And has anyone who replaced their unit under warranty found that the replacement behaved differently, or did the issue persist?

Any insight from the community — or from Neural DSP staff — would be really appreciated. Happy to share patch details or I/O settings if that helps narrow it down.

Thanks in adevance for any help.

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Do you experience this with several presets ? Fizzy, harsh, oversaturated are usually telltale signs of a lack of cab sim.

Maybe share the presets you’re experiencing this with, so we can check there’s nothing amiss with them.

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Thanks for the reply. This happens on any preset, but I almost always start from scratch with Input 1 (I’ve also tried Input 2), then add a capture — usually one of the CA MKII+ NDSP captures — followed by a cab block using the CA Std OS V30, maybe a room reverb, and then multi‑out or Outputs 1/2 into a pair of Kali monitors. That’s the entire signal chain.

It might be more noticeable on version 2 captures, and the IR block seems more prone to the fizzy tone than a virtual cab, though some of that is subjective. Because of this, I can’t provide a specific preset, but I’ve confirmed the issue also appears on factory high‑gain presets.

Fizz is just one part of the spectrum — the low end, mids, and other regions seem affected as well. The lows in particular feel more boosted. It may be more accurate to say the overall tone becomes overdriven or pushed in a way that shouldn’t be happening.

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seems like you’re getting some clipping at some point. Is the output block blinking red when you play ?

Also, do you have the same issues with the stock presets ? Try them, this might help pinpoint your issue.

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I think I’ve solved it!

Quick answer first: yes, I had hot output from the output block, and it seemed to be picking up extra gain in the IR block. The clipping was happening everywhere — factory and scratch presets alike — with anything beyond lo crunch.

I’d been using the QC as my interface, and after trying headphones with no perceived improvement, I swapped it out for a Motu M4 I had lying around. Ran the QC into the Motu’s 3/4 inputs and out on QC 1/2 — same output as going direct into my monitors — and everything sounded dramatically better. Built up a complex stereo scratch preset and it held up “clean” the whole way through. Went back to factory presets and they were seemingly night and day.

The weird part: why does it sound bad going directly into my Kali LP-6 V2 monitors, which have never given me any trouble? If I lower the QC output enough to clean up the tone, the volume is too low. The Motu is running into the same monitors on the same I/O except (TRS vs. XLR being the only difference), and the QC’s level into the Motu looks totally reasonable with the volume knob dimed for line level. Something unusual is going on with the direct connection specifically.

Not concerned long-term since this stays in my studio, but it’s strange. Happy to troubleshoot further if anyone has ideas.

Honestly the most frustrating debug session I’ve had in years — but worth it if it sticks. Fingers crossed it still sounds good next session! It is so subjective when the tone in your head is a clipping tone and you are trying to hear the difference between good and bad clipping.

Thanks to ScreamOfAnger for he;ping as best he could.

Cheers!

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what cables are you using to connect to the MOTU? Are they the same you used to connect the QC to the monitors?

Outputs 3/4 on the QC will deliver a +6dB boost if you use TRS cables connecting to another balanced input jack. That probably isn’t what’s happening here, but I sure can’t imagine what else would be causing it.

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The cabling is different now. The QC is currently connected to the Motu using XLR from QC Out 1/2. Previously, the QC was connected directly to the Kali monitors via XLR‑to‑XLR, and before that I had the QC on Out 3/4, which would have been TRS on both ends. I’d read that Out 3/4 runs hotter, so I switched away from it a while back.

What’s interesting is that when I connected the QC directly to the Kali monitors—regardless of which output pair I used—the clipping still happened even with the QC output level turned down significantly. All three cable types are known‑good; I’ve used them with my previous QC and the Nano without issues. And if the problem were simply overdriving the Kali monitors, I’d expect the signal going into the Motu to be hot as well, but it isn’t.

Thanks for thinking it through with me. I’ll play through the QC again soon to confirm that the issue really is resolved. If it is, at least I know the QC is fine and capable of good tone. From there, I’ll experiment with the Kali monitors directly to figure out what’s going on.

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It still sounds good!!! Phew! I uploaded a preset of the rig I built last night and thought it was worth giving others. It has some AI tone shaping ideas if anyone care. I am on the Cortex Cloud the same ID. The mystery remains as to why it freaks when used as a interface but, I’m happy I can play at least.

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Quick update: I rewired my QC to the Kalis yesterday using Out 3/4 (TRS) into XLR on the monitors, with the Motu completely out of the chain. The clipping was gone. The signal was hotter than through the Motu, but it was the right kind of hot—mid‑forward with good presence, which seems normal.

I spent some time tweaking the preset’s gain and EQ and ended up with a realistic, usable tone. So the clipping seems tied specifically to the XLR‑to‑XLR or TRS‑to‑TRS connections. It makes no sense, but for now this setup gives me the low‑latency signal chain I wanted.

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have you talked to support@neuraldsp.com yet?

I think I might be noticing the same issue. I recently started using an XLR connection in my studio, where I had been using either TS or USB. I’m definitely hearing a difference. I’m doing more testing, but I think there’s definitely something there

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I didn’t. I decided to try the forum first, mostly because I was relatively sure it wasn’t hardware but more likely firmware.

Try lowering the global out and compensate to the extent you can with the preset out. Don’t use Multi-out unless you need it. Pick a capture that has low enough gain that you can tell the difference between desired clipping and undesirable. I did that to verify that the only clipping/saturation I was hearing was from my capture and not something else. Also, check to see if your cab block/IR are adding in gain that might clip. I see that with some IRs if they are hot.

I’m not sure if it is an intentional decision or not but, the final output from the QC is a little mid-forward and not necessarily transparent. Knowing that will help you EQ more easily.

Cheers!

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I’m starting to think this thing is possessed.

I’m running the QC as my interface now, and the tone is solid using Out 3/4 into my monitors using XLR. So the original problem is mostly solved.

As part of testing, I grabbed a new power supply because the stock one is honestly the biggest disappointment of QC. I’m using a Mean Well GST60A12‑P1J (12V, 5A, 60W) with a Belker 5.5×2.1mm reverse‑polarity converter. Noise floor dropped, the PSU runs cooler, and the PSU itself doesn’t feel like it’s about to break in your hands. The stock wall wart is barely usable.

Now here’s the weird part: after the noise dropped, my Ditto Pro — plugged into QC Input 1 — randomly started playing back an old test loop. And I can hear it through my laptop’s built‑in speakers while the QC is OFF.

What the hell. The Ditto isn’t even on the same outlet as the QC. I’ve had that pedal for years and it’s never acted strange. And why can I hear a faint signal from it when the QC is off and I’m only using laptop speakers? Grounding issue? RF bleed? Ghosts?