Most guitar pedals obtain a high input impedance simply by using a large resistor at the input of the first opamp, but this generates a good deal of noise due to the input bias current. The Glass Blower avoids t his by using as my resistance (R2) which is bootstrapped by C2 to an effective value of tens of megohms. The total input impedance of the circuit is then set mainly by R1, which does not carry any DC bias current. Because most guitar pedals use a 9 V supply as standard, their output swing is limited to about 6 Vpp with ordinary opamps, and this is barely enough to cause clipping in the first stage of a tube amp. The Glass Blower doubles these figures without requiring a greater supply voltage, and so can produce ‘early Jesus & Mary Chain’ i.e. very high levels of additional tube overdrive. This is achieved by driving T1 and T2 with the output signal, which forces pins 4 and 7 of IC2 to follow the audio signal, effectively bootstrapping the power rails. With a rail-to-rail opamp for IC2, an output of 16 Vpp (!) can be obtained with an ordinary 9 V battery. The voltage across the opamp remains constant, however, so there are no worries about damaging the opamp even with supply voltages up to 30 V. To avoid instability at high gain and input levels, individual opamps should be used, not dual opamps.
R7 sets the maximum gain to
1 + R6 / R7
or 22 (27 dB) using the component values shown. For use with humbucker pickups, a value of about 1 kΩ for R7 may be more appropriate, to avoid clipping at maximum settings. Switch S1 is an ordinary, latching footswitch (e.g., Maplin # N84AR). The power supply is of the conventional type used in guitar pedals. Either a 9 V PP3 battery or mains power adapter can be used, and the pedal is only switched on when a mono guitar plug is inserted into the stereo input jack. The author’s prototype was built in a 116 × 64 × 30 mm aluminium enclosure (suggest Maplin LH71N and Rapid 303540, or Maplin GU62S and Rapid 303539 for the more experienced constructor!) The 2.1 mm DC socket must be an insulated type since the centre pin is grounded (e.g., Rapid 200980, Farnell 1137744, Maplin FT96E). The input/output jack sockets (6.35 mm) should ideally be of the insulated type, but non-insulated ones will do (e.g., Maplin HF92A or HF93B).
For guitar pedals, it is inconvenient to have all the sockets/controls on a single circuit board they are panel mounted and wired to the PCB by hand. The author’s design for a PCB and the associated wiring diagram may be downloaded from [1]. Compared to the schematic shown here, small differences exist in respect of component reference numbers.
[1] www.elektor.com/100165
