A question occasionally comes up about the way the rotary "EQ voicing" switch works in the LB v.2.0. So here's a rundown on the subject:
As described in the Beaver instructions, this 4-position rotary switch allows you to change the voicing imparted by the tone stack, which was a common modification that builders used to perform on the original v.1.0 kit. In a nutshell, the switch allows you to select different combinations of capacitors in the tone stack stage of the circuit, changing how mid-range frequencies are processed.
The Big Muff style tone stack is different than what you find in most dirt pedals, which is most commonly a simple treble cut control, i.e. as the tone pot is turned down, more and more of the high frequencies are shunted out of the signal path to ground. Instead, the Muff style tone stack uses two RC filters, one a high pass filter (HPF) that removes lower frequencies and the other a low pass filter (LPF) that removes highs. Both are wired into the same control pot, so that turning it has the dual effect of reducing one side of the frequency range while increasing the other. The choice of the ratings of the resistor-capacitor pair that makes up each filter determines its rolloff frequency
, beyond which a significant portion of the signal is scrubbed off. The farther beyond this point the signal frequency is, the greater the portion of it that is removed. For the HPF, frequencies below
the rolloff point are removed, and for the LPF, frequencies above
the rolloff point are scrubbed off. Consequently, the values of those two rolloff frequencies become critically important to how the tone stack functions.
For the Triangle version of the Large Beaver at the "stock" Muff EQ setting, the HPF rolloff point is 1046Hz, and for the LPF, it calculates as 408Hz. (You can see how this is calculated HERE
, or plug in the R and C values HERE
to have it done for you.) This means that, assuming that the tone pot is set at the midpoint, the tone stack is removing an increasing portion of the frequencies below 1046Hz, while also attenuating an increasing amount of the frequencies above 408Hz. Take note that the two attenuation ranges OVERLAP between 408Hz and 1046Hz, which means that within that interval, there will be an additive loss effect.
This creates a "notch" in the mid-range response and gives the Muff-based pedals their characteristic "scooped mids" sound. (For the Ram's Head version, the overlap is even greater and the scooping somewhat more pronounced.)
The v.2.0 Beaver gives exactly this behavior at position 1 of the rotary switch, which is the "stock Muff" position. At position 2, the HPF cap is changed, and the new rolloff value is 408Hz--the same value as the LPF rolloff. With the two rolloff points equal, the additive loss notch is eliminated, so this switch position gives a flat mid-range profile. In position 3, the LPF cap changes, and its rolloff frequency jumps up to 1046Hz. Now, the 408 - 1046Hz interval is getting little to no attenuation from either
filter, resulting in a mid-range "hump" in the signal. Think of it as "Big Muff meets the Tube Screamer"!
In switch position 4, the signal is bypassed around the tone stack completely, so an unfiltered signal goes directly into the final transistor stage, making for a very full and open sounding (and noticeably louder) output. Refer to the green-shaded "Tone Stack" portion of the schematic shown in the first post of this thread to see how this portion of the LB circuit is laid out, and how the switching between components is configured.
Here are a diagram and a couple of tables that summarize what's going on in the switch sequence, and the impact on the circuit and tonal output: