Rangemaster Modification / Variant: Input Capacitor Switch
The Dallas Rangemaster Treble Boost is one of the first and most legendary guitar effects ever made. Like many of the earliest effects, it was a very simple and crude circuit. It is basically a first-order passive high pass filter followed by a common-emitter transistor amplifier. The input high pass filter is partially responsible for the "treble boost" part of the Rangemaster’s sound. It is a standard passive RC high pass filter with its cutoff frequency determined by the input capacitor and the circuit’s input impedance.
A common modification that is often made to the Rangemaster circuit when it is used in new builds is to add a switch that allows for selection of the input capacitance. This will change the cutoff frequency of the high pass filter which determines how much of the low-end is let through. To understand how it works, let’s first take a look at the original Rangemaster circuit.
Figure 1: Typical Rangemaster circuit with 5n input capacitor
The input impedance of the circuit is set by the 470kΩ and 68kΩ input resistors in parallel with the input impedance of the transistor, which is roughly 12.5kΩ. The equivalent resistance of 470kΩ // 68kΩ // 12.5kΩ is ~10.3kΩ. The input impedance of the transistor can vary depending on transistor gain and bias point, but these numbers should give a ballpark idea of the cutoff for a typical Rangemaster.
With an input impedance of 10.3kΩ and an input capacitor of 5nF, the cutoff frequency is determined by the simple equation for a passive RC high pass filter:
$$F_c = \frac{1}{2\pi \times R \times C}$$Substituting in the values of ~R = 10.3kΩ~ and ~C = 5nF~, we get:
$$F_c = \frac{1}{2\pi \times 10.3\text{kΩ} \times 5\text{nF}}$$$$F_c = \frac{1}{2\pi \times (10.3 * 10^3) \times (5 * 10^{-9})}$$$$F_c = \frac{1}{2\pi \times (51.5 * 10^{-6})}$$$$F_c = \frac{1}{0.00032358404}$$$$F_c = 3.1 \text{kHz}$$The two variables we can change in the above equation are ~C~ (the capacitance of the input cap) and ~R~ (the circuit input impedance). There are ways to change the input impedance, but they would all have unintended effects on the rest of the circuit. The simplest way to modify the cutoff frequency of the circuit is to change the input capacitor. A change to the input capacitance will change the high pass filter cutoff frequency without affecting the rest of the circuit.
A change to the input capacitance is often accomplished with an ON-OFF-ON SPDT toggle switch in parallel with the input capacitor.
Figure 2: Rangemaster circuit with an input capacitor toggle switch mod
When the toggle switch is in the center "OFF" position, no capacitor is placed in parallel with the original 5nF input capacitor, and the Rangemaster has the original cutoff frequency of ~3.1 kHz.
If the toggle switch is set to one of the "ON" positions, either 10n or 5n is put in parallel with the original input capacitor. When capacitors are put in parallel, the equivalent capacitance is simply the sum of the parallel capacitances:
$$5\text{nF} // 5\text{nF} = 10\text{nF}$$$$10\text{nF} // 5\text{nF} = 15\text{nF}$$Where ~//~ denotes "in parallel"
With an ON-OFF-ON switch, you can choose between three different capacitances and thus three different cutoff frequencies:
$$C = 5\text{ nF} \Longrightarrow \frac{1}{2\pi \times 10.3\text{kΩ} \times 5\text{nF}} = 3.1 \text{ kHz}$$$$C = 10\text{ nF} \Longrightarrow \frac{1}{2\pi \times 10.3\text{kΩ} \times 10\text{nF}} = 1.5 \text{ kHz}$$$$C = 15\text{ nF} \Longrightarrow \frac{1}{2\pi \times 10.3\text{kΩ} \times 15\text{nF}} = 1 \text{ kHz}$$The frequency response of the three possible input high pass filters can be seen in Figure 3.
These capacitor values can of course be experimented with. It is fairly common for one of the capacitors to be a much larger value in order to provide a "full-range" boost where the high pass filter’s cutoff frequency is low enough that it does not affect the audible guitar frequencies.
