Jim Dunlop M133 MXR Micro Amp Pedal, White

Product Information

In this post I’ll get to know the circuits. In the follow-up posts I’ll do some SPICE analysis, do some breadboarding and then prepare the enclosure and complete the pedal on a protoboard. This one got too long so I thought to split this into several and publish them as they get ready. Getting to Know the Effects MXR Micro Amp

Setting your amp’s volume in the lower third of its range and dialing all three Micro Amp+ controls to noon (at these settings the treble and bass controls are flat) is like pushing up a fader on a mixing desk: Your guitar/amp sound is unmistakable, but with more body, more dimension, and that trace of edge and compression you get when you push the level just into the red. This is the sound many folks seek in a clean boost. If you keep your amplifier’s volume low, it’s remarkable how little color and grit are added. These levels are ideal for enlivening thin output from your E and B string (particularly from single-coils) without overdriving the third and fourth strings. 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.Looking at the highlighted part on the breadboard (going from right to left) R9 is connected to R8, connected to positive lead of C6. Then, there’s a jumper wire going to R3. The connection goes underneath R2 and it’s not connected to it. C4 has the same purpose as C3 in micro amp, to only allow AC signal to be boosted. The values were chosen so they do not affect the frequency response (10Hz cut-off). R3 has the same purpose as R2 in micro amp, to bring reference voltage to non-inverting input of the op-amp.

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. R9 is there to help with stability and help preventing oscillation driving capacitive load (guitar cable may produce about 100pF capacitance per meter). Final thoughts 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 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.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. The amplifier encircled in red is a non-inverting amplifier. This is a variable gain amplifier, and the gain is controlled by a reverse-log (reverse-audio) pot – R5. The reverse-audio taper is slightly un-common and typically used in variable gain circuits as here. See how this taper looks like in my post on pots.