Jim Dunlop M133 MXR Micro Amp Pedal, White

Product Information

Long a cherished secret weapon of tone-savvy pro guitarists, clean boost lets you always be at the right volume and sweeten your tone by hitting the front end of your tube amp just so. Put the MXR Micro Amp Plus on your pedalboard alongside killer overdrive, delay, and phase/flange pedals, and you've the sonic versatility for stage and studio domination. To create the Micro Amp Plus, MXR took its successful Micro Amp - a staple for many Sweetwater guitarists - and juiced it with specially-tuned 2-band EQ, true-bypass switching, and low-noise op-amps that keep your signal clean, even with boost cranked. The MXR Micro Amp Plus: consider it essential kit.

Highlighted in yellow are connections to the 2nd pin of the TL072 chip. On both, the schematic and the breadboard diagram, I can follow one lead of R4 connected to a lead of C5 to a blue wire going to pin 2 of the potentiometer to 2nd pin of the chip. The MXR Series 2000 sported six models. These original pedals in the series included the Dyna Comp, Distortion +, Phaser, Stereo Chorus, Stereo Flanger and Time Delay. In late 1983 MXR announced a digital sound effects pedal generator simply known as the Junior. The Junior was released in early 1984 and was added to the other six pedals in the Series 2000 line. The Junior boasted four sound effects: a laser, a clap, a snare and a high hat. The effects were samples stored on a replaceable 24-pin ROM chip that was produced by MXR. MXR continued to manufacture most of the Reference, Commande and 2000 Series pedals until they closed down in 1984. It's worth noting that the Micro Amp is mainly useful in the context of playing live, specifically with a tube amplifier. It's not likely to add much to your solo practice or writing sessions (although if you practice or write at a fairly loud volume, you may find that stomping on this guy when you arrive at a song's solo section fattens your sound a lot, which can be very inspiring) and when used in conjunction with a solid state or hybrid amp, you're unlikely to really be able to take advantage of the M-133's ability to add searing gain to an already-saturated amp sound. Front end overdrive channel: works as an amazing gain boost. It does not color the tone at all! This pedal is a perfect choice if you are tired of OD/Distortion pedals that always warp your tone…

Transparency is an overused (and generally inaccurate) way to describe most overdrives and boosts. Even the cleanest boost adds shading and a little dirt to your output as it drives your preamp. But if transparency is your aim, you’ll find that the Micro Amp+ gets you close to your goal. At times it seems almost polite about preserving an amplifier’s voice. The most basic boost pedal is just an amplifier stage, so the only variable there would be how much amplification (or gain) it delivers. Most of the time this will be listed as a decibel or dB value, and most dedicated boosts offer around 20dB. I didn’t have 22M resistor, so I used 10M for R1. This won’t make a difference in sound, it will lower the input impedance just a bit, but nothing significant. Trying it out

In this final build, when it comes to simple booster effects, I am going to use Op-Amps. An op-amp is an integrated circuit (a chip) containing several transistors inside of it to achieve a very large gain. But it does so in a way that makes amplifier design very consistent and predictable. Now, truth be told, I like to prepare everything in advance so it looks good on the videos and photos. I do clip the leads to a more appropriate size and I cut wires to size from a spool I have etc. But when it comes to rapid prototyping, I sometimes just go directly from the schematic and just plug in components and wires as I go along without much consideration for aesthetics. Bill of Material For example, I place all 7 resistors next to the board first. I mark them all R1 to R7 exactly as on the schematic and give them correct values. I then take them one by one and place them on the board until I have placed all of them. In the end here’s the result: Thunder on a breadboard The word ‘transparent’ is used to refer to boosts (and overdrives) that do not alter the general frequency content of your sound. Put equal parts treble, bass and midrange into a transparent boost, and expect similar ratios between frequencies out the other end. While a lot of clean boosts are also transparent, some do occasionally offer EQ controls to boost or cut certain frequencies. And some boosts are neither clean nor transparent… I've got a Mooer pure boost in the middle of my board which does give a very clean boost but it has a slightly clinical sound when on which the micro amps don't.I called my design … wait for it … Thunder. OK, slightly uninspiring name, but still, it should be a great circuit (oh wait, I already revealed it in the heading … doh). Firstly, a ‘clean’ boost is a boost pedal that doesn’t introduce any sort of limiting to the signal; it responds linearly to its input. With a clean boost, you double the input volume, and the output volume is also doubled. With a limiting boost, that’s not necessarily the case. That’s it as far as MXR Micro Amp is concerned. On one hand, the circuit looks intimidating compared to the ones using transistors I did before, it definitely has way more components used then the other circuits. On the other hand, with op-amps I can get more consistent results, and it is easier to predict how they are going to behave, even using different op-amps should not be a big deal. I used pen and paper before discovering the tool, and that is such a tedious job. With the tool it is really easy to move things around to your liking. Very subtle volume push, until you reach closely full pot, but instead adds gain frequencies and overdriving your signal. It is a nice transparent push when you like more of your Class A amp without messing on volumes, or having an overdriven setup and cleaning the signal with the guitar pot.

I forgot to give some more details on parameters for the pedals. Normally I do it after the analysis, so here it is before I dive into breadboarding stuff: Parameter The breadboard layouts look nearly identical. If you compare the schematic with the breadboard layout, component references are all the same, R1 on the breadboard corresponds to R1 on the schematic. MXR was bought by Jim Dunlop in 1987 and since then MXR has seen faithful reissues of classic designs as well as continuing to innovate with new designs that uphold the legendary MXR name. They are all built with reliable quality that can be expected from Dunlop products. Today you can find classic and new pedals alongside reliable power supplies, and professional grade instrument cables. Why Should I Choose MXR? It is similar to the MXR pedal even though it might not look like it. I’ll go through the parts and just reflect on differences and similarities between the circuits as I go. Non-Inverting AmplifierFor this, I just go straight from the schematic, I place the breadboard and the most important components first (in this case the op-amp chip). After that, I place all the other components next to the board. You left out the studio. If, like me, you often like to play solos on a vintage spec strat or tele into a an old-school tube amp with moderate amounts of preamp gain, a clean boost like this is an invaluable tool while recording. I used to be a dyed in the wool TS9 man (and later an 80s SD1 guy), often using them as a colored, dirty boost for solos with fender guitars (like everyone and their mother likes to), but in the 90s I got turned on to clean boosts, treble boosts and even the venerable MXR Dynacomp as ways to add gain and 'cut' (these pedals generally turn your hi-z guitar signal into a fairly low-z affair that drives the cable a lot easier which will add/preserve treble just like a buffer) for a solo sound without sacrificing the bandwidth and signature voice of my guitar. I do this more in the studio than live, believe it or not. Live I tend to dime an AC30 and control everything with my guitar volume knob. Maybe I'll run an always-on boost or buffer if I need a line-driver on a big stage.

All values I got through simulation and all are approximate. Both pedals look really good in terms of general characteristics. Breadboard Diagram Front end clean channel: incredible amount of volume boost while gradually pushing the preamp tube into clipping. It's fun to find that sweet spot where you have a little bite/grit. Great for RHCP-like cleans. Even at lower gain settings, it adds a sort of fullness to the sound. Reference voltage is the same as for Micro Amp, R8 and R9, form a voltage divider and C6 is there to filter out any power supply ripple. R8 and R9 have the same value so reference voltage is half of the power supply voltage: 4.5V. Coupling Capacitors Hopefully the two examples above help with following how the components are connected and how to even do it from scratch. Breadboarding

I like to use DIY Layout Creator to prepare for breadboarding. This step is not strictly necessary, but I prefer to have it done this way. Makes my life easier when it comes to planning, and it looks way more professional for my posts. Always-on boosts can also function as buffers. Without getting too far into the weeds of inductance, impedance, capacitance and so on, buffers are essential to retain high end in a rig with lots of patch cables, true-bypass pedals and long leads between your guitar, ‘board and amp. In short: an uninterrupted length of cable (including true-bypass pedals) works as a very weak capacitor, the same component used in your guitar’s tone control to filter out treble frequencies. And so, you might notice that when you’re running through your ‘board with all the pedals bypassed, your sound is a little duller than when you plug straight into your amp.