Black Keys's Big Muff: dealing with mids frequencies

Another Big Muff clone! This time, I was inspired by the Black Keys (if you do not know this band, go check it out, it is awesome!). Dan Auerbach, the guitarist/singer, uses a lot of fuzz effects, and especially a green russian Big Muff, and a Earthquaker Devices Hoof (a Big Muff variant with some interesting modifications as we will see later).

So I decided to basically mix these two variants in one Big Muff! Here it is, a 4-knobbed Big Muff:

There are the 3 usual controls of a Big Muff: volume, tone, sustain, and a supplementary one: mid control! As you may know, Big Muffs can be annoying because you loose a lot of medium frequencies when you use it, which as huge downside effect: you just disappear from the mix! It can be nice sometimes when you do not want to be heard too much (when heavy riffing with a singer, or to let your other guitarist do a solo...etc). However, when you want to solo, you are doomed to be silenced by the Muff!

To avoid that, many muff today incorporate variants of the AMZ tone stack that allows you to have a mid control. The Hoof has such a control, and as it is very useful, I decided to incorporate it. So 4 controls Big Muff it is!

I polished the enclosure with sandpaper, beginning with big one (180), and then sanding with a smaller one (480). I the varnish a bit the enclosure to avoid oxidation of the aluminium. I find the result very appealing, and it evokes my previous Big Muff build!
Big Muff clone Black Keys Dan Auerbach 
As usual, I used the laser engraved logos I made before.
Big Muff clone Black Keys Dan Auerbach
Again, I used a PCB from musikding, named "Der Muff". Honestly, this PCB is quite cheap (4.5 euros), but it is monolayer, and thus quite big. So it can only fit a 1590BB enclosure. Moreover, placement of the outputs / ground / battery.... pads is not optimal, and there is a lot of unused space on the board. I am currently designing a PCB that I will use for my next builds. I am planning to make it fit a 1590B enclosure instead.
Big Muff Black Keys Dan Auerbach inside
I mixed a russian green muff with the Hoof and Ram's Head values to have something nice. I used Panasonic SMF capacitors, and 2n5089 transistor (and a MPSA18 for the output transistor, which gives a high volume output). For the tonestack, I used the exact same values as the AMZ tonestack circuit.

I tried to use germanium transistors (AC187) for the clipping section, like in the Hoof, but I was not really satisfied with the result. You loose a lot of output volume (due to the low hfe of germanium transistors, but you can bias the transistors differently to get the volume back), and the result is not really different from a classic Muff. It makes sense because in the Muff circuit, transistors are not supposed to saturate, and the difference between silicon and transistor is not really obvious when only amplifying a signal.

You can see that, instead of the classic 1n914 diodes, there are LEDs on the board... Actually, LEDs are diodes, and can be used for clipping! They have a voltage drop of nearly 1.6V (for Red LEDs), which is a lot compared to silicon (0.6V) or germanium (0.35V) diodes! Actually, it is really nice sounding a Big Muff, the sound is brighter and less compressed, it really gives a breath to the Muff, which is less compressed, and more overdrive sounding, especially when turning the mid knob up!
 Big Muff LED clipping diodes
Voltage drop changes depending on the color of the LED. Red diodes have the lower voltage drop, and blue ones for instace have a huge one (2.8V!). The sound is really different when you swap for blue LEDs, in fact the transistors started to saturate, and the muff was sounding much more mushy, not really a nice compressed and saturated Big Muff sound... Red LEDs are better.

How does it sound?

Well, I am sorry again, I really need to buy some stuff to properly record my pedals - but no-money-effect - I cannot, so you will have to believe me again... First, it sounds really closer to the Hoof than a Green Muff. I think this is mainly because of the LED clipping diodes. It gives a really nice touch to the sound of this Muff, it is less compressed, less saturated, but still quite heavy. I really like it! It gives also more versatility to the Big Muff, if you roll back the Sustain knob, and increase the Mids, you really have something that sounds close to an overdrive, really interesting! I think I now completely understand why the Hoof was a success!

Funny thing: if you replace the LED by classic 1n914 silicon diodes, you get back to a classic green muff sound! I do not know why anybody did not make a switch for the clipping diodes already! I think I will try to do something like this for my next Big Muff clone.

However, I am a bit disappointed by the AMZ tonestack. Evenif the Mids knob works really really well, the tone knob is weird. I guess it is because of the logarithmic scale, but you do not have that much variation when turning it, and suddendly everything becomes trebly and agressive... I will try a classic linear knob next time, or maybe tweak a bit the values because I am not really satisfied with it now.

Overall, it is a really nice sounding device, and you can get many different sounds, from a more overdrivy sound to classic muff sounds when dialing with low mids and high sustain. Great thing for sure!

Circuit analysis: the Big Muff tonestack

I advise you to read the Big Muff circuit analysis first. Here we are going to see in detail a very important part of the Big Muff circuit: the tonestack.

The tonestack is a small part of the circuit, with 5 components,, that allows you to set the tone of the saturated signal, the amount of bass and trebles. One of the characteristics of this tonestack is that it scoops the mids of your tone.
Scooped mids setting big muff
It gives you a heavy, powerful tone. However, the huge downside to it is that when playing in a band, you can easily disappear in the mix. Indeed, the low frequencies are covered by the bass guitar, and the drums are really loud in the high frequencies... Sometimes, it is like your guitar has been muted!

