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Stereo signal processing Deep adjustable notch Q and frequency adjustments Silent power-on and power-off Flexible power requirements Power supply: 9-15V DC <at> <100mA Notch Frequency: 50Hz or 60Hz Notch adjustment: ±2% (±1Hz <at> 50Hz) Q adjustment: 7-14.5, with 10.5 recommended Notch depth: typically >30dB Mains Hum Notch Filter Project by John Clarke Long unbalanced audio signal leads can pick up significant mains hum. This stereo mains hum notch filter can help to reduce it to inaudible levels. SC7598 Kit ($50 + postage): includes the PCB and all onboard parts. You just need to add the case and power supply. W hen using long audio leads between a signal source and amplifier, most of the time, there will be mains induction in the leads from nearby power wiring. If balanced leads are used, the amount of mains signal pickup will be the same in the twisted pair signal wires and can be cancelled out at the receiving end. But with unbalanced leads, the hum pickup remains. Proper grounding methods and using balanced leads with unbalanced-­ tobalanced and balanced-to-­unbalanced converters at each end will prevent hum in most cases. Sometimes, full galvanic isolation between sections is necessary due to different signal grounds and can be accomplished using audio isolation transformers. These techniques are fully detailed at siliconchip.au/link/ac9f siliconchip.com.au So, with the correct Earthing methods and use of balanced leads and isolation transformers where necessary, you shouldn’t have any hum. But what if, for example, you are living in a house that is already wired up with audio leads and there is hum? The best method to remove it is to rewire it using balanced leads with converters at the signal source and receiver ends, but that may not be practical. That leaves the possibility of removing the hum with an audio filter. The effect on audio frequency response will be minimal, provided that the filter produces a deep and narrow notch that centres around the mains frequency. Note that the filter will only work if the mains hum is due to pickup in the interconnecting leads. It won’t necessarily help if the hum is caused by Australia's electronics magazine ground loops or the power amplifier is producing the hum. Our Notch Filter has a stereo input and output with a mains notch filter between them to reduce the hum signal component dramatically. The Notch Filter is connected using RCA leads at the power amplifier input, so that the signal passes through the filter before being applied to the power amplifier. It is powered from a DC plugpack that provides 9-15V DC. The power requirements are modest; a 100mA plugpack is more than adequate. The filter is housed in a small instrument-­style enclosure with the left and right channel RCA inputs at the rear and the outputs on the front. The DC input socket and power-on indicator are also at the front. The Notch Filter operates silently when power is switched on and off February 2026  53 0dB -6dB -12dB -18dB -24dB -30dB Q=7 | VR1, VR5 anti-clockwise Q=10.5 | VR1, VR5 centred Q=14.5 | VR1, VR5 clockwise -36dB -42dB -48dB -54dB -60dB 44Hz 46Hz 48Hz 50Hz 52Hz 54Hz 56Hz 58Hz 60Hz Fig.1: a simulation of the Fliege filter showing how the width and depth of the notch varies as the Q factor is adjusted using a variable resistance. using reed relays to keep the signal isolated from the circuitry until voltages stabilise. When switching it on, the relays remain off for about five seconds before being energised, preventing DC voltage swings at the output. At switch-off, the relays open immediately, preventing DC shifts in the audio output as the power decays. Filtering is achieved using what is called a Fliege notch filter. This has the advantage of being adjustable in frequency over a small range using a single trimpot. This simple frequency adjustment is not possible with a Twin-T filter. Both an active Twin-T and Fliege filter can be adjusted for filter Q with a single potentiometer. For more information on such filters, see www.ti.com/lit/pdf/slyt235 Fig.1 shows the simulated response of the notch filter. The notch is at 50Hz; three Q values are shown, covering the adjustment range of our filter. The higher Q values have a narrower notch and so less of the audio band is affected. A Q of around 10 usually provides the best compromise, allowing a small amount of variation in the mains frequency while maintaining a good notch depth. This filter could be used for an alternative purpose, such as nulling out mains control tones that may