The Gatekeeper is an overdrive pedal focused on utilising numerous BJT's in conjunction with hard clipping diodes. The Gatekeeper will offer perhaps a different sonic interpretation than your expected overdrive pedals. Offering 'chewy' and 'fuzzy' overtones on your driven signal this pedal offers a different flavour and colour for your audio palette.
The transistors implemented within the gatekeeper are BJT's configured in Common Emitter configurations to amplify the signal with an appropriate amount of gain to push the signal to its overdriven potential. The use of these circuits will add optimum levels of saturation, blooming with rich harmonics and an extended amount of sustain. In conjunction with the transistors amplifying there signal there are a pair of diodes shunt to ground to further clip the signal. There is then a transistor configured after this clipping stage to then amplify this signal.
NPN transistor are active deivces containnig three terminals, the BJT is a current controlled device. This alludes to an output current being controlled by an incoming current situated at the Base terminal. For a BJT transistor to conduct current a forward bias voltage needs to obtained at the base terminal, for a silicone transistor this is in the region of 0.6-0.7 volts. A guitar signal itself isn’t powerful enough to obtain this amount of voltage so a small amount of voltage is provided to the base allowing for this feat.
The resistor components found at the Collector and Emitter terminals, help determine several factors (biasing points, output impedance etc.) These were carefully managed regarding these aspects, with one of the main factors at the forefront when assembling being the control of gain the transistor displayed. This was carefully deciphered to add some fuzz elements but still keeping a mainly overdrive sound, if the gain was calculated too ‘high’ then this pedal may have found its way exclusively in fuzz territory.
Another feature of the circuit that would impact the sonic perception and interpretation of the pedals performance is the clipping diodes. The diodes are configured ‘shunt to ground’ (had clipping diodes.) After the incoming AC signal has been amplified by the noted BJT the signal is met by a pair of diodes configured in symmetrical clipping. Symmetrical clipping refers to the impact the clipping will have on the incoming signal, with both the negative and positive portion of the incoming AC signal receiving the same amount of clipping. Symmetrical clipping can be known for having a smoother sound.
Another important implication of the diodes is the material they are constructed of, as silicone, germanium, Schottky and Led diodes all have different forwards voltages and other characteristics that will impact the clipping characteristics on the signal. Germanium diodes are often associated with a vintage and warm tones. When their forward bias voltage is achieved or exceeded germanium diodes ‘turn on’ with a knee slope that is smoother and softer than their other diode counterparts.
As noted there are several BJT's being utilised in numerous stages, the first BJT doing the heavy lifting in terms of gain. This stage exhibits a potentiometer located at the Emitter terminal. This potentiometer is the gain 'knob' that can be altered external on the surface of the device. Varying the value of resistance at this terminal subsequently affects the amount of current flow through the BJT and affects the amount of gain displayed.
This now amplified signal is met with the noted 'clipping diodes' to clip the signal providing the 'overdrive' sound. Due to the amount of gain being provided by the transistor, without the clipping diodes the transistors would be driven into clipping regardless. However the clipping diodes are detrimental in provide the clipping characteristics of the pedal. This now clipped signals amplitude is at the amplitude of the diodes forward bias voltage (0.2 - 0.3 volts) with the waveforms edges being squared off, showcasing a waveform typical of a clipped/overdriven signal. There is a final transistor implemented to increase the amplitude levels after the clipping diodes, alongside perceived volume loss due to the tone control.
The tone control in the Gatekeeper consists of a passive low pass filter control, utilising a capacitor value fixed to ground in conjunction with a potentiometer altering how much high end frequency content is attenuated. As the value potentiometer value is increased with the tone ’knob’ externally being rotated counter clockwise, frequency from the midrange onwards begin to become attenuated with the filter point moving closer to the midrange as the ’knob’ continues further to be rotated.
Throughout the pedals circuit there are several filter stages implemented to attenuate some low frequency content. This is utilised to maintain some of the low end that could become too 'bloomy' 'flubby' as its run through this non linear circuit. Despite this factor, the overall frequency response isn't particularly treble heavy and still maintains a full tonal structure with an enrichment of low and low mid range with no 'midrange scooping.'
This factors perhaps leaves the Gatekeeper with a 'fuller' and more natural sound that your traditional overdrive pedals with many utilising filter characteristics that are either accentuating or over attenuating frequency points.
Some of the pedals parameters can affect the frequency response, this factor is regarding the Volume control. As your volume 'knob' is turn down this increases the resistance of the potentiometer seen by a coupling capacitor, which is forming an RC filter. This factor will lead to more bass attenuation as this 'Knob' Is 'turned down.' However, with the volume 'knob' at 'full value' there will be no resistance value and no extra attenuation on this frequency range.
WHERE TO FIND THE GATEKEEPER
If The Gatekeeper sounds like a flavour crafted for yourself and wish to purchase the device, this can be achieved here