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AUDIO OUT AUDIO OUT L R By Jake Rothman The Pre-driver guitar overdrive pedal We’ll now move on to the guitarist’s next favourite pedal after fuzz, which is overdrive. This is a preamp/tone control circuit used to overload/drive the guitar amp. This works most effectively if the guitar amplifier is a valve-based design. Typically, the first ECC83 triode stage in the amp can be overdriven, which the guitar alone can’t do. The amplifier’s master volume control can then be turned down, giving some of the sound of an overdriven valve amplifier. This avoids the ear-splitting levels that occur if the output stage is driven into overload. However, most guitarists and sound engineers still want the full AC30 amp overdrive sound from a Celestion G12 speaker cone breaking up along with push-pull EL84 output valve slope distortion, so hearing damage it is. When recording, I used to confine the guitarist and his amp to an isolation booth, but I still suffered hearing damage going in there to find the optimum mic position. Most overdrive pedals are fairly clean in themselves, with typically 0.2% total harmonic distortion (THD), although some do generate a bit of their own fuzz at high outputs. Guitar pedals have a unique culture of funny names, some rather rude. The Klon and Tube Screamers (variations on the Ibanez original) are popular. Two designs I have been involved with have been the Colorsound Overdriver (Photo 1) and the Powerboost (Photo 2), from which this circuit is derived. To avoid infringing anyone’s name, especially litigious Americans, I decided to call this pedal the Pre-driver. Yes, there is a pedal called something similar, but I used a smug hyphen, so I claim it. More gain, more pain Most guitar pedals work at a nominal signal voltage of -10dBu or 0.7V peak-to-peak. The “u” means “not referenced to any impedance”; it’s just the voltage level. This is because the high impedances involved don’t result in any significant power. Most studio equipment and devices like CD players use ‘line level’, 0dBu, which is 2.2V peakto-peak. Professional equipment often uses +4dBu, which is 3.5V peak-to-peak. It is often stated using the old telephone standard, dBm, referenced to 600Ω, but that is rarely the case in reality. The difference between guitar and studio levels often results in clipping if guitar pedals are used as studio effects. In this design, Circuit structure Fig.1 shows the basic system: an input buffer, preamp stage and tone control. There may also be a master volume on the output, allowing the pedal to distort if needed. The tone control is occasionally just a treble boost, as in the Electro-Harmonix Soul Food, but usually at least bass and treble controls are provided. There was a variation of the Electro-Harmonix pedal by JHS with a “meat” modification (to offend vegans?), but it did sound “meatier”. Photo 2: the Colorsound Powerboost is an overdrive pedal with an added JFET input buffer. Photo 1: the Colorsound Overdriver embodies all the standard functions of a good guitar preamplifier. 50 Practical Electronics | January | 2026 Practical Electronics | January | 2026 Baxandall tone control (TR4) Unity-gain buffer (TR1) Bass (VR2) Gain stage (TR2/TR3)) Input Output CW 0V Master volume control (VR4) Treble (VR3) Negative feedback gain control (VR1) + + Fig.1: the basic block diagram of most overdrive pedals. the gain of the preamp varies from More pot +17dB (7×) to +47dB (200×), while problems Photo 3: the gain control (VR1) needs to be an anti-log the tone control is ×1 or unity gain. A major taper pot, written R-LOG on this Omeg example. When Gary Hurst designed the modification Colorsound Powerboost, he based required to the Mullard circuit was a circuit using a normal log pot in it on the 1969 Mullard “Hi-Fi” to make the gain variable. This was Fig.3, but it had less gain. preamp in their Transistor and achieved by adding a pot, VR1, to Diodes D1 and D2 prevent DC Radio Circuits book (page 142 in the lower arm of the feedback net- bias shift when overdriven. The the 1972 2nd