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AUDIO OUT AUDIO OUT L R By Jake Rothman A silent stomp switch All the previous guitar pedals I’ve described over the last few months have had a large mechanical latching stomp switch with three double-pole/changeover sections, like the one shown in Photo 1. That setup is particularly neat and I want to do something similar for my future pedals. In case it isn’t obvious, it’s called a stomp switch because you press it with your foot, and depending on how passionately you are playing, you might be pressing it rather vigorously! Hence, such switches must be robust. This is an expensive and elusive component, which is prone to failure, especially regarding the latching mechanism. The way around this, used by volume pedal manufacturers such as Boss, is electronic switching using a cheap single-pole momentary switch. It usually works out cheaper as well, but alas not for the home constructor who cannot benefit from quantity discounts and automated assembly. The Boss circuit used in the FZ-2 pedal forms the basis of the design given here. It can be retrofitted into existing designs if required, space permitting, as shown in Photo 2. to note that he was barefoot the entire time on stage! I bet he prefers a soft stomp switch…] Silence is golden All mechanical latching switches, especially stomp switches in metal boxes, produce a loud acoustic crack when pressed. Some guitarists find this noise reassuring, although it should be obvious if the effect has been engaged, especially if it’s fuzz. These switch cracks can spoil a vocal recording if somebody is singing and playing at the same time. Grindle, the board designer and pedal maker, also informs me that there is a type of bedroom musician who just wears socks! They like the feel of a “soft” switch. [Editor’s note – I attended a performance by top UK musician Steven Wilson recently (who plays many instruments, including electric guitar and bass) with his touring band, and it was interesting Switching transients Switching audio waveforms usually produces audible artifacts. Mechanical switches ‘slice’ the waveform which, along with switch bounce, produces high-­frequency clicks. Electronic switches tend to make a ‘soft LF thump’ with a bit of delay as they ‘fade’ in/out. This is because most use JFET switching elements with a ramped control voltage. This design is no exception. Distortion and level Electronic switches have more distortion, and can only handle signal swings up to a couple of volts below the power rail. The low-order Photo 1: a 3PDT mechanical latching switch in an Electro Harmonix Holy Grail reverb. Photo 2: a Colorsound Overdriver pedal retrofitted with the silent stomp switch circuit. 4 Practical Electronics | March | 2026 harmonic distortion of around 0.1% generated by the JFETs is no problem with guitars, and is possibly desirable, depending on the effect the musician is going for. Hard-bypass switching Mechanical switches can offer the advantage To Audio of not stopping the sound Board FET if the power goes off, if inputs socket wired with an extra input switch section, called hard-bypass. Electronic switches need power to pass a signal regardless of their state. Hard-bypass avoids frantic unplugging of a faulty pedal while playing if the battery dies. Since JFETs are conducting when biased at 0V, it may be possible to design an electronic switch with hard-bypass. The trouble is that buffers are needed as well, and they won’t work without power. Input jack R26 1MΩ Fig.2: the switch board control circuit is a blast from the past. D3 1N4148 R25 1MΩ D4 1N4148 R24 1.8kΩ R16 56kΩ TR7 BC549 B A D1 1N4148 +9V ZD1 4.7V LED1 Highefficency C D2 1N4148 To Audio Board C15 100nF C16 220nF C10 470pF C11 470pF R17 100kΩ R20 100kΩ R18 47kΩ C12 220pF C13 220pF R23 1MΩ R19 56kΩ + C1 100µF 25V TR8 BC549 R21 47kΩ 0V To Audio Board C14 10nF R22 100Ω SW1 sistor’s base, formed the basis of many late-fifties and early-sixties computers. My grandad used to get me scrap computer boards full of these flipflops from Metrovicks back in 1971. They held lots of Mullard OC42 germanium transistors with long leads, worth money today. If resistors R17 and R20 are reThe system placed with capacitors, the physics It still shocks me how much teacher’s favourite circuit results: electronics is needed to replace a the flashing light multi-vibrator. simple analog mechanical switch. One problem with this symmetEven a guitar pedal stomp switch rical circuit is that its initial state needs eight transistors, or a few upon switch-on is undefined. Due chips, to do the job properly. This to transistor variations, such as is more than a typical fuzz box. The differences in turn-on voltage (Vbe), thing to remember is that commodone side will usually