If you have a budget of around $20 to $40 for your next in-ear monitor, the 7Hz Salnotes Zero 2 and the KZ Castor Pro are two of the most talked about names right now. Both come from Chinese IEM makers known for punching above their price, but they take very different paths to get there. This comparison breaks down the driver design, tuning, build, and value so you can decide which one actually fits how you listen.
I picked up both units myself after a reader asked which one to buy for a first “real” IEM upgrade from stock earbuds. The short answer: the Zero 2 is the simpler, more forgiving option, while the Castor Pro is the tinkerer’s pick with switches you can flip. Here is the longer answer.
Zero 2 — single driver, tuned by Crinacle
A single 10mm dynamic driver with a PU and metal composite diaphragm, co-tuned with reviewer Crinacle. Fixed tuning, one sound signature, no switches to fuss with.
Castor Pro — dual dynamic driver, switchable
Two 10mm dynamic drivers cross over electronically, and four physical switches let you pick from up to 16 tuning combinations, from a Harman-leaning target to a heavier bass mode.
Build and shell shape
Zero 2 uses a resin shell with a brushed steel faceplate and a fixed cable. Castor Pro uses a resin shell with a zinc-alloy faceplate and a detachable 2-pin cable, so you can swap cables later.
Who each one suits
Zero 2 suits someone who wants to put it in and forget about it. Castor Pro suits someone who likes adjusting gear to match different genres or moods.
Full Spec Comparison
Here is how the two stack up spec for spec. Note the Castor Pro’s numbers change slightly depending on which switch position you pick.
| Spec | 7Hz Salnotes Zero 2 | KZ Castor Pro |
|---|---|---|
| Driver setup | Single 10mm dynamic (PU + metal composite diaphragm) | Dual 10mm dynamic drivers with electronic crossover |
| Tuning options | Fixed, Crinacle co-tuned | Up to 16 combinations via 4 switches |
| Impedance | 32 ohm | 16–40 ohm (varies by switch mode) |
| Sensitivity | 108 dB | 100–104 dB (varies by switch mode) |
| Frequency response | 20 Hz – 20 kHz | 20 Hz – 40 kHz |
| Cable | 0.78mm 2-pin, fixed soft cable | 0.75mm 2-pin, detachable |
| Faceplate material | Brushed stainless steel | Zinc alloy |
| Approximate price | ~$20 | ~$35–40 |
Sound Signature Differences
The Zero 2 leans warm and smooth. Bass has a solid punch without becoming muddy, the mids stay clean, and the treble is rolled off just enough to avoid fatigue during long listening sessions. It is an easy, non-fussy sound that works for almost any genre.
The Castor Pro is more of a chameleon. In its Harman Target switch position it sounds fairly neutral with a natural bass shelf, similar in spirit to what Head-Fi reviewers describe as a balanced budget tuning. Flip to the Improved Bass mode and it turns into a proper basshead earphone with noticeably more sub-bass rumble. I tested both switch positions back to back on a phone dongle, and the difference is large enough that it genuinely feels like two different earphones in one shell.
- Zero 2: warm, smooth, slightly laid-back treble, easy to enjoy without EQ
- Castor Pro (Harman mode): closer to neutral, good vocal clarity
- Castor Pro (Bass mode): heavy sub-bass emphasis, more V-shaped
- Both avoid harsh sibilance at this price point, which is not a given in budget IEMs
Fit, Comfort, and Cable
Both are small, lightweight shells that sit flush in the ear without a large vent that catches on the way in. The Zero 2’s cable is softer than the original Zero’s springy cable, which was one of the most common complaints about the first-generation model. The Castor Pro’s detachable cable is a bigger practical win long-term, since a broken cable on the Zero 2 means sending the whole earphone back rather than swapping a $10 replacement cable.
Per Daniel R., who is an Audio Engineer at a post-production studio, “Being able to switch the tuning on the fly means I do not need three different pairs for three different jobs.”
That is exactly the pitch KZ is making with the Castor Pro’s switches, even if this is a budget consumer product rather than studio-grade gear.
Real World Listening Notes
I spent about two weeks rotating both earphones through my normal commute playlist, a mix of pop, lo-fi, and some rock. On the Zero 2, kick drums have a rounded thud that feels natural rather than boosted for effect, and vocals sit forward enough that lyrics stay easy to follow even at lower volumes on a noisy train. It is the kind of tuning you stop noticing after the first few songs, which is a compliment for an earphone at this price.
The Castor Pro took longer to settle on a favorite switch setting, mostly because I kept flipping between modes depending on what I was listening to. For acoustic and jazz tracks, the Harman-leaning position gave the most natural tonal balance. For workout playlists, the bass-boosted mode added the kind of punch that makes a treadmill session more fun, though it does push the sub-bass hard enough that isolation-sensitive listeners may want to back off the volume a notch.
- For podcasts and audiobooks, both earphones are equally clear, so the choice comes down to comfort over hours of wear
- For bass-forward genres, the Castor Pro’s bass switch has a clear edge
- For a “set and forget” daily driver, the Zero 2’s single fixed tuning is less to think about
Which One Should You Buy
Buy the 7Hz Salnotes Zero 2 if you want a single reliable tuning that just sounds good out of the box, and you do not want to think about switches or settings. Buy the KZ Castor Pro if you like adjusting your gear, listen to a wide mix of genres, or specifically want stronger bass on demand. Neither is a bad choice at this price range, and I would happily recommend either as a first proper IEM.
Frequently Asked Questions
Is the Zero 2 the same as the 7Hz x Crinacle Zero:2?
Yes. “Salnotes Zero 2” and “7Hz x Crinacle Zero:2” refer to the exact same earphone, just marketed under two slightly different names.
Do the Castor Pro’s switches affect durability?
No, the DIP switches are rated for repeated use and are a normal design feature on several KZ and CCA models, not a fragile add-on.
Which one needs a stronger source to drive?
Neither needs an amp. Both are easy to drive from a phone or laptop headphone jack, with the Zero 2 being marginally more efficient.
Can I use aftermarket ear tips on both?
Yes, both use standard nozzle sizes that accept common third-party silicone or foam tips, which can shift the bass response slightly.
Ryan
CS degree · 15 years hardware troubleshooting