encroach into your audio signal. These tones are superimposed on the mains and are used to control things like street lights and off-peak loads. The tones may enter the audio pathways within your preamplifier. Typically, mains control signals are at 492Hz, 750Hz and 1050Hz. Changing the filter components can provide a notch at any one of those frequencies. Performance Fig.2 shows the frequency response of the unit, which is quite flat except for the obvious notch centred around 50Hz. The response is +0,-1dB from 20Hz to 20kHz except between 45Hz and 55Hz (5Hz either side of the notch frequency on our prototype). If you look only at the response above the Fig.2 (left): except for the notch region, the circuit’s frequency response is flat within +0,-1dB from 20Hz to 20kHz. It’s only down by 1dB <at> 20Hz and is otherwise ruler-flat from 100Hz to 20kHz. Fig.3 (right): a close-up of the 40-60Hz region in Fig.2, showing the notch in more detail. We didn’t quite tune ours to exactly 50Hz but then it’s unrealistic to assume every constructor will tune it perfectly. It still has good attenuation at exactly 50Hz, showing why you don’t necessarily want to set the Q factor to maximum. Also, the mains frequency drifts a little over the course of a day. 54 Silicon Chip Australia's electronics magazine siliconchip.com.au is quite usable even with lower-level signals such as ‘line level’ (~775mV RMS), where the THD+N reading is around 0.004%. There are a couple of odd steps/ shelves in Fig.5, we suspect due to resonance effects from the notch filter (and possibly quirks of our test equipment). These deviations are not so large as to be concerning. Circuit details The rear of the Notch Filter case features the input connectors and little else. notch, it’s flat within a fraction of a decibel. Fig.3 shows a ‘zoomed in’ view of the 40-60Hz region so you can see the notch. This is with a Q set to the recommended value of around 10. You can see the depth is around 27.5dB; it could be set deeper with a higher Q value, but then the sides would be steeper, and it would need to be set more precisely. You can see that we set ours to around 49.8Hz, so even though it isn’t perfectly accurate, the attenuation is still over 20dB at 50Hz. Distortion performance is good, with measurements at a variety of signal levels shown in Fig.4. 2.3V RMS was chosen as it’s a typical value that you would get from a CD, DVD or Bluray player and with that signal level, it gives a THD+N reading below 0.002% across much of the audible frequency range. Performance at 2V and 1V is slightly worse because the signal is closer to the noise floor. The signal-to-noise ratio (SNR) with a 2.3V RMS signal is about 100dB. Fig.5 shows how the THD+N at 1kHz varies with the signal level. As you’d expect, the THD+N figure goes down as the signal level goes up due to the improving SNR. With a THD+N figure better than 0.01% for signals of ~350mV RMS and above, this unit The circuit of the Notch Filter is shown in Fig.6. It comprises nine op amps and a 555 timer IC. Eight of the op amps are within two quad op amp ICs. Some op amps provide buffering, some active filtering and another provides a low-impedance half supply. The timer is used to provide a delayed signal switch-on at power-up. The signal common throughout most of the circuit is set at half supply (~4.5-7.5V) so the signal can swing symmetrically within the supply rails. Having a positive ground reference means that we can use a single supply rail (a negative rail is not required), which can be provided by a standard DC plugpack. The half-supply rail is derived using two 10kW resistors connected across the main supply, resulting in a nominal 6V level when a 12V DC supply is used. This is then decoupled with a 100μF capacitor and buffered by IC3 Fig.4 (left): the distortion of this circuit is generally pretty low (the spike around 50Hz, in the notch, is to be expected). The best performance is at 2.2-3V, which is exactly what many DACs and CD/DVD/Blu-ray players will deliver. Fig.5 (right): at 1kHz, the THD+N figure is below 0.01% for all signal levels from 350mV RMS up to 3V RMS. siliconchip.com.au Australia's electronics magazine February 2026  55 to provide a low-impedance reference voltage from its output. The common reference half-supply voltage from pin 6 of IC3 is used in the filter circuitry for both channels. Only the left channel circuitry is shown on the diagram, with the right channel being identical except for the component labelling; the designators for the other channel are shown in brackets. The signal for the left