M238 ISO-Brick review by MXR

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  • Ease of Use: 8
  • Sound: 9
  • Reliability & Durability: 6
  • Overall Impression: 8
  • Reviewer's score: 7.8 Good
  • Users' score: 0 (0 votes)
MXR: M238 ISO-Brick

Price paid: £ 140

Purchased from: Online

Ease of Use — 8
Use it just like you would any other PSU. As long as you set the aforementioned variable voltage pots to the appropriate voltage and don't plug an 18v output into a 9v pedal, it's pretty idiot proof.

Sound — 9
Isolated power supplies are very utilitarian devices that do a pretty boring job. But depending on the sorts of pedals you're running, the number of pedals, the path the signal is taking, the amp you're using and how it is set up, down to how clean the power is coming from your wall socket, investing in one may become painfully necessary.

In my case, the daisy chain supply I was using was creating ground loops in my amp's effects loop, rendering the loop completely unusable for anything but just 1 pedal at any one time. Obviously given the setup I have, that isn't a viable option.

The pedals that I'm using in my rig (as of the time of this review) are the following:


  • Boss TU-3
  • Morley Bad Horsie Wah
  • Green Rhino MkIV
  • Keeley Phase 24
Effects loop:
  • MXR 10 Band EQ
  • Subdecay Starlight MKII
  • MXR Analog Chorus
  • MXR Carbon Copy
  • TC Hall Of Fame
  • TC Ditto
The amp in question is an EVH 5153.

Power supplies that are fully isolated and offer enough outputs to power a decently-size board all by themselves tend to be very expensive. But the MXR ISO-Brick is a relatively inexpensive solution that brings a lot of promise.

The ISO-Brick offers:
  • 6x 9v outputs: 2x 100mA, 2x 300mA and 2x 450mA.
  • 2x 18v outputs. Both rated for 250mA.
  • 2x variable 6-15v outputs, fully controllable through a trim pot on both outputs.
The purpose of the variable voltage outputs is to allow you to adjust the voltage level to either give a pedal additional headroom that powering it with 9v alone (or 12v if it's a 12v pedal) doesn't provide. Or to turn the voltage down to fully replicate the tonal effect that running a half-dead battery is known to have on certain overdrive, distortion and fuzz pedals. If neither option is desired, the voltage adjustment knobs can be set to 9v and it should run just like any of the other 9v outputs. More info on that later.

This pedal is almost unmatched in it's versatility at any price. And there's enough outputs that it is able to fully support a decently-sized pedalboard all by itself. The inclusion of 2x 18v and 2x 450mA outputs is something only commonly found on larger and much more expensive power supplies than this one. And variable voltage outputs are be extraordinarily useful for users who want the tone that a dying battery gives to their OD's and fuzzes in a way that is infinitely more reliable than actual batteries. Likewise with the additional headroom that. Let alone one in the price bracket of the ISO-Brick.

Noise reduction is another crucial function that the ISO-Brick must perform well in to be worth considering. On the surface, things do not look great. It uses an 18v wall wart and the pedal lacks the toroidal transformer found in other similarly priced power supplies, such as the venerable Voodoo Labs Pedal Power 2. This makes the ISO-Brick appear more akin on to the older (and pretty terrible) MXR DC Brick, which did not have any isolated outputs and was more akin to a glorified, overpriced daisy chain than a real PSU.

But as it turns out, this pedal truly is isolated like it claims to be, but it does this without using the toroidal transformer with multiple taps found in other pedal PSU's. Without getting overly technical, the ISO-Brick instead uses DC/DC conversion similar to an SMPS power supply, and the high efficiency that's inherent in DC/DC conversion allows for the use of small, individual transformers to provide isolated power for every output. This method of achieving full isolation also explains how the ISO-Brick can have so many different options in terms of voltage and current.

So having actually used the ISO-Brick in my rig, I am happy to report that this power supply does a great job at reducing noise to an enormous degree. It's very quiet regardless of whatever pedals I'm running at any one time, and the ground loop problems I was having before have all been completely eliminated. It simply does exactly as advertised.

Reliability & Durability — 6
As great as the pedal is at offering lots power options very quietly, an unbiased review would state that MXR customer experiences with the build quality of earlier ISO-Bricks raises a few eyebrows. Along with a couple of fair criticisms of the refinement of some of the ISO-Brick's features.

Some users are concerned that the trimpots which control the voltage of the variable outputs do not have a 'notched' setting on the 9v mark. Thus making the trimpot more prone to being accidentally bumped, potentially leading to the PSU suddenly giving more power to a pedal than what it was designed for and possibly damaging it. I 100% completely agree with this concern. Putting the power supply under one's pedalboard makes accidentally bumping the trimpots a lot less likely to occur, but for those who do not have that option, the ability to either 'lock' the variable voltage pot in a certain position that gives 9v, or offer a switch that bypasses the variable voltage pots altogether would've been a lot more considerate of MXR. While I would take some rather unfortunate circumstances to happen, relying on the user to not accidentally bump the trimpots is not a sufficient protection measure against potentially damaging pedals. MXR take note please.

Another complaint that's found on early examples of the ISO-Brick is that the graphics which represent the different voltages on the variable voltage trimpots are not precise enough. Sometimes on examples made shortly after the product's release, if you adjust the pots to 9v, measuring the voltage with a multimeter shows that the pedal is pushing more like 10v. However, my particular ISO-Brick is not an example from the early run and a multimeter shows that with the trimpots on exactly the 9v mark, the meter shows a voltage of 9.2v, which is perfectly equivalent to a fresh, strong 9v battery. If you are to buy an ISO-Brick, I would strongly recommend buying one new to help ensure that the one you're getting isn't an earlier one that could have inaccurate graphics indicating the voltage level on the variable voltage outputs. If one is in any doubt, set the voltage with a multimeter so you know exactly what power the PSU is feeding to your pedals.

Another complaint from some people is that the power cord isn't quite as beefy as they might've liked. I would somewhat agree with this complaint, and if you're really going to be absolutely ragging on the power cord with lots hard gigging, I would suggest pulling a full-length flexible plastic sleeve over the cable to give it additional strength and strain relief. Otherwise, it works fine and isn't noisy whatsoever.

Aside from those complaints, this pedal is built pretty tough. It comes in a tough brushed aluminium case that covers the PSU on all sides, and it gives plenty of strain relief on whatever cord you plug into it. The PSU also features a red and a white LED on the top of the PSU to indicate any fault with the power being fed to it. And individual blue LED's on each pedal output will go out to indicate if there's any fault with any of the PSU's outputs. Given that the pedal is fully isolated in nature, if any problem is to occur with any of its outputs, the performance of the other outputs will remain unmolested.

Overall, while accidentally moving the trimpots is a potential cause for concern, hot gluing the trimpot in place after you've set it works well. But I would prefer if some paranoid users did not feel the need to do that in the first place.

Overall Impression — 8
This power supply offers so much in terms of a wealth powering options, clever methods of noise reduction and sheer bang for the buck that even when considering that the trimpot functionality could be a bit more refined, this is still an excellent buy. It does everything as advertised. And that's all a PSU can really be asked of it.

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