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Building Maze Runner – a RAT clone with mods

Another pedal that has been out there since ’70s (well, late ’70s) is RAT. A legendary pedal used by likes of Jeff Beck, Dave Gilmour, Joe Perry, James Hetfield and Cobain just to name a few (Stylistically, a surprisingly diverse bunch).

While similar in design to MXR Distortion+, a bit more complex, more versatile and a very rewarding DIY effort.

I decided to do the full build instead of splitting it into multiple posts. Things covered are the basic schematic for the circuit, with more cover of some interesting details, there’s even a SPICE simulation if you want to do some analysis yourself. I did a breadboard ad-hoc without a diagram (I just felt like it), but I still included a list of things to try-out.

Customarily, I’m including bill of material, then do the soldering layout for a protoboard. I’ll plan out the enclosure, and finalize the enclosure. I’ll finally assemble it and do test and demo.

If you’re interested in only the final result just jump right down to test and demo section.

This is a only a slightly more complex pedal than my previous build (MXR Distortion clone), but also very easy to modify. Let’s get to it then.

Schematic

I got the basic schematic from Electrosmash, here’s my version:

Maze Runner schematic - a Pro Co Rat clone.
Maze Runner schematic (click for the full size image)

Differences from the original schematic

I refer to Electrosmash’s schematic as original schematic – which is based on the first versions of RAT (Juggernaut). There are several differences in my version

I’m using OP7 op-amp instead of obsolete LM308. OP07DP is currently used in commercial RATs. The filter pot is wired opposite way in my version – in line how the current RAT versions are wired-up (and is named filter instead of tone). Different JFET in output stage is used – I used BF256B which is more readily available than the obsolete 2N5458, but since this is just a buffer it should not make an observable difference.

Interesting Bits

For in-depth analysis go to Electrosmash, I can’t do anything better 😊. I’ll just point out some interesting things about the circuit.

I’ll briefly cover three main sections – amplifier and clipping circuit, filter and output section, and mostly in context of difference to the MXR Distortion +.

Gain and Clipping Stage

Looking at amplifier and clipping section – if we, for a second, ignore multiple clipping options – the diagram looks pretty close to my previous build (MXR Distortion+ clone):

Schematic of input gain and clipping stage of the Maze Runner pedal
Gain and clipping stages (Click for larger image)

This is a typical non-inverting op-amp circuit – pretty similar to the MXR pedal. The main difference really are two resistor/capacitor pairs: R4/C5 & R5/C6. They shape the frequency response of the gain stage and that gets clipped by the clipping stage.

Minimum gain is 1 when distortion pot is completely rolled-off. Maximum gain of the amplifier would be when the distortion pot is at maximum and gives over 67dB of gain (100K pot and R4 and R5 in parallel determine this). Compare this to 46dB gain of MXR Distortion+ pedal.

Of course, it will be hard to reach that gain since we’ll hit various op-amp limitations before that happens. Of course we’ll never be able to pass 9Vpp of our power supply, but more realistically we’ll hit op-amp limit of 6 to 7Vpp due to op-amp not being rail-to-rail. This really means that signal as high as 3mVpp from guitar will reach this limitation.

Frequency response

Frequency response of this stage is shaped by two resistor-capacitor pairs. R5 and C6 on the op-amp’s inverting input form a high pass filter with cut-off frequency of about 60Hz, but in parallel, R4 and C5 form another high pass filter with cut-off frequency of about 1.5KHz.

Max gain thus, over 1.5KHz is about 67dB. If we look at isolation R5/C6 filter – max gain for it (over 60Hz) is 1+100K/560 which is still respectable 45dB. What this really means is that R5/C6 helps with boosting somewhat low frequencies which would have not been boosted as much with just R4/C5. With just R4/C5 alone, 45dB gain would have been realized around 130Hz.

Anyway, here’s the frequency response plot of the amplifier part.

Frequency response plot of RAT amplifier section
Frequency response of RAT amplifier section (click for full image)

You’re probably noticing that gain reaches maximum just after 600Hz, levels off and after 800Hz or so starts dropping (despite me saying 1.5KHz cut-off and never reaches 67dB). This is because of the closed loop bandwidth of the op-amp which is typically 600KHz. Based on the data sheet, open (and closed) loop gain drops under 60dB just after 1Khz.

Btw, LM308’s bandwidth depends on C3 and since it seems to be slightly greater, the frequency response of the LM308 version is very, very slightly different. Also, C3 in OP07 version is most likely useless since serves different function, probably left there for compatibility with LM308.

Slew rate between LM308 (with 30pF compensation cap) and OP07 is similar to around 0.3V/us. But output voltage swing of OP07 is typically slightly lower than LM308. If we assume output swing of 6V, slew rate will start adding to the signal distortion over 6KHz (we’re talking about overtones that are way lower level at this frequency as well). Slew rate might not have such a huge impact (if any) on the sound after all.

