Innovative Input Impedance Enhancement in Guitar Pedals
Traditionally, guitar pedals employ a large resistor at the first opamp input to achieve high input impedance, but this method introduces noise due to input bias current. The Glass Blower, however, sidesteps this issue by utilizing a resistance (R2) that is effectively bootstrapped to tens of megohms through C2. The circuit’s primary input impedance is predominantly determined by R1, which carries no DC bias current. Standard guitar pedals operate on a 9 V supply, limiting their output swing to approximately 6 Vpp with ordinary opamps. This limitation barely causes clipping in the first stage of a tube amp. The Glass Blower overcomes this constraint without requiring a higher supply voltage.
It achieves this feat by driving T1 and T2 with the output signal, compelling pins 4 and 7 of IC2 to track the audio signal. This innovative technique bootstraps the power rails, enabling remarkable enhancements. With a rail-to-rail opamp for IC2, an astounding output of 16 Vpp can be attained with a conventional 9 V battery. Crucially, the voltage across the opamp remains constant, eliminating concerns about damaging the opamp, even with supply voltages up to 30 V. To prevent instability at high gain and input levels, it’s recommended to use individual opamps instead of dual opamps.
Tailoring Maximum Gain with R7
R7 plays a pivotal role in setting the maximum gain, adhering to the formula 1 + R6 / R7 or a remarkable 22 (equivalent to 27 dB) with the specified component values. However, for applications with humbucker pickups, a more suitable value for R7 might hover around 1 kΩ. This adjustment helps prevent unwanted clipping at maximum settings.
Efficient Power Control and Supply
Switch S1 serves as an ordinary latching footswitch, such as Maplin # N84AR. The power supply adheres to the conventional standards commonly found in guitar pedals. It accommodates both a 9 V PP3 battery or a mains power adapter. The pedal activates solely when a mono guitar plug is inserted into the stereo input jack.
Enclosure and Socket Considerations
The author’s prototype is housed within a compact 116 × 64 × 30 mm aluminum enclosure, for which suggestions include Maplin LH71N and Rapid 303540, or Maplin GU62S and Rapid 303539 for those more experienced in construction. The 2.1 mm DC socket is of the insulated variety since the center pin is grounded, offering options like Rapid 200980, Farnell 1137744, or Maplin FT96E. While the ideal choice for input/output jack sockets (6.35 mm) is of the insulated type, non-insulated alternatives, such as Maplin HF92A or HF93B, are also acceptable.
Flexible Component Mounting for Enhanced Utility
In the context of guitar pedals, it’s cumbersome to have all the sockets and controls directly integrated into a single circuit board. Instead, these elements are panel-mounted and meticulously wired to the PCB by hand. The author provides a downloadable PCB design and the corresponding wiring diagram, offering flexibility and customization. It’s worth noting that slight variations in component reference numbers may be encountered when comparing the provided schematic to the downloadable resources.
[1] www.elektor.com/100165