edition) and its many work. Originally this was a 4.7kΩ input impedance is lower than the variants published in Practical linear pot, which gave an abrupt original design, so the input buffer Wireless. boost in gain at the end of its rota- needs a 2.2kΩ source resistor. This had a two-stage DC-­coupled tion. That made it difficult to use Another problem with gain congain stage and a single-stage and could result in uncontrolled trol pots is getting the gain to go low Baxandall tone control running acoustic feedback if not adjusted enough. This can’t be done unless on 18V. It used the silicon planar, carefully. +9V metal-cased Mullard BC109 tranLater on, this was changed + 22µF sistors, which were then the latest to a 2.2kΩ anti-log pot (“C” 75kΩ 1kΩ Metal Metal thing. taper), needed because the film film 0V My take on the later Dick Denny gain goes up as the resistance BC549C production circuit, shown in Fig.2, gets smaller, the opposite Input (from 2.2µF BC549C 10µF is very similar but with the resis- to a normal volume pot. FET buffer) Tant tor values reduced to reflect the Unfortunately, anti-log or lower supply voltage. Colorsound reverse-taper pots are hard to Overdrivers made from 1972 used get and are normally specially 100Ω 470Ω a modern plastic-encapsulated made (by Omeg in the case equivalent of the BC109, the Texas of Colorsound; see Photo 3). Instruments BC184L (which had a Tayda makes Alpha antilog 0V central collector pin). pots, but these are difficult to We’ll use the latest incarnation mount. A solution to this is to Fig.3: the 2x 1N4148 of the BC109, the BC549, made put the pot in the upper arm preamp by Philips and later by Fairchild. of the feedback network, al- stage 470kΩ 10µF Tant Log They are still the go-to low-noise, lowing a normal log pot to be using a log low-voltage, high-hFE small-signal used. This can’t be done here, pot. CW audio transistor, and they work out since it would upset the DC about 2p each if you buy a bag of 100. bias conditions. I developed 470kΩ Output 0V + + R1 3.3kΩ Input C2 10nF +2.9V C1 470pF R3 4.7MΩ (2.7MΩ) R8 1.8kΩ (2.4kΩ) R5 180kΩ TR1 2N5457 TR2 +1.44V BC549C R4 22kΩ C3 220nF C7 10nF R13 5.6kΩ +0.81V + +0.26V R6 6.8kΩ C4 22µF + C15 22µF R9 470Ω R10 12kΩ Treble CW +5V TR3 BC549C R7 150kΩ +0.8V +4.5V C5 560pF C6 10µF VR3 100kΩ Lin R14 39kΩ R11 4.7kΩ CW C8 10nF C10 220nF VR2 100kΩ R12 Lin 4.7kΩ Fig.2. the Pre-driver circuit. Use the resistor values in brackets if the supply voltage can exceed 9V. VR1 2.2kΩ Anti-log C9 47nF CW Practical Electronics | January | 2026 Gain Supply voltage 9V Supply current = 5.5mA R15 150kΩ R18 1.8kΩ C14 R19 220nF 100Ω Bass 0V C11 1µF + 0V C13 100µF R2 4.7MΩ + +9V TR4 BC549C +5.35V +1.1V VR4 47kΩ Log R17 220Ω + Output to switch Volume +0.46V R16 33kΩ (22kΩ) CW C12 100µF R20 82Ω 0V TR1 Top view 2N5457 G D S TR1 alternate Top view MMBFJ202 D G S SMD TR2,3,4 Top view BC549C E B C 51 Tone control boost cut (dB) +10 0 –10 –20 –30 –40 20 50 100 200 500 1k 2k Frequency (Hz) 5k 10k 20k Fig.4: the tone control curves. Note the scale is compressed by the use of the 50dB scale. The boost and cut is ±14dB at 50Hz and 10kHz. the CW terminal of the 2.2kΩ log pot is switched out to give full negative feedback. A gain of 4.4dB (1.66×) is then obtained. You try getting an anti-log pot with a switch! I do have some dual-gang 5kΩ anti-logs with switches that I had specially made for mic preamps. With the two gangs connected in parallel, they work well here, but their body is 40mm deep, which means that they won’t fit in a low-­ profile pedal box. They would have to go on the side; it’s simpler to put in a separate switch. At the moment, you have to cut the pot tag, which is a bit messy. I’ll put a proper switch connection on the next batch of boards. Input buffer I think guitars sound better feeding high-impedance JFET inputs, so I added a JFET input buffer to the circuit. The Colorsound Overdriver is actually the original Powerboost, having no buffer, while the current Powerboost has one. The “Aussie” Powerboost variant, made for Pink Floyd tribute bands, generates a spike at the