channel comes via CON1 and is biased to 0V by a 100kW resistor. This discharges any AC coupling capacitor that could be in the signal line before CON1, and makes the signal swing about ground. The ground connection for CON1 is via a 10W resistor to reduce possible Earth loop currents between interconnecting leads. The ferrite bead (FB1) and 150W resistor provide high-­ frequency attenuation of radio signals that could otherwise be picked up and accidentally demodulated to produce spurious audio signals. Following the 150W stopper resistor, the signal is AC-coupled to the non-­ inverting input of IC1a. This input is biased at the half supply via a 100kW resistor. The output from IC1a’s pin 1 drives the notch filter that comprises op amps IC1d, IC1c and IC1b, several resistors and the two 47nF capacitors, Cx and Cy. The Rx and Ry resistances are formed using either VR3 and VR4 or the fixed-value resistors, R3a/R3b and R4a/R4b. Assuming Cx = Cy and Rx = Ry, the notch filter frequency is 1 ÷ 2πRxCx. For a 50Hz notch and 47nF capacitors, Rx and Ry should both be 67.7255kW. This resistance is made up using a 62kW and 5.6kW resistor in series, or using VR3 and VR4 set to this resistance. For a 60Hz notch, the resistances are different, as shown on the circuit diagram. The resistance values are suitable when the 47nF capacitors are actually within ±1% of 47nF (about 46.5~47.5nF). If you don’t use 1% capacitors, their values could differ by 5%. The capacitors need to be chosen so that they are within 1% of each other, but not necessarily within 1% of 47nF. VR3 and VR4 are then adjusted to set the notch to the correct frequency. How these are adjusted is described towards the end of the article. Once the notch filter is adjusted correctly, a small frequency adjustment is also available using VR2. This provides a frequency trim to get the best null from the notch filter. The adjustment uses feedback from the notch output at pin 14 of IC1d back to the filter components. VR2 adjusts the signal level difference between the filter input and output, with the 22kW resistors setting the frequency range adjustment limits. The circuit only works with a frequency adjustment over a small range, so the notch depth remains relatively unchanged over the adjustment range. The filter Q is adjusted with VR1. This sets the narrowness of the notch. The higher the Q value, the narrower the frequency range over which the notch will attenuate the signal. A narrower notch will affect the audio signal less, but allows for less variation in the signal frequency you want to Fig.6: only the left channel is shown here; the right channel is identical, with the corresponding designators shown in brackets. The signal chain includes a buffer (IC1a), half-supply generator (IC3), the Fliege notch filter (IC1b/c/d), output isolation reed relay (RLY1), a timer to drive the relay (IC4) and a regulator to power the relay (REG1). 56 Silicon Chip Australia's electronics magazine siliconchip.com.au remove before it will go outside the notch region. Typically, a Q of around 10 is a good compromise. VR1 allows a Q adjustment of between about 7 and 14.5, with 10.5 being at the centre position. For typical mains frequency excursions from 49.75 to 50.25Hz, the notch filter provides a minimum attenuation of 16dB for a Q of 14.5 and 23dB for a Q of 7. The attenuation in the centre of the notch stays constant over any of the Q settings. Two series 10kW resistors reduce the filter signal level input by half. Fliege filters typically then apply this signal to the other side of the Cx capacitor, and the resistance values are much higher than the 10kW values we use. To adjust the Q, both these resistors need to be changed to alter the overall resistance, but they must also maintain the same ratio. The Q value is the parallel resistance of the two divider resistors divided by Rx. So, for a Q of 10, the divider resistors need to be 20 times larger than Rx. Our Fliege filter has a modification where the half-signal level of the filter input is applied to the input of buffer IC1b. Since the divider ratio is maintained at ½, the Q is adjusted using a single resistance change following the buffer output. For this arrangement, the Q is calculated as the RQ value over the Rx value. Following the filter at pin 14 of IC1d, the signal is AC-coupled using a 1μF capacitor. The 100kW resistor to ground makes the output signal swing around 0V. The RLY1 contact connects the signal to the output via a 150W stopper resistor. This prevents IC1d from oscillating should capacitive loads be connected to CON3. Power Power for the circuit is supplied via CON5 with a 9-15V DC supply from a DC plugpack. Reverse polarity protection is provided by diode D4, and the supply is then filtered by a 470μF 16V capacitor. This voltage, labelled V+, powers all the op amps. REG1 is a 5V regulator to supply the 555 timer, IC4. REG1 includes a 100μF capacitor at its output to prevent regulator oscillation and improve transient response. The 555 could run off V+ but then its output would need to be regulated to 5V to drive the relay coils, so it’s easier to just regulate the voltage applied to IC4. siliconchip.com.au Parts List – Mains Hum Notch Filter 1 double-sided, plated-through PCB coded 01003261, 129 × 101.5mm 1 140 × 110 × 35mm plastic instrument enclosure [Jaycar HB5970, Altronics H0472] 2 red PCB-mounting RCA sockets (CON2, CON4) [Altronics P0145A] 2 white or black PCB-mounting RCA sockets (CON1, CON3) [Altronics P0147A] 1 PCB-mounting barrel socket (CON5) [Jaycar PS0520, Altronics P0621A] 2 small ferrite beads (FB1, FB2) [Jaycar LF1250, Altronics L5250A] 2 SPST 5V reed relays (RLY1, RLY2) [Jaycar SY4036] 2 500kW top-adjust, single-turn trimpots (VR1, VR5) 2 1kW top-adjust single-turn trimpots (VR2, VR6) 2 14-pin DIL IC sockets 2 8-pin DIL IC sockets 4 No.4 × 6mm self-tapping or M3 × 5mm panhead machine screws Semiconductors 2 TL074 quad JFET-input op amps, DIP-14 (IC1, IC2) 1 TL071 single JFET-input op amp, DIP-8 (IC3) 1 555 timer, DIP-8 (IC4) 1 78L05 5V 100mA linear regulator, TO-92 (REG1) 1 BC337 45V 0.8A NPN transistor, TO-92 (Q1) 3 1N4148 75V 200mA signal diodes (D1-D3) 1 1N4004 400V 1A diode (D4) 1 3mm or 5mm LED (LED1) Capacitors 1 470μF 16V PC electrolytic 2 1μF 16V PC electrolytic 5 100μF 16V PC electrolytic 2 220nF MKT polyester 2 10μF 16V PC electrolytic 3 100nF MKT polyester Resistors (all ¼W axial ±1%) 1 470kW 7 10kW 2 470kW (for 50Hz notch) 1 4.7kW 2 390kW (for 60Hz notch) 1 620W 8 100kW 4 150W 4 22kW 2 10W Extra parts for the 1% capacitor version 4 47nF ±1% polypropylene capacitors [RS Components 166-6465] 4 62kW ±1% ¼W axial resistors (R3/4/7/8a) for 50Hz 4 5.6kW ±1% ¼W axial resistors (R3/4/7/8b) for 50Hz 4 56kW ±1% ¼W axial resistors (R3/4/7/8a) for 60Hz 4 430W ±1% ¼W axial resistors (R3/4/7/8b) for 60Hz Extra parts for the 5% capacitor version 4 47nF ±5% MKT polyester capacitors with closely matched values 4 100kW top-adjust multi-turn trimpots (VR3, VR4, VR7, VR8) Power indicator LED1 is supplied via a 620W series resistor and provides a consistent light output regardless of the input supply voltage, provided this is between 9V and 15V, sufficient to keep REG1 in regulation. Several additional capacitors are used to bypass the supply for the four ICs. Relay operation The two relays (RLY1 and RLY2) switch the output signals to prevent thumps (large voltage excursions) at power-up and power-down. IC4 delays relay switch-on after power up to allow everything to stabilise first. IC4 is connected as a monostable timer, with the pin 3 output high (5V) Australia's electronics magazine at power-up. This is because the pin 2 (trigger) input is lower than 1/3 of the supply voltage due to the 10μF capacitor being initially discharged. The output at pin 3 stays high until the 10μF capacitor voltage at pins 2 and 6 rises to above 2/3 of the supply voltage, whereupon the pin 6 (trigger) input signals the pin 3 output to go low. This time period is around five seconds due to the time constant of the 470kW resistor charging the 10μF capacitor. At this point, the relays switch on due to IC4’s pin 3 output going low, while transistor Q1 is switched on due to the incoming supply voltage being applied to its base via a resistive divider. Q1’s collector February 2026  57 is connected to the 5V supply, so any voltage 0.7V above this will cause base current flow, switching Q1 on. So the relays are energised a few seconds after power is applied. When the incoming voltage drops, Q1 loses its base current and so disconnects power from the relay coils. This therefore disconnects the output signals immediately from CON3 and CON4. Diodes D1 and D2 across the relay coils clamp the reverse voltage developed when the relays are switched off, and this charges the 100nF capacitor. The diodes prevent excess voltage from damaging Q1. Diode D3 is used for reverse-polarity protection since this part of the circuit is powered from before diode D4. That diode also prevents the 470μF filter capacitor from holding up Q1’s base at switch-off. Capacitor selection As mentioned, the 47nF capacitors for the notch filter need to be selected so that the values