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Power Amplifier Clipping Indicator Ensure your loudspeakers are protected from being overdriven and possibly damaged or destroyed by building this Power Amplifier Clipping Indicator. Not only does it show when an amplifier clips (however briefly), it can also reduce the signal level applied to the amplifier to limit subsequent clipping, protecting the speakers. By John Clarke A mplifier clipping occurs when the output flat-tops because it cannot increase the output voltage any further due to power supply limitations. This means that the amplifier has reached its limit to deliver power to the loudspeakers. It also means the sound becomes vastly distorted, leading to the sound quality suffering. If clipping is allowed to continue, loudspeakers can be damaged or destroyed. With suitable volume levels, the amplifier reproduces the audio signal faithfully. But if the amplifier is turned up too much and clipping starts to occur, you get a compressed signal 30 Silicon Chip that causes the overall power delivered to the loudspeakers to be greatly increased, causing them to overheat and burn out. Woofers and, to some extent, midrange drivers are less prone to damage than tweeters. This is because they utilise more robust and larger diameter wire in their voice coils than their tweeter counterparts. The tweeter is more delicate, using a thinner, smaller and lighter voice coil so it can move quickly to reproduce higher frequencies. When loudspeakers are overdriven, the voice coil windings can burn out Australia's electronics magazine or the voice coils can soften and permanently distort. Continued excessive overdrive can result in the loudspeaker catching fire and/or fusing. If you want to explore more about loudspeaker damage due to amplifier overloading, see www.sound-au.com/ tweeters.htm and www.sound-au. com/clipping.htm We have previously published three Power Amplifier Clipping Indicator projects, one as a full project and two Circuit Notebook entries. These all detect clipping based on whether signal level peaks approach a fixed voltage difference from the amplifier’s siliconchip.com.au Features & Specifications Detects genuine signal clipping and gross distortion Doesn’t require opening up the amplifier Optional automatic signal reduction at clipping Separate left and right channel clipping indicator LEDs Momentary clipping is shown on LEDs with a minimum 50ms duration Suitable for inverting, non-inverting and bridge-tied amplifiers Uses commonly available components Easy to solder Power supply requirement: 15-24V DC at 200mA supply rails. Typically, this was set at somewhere around 4.7-6.2V less than the supply rails. This level is not necessarily the point of clipping. It depends on the type of driver devices used in the amplifier (whether Mosfets or transistors), the driver device’s temperature, and the loudspeaker impedance. Additionally, to install such a power amplifier clipping indicator requires access to the inside of the amplifier to tap into the power supply rails. This new Clipping Indicator is designed to detect when the amplifier is actually clipping. Only externally available amplifier connections need to be accessed: the input sockets and the speaker outputs. How it works The Clipping Detector works by comparing the signal applied to the amplifier with the output from the amplifier. If the amplifier is not clipping, the two signals should be identical in shape, only differing in voltage magnitude. We compare the signals after reducing the amplifier output level so it matches that of the input. That way, any differences between the two waveform shapes can be detected. A summing amplifier compares the two signals. Typically, the input and output signals of an amplifier will be in phase. So when the input goes positive, the output also goes positive. If we invert the amplifier output signal and then sum the two signals, they should cancel out. If there are any differences, such as phase changes or clipping, the summing amplifier (or ‘adder’) will produce an output that becomes a difference or error signal. We simulated this in LTspice, as shown in Fig.1. The blue trace is the amplifier input, the red trace is the attenuated and inverted amplifier output, while the green trace