become active ity ‘jelly bean’ components are very Switch latching circuit before the other, but you won’t cheap, especially in surface-mount The circuit for this board is know which. packages. shown in Fig.2. The switch latching One way to define it is to add a This circuit also has a JFET input part of the circuit is based on the deliberate imbalance or a start-up buffer and an output buffer, which classic two-transistor discrete flip- circuit. In this case, because one will enhance the performance of flop or monostable. This instantly side (TR7) drives the LED, and it most pedals. This was the case recognisable configuration, using has 10× more collector current, it c r o s s - c o u p l e d needs more voltage to switch on, Audio Board (upper) Bypass buffer feedback paths resulting in the other side (TR8) (TR3) Bypass JFET from one tran- switching on first. It’s theoretically switch (TR4) sistor’s collector not 100% guaranteed, though. Input buffer Output buffer to the other tranGuitarists generally prefer the (TR1) (TR6) circuit to start up with no effect engaged (with the LED off) until Output FX input FX output jack the switch is pressed, as happens switch (TR2) switch (TR5) 0V here. If you have trouble, the +9V circuit can be deliberately unbalTo buffers V+ anced by altering the values of the OUT A IN B C 0V V+ trigger capacitors C12 and C13. Switching Board (lower) FX C10 and C11 are speed-up capacIN OUT itors, which make the triggering LED Fig.1: the block more definite. Going to the switch diagram of the part of the circuit, C14 is charged silent stomp up by R23. When pressed, the switch system. It switch quickly discharges C14 to Ramp is split into two trigger the circuit. R22 is Bistable PCBs: the switch Push switch needed to limit the current (TR7/TR8) to change board and the through the switch contacts. Ramp audio board. R25, R26, C15 and C16 form ramp networks to smooth the control voltage to the Practical Electronics | March | 2026 with the Colorsound Overdriver, which has an input impedance of 50kΩ and an output impedance of 25kΩ, both non-optimum values for guitar use. These are improved to 1MΩ for the input impedance and 200Ω of output impedance with this circuit. The block diagram is shown in Fig.1, with the system being split into two PCBs, one for the switch latching circuit and the other for the audio circuit. This partitioning allows each board to be used in its own right in other systems. It also enables the boards to be stacked on top of each other, with the switch underneath, for compactness. 5 +9V + TR2 J113 C5 10µF +4.5V 10V IN +5.7V +4.5V Half-rail R2 47kΩ +4.5V R4 1MΩ + C2 100µF 10V R5 10kΩ R6 1MΩ R7 1MΩ A Cont input R9 1MΩ FX TR4 J113 C7 TR5 1µF 10V +4.5V J113 +4.5V + +4.5V + C4 1µF 10V R8 10kΩ Power supply The circuit has the basic power supply housekeeping, consisting of the main decoupling capacitor C1, which is also used to feed the audio board. Reverse polarity protection is provided by diode D1. This is not really necessary, but I’ve known guitarists stick all sorts of power supplies into pedals, including a 33V Hewlett Packard printer supply of reverse polarity, causing the decoupling capacitor to explode! 6 TR6 BC549C +3.3V C9 10µF +4V R10 1MΩ R11 1MΩ R12 1MΩ R15 100Ω R13 10kΩ R14 100kΩ Output B Cont input 0V This part of the circuit is shown in Fig.3. Since the circuit uses a relatively low voltage (+9V) rather than the more normal audio ±15V (30V total), we have to use a lower voltage pinch-off (Vp) JFET for the audio switching. The normal J111 and J112 used in professional audio have too high a maximum Vp to work. So, we use the J113 here, which is very cheap and is cut off at a maximum of -3V (relative to the half-supply rail). This allows the circuit to work down to 6V, where the battery is getting a bit flat. It is an inferior switching device, having an on resistance (Ron) of around 100Ω. That’s three times more than the J111, which means three times more distortion, but this is not a Hi-Fi application, so it’s no problem. The original Boss and Roland pedals used the 2SK30, which is now rare and expensive. A strange choice, since its main attribute is a Audio circuit This board contains the half-rail bias generator, consisting of the divider formed by resistors R1 and R2, decoupled by capacitor C2. Photo 3: this cheap momentary push button from Rapid saves almost £2.50 compared to the 3PDT switch. C8 1µF 10V OUT 0V switching JFETs in the audio board, preventing clicks. Interestingly, having two different ramp times for the bypass JFET (TR4) and the effect JFETs (TR2 and TR3) gave minimum thump. Diodes D2, D3 and D4 protect the JFET gates from possibly being destructively forward-biased under fault conditions. They also seem to reduce distortion, especially if used with a