An alternative to the 3-position toggle switch is a rotary switch. A rotary switch can provide more filter options since it’s fairly easy to find rotary switches with 4, 6, or 12 positions. In this particular Rangemaster build, we are using a 2P6T rotary switch, and only using one of the poles. The Rangemaster circuit with an input cap rotary switch looks like this:
Figure 4: Rangemaster circuit with an input capacitor rotary switch mod
With these capacitor values, the potential input capacitance values are:
$$1\text{nF} // 2\text{nF} = 3\text{nF}$$$$1\text{nF} // 3.9\text{nF} = 4.9\text{ nF}$$$$1\text{nF} // 10\text{nF} = 11\text{ nF}$$$$1\text{nF} // 20\text{nF} = 21\text{ nF}$$$$1\text{nF} // 47\text{nF} = 48\text{ nF}$$$$1\text{nF} // 220\text{nF} = 221\text{ nF}$$And the cutoff frequencies are:
$$\frac{1}{2\pi \times 10.3\text{kΩ} \times 3\text{nF}} = 5.15\text{ kHz}$$$$\frac{1}{2\pi \times 10.3\text{kΩ} \times 4.9\text{nF}} = 3.15\text{ kHz}$$$$\frac{1}{2\pi \times 10.3\text{kΩ} \times 11\text{nF}} = 1404.7\text{ Hz}$$$$\frac{1}{2\pi \times 10.3\text{kΩ} \times 21\text{nF}} = 735.8\text{ Hz}$$$$\frac{1}{2\pi \times 10.3\text{kΩ} \times 48\text{nF}} = 321.9\text{ Hz}$$$$\frac{1}{2\pi \times 10.3\text{kΩ} \times 221\text{nF}} = 69.9\text{ Hz}$$The frequency response of these 6 possible filters can be seen in Figure 5.
The 221nF input cap creates a filter with a cutoff frequency (69.9Hz) below the fundamental frequency of a guitar’s low E string (82.41Hz). The nominal cutoff frequency of this type of high pass filter specifies the point at which the signal is attenuated by 3dB, so there will still be a very small amount of attenuation (<3dB) of the lowest frequencies. If that is a concern, a larger cap in place of the 220nF cap will move the cutoff frequency even lower. A 470nF cap there will result in a cutoff frequency of 33Hz.
Earlier, it was mentioned that this high pass filter is "partially" responsible for the treble boost effect. The filter itself does not boost anything. It only cuts the low frequencies. However, the rest of the Rangemaster circuit is configured as a single-stage common emitter amplifier, which boosts the signal after the high pass filter. The lowest frequencies tend to get boosted back up to around unity gain with the original input capacitor, and the gain of the pedal increases for higher and higher frequencies.
The result is a sound with more high-end presence, which is great for getting the sound of a dark amplifier to cut through the mix. The Rangemaster also provides some asymmetrical soft clipping which can sweeten up your tone in a way that is responsive to pick attack and frequency, since it is the higher frequencies that get boosted the most and thus clip the most.
There is one other change that is often made on newer Rangemaster builds, and that is to add 1M pulldown resistors to the input and output. The original Rangemaster was an amp-top effect that was usually left either on or off. When moving the circuit into a true bypass guitar pedal, the 1M pulldown resistors help to prevent pops or clicks that might otherwise occur when the effect is engaged. The reason these pulldown resistors are required is that when the effect is dis-engaged, the input and output are floating (not connected to anything) if they do not have the 1M pulldown resistors. The input and output capacitors will leak a small amount of DC current, and over time a DC voltage will appear on the input and output contacts. When the pedal is re-engaged, this voltage is usually discharged via the connected stage, which can cause a popping sound.
Figure 6: The schematic seen in Figure 4 with added input and output pulldown resistors.
With the pulldown resistors, any leakage current will flow to ground via the 1M resistors, and the input and output contacts will be kept at 0V. Note that this modification does not have anything to do with the input capacitor switch, but it is a recommended modification if the Rangemaster is going to be used in a true bypass build (whether it uses an input capacitor switch or not).