is the adder output. We have clipped the positive output of the amplifier output a little at the peak of the positive excursion to show how the adder responds to a signal difference. There is a rise in the adder signal level when the two waveforms differ. When the two signals are the same except for being inverted (as shown for the negative excursion), the adder output remains close to 0V. We use the adder output to gauge the amount of difference between the two signals. A window comparator detects when the adder produces a large enough difference signal to trigger the clipping indication. To verify this, Scope 1 shows the amplifier output waveform (yellow trace) at the point of clipping at 104V peak-to-peak. The lower cyan trace shows the output from the summing amplifier, IC2c. The summing amplifier begins to produce a difference signal at the point of clipping on the output waveform. Fig.1: a SPICE simulation of a summing amplifier fed with the inverted input and non-inverted output signals of an amplifier just starting to clip. The output of this ‘adder’ only varies from zero during clipping. siliconchip.com.au Australia's electronics magazine The clipping is asymmetrical, meaning that the degree of clipping in the positive portion of the waveform is greater than in the negative half. This is due to the differences in the power amplifier output transistors used for the positive and negative output drive. Block diagram The block diagram, Fig.2, shows the main sections of the Clipping Indicator. We will describe the sections briefly since there is more detail in the circuit description. The diagram shows the left channel only, and we’ll explain just that channel; the right channel designators are shown in brackets. The signal input at CON1 is from a signal source such as a CD player or preamplifier output. This signal is buffered by IC1a and goes through a high-pass filter to remove signals well below the audible range (sub-20Hz). Following this is the variable attenuator. This acts to reduce the signal level should extended clipping occur. It uses a light-dependent resistor (LDR1) and LED2. The LDR forms a voltage divider with a fixed resistor, providing more attenuation when the LED light intensity increases. Normally, without any light from the LED, the LDR has a very high impedance, so there is minimal attenuation. Thus, this section has no effect on the signal except when the amplifier is driving into clipping. Another buffer follows (IC1d) before the signal is applied to the power amplifier input via CON3. The output from buffer IC1d also goes to two phase-adjustment filters, one for the high-frequency end of the audio spectrum and the other for the low-frequency end of the audio spectrum. These are adjusted to match the phase shifts that occur in the power Scope 1: the amplifier output waveform (yellow) and output from the summing amplifier IC2c (cyan). May 2026  31 amplifier at the lower and upper frequency extremes. These are inherent to most audio amplifiers due to capacitors in the power amplifier causing low-­ frequency roll-off where the input and feedback signals are AC-coupled, and high-frequency roll-off due to the compensation capacitor used to ensure the power amplifier’s stability and possibly other RF/noise filters. We need our clipping detection signal path to have the same phase shift characteristic as the amplifier so we can compare the two signals. Otherwise, they will be different even if there is no clipping, possibly causing false triggering. The output from the phase adjustment filters is applied to the summing amplifier input. The power amplifier output connects to either the CON5 non-­inverting or inverting amplifier input. Most power amplifiers are non-inverting, so the non-inverting input is typically used. The inverting input is mainly included so that you can use this device with a bridge-tied load (BTL) amplifier, where there are two amplifiers driving the loudspeaker with one producing an inverted signal compared to the other. Having the two input options allows for both amplifier outputs to be monitored for BTL amplifiers. The signal level from CON5 is controlled using trimpot VR4 or VR5, or both in the case of a BTL amplifier. The signals are buffered following the attenuators, and in the case of the non-inverting amplifier signal, it is inverted by another op amp, ready for comparison in the summing amplifier using IC2c. IC2c is the summing amplifier described previously, and the resulting summed signal is monitored by a window comparator (IC3). Normally, this signal level will sit close to 0V when there is no clipping. When the summing signal reaches a set level (beyond ±1.25V), the window comparator triggers timer IC4. IC4 provides a 50ms minimum output to drive the clipping LED via transistor driver Q2, ensuring that the flash is visible even for very brief clipping events. This driver also provides a fast attack and slow decay voltage that drives LED2 via transistor Q3. This reduces the resistance of LDR1, attenuating the signal that ultimately is applied to the power amplifier input. Circuit details The full circuit is shown in Fig.3. It comprises four quad op amp ICs and three single op amp ICs for a total of Fig.2: the Clipping Indicator is connected between the preamp (CON1/ CON2) and power amplifier (CON3/CON4). A current-controlled attenuator can reduce the signal going to the amplifier when clipping is detected. After phase adjustments, the input signal is fed to the adder, along with the amplifier signal(s). Its output goes to a window comparator that detects clipping, then a pulse-stretching timer to drive the LEDs. 32 Silicon Chip Australia's electronics magazine 19 op amps, plus three 555 timers and two dual comparators, along with two reed relays and associated diodes, a regulator, resistors and capacitors. Three indicator LEDs indicate clipping in each channel and show when the power is on. As with the block diagram, only the left channel is shown, with the right channel being identical; its alternative designators are shown in brackets. Some op amps provide buffering, some active filtering, while another (IC12) provides a low-impedance half supply. Looking at the audio signal circuitry first, through the circuit the signal common is set at half supply (Vcc/2) so it can swing symmetrically between GND and the 15-24V DC supply rail. This means it can run from a standard DC plugpack without needing a supply voltage inverter section. The signal comes in at the CON1 RCA socket and is biased to ground by a 100kW resistor. This discharges any AC-coupling capacitor that could be in the signal source. A 150W series resistor acts as an RF stopper to prevent radio signals entering the first buffer op amp. Following this, the signal is AC-­ coupled via a 10μF capacitor to the non-inverting input of IC1a. This input is biased to the half supply via a 100kW resistor. A 13Hz high-pass filter rolls off very low frequency components of the signal at 40dB/decade; it is 6dB down at 13Hz. The reason we set this roll-off at 13Hz is so the signal is only 3dB down at the lowest audible threshold at 20Hz. Following the filter is the current-­ controlled attenuator. This comprises a 10kW series resistor in conjunction with LDR1 and trimpot VR2 shunting some signal to the Vcc/2 reference point. Normally, the LDR is in complete darkness and its resistance is around 0.5MW (500kW). In this condition, it produces negligible signal attenuation until clipping is detected. IC1d acts as a buffer for the attenuator. At IC1d’s output, the signal is diverted two ways. One is to the output to the amplifier. This is AC-­ coupled to the relay contact and the 100kW resistor in conjunction with the 10μF capacitor sets the output signal to swing about ground (0V). In other words, the 10μF capacitor removes the Vcc/2 DC bias voltage from the signal. siliconchip.com.au Fig.3: the full circuit with only the left channel shown – the right channel components are identical and their designators are shown in brackets. The sections that are common to both channels are the half-supply rail generator, power supply (including REG1), on-delay and off-delay sections. Following the relay contact, the signal is sent to the RCA socket (CON3) via a 150W resistor. This provides a small series impedance for the op amp so it won’t oscillate when there is a capacitive load connected, such as screened audio cable. siliconchip.com.au The second signal path from IC1d is to the high-frequency phase adjustment circuit. This is a low-pass filter comprising a series 4.7kW resistor, 100kW trimpot (VR2) and a 22pF capacitor. It produces an overall high-frequency roll-off that has an Australia's electronics magazine adjustable -3dB point from 4.6kHz to 102kHz. It is used to match the phase shift within the power amplifier at higher frequencies. IC1b buffers the signal from this filter. Following this is the low-­frequency phase adjustment circuit. It comprises May 2026  33 The board is designed to fit into a UB2 Jiffy box although it also can be incorporated into other equipment, such as a power amplifier. a high-pass filter using a 1μF capacitor and 100kW trimpot in series with a 4.7kW resistor. The range of adjustment for the -3dB point is from 1.6Hz to 34Hz. This allows it to match the power amplifier output phase at low frequencies. IC5 buffers the output of this filter and its output is applied to the summing amplifier (IC2c) via a 10kW mixing resistor. Power amplifier monitoring The power amplifier’s output(s) is/ are connected via CON5 (as well as going to the loudspeaker[s]). There are two inputs. One is the non-inverting amplifier input, while the other is the inverting amplifier input. Most amplifiers are non-inverting, so that input is the most likely the one to use. If you have an inverting power amplifier, simply use the inverting input instead. If your amplifier has a bridge-mode output, where neither terminal is connected to ground but both are actively driven in anti-phase, connect both outputs to the two inputs. In this case, a couple of the summing amplifier resistor mixing values will need changing – more on that later. Trimpots VR4 and VR5 are used to set the signal levels for the non-­ inverted amplifier input and inverted amplifier input, respectively. These are set to match the signal level that is applied to the amplifier input. If one of the inputs is not used, the attenuation is set to maximum to minimise noise. The signal for each input is AC-­ coupled to buffers IC2a and IC2d. The 34 Silicon Chip inverted amplifier signal is applied to the adder via a 10kW resistor, while the non-inverted signal is inverted using the IC2b unity-­ gain inverter first. We invert the non-inverting power amplifier output and don’t invert the inverting power amplifier output so that when the power amplifier input and output signals are summed, the output will be zero. That’s because we are adding two waveforms that are 180° out of phase. The adder sums the signals from the IC5 output and the IC2b and IC2d outputs. When these signals sum to zero, the adder output sits at Vcc/2. Should any of the signals applied to the adder cause a difference output, once that reaches a sufficient level, it will be detected in the following window comparator. Window comparator IC3, a dual LM393 comparator, is connected as a window comparator detecting excursions 1.25V above and 1.25V below the Vcc/2 voltage. The 470W resistor and 1MW feedback resistors add hysteresis so the comparator output does not oscillate when signal at the inverting input (pin 2) of IC3a is close to the +1.25V reference. Resistors of the same values for IC3b prevent this comparator from oscillating if the input at pin 5 via the 470W resistor is close to -1.25V. Australia's electronics magazine IC3a and IC3b have open-­collector outputs, so they can be connected together. These outputs are pulled high via a 10kW resistor to the 12V supply. They remain high if the voltage from the adder remains within ±1.25V of Vcc/2. This is because the inverting input of IC3a is lower than Vcc/2 + 1.25V at the non-­ inverting input, and the non-­inverting input of IC3b is higher than Vcc/2 – 1.25V at its inverting input. If the adder output goes above 1.25V, the IC3a output will go low (near 0V); if it goes below -1.25V, IC3b’s output will go low. In either case, this pulls trigger pin 2 of 555 timer IC4 low, and its pin 3 output goes high. The low voltage at pin 2 also pulls the base of transistor Q1 low, preventing the 1μF capacitor at pin 6 of IC4 from charging via the 47kW resistor. When the comparator outputs go high again, Q1 switches off and the 1μF capacitor can charge. When this voltage reaches 2/3 of the 12V supply (about 8V), the threshold input at pin 6 detects this, and the pin 3 output and pin 7 discharge output go low. The 1μF capacitor is discharged via the 100W resistor at pin 7. The output remains low for some 50ms after the pin 2 input is taken siliconchip.com.au high, extending the clipping indication by 50ms. This allows very short time periods from the comparator to be seen by the user by lighting the LED for long enough to make it visible. While the pin 2 input of IC4 is pulled high via the 10kW resistor to 12V, it does not reach 12V because transistor Q1’s base-collector junction breaks down like a zener diode at about -5V. So the maximum voltage at pin 2 is about 5V. This is more than sufficient voltage to allow trigger operation at pin 2, since the trigger voltage needs to go below 1/3 of the 12V supply (about 4V) to be triggered. When pin 3 is high, it drives transistor Q2 via a 2.2kW resistor, which in turn drives the Clipping Indicator LED (LED1) with its current limited by an 820W resistor from the 12V supply. The high level at pin 3 also begins to charge the 10μF capacitor at Q3’s base via the 10kW resistor, diode D1 and the 100kW resistor. The emitter of Q3 follows the base voltage but 0.7V below the base, and this drives LED2 via an 820W resistor. The longer pin 3 of IC4 is high, the higher the voltage at the emitter of Q3. This means LED2 is driven with a varying current depending on the charge at the 10μF capacitor. When pin 3 of IC4 goes low, the 10μF capacitor at Q3’s base discharges via the 100kW resistor over about one second. When lit, LED2 lowers the resistance of LDR1 at pin 12 of IC1d, so the audio signal is reduced via the voltage divider comprising the 10kW resistor, LDR1 and VR1 to the Vcc/2 reference. VR1 is adjusted for the required amount of attenuation to reduce signal clipping but not so that the signal level drops unnecessarily low. Power supply Power is from a DC plugpack ranging from 15V to 24V. There is no power switch; an inline switch can be used at the DC plugpack output if required, instead of controlling power via the same mains outlet as the power amplifier. Diode D5 provides reverse-­ polarity protection. The supply at its cathode is labelled as Vcc and is typically about 0.7V below the DC input supply voltage. The Vcc/2 supply is derived using two 10kW resistors across this rail, feeding pin 3 of IC12 and bypassed with a 100μF capacitor to ground. IC12 is connected as a unity gain amplifier to siliconchip.com.au Parts List – Power Amplifier Clipping Indicator 1 double-sided, plated-through PCB coded 01104261, 185.5 × 101.5mm 2 white right-angle PCB-mount RCA sockets (CON1, CON3) [Altronics P0147A] 2 red right-angle PCB-mount RCA sockets (CON2, CON4) [Altronics P0144A] 2 3-way PCB-mount screw terminals, 5.08mm spacing (CON5, CON6) 2 2-way PCB-mount screw terminals, 5.08mm spacing (CON7, CON8) 1 PCB-mount DC socket (CON9) [Altronics P0621A, Jaycar PS0520] 2 12V DC coil PCB-mount reed relays (RLY1, RLY2) [Altronics S4101/S4101A, Jaycar SY4032] 2 500kW/2-10kW LDRs (LDR1, LDR2) [Altronics Z1621A, Jaycar RD3485] 6 100kW miniature top-adjust trimpots (VR1-VR3, VR6-VR8) 4 5kW miniature top-adjust trimpots (VR4, VR5, VR9, VR10) 4 14-pin DIL IC sockets 8 8-pin DIL IC sockets 1 50mm length of black 6mm heatshrink tubing 1 strip of Blu-tack or similar non-drying putty Optional case mounting parts 1 197 × 112 × 63mm UB2 box [Altronics H0152/H0202, Jaycar HB6012] 4 25mm M3-tapped standoffs 8 M3 × 6mm machine screws 4 cable glands to suit 3-6mm cable Semiconductors 4 TL074 quad JFET-input op amps, DIP-14 (IC1, IC2, IC6, IC7) 3 TL071 single JFET-input op amps, DIP-8 (IC5, IC10, IC12) 2 LM393 dual single-supply comparators, DIP-8 (IC3, IC8) 3 555 timers (not CMOS types), DIP-8 (IC4, IC9, IC11) 1 7812 12V 1A linear regulator, TO-220 (REG1) 1 LM336-2.5 2.5V reference, TO-92 (REF1) 5 BC337 45V 0.8A NPN transistors, TO-92 (Q2, Q3, Q5-Q7) 2 BC327 45V 0.8A PNP transistors, TO-92 (Q1, Q4) 4 1N4148 75V 200mA signal diodes, DO-35 (D1-D4) 1 1N4004 400V 1A diode, DO-41 (D5) 5 5mm high-intensity red LEDs (LED1-LED5) Capacitors (all 16V radial electrolytic unless noted) 1 470μF 25V 5 100μF SC7649 Kit ($95 + postage) 3 47μF 25V Includes the PCB and all onboard 1 22μF parts. The case and power supply 1 10μF 25V are not included. 10 10μF 2 1μF 2 1μF non-polarised electrolytic 4 220nF 63/100V MKT polyester 13 100nF 63/100V MKT polyester 2 22pF 50V NP0/C0G ceramic Resistors (all ¼W ±1% axial) 4 1MW 4 22kW 5 820W 1 470kW 18 10kW 4 470W 17 100kW 5 4.7kW 4 150W 4 56kW 2 2.2kW 2 100W 2 47kW 4 1kW 2 10W 4 20kW (only for use with bridge-tied load amplifiers) Australia's electronics magazine May 2026  35 buffer this half-supply rail so that loading on that rail doesn’t affect the voltage much. In other words, IC12’s output provides a low impedance source for the components fed from this rail. We use REF1, a 2.5V reference, to provide the Vcc/2 + 1.25V and Vcc/2 – 1.25V reference voltages for the window comparator. So if Vcc/2 is 7.5V, the resulting reference voltages will be 8.75V and 6.25V. REF1 is supplied current via a 1kW resistor from Vcc to the plus (+) terminal of REF1 and another 1kW resistor from the negative terminal to ground. The resulting 2.5V (actually 2.490V) reference is across the Vcc/2 supply using two more 1kW resistors to ensure it’s centred on Vcc/2. There are 100nF bypass capacitors for the Vcc/2 + 1.25V and Vcc/2 – 1.25V rails. The Vcc supply is bypassed with a 470μF capacitor and feeds the input of a 12V regulator (REG1) that supplies 12V to the 555 timers and relays. IC11 is a 555 timer that is used to switch on the audio outputs about 10s after power is switched on. This prevents large voltage excursions in the audio signal by waiting to connect the signal until all the voltage levels have stabilised. IC11 is connected as a monostable timer. At power-on, the discharged 22μF capacitor at pin 2 triggers the 555 so that the pin 3 output goes high (12V) and so the bottom connection of each relay coil is at 12V. At the same time, transistor Q7 is switched on due to its base being supplied with current from the Vcc supply. There is 12V at each end of the relay coil contacts, so the relays remain off. This keeps the relay contacts open and prevents any signal at the audio outputs. After about 10s, the 22μF capacitor charges to about 8V and the threshold input of IC11 detects this as being over 2/3 of its supply voltage and takes its pin 3 output low. This energises the relay coils, closing the relay contacts and allowing audio signals to pass. At switch-off, the 4.7kW resistor supplying current to the base of Q7 does not have voltage, so Q7 switches off due to the 100kW pulldown resistor. That removes power from the relay coils. Diode D4 clamps the back-EMF produced by the coils, preventing damage to transistor Q7 from an excessive voltage transient across the collector and emitter. Construction The Power Amplifier Clipping Indicator is built using a double-sided, plated-through PCB coded 01104261. It measures 185.5 × 101.5mm. You can install the assembled PCB within existing equipment, or it can be fitted into a UB2-size plastic utility box that measures 197 × 112 × 63mm. Follow the overlay diagram, Fig.4, first by installing the resistors and five diodes. Check the value of each resistor before installation by checking its colour code and/or measuring with a multimeter (the latter is less prone to errors due to similar colours). Make sure all the diodes are orientated with their cathode strips as per Fig.4. There are four 10kW resistors below IC2 and IC7 that are marked with an asterisk. These all need to be changed to 20kW after setup if you are applying signal to both the inverting and non-inverting amplifier inputs, such as when connecting to a BTL amplifier. So you may wish to install these 10kW resistors above the PCB surface to make them easier to remove later. We recommend using 10kW first since setting up is easier if each input is connected independently and adjusted for level initially. They can then be changed to 20kW. Next, install the sockets for the ICs, taking care to orientate them with the notches all towards the top of the PCB as shown. The two relays can be installed now as well. Next on the list are the screw terminals (CON5-CON8), RCA sockets (CON1-CON4) and DC socket (CON9). Fig.4: fit the components to the PCB as shown here. Watch the orientations of the ICs, diodes, LEDs, transistors & regulator. 