Posted by & filed under Distortion, Overdrive, and Fuzz, Stompbox stuff.

The Boss MT-2 Metal Zone distortion pedal is one of the most complex drive pedals there is, and has probably the most sophisticated (and least understood!) tone control circuit on any mass-market pedal. It’s also quite a divisive beast, with some loving it, and others hating it. It’s a great pedal for modifications – easily available, cheap, and rugged. This article is a technical analysis of the different stages in the pedal, finding out what each part does to the tone. This process should suggest various ways you can tweak it to get your own high-gain version of heaven.

First, take a look at the schematic. We’ll break it down into functional blocks, and to keep things reasonably simple, we’ll ignore the power circuitry and the typical Boss noiseless FET-bypass. That’s another thing for another day.

Boss MT-2 Metal Zone Schematic

Ok, reading from top-left to bottom-right, we’ve got the input buffer, the pre-distortion tone shaping circuit, the gain stage and clipping diodes, the post-distortion tone shaping circuit, the High and Low tone controls, and the sweepable Mid tone control. All this is finished off with an output buffer. See, I told you it was complicated! Let’s have a look at those parts one by one.

Input buffer

MT-2 Metal Zone Input BufferThis is just a simple stage to get us warmed up! It’s a FET buffer. The input impedance is largely down to R058, so we’ve got 1MOhm. It’s connected to “black up arrow” which is the symbol they’re using for the 4.5V midpoint voltage. C042/47n is a AC-coupling capacitor (or “DC blocking”, whichever way you want to see it).

Pre-distortion tone shaping

Pre-distortion toneNow things start to get interesting! Our input signal comes in through C033/15n to the +ve input of the op-amp, which is biased to the midpoint voltage with R043/100K. These two components form a highpass filter with a cutoff at 106Hz, just enough to reduce any mains hum a little without heavily affecting the input signal. Op-amp 3b is a non-inverting amplifier, but instead of a simple potential divider to set the amount of feedback to the negative input (and hence the gain) there’s a lot of extra stuff and a transistor. The feedback resistor R044/220K is simple enough, and in combination with C032/100p, it gives a gentle 6dB roll off of high frequencies above 7.2KHz, which is about the top of the guitar’s range. You can check this 220K/100p calculation on the RC filter calculator page.

The “extra stuff” is a transistor-based gyrator stage. As usual, Rod Elliot has the best page about gyrator filters. Here it acts as a bandpass filter, centred around 1KHz. Either side of this, the gain starts to drop off. Additionally, R045 and R042 form a 10K/10K potential divider on the output of the stage, and R045 and C031 form a lowpass filter with a cutoff at around 340Hz. This rolls off the highs even more, and helps smooth things out a little bit.

Boss Mt-2 Metal Zone pre-distortion tone shaping frequency response

Metal Zone pre-distortion tone shaping: Red line shows output from op-amp, blue line is response after the lowpass filter.

This stage gives a notable “mid hump”. If you try putting a wah pedal in front of a distortion pedal, you’ll find you can alter the tone of the distortion by moving the wah. This is similar to what’s going on here – a peak in the frequency response is being created to boost mid-range and shape the sound of the distortion.

Gain stage and clipping diodes

MT-2 Metal Zone Gain and ClippingWe start the next stage with an AC-coupling cap C029/33n. With R040/100K, that input cap forms a high-pass filter at 48Hz, plenty low enough to get all guitar tones. The gain is provided by another non-inverting op-amp, 3a. R051 and VR1 give a minimum and maximum gain of x2 and x252. C028/47p is small enough that even with VR01 at maximum resistance, it only rolls off the top end above 13KHz. Its role here is to limit gain for noise outside the audio band. Note that with a typical guitar input of around 1Vpp, there’s no way an op-amp running on 9V can provide an output of 252V when the gain is turned up to full. Instead the signal will get heavily clipped by the op-amp as it hits the power rails, usually at around 8Vpp for typical op-amps. Since different op-amps handle this differently, the choice of op-amp here might have some bearing on the tone, although given everything else that happens to our poor abused input signal, I doubt it’s a major factor. Following the gain stage is another AC-coupling cap and then a pair of clipping diodes. These are common-or-garden silicon diodes, so they clip the signal to +/-0.6V or 0.7V. That’s another heavy clip for our 8Vpp output from the gain stage. R033/2K2 helps to soften the edges of the clipping a little, as described here.

Looked at like this, the circuit is really a two-stage clipping circuit. Even at minimum gain (x2) there’ll be just enough signal to make the diodes conduct, so we can expect a light crunch. As the gain gets up to x8 or more, the op-amp will start to clip too, and its boosted output will be clipped hard by the diodes. And that gain control doesn’t just go to 11, it goes way up to 252. By that point, all that is left is a very heavily clipped signal.

After the diodes, we’ve got our raw distortion, with as much gain and crunch as we could possibly want. From here on, it’s a question of trying to sculpt that sound into our final tone. The first step in that process is the potential divider/ lowpass filter created by R032, R031, and C023.The filter gives a soft lowpass action and removes some treble. The maximum effect of this filter is only -10dB, so it’s not a huge effect.

Boss MT-2 Metal Zone Clipping Diode Filter

Boss MT-2 Metal Zone: Lowpass filter after clipping diodes

Post-distortion tone shaping

MT-2 Metal Zone Post-distortion toneThis stage shouldn’t be quite so unfamiliar this time around. It’s a more sophisticated version of the pre-distortion tone shaping circuit. This one has two gyrators, and therefore two peaks in the frequency response. We can plug the component values into the everso-helpful Bandpass EQ calculator over on and we get the following:

R1 R2 C1 C2 Freq Q
Left Gyrator 47K 1K 15n 1n5 4898Hz 2.2
Right Gyrator 470K 470R 220n 47n 105Hz 14.6


Note the extremely high Q figure for the right-hand gyrator. This implies a bandwidth of 1/10th of an octave. In practice, the transistor’s gain isn’t sufficient to achieve such a boosted response. It may have been designed to allow for some slack from the transistor. Plotting the frequency response of the stage in LTSpice is instructive, and gives a reasonable result.

Boss MT-2 Metal Zone post-distortion frequency response

Metal Zone post-distortion tone shaping: double-peaked mid -scooped frequency response

The final op-amp 4a is a simple unity-gain inverting stage. To be honest, I’m not sure what this is doing here. Perhaps Boss were concerned that the pedal should be non-inverting overall (is it? I haven’t checked) or perhaps they had one op-amp left over. Suffice to say, it doesn’t add anything, just flips the signal the other way up.

High and Low tone controls

MT-2 Metal Zone High and Low tone controlsNow we get onto the tone controls. The first part of the tone control circuit is the High and Low controls. This is done with a pair of cut/boost-type controls. This circuit is related to the op-amp differential amplifier, and with the controls in the centre, the signals to the +ve and -ve inputs of the op-amp cancel out and there’s no overall effect. As the controls are moved away from the centre position, either the positive or the negative signal will dominate, and there’s a resulting cut or boost. This circuit can give +/-20dB, so it’s a substantial cut or boost too.

Boss MT-2 Metal Zone High tone control frequency response

Metal Zone: High/Low tone control frequency response

The High control is simpler, being just a resistor and a capacitor to make a high shelving. The Low control is another gyrator, this time with an op-amp as the active element rather than a transistor.

R1 R2 C1 C2 Freq Q
Low tone control 100K 2.2K 220n 47n 105Hz 3.1

The Low tone control has the same 100Hz centre frequency as one of the post-distortion tone shaping gyrators, but this time it’s got a much lower Q, giving it a much wider, shallower action. It’s more of a broad brush, and not such a “peaky” sound.


Sweepable Mid tone control

MT-2 Metal Zone Parametric Mid tone controlThe second part of the tone control circuit is the sweepable Mid tone control. It isn’t a fully parametric EQ because although you can change the amount of cut or boost and sweep the frequency, you can’t change the “Q factor”, which determines the bandwidth, or how wide the effect of the stage is. We can call it a “semi-parametric EQ” if you like, but “Sweepable Mid control” is just as good.

The heart of the tone control is the Wien bridge network formed by C036/22n, C043/8n2, R048/2K2, R062/2K2 and the dual-gang pot VR02/50K. A Wien bridge network always needs a dual-gang pot to alter its frequency. If you want to know more about Wien bridge circuits, you could read the ESP page about Wien Bridge EQ. Most of the stuff about them deals with the Wien bridge oscillator, which is a standard sine wave oscillator circuit.