are within ±1% of each other. Typically, if you buy standard ±5% capacitors on a bandolier (paper/cardboard tape), the adjacent components will have a similar value. We found that four capacitors of the same marked value in a row wouldn’t necessarily be within ±1% of 47nF, but whatever value they were, three would be within ±1% of each other. You may need to get more than four capacitors so that at least four will be of a similar value. That’s a lot cheaper than purchasing 1% capacitors, although 1% capacitors can be used if you want. If you have a capacitance meter, the values can be measured and compared. Alternatively, if you have an oscilloscope or frequency meter, the capacitors can be tested using a standard astable oscillator made with a 555 or 7555 timer. The frequency of oscillation will be inversely proportional to the capacitance of the timing element. Fig.7 shows the circuitry required. Using 10kW for RA and RB, the oscillation frequency would be around 1023Hz for a 47nF capacitor. Note that the oscillator does not allow you to accurately determine the exact capacitance value. However, it is suitable for comparing the values of several capacitors as long as you make the measurements at around the same time, so there are no ambient temperature change effects affecting the readings. Select capacitors that run at the same frequency to within 1%. A 1% variation in capacitor value will mean that the oscillator frequencies will be within about 10Hz using this circuit. Construction The Audio Notch Filter is built using a double-sided, plated-through PCB coded 01003261 that measures 129 × 101.5mm. It is housed in a plastic instrument enclosure measuring 140 × 110 × 35mm. All the parts are through-hole types that mount on the top side of the circuit board. Some resistor values depend on whether you are setting the notch filter to 50Hz or 60Hz. The resistors that vary are R1, R2, R3a, R3b, R4a, R4b, R7a, R7b, R8a and R8b. There are two options when building the notch filter. One is to use ±1% 47nF capacitors and fixed 1% resistors for R3a, R3b, R4a, R4b, R7a, R7b, R8a and R8b. Alternatively, use similar-­ value 47nF capacitors and adjustable resistors (trimpots) VR3 and VR4 for the left channel and VR7 and VR8 for the right channel. This allows for trimming of the notch frequency. 47nF ±1% capacitors are hard to find and expensive, so our kit includes the trimpots. If using fixed resistors, their values are shown on the circuit diagram for 50Hz and 60Hz notch frequencies. Do not use both the trimpots and fixed resistors. Follow the overlay diagram (Fig.8) and begin construction by installing the resistors and four diodes. Check the value of each resistor before installation by measuring with a multimeter (they have colour-coded stripes but it can be hard to distinguish some colours). Fig.7: a simple circuit for an oscillator that produces a signal frequency proportional to the capacitor under test. Fig.8: follow this overlay diagram to assemble the PCB. This shows all the fixed resistors and trimpots fitted, but you should either install VR3/4/7/8 or R(3/4/7/8)(a/b), not both sets. Take care with orientations of the ICs, diodes, LED, trimpots and electrolytic capacitors. 58 Silicon Chip siliconchip.com.au Diodes D1, D2 and D3 are small glass-encapsulated 1N4148 types, while D4 is a larger, plastic-cased 1N4004 diode. Ensure these are all fitted with the orientations shown in the overlay diagram and PCB screen-­ printing. Mount ferrite beads FB1 and FB2 using resistor off-cut wires fed through the centre hole and then bent to insert into the PCB holes. Now install the sockets for the four ICs, taking care to orientate them correctly, with the notches facing as shown. Next are RCA sockets CON1 to CON4 and the DC socket, CON5. We used white for the left channel and black for the right channel. Red sockets should be used for the right channel sockets, not black as in the photos, as this is the standard colour for the right channel. However, at the time we purchased these, the red sockets were out of stock at Altronics and Jaycar only sells the black type. Trimpots VR1/VR5 (500kW) and VR2/VR6 (1kW) can be installed now. If VR3, VR4 and VR7 and VR8 are being used, the adjustment screws need to be orientated as shown. That’s so the resistance changes with clockwise direction as indicated on the circuit. For the 500kW and 1kW trimpots, be sure to place the correct value in each position. The trimpots will have printed codes, but you can also check the value by measuring the resistance between the outer two leads. Transistor