Here is the schematic of the tonestack (I used the Big Muff Page's annotations for the parts names):
Big muff tonestack schematic

It is composed of two passive RC filters, a high pass and a low pass filter. The tone potentiometer mixes the two filters to let more or less bass / treble going through.
Big Muff tonestack circuit analysis

High pass filters let frequencies pass when they are above a fixed cutoff frequency, and low pass filters let frequencies below a fixed cutoff frequency pass (tip to remember: "low pass = below"). Here are examples for both low pass and high pass filters:

Low Pass High Pass filter

So basically, high pass lets trebles go through, and low pass let the bass frequencies go through. As you can see, the sound is not "cut" directly, but is reduced progressively. So when you turng the "tone" knobNow, why are the mids taken out when passing through this tone stack? To know that, we can calculate the cutoff frequency of these 2 filters. The formula to calculate it is:
passive filter cutoff frequency formula 
Beware: R is in Ohms, and C in Farads. Lets take the Violet Era Ram's Head Big Muff as an example. The values are: 
Ram's head tonestack big muff schematic
 We can calculate the cutoff frequencies of the high-pass (HP) and low-pass filters (LP):
Here we have the cutoff frequencies of the high pass and low pass filter! Sorry about these calculations, I know they can seem a bit boring... If you want to spare you the pain of doing them, there is a great tool on AMZ website to calculate cutoff frequencies without having to use you calculator! The cutoff frequencies are clearly in the midband range (between 200 - 2500 Hz approximately) Lets trace the graphical representation of these filters, and combine them:

Big Muff mids scoop schematic

Boom! Here is the mid scoop that we hate when playing in a band! How to change it? As we have previously seen, the value of the cutoff frequency depends on the value of the components (R and C). So changing these components will change the mids scoop! Actually, as the Big Muff circuit changed a lot during its history, the mid cuts is more or less pronounced depending on the era of the model. Green russian Big Muff for instance have a less pronounced mids cut. Here are the tonestacks of all versions:
  • Triangle Big Muff: R5: 22k , R8: 22k , C8: 0.01uF, C9: 0.004uF (723 / 1809 Hz)
  • Ram's Head Big Muff (73 version): R5: 33k , R8: 33k , C8: 0.01uF, C9: 0.004uF (482 / 1206 Hz)
  • Ram's Head Big Muff (75 version): R5: 22k , R8: 39k , C8: 0.01uF, C9: 0.004uF (408 / 1809 Hz- big scoop of the mids)
  • Big Muff "Civil War": R5: 22k, R8: 20k, C8: 0.01uF, C9: 0.0039uF (796 / 1855 Hz)
  • Big Muff Russian "Tall Fonts": R5: 22k, R8: 20k, C8: 0.01uF, C9: 0.0039uF (796 / 1855 Hz)
  • Big Muff Russian "Buble Fonts": R5: 22k, R8: 20k, C8: 0.01uF, C9: 0.0039uF (796 / 1855 Hz)
  • Flat mids: R5: 22k, R8: 39k, C8: 0.01uF, C9: 0.01uF (408 / 723 Hz)
  • Mids bump: R5: 22k, R8: 39k, C8: 0.0056uF, C9: 0.01uF (729 / 723 Hz)
OK so we can have different mids scoop depending on the value of the components... But is there a way to dose the mid scoop with a knob? Yes!

AMZ fx had the really great idea of using a resistor plus a potentiometer for R5. Thus, you can modulate the cutoff frequency of the high pass filter. If you diminish the value of the resistor, the cutoff frequency will be lower, and thus you will get more mids.

You can modulate the values of C8, R8, C9 or R5-1 depending the kind of sound you want. Generally, a small value is used for R5-1 (like 2.2k), and the Mids knob is a 25k linear potentiometer. This is what I used for the Big Muff "Black Keys" build! A lot of manufacturers and builders use this technique to add a "mid" knob to their Big Muff. Here are a few examples:
  • Earthquaker Devices Hoof fuzz: R8: 39k, C8: 0.0068uF, C9: 0.0068uF, R5-1: 2.2k
  • AMZ fx v1: R8: 39k, C8: 0.01uF, C9: 0.012uF, R5-1: 3.3k
  • AMZ fx v2: R8: 470k, C8:1500pF, C9: 0.015uF, R5-1: 3.3k, tone: 250kA (the one I use here)
  • Earthbound Audio Supercolider: R8: 39k, C8: 0.01uF, C9: 0.01uF, R5-1: 10k
Another technique that is used to bring back the mids is to use a 100k potentiometer to blend in a high value capacitor that will lower the cutoff frequency of the high pass filter:
Big Muff tonestack mids
This system is also used in a few Big Muff clones:
  • Blackout Effectors Musket Fuzz: R8: 10k, C8: 0.047uF, C9: 0.0033uF, C9bis: 0.047uF R5-1: 56k
  • Mojo Hand FX Colossus: R8: 10k, C8: 0.047uF, C9: 0.022uF, C9bis: 0.056uF R5-1: 47k
  • Mojo Hand FX Iron Bell: R8: 18k, C8: 0.01uF, C9: 0.0068uF, C9bis: 0.033uF R5-1: 33k
I actually prefer the AMZ Fx system because you can reverse it easily to a classical Big Muff tonestack. If you put a jumper on the potentiometer, and use a "normal value" resistor, you get a classical Big Muff tonestack. It also adds only one component, compared to 2 for the capacitor blending. (moreover, high value capacitors are quite big)

Have fun experimenting! You liked this post? Thank me by liking Coda Effect's Facebook page!

    To go further:
    AMZ Big muff tonestack: great explanations and tweakings around the Big Muff tonestack
    AMZ RC filters cutoff calculator: great if you do not want to use your calculator for cutoff frequencies calculations
    Big Muff mods and tweaks: tonestack mods
    Modding the Big Muff tonestack (guitarkitbuilder)

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    2 Comment
    This comment has been removed by the author. - Hapus

    Hi, nice article.All good relevant info in one page.

    Only one thing that I would say is that the Pharaoh has a diode selection switch. GE(asm) / Si (sym)

    Check it out..

    Good article. Bookmarked.