top of page

Cornerstone Zefiro Compressor Review

Cornerstone Music Gear is a family owned company producing handmade guitar and bass effect pedals in Italy. The Zefiro is an optical compressor and is designed for people "who don't like compressors." I am sometimes suspect of phrases that seem like they might be a gimmick. But I'll tell you what, I can see people really like this one who have shrugged off compressors before.

If you are looking for tone shaping and sustain you should definitely consider the Cornerstone Zefiro optical compressor.

There are just four controls in total. Well, five if you count the foot switch. The Zefiro is incredibly easy to dial in and is full of great sounds.

The Comp control determines the amount of compression with a range of very, very subtle to a fair amount of squashing. But there is no dip-and-swell effect here. I do not know what the comp range is but even at high compression levels the compressor remains quite usable and can still breathe some. I'm guessing something like 1:1 to a max of 10:1. It is not a device I would recommend to serve you as a limiter.

The Sensitivity control acts a lot like a threshold control. However, it really effects the overall dynamic response of the device. Across the entire range of the Sensitivity dial you are getting the same actual level of compression, but the feel and response changes quite a bit. All the way clockwise is less of an effect and more open with more transient attack. More counterclockwise changes the feel to be less dynamic and more even.

I found a real nice sweet spot with the Sensitivity dial between noon and 1:00 for what felt like obvious compression but still very dynamic. Nice!

The Volume control acts as a master output level control and there is plenty of gain on tap. Not boost pedal gain, but more than enough to make up for even heavy compression.

The Tone knob is a tilt-eq type, just like the Diamond BCP-1 or Diamond Bass Comp JR, and Becos compressors, and JHS Pulp 'N Peel. It changes both the bass and treble frequency response at the same time: the more treble the less bass and vice versa. The pivot frequency is set at 1kHz. If I had my choice, I'd prefer the tilt pivot frequency set a bit lower — maybe 900hz like the Diamond — but this is a nitpick. In practice, the Tone knob is very useful and delivers a ton of tonal variety.

Then you have a true bypass foot switch and white LED that illuminates when the pedal is activated.

That's it. In the tone department though there is a lot to talk about. Why?

In many ways, I think this is the closest comparable to the Diamond I've encountered to date.

There is a lot of love for the Diamond BCP-1/Bass Comp JR and rightfully so. It has been a staple for a long time, has proven itself to be reliable, and is a real tone monster.

I spent a fair amount of time comparing the Zefiro and Diamond side-by-side with different basses and I was able to dial in the Zefiro to sound very similar to the Diamond, at least to my ear. The Tone knob works a lot like the Diamond EQ knob. The feel is very similar. Turning the tone knob more counterclockwise produce a bigger, beefier disposition. Some real fattening of your signal. Turning more clockwise produces a brighter tone. Right at noon is a fine balance of full and punchy with strong presence.

The the sensitivity dial is what gives the Cornerstone Zefiro the edge. But unlike the Diamond, the Zefiro provides far more control in how the compressor responds. You can dial in more compression than with the Diamond. In this sense the Zefiro makes for a pretty good compressor in addition to delivering "tone mojo."

In my review of the Diamond Bass Comp JR I said,

"the Diamond definitely adds a fattening to your tone, but that comes at a price of altering your original tone."

That statement is also true for the Zefiro. It isn't a transparent compressor. If that is what you want, look elsewhere. But if you are looking for a lot of the Diamond "tone magic" but want more control, check out the Cornerstone Zefiro.

What separates the Zefiro from other compressors with a tilt EQ function is two-fold:

1. Because of the combination of Comp and Sensitivity dials you can make the Zefiro deliver smooth compression. 2. It's got this inherent tone mojo going one. It's one of those devices that you want to always leave on in your chain.

There is nothing clinical about it. It sounds especially nice in front of my tube preamp for some reason. The Zefiro is also very quiet.

However, I did run into headroom issues with a couple of my basses with onboard preamps. At higher compression settings and lower sensitivity I could clip the pedal producing unpleasant distortion. In talking with Emilio at Cornerstone he suggested I try running the pedal at 12 volts, 15 volts, or even 18 volts. Bingo, higher voltages solves the problem. Emilio says it is entirely safe to run the pedal up to 18 volts (or even a bit more) with no concern. At 18 volts the Zefiro is a tank handling anything I could throw at it.

In talking with Emilio we discussed the possibility of releasing a bass specific version of the Zefiro with some modifications. Possibilities could include a different tilt point and other changes for headroom for 5 string basses. Sounds like he is interested at some point in the future. In the meantime, the Zefiro is an excellent compressor for adding warmth and balance to guitar or bass.

The enclosure is an attractive sky blue color. All knobs turn with authority and the pedal appears to be well made.

Input and output jacks are side mounted. Power input is mounted on top.

There is no LED indicating compression/gain reduction. Points to the Diamond on that one.

It will run on 9 volts to 18 volts with a standard Boss style plug.

There are other compressors that offer far more control over parameters like attack, release, ratio, and knee (not to mention things like side chain functionality, drive/saturation, input gain, etc.). I don't think that's the target customer for the Zefiro. For an always on, tone enhancing device the Zefiro is one to consider.


  • Tone fattening/mojo

  • Tilt EQ

  • Sensitivity control and how it changes the feel of the compressor

  • Price

  • Pedalboard friendly


  • No gain reduction LED

  • Not the most versatile on the market

  • More of a tone enhancer than compressor (though you will never want to turn it off)

  • No control of attack and release

Retail price: $204 USD (at time of writing)


bottom of page