verge of clipping achieved by leaving off C15. This ‘upsets’ the distorting guitar amplifier in an interesting way. This helps achieve that David Gilmour (of Pink Floyd fame) sound. Queen guitarist Brian May is famous for using a simple germanium treble booster to drive a tiny transistor amplifier built by bassist John Deacon in 1972. The half-watt amp, based on a Class-AB germanium output stage and fitted with a six-inch Elac speaker, produced a surprisingly rich, saturated tone when overdriven. May used it extensively for layered guitar harmonies on Queen recordings. The unit became known as the Deacy Amp and it remains an integral part of May’s distinctive sound (I described the Deacy Amp in some detail in June 2015). 52 Volume control A master volume control was added to the Powerboost and Overdriver around in 1990, mounted on the side of the box. This is done here as well, using VR4. It enables it to be used as a fuzz box to a degree, but it is not as good as a dedicated unit because the high negative feedback used means the onset of distortion is abrupt. The Pre-driver works well driving power amps directly to make a complete guitar amp. Tone control The Baxandall tone control stage is completely conventional, although the 100kΩ pots would be considered too high in value today in a hi-fi preamp because of the high Johnson noise generated. The value used in the original circuit was 220kΩ. I found that using more readily obtainable 100kΩ potentiometers made no difference. The tone control curves are shown in Fig.4. A single-transistor stage would also be frowned upon in hi-fi these days because the THD at full boost is around 1%. However, for electric guitarists, it doesn’t matter because the hum and noise of the high-­ impedance magnetic pickups completely dominates the noise floor, and distortion is actually a goal. I suspect some guitarists even like ‘resistor grunge’, with some pedal makers using original carbon-­ composition resistors. I spend a lot of my time removing them from old audio equipment. One advantage of using high-­ value resistors, of course, is low current consumption. The Pre-driver draws just 5.6mA from the battery, leading to a long battery life, which is important for those extended jam sessions! Power supply It’s standard for most guitar pedals to run on a 9V ‘PP3 style’ block battery, which is the minimum voltage to avoid signal clipping at standard guitar signal output levels. In this case, clipping occurs with a 5V peak-to-peak signal level with the tone controls flat, the gain set to its minimum and the volume control at its maximum. Increase the supply voltage for a more effective overdrive and less chance of hard transistor clipping. A higher voltage can be obtained by clipping two PP3s together in series, or by using an external power supply, like a plugpack (eg, 12V DC or 15V DC). Colorsound offers an 18V Photo 4: the master volume control is mounted on the right-hand side of the box (viewed from the top), off the PCB. Practical Electronics | January | 2026 original version of the Powerboost built by Stu Castledine for Macari’s. Some pedals use an up-converter, but this can cause switching noise break through and it will also flatten a PP3 battery much more quickly (a higher output voltage implies a higher input current; there’s no such thing as a free lunch!). This design is re-biased so that it can be used at either voltage. The original 9V values are on the PCB, with the values for a higher supply voltage (up to 18V) shown in brackets on the circuit diagram in Fig.2. The current consumption at 18V increases to 13mA. Building it The only extra bit to the construction compared to the Transmanium fuzz box, described over the last two issues, is the master volume control potentiometer mounted on the side of the box, as shown in Photo 4. We’ll get to assembly of the main circuit board soon, but first… Drilling The best way to drill the PCB mounting and pot holes is to mark their positions on the case using the PCB as a template. I find I’m more prone to errors when measuring them with a ruler. I have also prepared