Clipping

I added a slide switch (SP3T) to choose between 3 different clipping options. This gives a bit of added versatility – again very similar to my previous build. There are 3 options to choose from – symmetrical silicon diode clipping (like RAT 2), symmetrical germanium diode clipping (like You Dirty RAT) and asymmetrical silicon diode clipping.

There’s a slight difference from the MXR pedal – there’s no capacitor acting as a filter – but there’s a whole adjustable filter after it. So let’s get into that part.

Filter and Output Stages

Here’s the next couple of stages:

Filter and output buffer highlighted on the Maze Runner schematic
Maze Runner – Filter and Output buffer stages (click for full image)

Filter is a simple adjustable low-pass filter formed by the pot, R7 and C8. The cut-off frequency goes from 32KHz to about 475Hz – so turning the knob clockwise rolls off some high frequencies.

The pot is wired-up in reverse from the Juggernaut version and it’s how the current versions of RAT are wired up. This filter really filters out some of the harshness of the higher order harmonics introduced by clipping stage.

I find the sound too harsh for my liking when the pot is fully anti-clockwise, also, probably first 30-40% of rotation doesn’t do much for me (remember the starting cut-off frequency is 32KHz – way over audible frequencies, and log pot slowly increases resistance).

Output buffer is just that – source follower with about unity gain. JFET I used here should be just fine for the role.

In the end, here’s the full frequency response with distortion on maximum and sweeping the filter pot:

Diagram of frequency response of the filter
Filter output effect on amp stage output

SPICE Files

Here are the SPICE files if you want to try out these analysis for yourself. I have 3 versions – RAT with LM308, RAT 2 with OP07, and also – an alternative with TL071. TL071 allows for a bit more highs it looks like, maybe something to try out.

NOTE: TL072 is used as part number in the file, makes no difference for the simulation – op-amps in TL071 and TL072 have the same specs, but the pinout of the chips is different – use TL071 if you want to try this out since it’s pin compatible in this circuit with OP07.

Things to Try Out

An obvious choice might be different clipping options – if you look at my previous build, there’s quite a few options there. Earlier, I described the three options I went with, but there are other options, like LEDs used in Turbo RAT, or even removing diodes completely for op-amp clipping 😮.

Op-amp options

Using a DIL socket for the op-amp makes it easy to try different op-amps – LM308 if you have it, is a drop in replacement (errr, or rather OP07 is a replacement for LM308, depending on how you look at it 🙂). TL071 should also work, since it has compatible pinout, as will probably a lot of other single op-amp chips like LM741 for example. When I say compatible – it’s not the same pinout but will work. Or you can go fancy with OPA134 – it has the same pinout as OP07.

Caution needs to be taken though – always check out the datasheet of the components you want to use – for example NE5554 has a different pinout and might not work (might not be stable at unity gain without compensation cap but uses different pins for that than LM308). I suggest you breadboard circuit first before committing to anything.

Tone Shaping Options

Another few options you might try out – removing R4/C5 pair or R5/C6 which will give a different frequency response, or change their values. Removing R4/C5 will lower the maximum gain and bring it closer to the response of MXR Distortion +, removing R5/C6 will remove boost for lower frequencies so some low frequencies will be shaved off. Give it a try.

If you want to change values for R4/C5 or R5/C6 – you can use tools like this online high pass filter design tool and plug in different values into it and see what you get. Or just breadboard it and enjoy the sound.

Also, the low pass filter formed by Filter pot and R7 & C8 can be also tweaked. As I noted, when the pot is totally rolled off cut-off frequency is 32KHz, which makes first 30% of revolution of the pot totally useless (it being a log pot). For more control, maybe increase R7 a bit (try maybe 3K, 10K), possibly use a linear pot for more control, tweak the cap … plenty to be playing with. See what works best for you. Use the tools from the above mentioned site to calculate how to get desired frequency cut-offs (there’s also a low pass filter calculator tool there).

Bill of Material

Here’s the final list of components I used:

DesignatorsComponentNotes
1590B HammondEnclosure
ProtoboardElectrocookie quarter breadboard protoboard (x2)
Jumpers and wires24 AWG solid core wire and 28 AWG stranded wire (24 AWG might be slightly easier to use)
Anything thicker might be hard to stick into the protoboard.
J3 Cliff FC681473DC Power Socket.
J1Switchcraft 12B
(Alternative: Switchcraft 112BX, 11, 111X)
In – mono Jack (I didn’t use battery so I could use mono. In the build actually used stereo, it’s fine, I just ran out of mono jacks)
J2Switchcraft 112BXOut mono Jack (I used stereo, it’s fine, I just ran out of mono jacks)
RV1, RV2, RV3Alpha 9mm 100K log potDistortion, Filter and Output pots: 100K log pot (PCB pins, metal shaft and bushing). Alternative is 16mm pots solder lugs, but that might be hard to fit into the enclosure.
SW1SF17020F-0302-21R-LTaiwan Alpha 3PDT latching foot switch
KnobsThis depends on what pots you use. I used 6mm straight shaft knobs with screw. If using 16mm Alphas you might need knobs that fit 6.35mm straight shaft for example, or if using split shaft – you’d need push on split shaft knobs.
SW2Alps SSSF014800 SP3T slide switch
C1, C922n (0.022uF) Metal film PET cap
C21nF (0.001uF) Metal film PET cap
C330pF Ceramic C0G (NP0) – optional if using OP07 op-amp, required if using LM308
C4100pF Ceramic C0G (NP0)
C52.2uF Electrolytic cap 25V
C6, C74.7uF Electrolytic cap 25V
C83.3nF Metal film PET cap
C10, C131uF Electrolytic cap 25V
C11100uF Electrolytic cap 25V
C12100n (0.1uF)Ceramic X7R
D1-D51N914Small signal diode (1N4148 is equivalent option)
D6, D71N34AGermanium diode
D81N4002Reverse polarity protection
D9LEDLow current red LED
R1, R22.2M 1% 250mW metal film
R3, R61K
R4, R1047 ohm
R5560 ohm
R71.5K
R81M
R9, R1310K
R11, R12100K
R144.7K LED current limiting (and brightness determining) resistor – choose value to your liking
Q1BF256BN JFET
U1OP07OP07CP
DIL Socket8 pin DIL socket so I don’t solder IC directly to the board (optional, I did not use it in this build and I soldered the op amp directly – talking about living on the edge)
List of components for Maze Runner

Board Layout Plan

As usual, I prepared one possible layout with DIY Layout Creator:

Protoboard layout for Maze Runner
Maze Runner – one possible layout (click for full size image)

It’s a big diagram. The plan was to solder the boards together using header pins and keep controls on the upper board so I can mount all the potentiometers and slider switch to the enclosure, and the rest would be on the bottom board.

The black rectangles I put there are for me to remember that there are mounting tabs/legs that provide support for potentiometers and keep them in place for soldering. Since the pots are in contact with the enclosure and the enclosure will be grounded, those lines will be effectively ground.

I had a stomp switch breakout board with LED on it – I pre assembled it a while ago, so thought, why not use it. The wiring is no different than in my previous builds (and you can just follow same colored wires).

In theory, this should provide for a neat build.

Here’s the DIY file if you want to use it:

Enclosure Planning

I did usual enclosure planning in Inkscape, I just used a different template:

Diagram of the drilling template for the Maze Runner
Maze Runner – Drilling template

Looks like I can fit everything comfortably in. I did have to be careful of where the pots and slide switch are going to go. They are going to be soldered directly onto the board, so I can’t move them as I could with free-standing 16mm Alphas.

I printed out this several times and pre-populated my board to make sure it will fit. Would be kind of a bummer to drill everything, solder everything and then it does not fit!

Photo showing checking the fit of the template
Checking the fit

Here’s the original SVG I used for this:

Preparing Enclosure

This time around I actually did soldering the boards first – since I can’t move pots and the switch after soldering I wanted to make sure again that the template will fit properly.

So after assembling the board, I did standard stuff with drilling the enclosure first using the above template. I used centre punch to mark the place where to drill holes, did pilot holes and then used unibit to get the holes to the right size.

Once done I made sure everything sill fits (you need to have a steady hand if you don’t use a bench drill).

I used printable vinyl for this. Mistakenly I ordered white instead of transparent one … doh! But the final result still looks good. Here are some photos from the process:

Here’s the svg ready for print:

Final Assembly

For the assembly, I did have to drill a few holes into one of the protoboards. To be able to fit pots with their little support legs and also for slider switch. Also, pins didn’t exactly align with the holes for the slider switch, but after some gentle persuasion (read: pin bending) I was able to slot the switch in.

The final assembly was also a bit tricky due to having two boards. Surprisingly, the complexity was how to solder them together with header pins. It was a bit tricky and I didn’t get the boards exactly parallel but in the end I managed it.

One tricky bit was getting a bit more separation between boards. I just put a couple of resistors in between – made a sort of a sandwich. Anyway, final result looked pretty good:

One thing that is important to me is to perform regular tests during assembly – for my piece of mind. It is easy to make a small mistake and the fact that boards are soldered one to the other makes any troubleshooting very difficult. So it pays off to be overly cautious and test few times during assembly.

I did it after finishing the first board, then after I finished both boards (before I soldered them together) and then when everything was soldered, before putting everything into the enclosure. It takes 5 minutes but saves hours of frustration 😊.

Test Ride

Here’s how the finished pedal looks like:

Finally, here’s how it sounds:

Maze Runner in Action

Sounds great I think, it really got me wanting to play more – great stuff!!!

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