Boss managed to keep the Q response constant across the range of the sweep. This is probably why they chose the Wien-bridge circuit rather than use another gyrator. It’s hard to keep Q within reasonable limits when you make the gyrator’s frequency variable.

Boss MT-2 Metal Zone Sweepable Mid Tone Control

Boss MT-2 Metal Zone Sweepable Mid Tone Control

The “Mid” control gives a cut or boost of 15dB. Note that there is a slight asymmetry between boost and cut. The “Mid Freq” control goes from 240Hz in both cases, but when boosted the top end is 4.7KHz, whereas when cut it is 6.3KHz.

We should also note that the “Mid Freq” control response is a long way from linear. The plotted values above are 0, 5K, 10K, 30K and 50K for the Mid Freq dual-gang pot. If you’re building one, a reverse-log taper would perhaps give the best response. Good luck finding a dual-gang reverse log pot – it’s possible, but not easy.

These tone controls are one of the reasons why some people hate this pedal, and why others love it. The four controls (High, Low, Mid, Mid Freq) give you a huge tonal variety to explore, but also make it difficult to find your perfect setting unless you know from the outset what you want. This is a pedal which maybe doesn’t reward random knob twiddling. It’s a bit too complicated for that.

Output buffer

OutputBufferPhew! We’ve finally made it to here! Another simple one to finish off. This is a simple emitter follower buffer. R004/100K ties the base to the bias voltage of 4.5V. C002/10μF AC-couples the input, and C001/10μF does the same for the output. R003/100K ties the output to ground to prevent (ok, “reduce”) thumps when you plug in, and R001/1K protects the output against shorts and people getting the input and output mixed up.

Why does the Metal Zone sound like it does?

I’ve already commented on the sophistication of the MT-2 Metal Zone’s EQ, and it does give you a lot of options. But it’s not the only thing defining the sound of the pedal. The pre- and post-distortion tone shaping circuits have a massive influence, and beyond what you can tweak on the EQ, they are the sound of the pedal. Swapping the gain op-amp or changing the clipping diodes might have a subtle effect, but changing the frequency-determining components in those two tone shaping stages will give much more radical results. Whereas on many other distortion or overdrive pedals, the op-amp used or the clipping diodes have a major influence on the final output, in the MT-2 it’s really all about frequency response. It’s heavily shaped every step of the way, and those frequency responses determine the sound of the pedal, much more than diode choice ever would.

That initial mid-hump before the distortion, high-gain double clipping, and then the scooped-mid response after it give the pedal its tone. Add to that the high degree of tweakability that comes from the powerful EQ and you’ve got a successful pedal.

And that is the gory detail of the guts of the MT-2 Metal Zone. Thanks for reading and I hope it inspires you to have a tweak at this remarkable pedal, or maybe design your own with a little inspiration from those clever Boss engineers of the 1980’s.


This article owes a lot to the extremely helpful folks over at, where I raised my questions. Thanks to Paul R in particular for his help in understanding the post-distortion tone shaping.


26 Responses to “Boss MT-2 Metal Zone pedal analysis”

  1. DevilSomething

    Thanks for doing this writeup! I’ve always thought that this pedal has way more potential than people give it credit for; nice to see a similar argument and the analysis to justify it. Much appreciated!

  2. Porchbass

    Thanks for this breakdown! Very eloquent. I’m an MT2 fan and it completely blows the MT3 away. For a lot of guitarists I fear there are too many op-amp stages and EQ – there is a lot of negative feedback, filtered at that -it’s easy to get a very phasey scooped tone.
    But it’s a great 303 mangler, listen to some chemical brothers!

  3. Toby Moorhouse

    Fantastic write up.. it really helped me unlock the sound I needed without relying on the often quoted mods suggested in the forums (which also create an unusable live sound based on most YouTube videos). Thank you for saving me from that horror!

  4. Patrick Richardson

    Thank you for this EXCELLENT writeup; very accessible and transparent (unlike the stock MT-2 tone).
    This was very helpful to show my students in my Audio Electronics class, and it’s motivated me to cut into my own old MT-2.

    I’d like to disable the 1 pre- and 2 post-distortion gyrators.
    Would you recommend I clip the C34 and/or C35, for example ?

    • Tom Wiltshire

      It seems like a reasonable plan to me. Those stages are basically a non-inverting op-amps, if you ignore the gyrator. So clipping C34 out disconnects the gyrator and leaves you with a unity gain op-amp buffer, more or less. I’d certainly give it a try. If it didn’t work, I’d try soldering a wire across R044/220K, which then turns it into absolutely a op-amp buffer. The post-clipping gyrators can be disabled the same way, but certainly have a listen (and record a sound file for posterity!) without the first gyrator stage before you attack the other two as well.
      Note that you’re definitely losing gain though (that first gyrator has a gain of *36dB* at it’s peak, so an alternative to soldering a wire across R044 would be to add a resistor from the R044/C034 junction to Vbias to give a flat boost across the whole audio range.
      Another approach would be to experiment with a few values in the gyrator calculator and make the gyrators very low Q (wide and shallow, instead of very peaky). That would effectively boost most of the spectrum, but without becoming completely flat.

      Let us know if you come up with anything good!


  5. doni

    i’m very confused about the mid freq knob, sometimes i get the tone and on the other time it’s stressed me out, can you explain the mid freq knob function into the detail ? thanks

    • Tom Wiltshire

      The “Mid” control is sweepable – that is to say, it’s not at a set frequency. The “Mid Freq” control sets where it takes effect, from about 300Hz up to about 7KHz. That’s a very wide range. The other Mid control then sets the cut or boost at the frequency selected. The graph in Mid Tone control section visualises what’s going on – any of those lines are possible responses from this section of the circuit.


  6. M.Latimer

    Ive never really understood why people would buy a Metal Zone and then expect, and desire it to sound like tube screamer, all whilst the gain is ‘dimed'(on mt-2) inevitably.
    In the future when people berate the pedal I will cite your, great site and informed article and drop some knowledge on them suckas.
    Truth be told.
    Myself MT-2 has always been a favorite.
    I love the hyper drive gain, which unlike many other so called ‘hi gain’ pedals, the gain really should rarely if ever de dimed. Its simply too much, for most situations….and I love gain more than most. But, even with more modest gain settings and careful eq use I think just about anyone should be ale to get a punishing metal tone out of the ol MT-2.

  7. Dr. No

    This is really similar to what Dimebag did with his entire rig, boost the mids into his amp then use a parametric eq to scoop the mids after the amp.
    The curve of that post distortion EQ remins me of james hetfield’s settings on the graphic EQ of his favorite amp.
    cool stuff

  8. Olivier V

    Great! Would it be possible to wire the eq to shape the sound before the distortion? Would that work even with the signal inverting eq input?

    • Tom Wiltshire

      It would be possible to hack the circuit to put the EQ before the distortion, yes. I don’t know how easy it would be. It’d involve cutting some tracks and wiring some jumper wires in here and there.
      It will certainly work despite the “extra” inverting op-amp, and it shouldn’t affect anything, since you wouldn’t be increasing or decreasing the number of inversions the signal undergoes – just shuffling them around!


      • Toby Moorhouse

        I’ve put the EQ before the gain section and it sounds brilliant.. I put the sweepable mid-range pre-gain but kept the treble and bass controls post-gain. This combination makes the pedal very useable in a live set-up (using a valve amp pushed to break-up); IMO this only works properly if you disengage all the gyrators and start with a good quality flat base tone. I will put the details of the entire mod I did below as I believe it is too good to keep to myself.

  9. Arnaud Labelle

    Thank you so much for the breakdown Tom. It was super interesting and informative.

    I’m working on an homebrew mod and I was wondering what would be the best way to diminish the amount of gain in the gain stage. Reducing the value of R51? Also if I jumper the the buffer between R28 and R14 can I jumper also one of those resistors?

    Thanks again.

    • Tom Wiltshire

      Reducing the value of R51 *does* decrease the gain, but it’s so small compared to the 250K pot VR1 that it’s not really worth it. A better course of action is to *increase* the value of R41 up from 1K. The total gain here is:
      gain = ((VR1+R51) / R41)+1
      So doubling the value of R41 to 2K2 would halve the maximum gain and back things off quite a bit. This would make sense to do if you find you only ever use settings in the first half of the Dist knob’s rotation.