boot-strapping resistor from drain to gate, but I’ve not tried this in a single-rail circuit (such as this) yet. Fig.3: the audio board circuit; nary an op amp in sight, just discrete transistors. + Input C3 R3 10kΩ 47nF TR1 J113 + R1 47kΩ C6 1µF + TR3 BC549 C Cont input Photo 4: this Tayda switch is rugged and looks good. The switch board has a special hole to accommodate it. high Vds rating of 50V, but the Vp is also low at -0.5 to -2V, while Ron is high at 300Ω. JFET series switches give acceptable results when driven from a low impedance and loaded by a high impedance. Buffer transistors TR1, TR3 and TR6 provide these conditions. TR1 uses a JFET to provide a better match for the guitar input. TR6 should be a high-hFE (>500) device, such as a BC549C, to maximise the input impedance and minimise the voltage drop on R12. It would offer a slight improvement to use another JFET, but they cost ten times as much. It may look as if the buffer (TR3) on the bypass path is superfluous, but it is necessary to provide isolation to prevent positive feedback occurring around high-gain non-inverting effects, such as Big Muff fuzz units, during switching. With some unity-gain effects, such as chorus, TR3 can be linked out and the emitter resistor R8 omitted. Another bit that is optional in some effects is the input switch TR2. It does little damage if the input to, say, a phaser is left connected during bypass. It is a different matter with a fuzz unit, though, because the harsh square wave can couple through into the clean bypass. Further simplification Boss did many variations of their switching circuit. The one used in some chorus pedals like the CE-2 is shown in Fig.4; it is greatly simplified, using only two JFETs and an op amp. It was previously shown in Audio Switching Part 4, September 2024, in Fig.81 on page 50. The circuit still needs the switch board to provide the JFET control voltages, however. It will be worth getting an alternative audio board together for this in a later pedal. Practical Electronics | March | 2026 +4.5V bias 1MΩ + + + R21 220kΩ SW1 R16 C12 C11 – 1N4148 10kΩ C13 + ZD1 LED1 R17 + TR8 50kΩ 1N4148 R24 0V R18 + R19 1µF Effect Off 0V Switch selection One has to be careful in selecting the switch. I bought some Alpha momentary foot switches from Rapid (78-0743) that made a loud click when pressed, defeating the whole point. It is possible to save money by using large generic momentary push buttons, such as the SCI R13507MA (Rapid 78-0183), shown in Photo 3, which cost £0.52. They will need an extra nut to make them tall enough for foot operation, though. The best one is the A-1091 from Tayda, which looks like a proper stomp switch, shown in Photo 4. Sadly, it’s relatively expensive for a momentary switch, at around £1.97. Soldering the switch is quite D C 4 B D A 3 D C 16 2 Fig.5: the switch board overlay. The odd-shaped hole took some work. 0V On R25 R26 22kΩ 220kΩ C1 R20 Mix 2SK30 C10 4.5V 1µF FX C 15 TR7 Output V+ supply (red) 0V (black) D 1 R23 4558 Buffer Input R22 10µF 2SK30 +4.5V bias C14 +9V to Audio PCB 0V to Audio PCB 0V to Audio PCB + Fig.4: the simplified Boss switching circuit used in some chorus pedals. 1MΩ It removes one JFET and has no buffer transistors. Note the effect 10µF mix control. TR1 R R R TR2 1 2 5 +9V from Switch PCB 0V from Switch PCB 0V from Switch PCB tricky, with its tags fitting into slots on the board; the PCB may need a bit of filing to get them to slide in. A l s o , i t ’s i m p o r t a n t t h a t everything is straight and aligned before soldering, so that the board is level when the switch assembly is bolted in. Finally, I’ve found that some of these switches I’ve stored in the damp British air for a year or so need cleaning or they won’t solder. Remember that the thermal capacity of the tags is much greater than the PCB, so hold the iron’s bit to the tag first and let the tag’s heat do the soldering to the board. Because of the heat retention, the solder will take a long time to solidify, so don’t let it move while Photo 5: the completed switch board. You may need to connect the LED via wires depending on where it’s... C2 + C4 R8 R3 R4 R7 R 6 + TR3 Input jack socket (violet) C3 C5 + + C7 TR6 V+ R R R R R R R 9 10 11 12 13 14 15 TR4 C6 + C9 TR5 + C8 + To FX input (orange) 0V (black) From FX output (pink) C control input from Switch PCB B control input from Switch PCB A control input from Switch PCB Output jack socket (grey) Fig.6: the audio board overlay with optional SMT pads for the JFETs. it’s cooling. The finished switch PCB is shown in Photo 5. Remote control It is a simple matter to wire a transistor across the switch contacts to obtain a control input. This can be very useful if a particular configuration of effects is required to be auto-switched. There also