Big Muff tonestack : dealing with mids frequencies

 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!

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 that cause this effect: 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.

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):

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.

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:

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:

 

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: 

 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:

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:

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!

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
Frequency response of different models of Big Muff
Modding the Big Muff tonestack (guitarkitbuilder)