Q1 and the 5V regulator (REG1) can be mounted now, taking care to orientate these correctly. They are in the same TO-92 package, so check the correct one is placed in each position before soldering. Relays RLY1 and RLY2 can be installed now as well. The capacitors are next. Electrolytic types need to be orientated with the correct polarity; the longer lead goes into the pad marked +, with the striped (negative) side of the can near the opposite pad. The MKT and ceramic types can be installed either way around. LED1 sits horizontally with the leads bent at 90°. Position the LED so that the top of the lens dome is 12mm in front of the PCB edge, and the centre of the LED is 5mm above the top surface of the PCB. When bending the leads, make sure the anode and cathode leads will go into the correct pads on the PCB (the longer anode lead goes to the pad marked ‘A’). siliconchip.com.au Removing mains harmonics In many areas, the mains voltage is not a reasonably shaped sinewave. Typically, the waveform is distorted and has a flattened top, as shown at the top of Fig.a. This shows a typical mains voltage waveform. The flattened top is mainly due to industrial and household appliance power supplies that draw power from the peaks of the mains waveform. Nonlinear loads will also cause flat-topping. Note the difference in shape between the measured mains waveform in yellow and the cyan sinewave trace. So, while the main fundamental mains frequency is 50Hz (or 60Hz in some other countries), the distorted waveshape means that the waveform includes harmonics of that frequency. These are primarily the odd harmonics (3rd, 5th, 7th etc), which are at 150Hz, 250Hz, 350Hz etc for a 50Hz mains (or 180Hz, 300Hz, 420Hz etc for 60Hz mains). Fig.b shows the frequency components and levels that are present in the distorted mains signal. The horizontal axis is 50Hz per division, while the vertical axis is 10dB per division. The fundamental at 50Hz is followed by harmonics at 150Hz, 250Hz, 350Hz and 450Hz. The third and fifth harmonics (150Hz and 250Hz) are only about 26dB below the 50Hz fundamental. You may need to notch out these harmonic frequencies as well as the fundamental if they are intrusive. This can be done with more notch filter circuits, connected in series, with one set for the fundamental (50Hz or 60Hz) and further notch filters tuned to the harmonic frequencies. If building such a system, the two relays and 555 timer and associated circuitry (such as Q1, D1-D3 etc) are only required in the final filter circuit, to disconnect the output during power-up and power-down. The power supply can be paralleled from one notch unit to the other, provided the plugpack can supply the extra current. Wire links would need to replace the relay contact positions on the PCB. These could all be installed in the same, larger box with fixed wiring from the output of one stage to the input of another. The filter component values to change to notch different harmonics are listed in Tables 1 & 2. We show the capacitor and resistor values for the various fundamental and harmonic notch components. Resistors R1 and R2 are unchanged at 470kW regardless of the notch frequency. Freq. Cx & Cy R*a R*b Freq. Cx & Cy VR3/4/7/8 Initial 50Hz 47nF ±1% 62kW 5.6kW 50Hz 47nF 100kW 67.73kW 150Hz 15nF ±1% 68kW 2.7kW 150Hz 15nF 100kW 70.74kW 250Hz 10nF ±1% 62kW 1.6kW 250Hz 10nF 100kW 63.66kW 60Hz 47nF ±1% 56kW 430W 60Hz 47nF 100kW 56.43kW 180Hz 15nF ±1% 56kW 3.0kW 180Hz 15nF 100kW 58.95kW 300Hz 10nF ±1% 51kW 2.0kW 300Hz 10nF 100kW 53.05kW Table 1: fixed components for mains harmonics Table 2: adjustable components for mains harmonics Fig.a: while the mains waveform is theoretically a sinewave (and is produced as a sinewave by the steam turbine alternators in large-scale power plants), by the time it reaches you, it will usually be flat-topped like this (top yellow trace). Compare it shape to the pure sinewave in cyan below. Fig.b: the spectrum of the mains waveform shows the 0dB fundamental at 50Hz with a series of harmonics at lower levels: the third (150Hz, -26dB), fifth (250Hz, -28.5dB), seventh (350Hz, -44dB) etc. Australia's electronics magazine February 2026  59 The ICs can now be inserted into their sockets, making sure that the pin 1 dot or notch is near the notch on the socket in each case. Also ensure that the leads don’t fold under the body during insertion, instead going into the holes on the socket. Panel cutouts The required holes in the panel pieces are as specified in Fig.9. It shows the positions of the holes for the LED (3mm diameter), RCA sockets (9mm diameter) and the DC socket (12mm diameter). Fig.10 shows the panel labels. You can download these as a PDF from siliconchip.au/Shop/11/3584, print them onto vinyl labels (or similar), ready to attach to the panels. Holes can be cut out with a sharp craft knife. For more information on making panels, see siliconchip.au/Help/FrontPanels Once the panels are completed, place the front and rear panels onto the RCA and other protruding components and slide the panels with the PCB into the baseplate of the enclosure. Secure the PCB to the enclosure base with No.4 self-tapping screws (short M3 machine screws can be used; the threads will self-tap the plastic posts). Setting it up SC7598 Kit ($50 + postage): includes the PCB and all onboard parts. You just need to separately purchase the case (shown above) and power supply. Initially, set VR1, VR2, VR5, VR6 at their mid positions. If using ±1% capacitors and fixed resistors, then skip to the section titled “Adjustments”. Adjust VR3, VR4, VR7 and VR8 to 67.73kW for a 50Hz notch or 56.43kW for a 60Hz notch. You can measure this resistance using a multimeter across the test points: TP3a/b for VR3, TP4a/b for VR4, TP7a/b for VR7 and TP8a/b for VR8. Connect a 9-15V DC plugpack and check that LED1 lights with the power switch on. Disconnect the power and insert IC1, IC2, IC3 and IC4 into their sockets. Be sure to orientate each correctly; IC4 is the 555. Apply power and measure the supply current, which should be less than 50mA. If your 47nF capacitors are all outside the 1% tolerance of 47nF (below 46.5nF or above 47.5nF), then VR3, VR4 and VR7 and VR8 will require trimming for best nulling of the mains frequency. You can use a signal Australia's electronics magazine siliconchip.com.au The completed circuit board housed in the case, with the lid off. This prototype used trimpots and ±5% capacitors rather than fixed resistors and ±1% capacitors. 60 Silicon Chip Fig.9: drill the holes in the front and rear panels as shown here. Fig.10: the panel labels for the front and rear of the device. The holes are drawn undersized here to allow for slight misalignment; use the holes in the panels as a guide to cut them out after attaching the label. generator set at 50Hz (or 60Hz) with a level of 1V RMS or similar. Alternatively, without a signal generator that is accurate enough, you may need to feed a signal with mains hum into the input, listen to the output and adjust the trimpots to minimise the audible hum. An alternative approach is to attenuate the output of a low-voltage AC plugpack (eg, 9V AC) with a resistive divider, say 100kΩ and 1kΩ. Connect the centre of the divider to one of the inputs and the other end of the 1kΩ resistor to the RCA shell/ground. You can use an oscilloscope or audio millivoltmeter to monitor the signal at the CON3 output, or an amplifier and headphones, earbuds or a loudspeaker to listen to it instead. If using an amplifier, make sure the volume control is turned down to minimum initially, then turn it up slowly when you apply power until you can hear the hum signal to avoid overload. Adjust VR3 and VR4 by small amounts each (either way) to minimise the mains hum in the left channel. siliconchip.com.au Similarly, adjust VR7 and VR8 in the right channel to minimise hum. Try to maintain the same value for each trimpot. Adjustments Adjust VR1 and VR5 to set the Q; higher settings will give a deeper notch but with less allowance for mains frequency variations. You could adjust the Q while monitoring the actual signal you want to remove hum from, allowing you to select the minimum setting that removes audible hum so as to avoid affecting ~50Hz bass in the actual audio signal too much. VR2 and VR6 are for the frequency adjustment for the left and right channels. These allow the notch frequency to be trimmed, they also affect the notch width. The frequency adjustment will be most useful when you are using the ±1% capacitors with fixed resistors. It is usually easier to adjust the frequency when the Q is set to a low value first (VR1 and VR5 set clockwise) before adjusting the Q higher as you Australia's electronics magazine further adjust the notch frequency. A midpoint setting for VR1 and VR7 (a Q of around 10.5) gives a good compromise between notch depth and a wide enough notch to allow for slight SC mains frequency variations. Mains Power-Up Sequencer February-March 2024 Hard-To-Get Parts SC6871: $95 siliconchip.au/Series/412 The critical components required to build the Sequencer such as the PCB, micro etc. Other components need to be sourced separately. February 2026  61