a drilling diagram, available at https://pemag.au/link/ac9b I use 10mm holes for the potentiometers rather than the ‘correct’ ⅜-inch/9.5mm to give a bit of clearance/wiggle room. To prevent the drill bit wandering, make an indentation with a centre punch (or, in a pinch, a nail hit with a hammer). It’s always wise to use a deburring tool on the holes to remove sharp ridges (you can use a larger drill bit, twisted by hand, as long as it’s sharp). It is especially important to remove any paint on the edges of the holes to ensure good Earthing of the component bodies, since the case is grounded through the pot bushes from the PCB. Also, touching the knob of a floating pot can cause hum to be injected into the signal. Even the 12.5mm switch and 10mm master volume holes must be clean to Earth their metal bodies. Marking out for the jack board is more difficult, especially the 8mm DC connector hole, which is 1.5mm higher than the jack holes (11mm). Jack board I took this second pedal as an opportunity to redesign the jack board. By making the holes for the Practical Electronics | January | 2026 Parts List – Pre-driver pedal 1 main PCB, 84 × 74mm, coded AO-JAN26-1 1 jack board MkII, 72.5 × 25mm, coded AO-JAN26-2 1 Tayda 1590DD 190 × 121 × 37mm diecast aluminium case 1 PP3 battery snap/connector with 200mm-long wires [Rapid 18-0093] 1 battery mounting spring clip [Rapid 18-3480 – Comfortable BH980] 1 PCB-mounting 2.1mm ID barrel socket [Rapid 20-0970 TruConnect, Tayda A-4118] 2 stereo 6.3mm PCB-mounting jack sockets [Tayda A-5079] Hardware & wire 6 80 × 3mm cable ties [Rapid 04-0630] 1 cable tie mounting clip/eyelet [Rapid 04-0600] 10 500mm lengths of 7/0.2 red, orange, yellow, green, blue, purple, pink, brown, grey & black equipment wire 3 Tayda A-7023 potentiometer shaft adaptors 3 ⅜-inch potentiometer nuts [Mouser 594-511-3002, MPN Vishay ACCTR511-3002] 3 M7 Alpha potentiometer nuts [Tayda A-5037] 3 M3/M4 potentiometer grub screws (if needed) [Tayda A-7804 or A-7805] 6 12.5mm rubber hole plugs [Farnell 896603] 6 M3 × 12mm countersunk head screws and hex nuts Semiconductors 1 2N5457 N-channel JFET (TR1) 3 BC549C NPN bipolar transistors (TR2-TR4) 1 3mm orange 3mm high-efficiency LED [Tayda A-264, Rapid 72-8972 (preferred) or Kingbright L-7104SEC] Capacitors 1 100µF 25V electrolytic (C13) 1 100µF 6.3V electrolytic (C12) 2 22µF 10V electrolytic (C4*, C15) 1 10µF electrolytic (C6*) 1 1µF 10V electrolytic (C11*) 3 220nF polyester ±20% (C3, C10, C14) 1 47nF polyester ±20% (C9) 3 10nF polyester ±10% (C2, C7, C8) 1 560pF ceramic disc ±10% (C5) 1 470pF ceramic disc ±10% (C1) * can be replaced with tantalum for lower pot rotational noise Potentiometers* (all ⅜in 10mm-long bush, ¼in shaft, >20mm diameter body) 2 100kΩ or 220kΩ single-gang linear (VR2, VR3) 1 22kΩ or 47kΩ single-gang log (VR4) 1 4.7kΩ or 2.2kΩ single-gang anti-log (VR1) 4 pointer knobs to suit VR1-VR4 (four different colours if possible) [RS or Tayda] * preferably Omeg (best) or Alpha (good) brand Resistors (all ¼W ±5% metal or carbon film) 2 4.7MΩ (R2, R3) 1 33kΩ (R16) 1 5.6kΩ (R13) 1 180kΩ (R5) 1 22kΩ (R4) 2 4.7kΩ (R11, R12) 2 150kΩ (R7, R15) 1 12kΩ (R10) 1 3.3kΩ (R1) 1 39kΩ (R14) 1 6.8kΩ (R6) 2 1.8kΩ (R8, R18) 1 470Ω (R9) 1 220Ω (R17) 1 100Ω (R19) 1 82Ω (R20) Substitutions for 18V operation: R3: 4.7MΩ → 2.7MΩ R8: 1.8kΩ → 2.4kΩ R16: 33kΩ → 22kΩ All parts listed here are available individually from the AOShop (see page 49). 53 Photo 5: the revised jack board has some minor improvements. Photo 6: the main PCB, with reassuring symmetry and pretty C280 capacitors. This is an interesting example of where poor-quality components can boost sales! power socket slot-shaped, it was possible to put the LED in a much better position, avoiding the need for wiring. This should be soldered after the board is mounted to prevent strain. I also reversed the input and output sockets, in line with guitar convention. The new Mk II jack board overlay is shown in Fig.5 and is seen assembled in Photo 5. It is not essential to use the jack board; some constructors may prefer to hardwire it. Nothing’s ever completely finished in electronics. I’m now doing a Mk III jack board because I want the battery wires on the left where the battery clip is. Right now, the 200mm battery connector is only just long enough. I’ll show this board next time. Main PCB The main PCB overlay is shown in Fig.6 and you can see what the assembled board looks like in Photo 6. An SMT/SMD JFET option for TR1 is provided, visible next to the yellow-­topped capacitor. For a while, discrete leaded JFETs became very expensive. However, I noticed that the situation has abated somewhat; for example, BF256s now cost just 16p at Rapid. The SMT devices are still cheaper, though. C8 1 Amplifier Blue Yellow Output 3 R R 20 19 R14 R7 C 3 C 9 Pink To switch (S2) output Black Gnd Black PCB Gnd Red To PP3 battery clip Red Grey Switch (S2) Brown E B C TR2,3,4 BC549 C1 R 1 TR3 TR4 C 10 C 14 C 7 C 11 + R R R R 4 6 5 8 VR1 Gain C4 C13 PCB 9V Switch (S3) Fig.5: the new jack PCB. Some early boards may be marked for positive Earth. Ignore that; follow this overlay. 54 G D S TR1 2N5457 + Violet Switch (S1) LED TR1 TR2 C15 CW C6 + Gnd (spare) LED under board C 5 + + C 2 R13 R9 + + – R R 3 2 VR3 Treble CW + DC input (centre negative) R R 11 10 VR2 Bass VR4 Volume CW Guitar CW R12 C12 R18 R17 R15 R16 20 swg link Fig.6: the main PCB overlay. Input V+ 0V 0V Practical Electronics | January | 2026 Photo 7: the pedal wiring. Note the use of cable ties; red wire is for +, black and green for Earth. I used the J201 from Tayda, which has lower noise (the J202 is also suitable). Although source-­followers are fairly insensitive to different JFETs (because of the 100% negative feedback), it still might be necessary to adjust R2 or R4 for maximum headroom. It is worth checking if you use something very different from the ubiquitous 2N5457. Be careful with the pin-out for the gate. Sometimes it’s the middle, upper or lower pin. Since JFETs are symmetrical at audio frequencies, the drain and source pins can be swapped, making substitution easier. The assembly procedure is virtually the same as the fuzz pedal given last month, except that the component – 9V values and positions are different, of course. Pay attention to the orientations of the transistors and axial electrolytic capacitors; they must all be orientated as shown in Fig.6 for the circuit to work properly. Wiring up The need for screened leads can be avoided if the purple input lead is well tucked in along the corner of the metal box. Instability is avoided by having the output lead run up the other side. The inverting nature of the circuit also helps. This is all shown in Photo 7. A little trick here is using a loop of thick (20SWG; 0.9-1mm diameter) wire between the two Earth connecFX output from VR4 wiper + Black A switch wiring close-up is shown opposite. Red NC switch contact Centre pin Outer 2.1mm DC connector Input from jack board Red LED jack board 24 SWG link tions on the lower-left of the PCB to act as a tie point for the battery leads. Finishing up Unpainted diecast boxes labelled with Dymo tape are functional but have a rather austere look to them. To brighten things up a bit for Christmas, I used some coloured knobs from RS! (See page 2). Next time The next pedal will be a silicon diode fuzzbox. The PCB is being designed by Mike Grindle to take conventional cheap 16mm pots, so no more nutty struggles. The circuit allows for Silver Tonebender and Big PE Muff variations to be made. Pink Grey S2 Violet S1 Brown V+ 24 SWG link 0V S1 Orange Pre-driver Input Output S2 Ring Tip Jack stereo socket (switch contact not used) (power switched here) 0V 0V To jack board ground (black) 0V Blue Ground to switch (green) 5.6kΩ 0.25W Output Pink Tip NC VR4 47kΩ Log Brown V+ Practical Electronics | January | 2026 CW Rear of 3PDT switch (Poles are centre pins in eacch switch section) Grey Yellow Jack sleeve FX input from main board S3 Green 0V Input Orange Output from jack board S3 Effect On 0V Jack sleeve Green 0V Fig.7: the wiring diagram. This is similar to last month’s fuzz box, but uses a conventional negative Earth and with a master volume control added. 55