seems to be a recent convergence of Eurorack synthesists and guitarists who would like the idea of gated fuzz. Assembly and components Fig.5 shows the overlay for the switch board and Fig.6 is the audio board. They are both fairly compact by guitar pedal standards, so require more care in construction. Note that the square pads on electrolytic capacitors do not necessarily denote the positive leads; check the + symbols in Figs.5 & 6 rather than trusting the pad shapes. to be mounted. In the Overdriver, it’s at the top of the box. Practical Electronics | March | 2026 JFET selection Provision is made on the audio board for the use of SMT JFETs, which are cheaper than the leaded types. A common problem with JFETs is the orientation of the gate. On most packages, it is not the 7 middle pin as one would hope. The other pins, the drain and source, can be interchanged because JFETs are symmetrical. The finished audio PCB is shown in Photo 6. Note the bent-over electrolytic mounting to obtain a low profile. Grindle and I use this technique in many of our commercial designs. It also allows an upgrade to tantalum bead capacitors if needed (or desired). Testing Get the switch PCB working first before attaching to the audio section. It is essential to check the DC voltages, the power rail (especially if using a battery), the 4.5V bias (Vb) and the others marked on Fig.3. After that, check the current consumption, which should be reasonably close to 1.7mA while off and 3mA while on, for the whole unit. If desired, the unit can be powered with an 18V supply, but the current then rises to 3.2mA and 9.8mA, a bit high for battery use. If there are problems with the audio transmission, it is usually the JFETs being incorrectly wired. I have become dependent on using Photo 7: the finished stomp switch assembly. Note the two boards plug into each other using pins and headers. the Peak DAC75 analyser to identify pinouts. The pinout diagrams in data sheets/books are often wrong or unclear as to whether is a top or bottom (pin) view. Installing the unit Photo 7 shows the ‘sandwich’ construction, with the audio board plugging into the switch board. Later boards may have some mechanical mods to stop the audio board Photo 6: the completed audio board. This was the prototype board so there are some minor differences... ... such as some + markings on the capacitors changing, plus TR3/TR6 being mounted in a different orientation. Follow the overlay diagram, Fig.6, as it will correspond to supplied boards. 8 Practical Electronics | March | 2026 falling off. If the board-to-board connection were hard-­ s oldered rather than connected via a plug and socket, that would not be a problem, but then service and testing would be. An extra switch-to-board mounting plate to improve rigidity might be a future mod. One thing that worries all pedal makers is that the main switch nut becomes loose, and the user keeps stomping on it, breaking the PCB underneath. The way to avoid this is to use the supplied lock washer between the switch body and the case. Always tighten with a box spanner rather than pliers to get sufficient torque and to avoid scratching the box. Pliers are not the right tool for the job as the jaws do not open parallel. Putting it to bed So there you have it. I’ve replaced over 20 mechanical parts in the switch with 50 extra electronic bits to avoid a click. Does this count as technological progress? I must make a silent pull-switch for our bathroom light so I can get PE some sleep. Parts List – Silent Stomp Switch 1 double-sided switch PCB coded AO-MAR26-1, 40 × 56mm 1 double-sided audio PCB coded AO-MAR26-2, 32 × 56mm 1 momentary ‘soft’ foot switch [eg, Tayda A-1091] 2 3-way 0.1-inch square-pin PCB plug and header pairs Semiconductors 4 J113 N-channel switching JFETs (TR1, TR2, TR4, TR5) 4 BC549C NPN high-gain bipolar signal transistors (TR3, TR6, TR7, TR8) 4 1N4148 or similar small-signal diodes (D1-D4) 1 BCY88C4V7 4.7V 400mW Zener diode (ZD1) 1 high-brightness 3mm red LED (LED1) Capacitors (all ±20% tolerance or better) 1 100µF 25V radial electrolytic (C1) 1 100µF 10V radial electrolytic (C2) 2 10µF 10V radial electrolytics (C5, C9) 4 1µF 10V tantalum bead or radial electrolytics, 2.5mm pitch (C4, C6-C8) 1 220nF polyester film, 5mm pitch (C16) 1 100nF polyester film, 5mm pitch (C15) 1 47nF polyester film, 5mm pitch (C3) 1 10nF polyester film, 5mm pitch (C14) 2 470pF 50V ceramic, 5mm pitch (C10, C11) 2 220pF 50V ceramic, 5mm pitch (C12, C13) Resistors (all ¼W ±5% carbon film or better) 10 1MΩ (R4, R6-R7, R9-R12, R23, R25-R26) 3 100kΩ (R14, R17, R20) 2 56kΩ (R16, R19) 4 10kΩ (R3, R5, R8, R13) 1 1.8kΩ (R24) 4 47kΩ (R1, R2, R18, R21) 2 100Ω (R15, R22) JTAG Connector Plugs Directly into PCB!! 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