Mini MicroAmp Build

With each new ped­al build, I try to focus on some aspect that makes it a new chal­lenge or some­thing new to learn. My first ped­al build ever (about 18 months ago) was a boost ped­al. I decid­ed I’d build anoth­er boost: this one using the MXR MicroAmp cir­cuit. I used the Gen­er­al Gui­tar Gad­gets MAMP PCB, which in addi­tion to sell­ing the PCB sells entire kits and has excel­lent doc­u­men­ta­tion1. Since it’s a rel­a­tive­ly sim­ple cir­cuit and, there­fore a fair­ly small PCB, I want­ed to try to fit it into a “mini” enclo­sure (i.e., a 1590A for­mat). This means hav­ing to real­ly think ahead about aspects of the build so that every­thing can squeeze into such a rel­a­tive­ly small enclo­sure.

The com­plet­ed enclo­sure, includ­ing the mis-aligned hole for the input jack on the right side

The first thing is that this ped­al for­mat can’t uti­lize a bat­tery for pow­er; the ped­al will be AC pow­ered only. That’s fine as I don’t use bat­ter­ies in any ped­al any­way and only ever added a bat­tery snap to that first ped­al build. Sec­ond­ly, the height of the com­po­nents real­ly mat­ters. The taller com­po­nents (gen­er­al­ly, the capac­i­tors) had to be bent over. For the elec­trolyt­ic capac­i­tors, I had to remove and replace a cou­ple in order to facil­i­tate this (I had planned ahead oth­er­wise — as my sketched notes on the wiring dia­gram shows below, but I am just so in the habit of sol­der­ing the com­plete­ly ver­ti­cal I for­got!). In the end, the tallest com­po­nent off the PCB was the inte­grat­ed cir­cuit (IC), as it was mount­ed in a sock­et. This way I can poten­tial­ly swap out ICs in the future. Speak­ing of ICs, I went with a low-noise TL071 op-amp (in place of the orig­i­nal ped­al’s TLo61 — which con­sumes less cur­rent but, again, I’m not using a bat­tery so I don’t real­ly care about that). The only oth­er mod­i­fi­ca­tion I made to the GGG cir­cuit was that I swapped out a 10MΩ in place of the 22MΩ pull-down resis­tor (R1). Real­ly, any fair­ly large (<1MΩ) resis­tor val­ue will do here and 22MΩ are a lit­tle hard­er to find.

The com­plet­ed wiring. This was a tight fit! Notice all the taller capac­i­tors look like a strong wind came through.

Last­ly, the arrange­ment of the larg­er off-board com­po­nents such as the footswitch, jacks, LED bezel, and pot real­ly came down to mil­lime­ters. I had to use calipers to mea­sure every last item and metic­u­lous sketch it out on a print­out of the enclo­sure. I still man­aged to mess up drilling one of the jack holes (I locat­ed it 1/2 the diam­e­ter off, which s about the worst place to mess it up!). I was able to re-drill the hole thanks to hav­ing a drill press and some clamp­ing blocks. It’s a bit ugly and the jack­’s nut is a bit crooked, but it worked out fine.

Re-drilling a hole for the out­put jack. Drill press & clamps absolute­ly required to fix this sort of bone-head­ed mis­take.

The ped­al works great. I mean, it’s about as sim­ple an effect as you can get. It sim­ply takes the gui­tar sig­nal and makes it a lot loud­er (prob­a­bly around the order of 20–25db). I’m pret­ty pleased with how clean the wiring worked out, as well.

My build cost around $27 for the parts I had to pur­chase. That’s not includ­ing resis­tors, capac­i­tors, diode, and LED (nor hookup wire and sol­der), all of which I already had in my parts bins but would run you around $3 in total. I also had to pay around $9 in ship­ping. The PCB from GGG for was about $3.50 to ship. I bought parts for sev­er­al builds at once in a large order from Mam­moth Elec­tron­ics (my parts sup­pli­er of choice), but small­er orders from there tend to ship for around $5. They have high-qual­i­ty pow­der-coat­ed enclo­sures for real­ly great prices, along with gen­er­al­ly good prices on oth­er parts and kits. So, in total, this build is roughy around $39 in cost (and I still haven’t added any art­work, so con­sid­er what slide decal or oth­er for­mat might cost).

That being said, unless you real­ly want to build your own, I would not rec­om­mend this build to any­one else. You can pur­chase a TC Elec­tron­ic Spark for about $35 used on Reverb.com (plus ship­ping) right now. It has the exact same size as my build, but has their amaz­ing non-latch­ing (relay) footswitch and essen­tial­ly the same amount of clean boost. If you don’t care about size, you can pur­chase a used MXR MicroAmp for around $49 on Reverb (plus ship­ping). Both of those are sol­id choic­es if you real­ly just want a boost ped­al and are less inter­est­ed in prac­tic­ing your sol­der­ing skills or learn­ing how to lay­out a small ped­al form fac­tor. And hon­est­ly, as much as I think this ped­al sounds great so far, those prob­a­bly sound even bet­ter and have less noise at full gain.

But over­all, I’m pleased with this build. On the clean chan­nel, it just gets loud­er with­out adding any­thing else notice­able. Best of all: with the knob set to about 3 o’clock, it makes my Black­star HT-5R head­’s gain chan­nel absolute­ly breathe fire!

  1. I think I could have pret­ty eas­i­ly build this cir­cuit on perf­board, but prob­a­bly not to fit in the this small of an enclo­sure. So for a bit more cost I opt­ed for the PCB, which has a fair­ly small foot­print. []

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