  10. Toby Moorhouse

    Transparent Distortion mod with Adjustable Drive Character.

    After various incarnations, and a couple of wasted pedals, this is the definitive mod for the MT-2 in my opinion (of course I’m going to say that) and what better page to share it on! You do not need to buy any more components for this mod; you just need a soldering iron, solder and some good quality wire.

    The main points I set out to achieve:
    1. Good quality transparent base tone when tone dials at noon – (EASY)
    2. Less compressed, dynamic sound with good pick articulation – (TRICKY)
    3. Flexible sound with different guitar/amp set-ups – (HARD, but worth it)
    4. Improved noise floor – (probably UNNECESSARY)

    This was achieved by removing a shed-load of components and switching the sweepable mid-range to in front of the gain stage. Ideally you need to start with a first or second generation MT-2 (M5218AL ICs) as these sound best in my opinion:

    1. Good quality transparent base tone when tone dials at noon – (EASY):
    Remove: C034, R042, C031, C024, C020, C023, C010, C026

    This disengages all of the gyrators and the low pass filters between, and during, op-amp stages. It refines the top end of the distortion and provides a good base tone. Please note: this will decrease the gain of the pedal. This sounds great with bridge pick-ups but is a little too flabby-sounding with neck pick-ups for my tastes. The third stage tightens this up and recovers the gain nicely.

    2. Less compressed, dynamic sound with good pick articulation – (TRICKY):
    Remove: R033, D004, D003, IC4
    Bridge: gain stage side of R033 gap to point 1 of IC4 gap

    I personally like the sound of op-amp distortion, particularly the M5218AL, and feel that the diodes add a nasal character. Removing the diodes boosts the output significantly too, which is great for pushing a valve amp to oblivion. I experimented with various diode combinations (symmetric/asymmetric +/- silicon/germanium) before removing them altogether; the sound is much less compressed and the pick articulation is better as a result.

    3. Flexible sound with different guitar/amp set-ups – (HARD, but worth it):
    Remove: C033, R043, R045
    Remove and keep (note the polarity): C011, C005
    Bridge: (a) signal side of R043 gap to mid-range side of C011 gap, (b) mid-range side of C005 gap to signal side of C031 gap
    Connect: (a) capacitor from C011 to C033 gap (same polarity as earlier), (b) low/high side of C011 gap to output side C005 gap via the C005 capacitor saved from earlier (same polarity), I mounted this on extension leads

    This puts the sweepable mid-range in front of the gain stage and lets you control the character of the distortion (dial in/out the “honk”); it is the key to this mod’s versatility. It will make single coils sound like humbuckers and vice versa, as well as outrageous sounds I have only ever got by stacking gain pedals together.

    Keeping the high and low controls after the gain stage makes this pedal useable in live situations: dialling out the bass can help you to be heard without sounding louder. Keeping control over the presence helps add clarity when playing with a band.

    4. Improved noise floor – (probably UNNECESSARY):
    Remove: Q007, Q008, Q010

    This was partly done by removing IC4 earlier, but if you have the soldering iron out you might as well remove these as they are not doing anything but putting noise to ground.

    I had great fun developing this mod, the information on this page was invaluable in the design process. Thanks Tom! This pedal is now my drive pedal of choice, it easily trumps all the other Boss distortion pedals and a good number of boutique overdrives and fuzz pedals I own.

  11. Daniel Devinson

    In my experience, diode-to-ground configurations tend to have the characteristic that some of the unprocessed clipping sound is still heard through all filters and level controls. DS-1 for example. So try unsolder the diodes and connect them to pin 2 and 3 of op amp 3a. Yes, this is a tube screamer configuration. But it allows the tone controls to do their job fully.

    • Tom Wiltshire

      Changing to tube screamer configuration is valid enough if that’s what you want to do, but I’m afraid I can’t agree with your analysis. The sound is clipped after the diodes-to-ground. There’s no unclipped sound in the signal.

  12. Sam

    Does anyone know if it’s possible to mod an MT-2 to have the mid EQ frequency sweepable via external expression pedal?

    • Tom Wiltshire

      It would be pretty difficult because the Mid Freq control is a dual-gang pot. It might be a customised taper too. In the original pedal, it’s part of a special concentric pot with the Middle level control – so the single pot has *three* sections!



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