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Mozztronics CO-2 Opto-Comp Review

Andrew, the mastermind behind Mozztronics, produces a small number of hand-made pedals in Melbourne, Australia. He has no interest in producing clones of other pedals and seeks to create something a little different. You will not find any 9-volt batteries in any of his designs because Andrew is dedicated to keeping batteries out of the landfill.

In designing the CO-2 Opto-Comp, Andrew set out to capture the essence of old vacuum tube compressors but developing a solid state circuit that captured how these old compressors sounded.

Other goals for the design included:

1. Must be an optical element for compression

2. The compressor must be useful for guitar and bass

3. It must be different from what is already on the market

At this point you are probably wondering whether all of this is just a lot of talk and hype or whether any or all of it was accomplished. Read on.

In looking at the Mozztronics CO-2 Opto-Comp it is obvious that there is a full array of controls. Sometimes more control makes a compressor more confusing to use or seem less intuitive. I can see that, but before writing this one off, let me assure you that this one compressor that offers complete control and yet manages to be easy to dial in.

Let's take a look at the controls.

Starting from left to right, the Gain dial is there to deliver makeup volume lost while increasing the amount of compression. It works exactly as you'd think. Rotating more clockwise increases the amount of makeup gain. There is plenty of gain on tap.

The control labeled DYN stands for DYNamics and is essentially a parallel control for blending a little or a lot of your dry signal. I personally really like compressors that offer this kind of control because it can allow you to dial in a higher amount of overall compression, but bringing back some dry signal to make the overall sound less "effecty" with more natural punch and attack.

Andrew says,

"With the DYN knob fully clockwise you get about 70% dry added to the compressed signal. Compressed signal is always there, we just add some dry to it, to add back a little of the dynamics. You shouldn't need to go over 50% DYN but I allowed for more if people like it like that."

I found the sweet spot for me to be around noon to 1:00 and set like that I was able to bring up the Ratio of compression higher than I might otherwise typically tend to like with compressors lacking a blend control.

Andrew goes on to say,

"Once you get away from bass needs 4:1 and -20dB threshold, and use your ears, you wont go back to the cookie cutter approach..."

That leads us to the RATIO dial which is 1:1 with the dial fully counterclockwise to approximately 20:1 fully clockwise. When rotated past 3:00 things get pretty squishy. It would be usable as a limiter.

THReshold adjusts the point at which the compressor starts to operate. Threshold is lowered the more you rotate the dial clockwise. Mozztronics documentation states that there is also an "interactive release" feature in the circuitry that works in parallel with the Threshold dial that serves to vary the release based on input signal. I'm not sure I could notice any of this in practice, but whatever is going on sounds real good.

ATTack adjusts the speed at which the compression starts to operate. Rotating more clockwise slows the attack, letting in more initial dynamics. I really like how the attack feels with the CO-2. You can dial in anything from killing the initial transient to keeping the punch and sparkle many of us desire with what we call "subtle" compression. It works really well on this one. Minimum attack time is fully counterclockwise

RELease controls the speed at which the compression stops operating (minimum release time is anticlockwise). There is a pretty big range here and I think it will appease most anybody.

Finally, you have the RANGE switch. Flipped up, the circuit is essentially working as a form of High Pass Filter limiting the lower frequencies from triggering the compression circuit. It works. It works really well. You will still experience compression in the lower range of your instrument, but that squashy feel you often notice in the E or B string on bass guitar is substantially reduced. It is somewhat like the HPF in the Cali76 CB but instead of a dial, it is a simple switch. I don't know what the frequency point is that is filtered but I definitely preferred the Range control activated (flipping the switch up) which kept my B string tight and full sounding.

There is a red LED that illuminates just above the footswitch when the pedal is activated. Another red LED is located in the top right of the enclosure and serves to indicate when the signal is over threshold. That's a real nice feature. It illuminates more intensely based on the degree to which a signal is over threshold.

In reality, how does it all come together and perform?


  • Easy to dial in (despite the many controls)

  • Big punchy sound

  • Highly versatile

  • Relatively small form factor

  • Threshold LED

  • Range switch is very nice and helps keeps a 5 string bass feeling big and full, yet tight and controlled


  • Not a whole lot really

  • Some would probably prefer top mounted jacks

  • Threshold LED could be brighter

I really like this compressor. It just works so well and its one of those devices that is hard to make sound bad.

It delivers the bigger, fuller, tighter form of compression with ease.

On the other hand, it smooths out your tone, evens dynamics, and manages to do all of that while sounding natural. That said, it isn't 100% transparent. It's almost a blend of the "tone magic" compressors we talk about and one focusing more on clean versatility.

Compared to the something like the Diamond or Mad Professor Forest Green, there is no EQ feature so the Diamond and Forest Green can sound more big and full but are also more colored. Compared to the Empress or Keeley the Mozztronics CO-2 has an inherent tone sparkly that is addicting. Less sterile would be another way to put it.

It is pretty quiet overall. Not in the far low end of the spectrum but not at all bad enough to even call it a problem. Really, not an issue. There's just a touch of noise added when engaged. As the gain is increased, there is more audible noise but that is to be expected. It should also be noted that it doesn't react with as much inherent noise when noisy pedals are placed in front of it.

If I had an official Top 10 list of compressors, I'd probably include the Mozztronics CO-2 Opto-Comp in the list.

It checks many boxes, sounds great, has a pretty good form factor, is easy to dial in and I consider it to be quite versatile. The price isn't bad either, even for those in the USA when factoring in shipping.

I've used it live several times and never found myself fighting it. It just works well and left me smiling every time.

The enclosure is the size of the original Diamond BCP-1 and is a glossy orange paint that appears to be durable. The graphics and lettering on top is actually a decal. Mine had a few air bubbles under the decal here and there. The knobs all turn with authority, though they are relatively small and placed pretty tight together.

Footswitch is true bypass and doesn't suffer from "popping" when activating the pedal. The pedal runs on 9 volt to 12 volt center negative barrel and worked just fine with a Onespot and my CIOKS DC7. I noticed not audible difference operating the pedal at 12 volts. There is plenty of headroom at 9 volts.

I like how the circuit board actually says "Turn it up loud".

The Mozztronics CO-2 ships in a plain white box.

Retail price: Approximately $170.24 USD at time of writing

The CO-2 Opto-Comp matched up with the Mozztronics BF-1 Bass Fuzz and BP-2 Bass Preamp is a killer package. But that’s another review for another day.

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