36 Silicon Chip Australia's electronics magazine siliconchip.com.au Make sure the screw terminal openings are towards the nearest outside edge of the PCB. For the RCA sockets, we used white for the left channel and black for the right channel. Red sockets can be used for the right channel sockets instead, 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 black. We sell red and white pairs on our website at siliconchip.au/Shop/7/2615 because they can be hard to obtain at times. Still, the colour is not absolutely critical as you can tell which inputs and outputs correspond. There are two different values for the trimpots, which are all standard vertical adjust single-turn types. VR1-VR3 and VR6-VR8 are 100kW, while VR4, VR5, VR9 and VR10 are 5kW. These can be installed now. Be sure to place the correct value in each position. The 100kW trimpots may be labelled with code 104 (10 × 104) and the 5kW trimpots with code 502 (50 × 102). The transistors, REF1 and the 12V regulator (REG1) can be mounted now, taking care to orientate them correctly and not get them mixed up. Q1 and Q4 are BC327s, while the remaining transistors are BC337s. REF1 is in the RIGHT IN Once the assembly is ready, shrink down the tubing with a hot air gun. Make sure the LED and LDR leads are orientated on the same plane. The LED leads can then be bent over 180° to be installed into the anode (A) and cathode (K) holes on the PCB, with the LDR leads inserted into their corresponding holes (the LDR is not polarised). Now insert the ICs into their sockets, taking care to match up their pin 1 indicators with the socket notched ends. Also be careful not to mix up the 8-pin ICs as there are TL071, LM393 and 555s in the same package that need to be placed in their correct positions. Panel cutouts If you are planning on installing the Power Amplifier Clipping Indicator in a UB2 enclosure, we have provided a drilling guide diagram (Fig.7). The height positions assume that the PCB is on 25mm standoffs. If you prefer to use a different standoff size, you can move the holes up or down to suit. Cable glands can be used to secure the leads for LED2 & LED4. Setting up The Power Amplifier Clipping Indicator needs to be connected to a power amplifier for setup (one that does not have any tone controls or preamplifier). If you use a preamplifier, connect LEFT OUT RIGHT OUT 15-24V DC LEFT IN same type of TO-92 package as the transistors. Install the capacitors next. The electrolytic types (in cans) need to be orientated with the correct polarity, and the 25V-rated capacitors must be placed where indicated. An electrolytic capacitor’s longer lead is positive, so goes into the pad marked +. The MKT and ceramic types can be installed either way around. LED5 can be installed horizontally with its leads bent by 90° so it can shine through a hole in the side of the case. The LED is positioned so the top of the LED dome is 12mm in front of the PCB edge and the centre of the lens is located 5mm above the top face of the PCB. When bending the leads, make sure the longer anode and shorter cathode are inserted into the correct pads on the PCB. The clipping indicator LEDs (LED1 and LED3) are intended to be wired to two-way screw terminals, either mounted onto the side of the enclosure or remotely using figure-8 wire into a hole in the amplifier or loudspeaker. LED2 and LED4 are used in conjunction with LDR1 and LDR2. These are within lightproof housings made from 20mm lengths of black 6mm diameter heatshrink tubing with Blu-tack sealing out external light at each end. This arrangement is shown in Fig.5. Fig.6: you can download this front panel artwork from siliconchip.com.au/Shop/11/3623 siliconchip.com.au Australia's electronics magazine May 2026  37 Fig.5: the LEDs and LDRs are sealed in heatshrink tubes using Blu-tack at each end so external light can’t get in. The photos show this done for LED2 & LED4. its output to the Power Amplifier Clipping Indicator input, and the audio signal output from the Clipping Indicator to the power amplifier’s inputs. Sometimes, the preamplifier and power amplifier are separate units. However, if you have an integrated power amplifier with an input selector, tone controls and preamplifier included with the power amplifier (eg, a receiver), the preamplifier output and power amplifier input will need to be accessed. These outputs and inputs are usually available. They are often joined with a curved loop of 3.5mm diameter plated brass inserted between the RCA sockets of the preamplifier output and power amplifier input. Older units may have a tape monitor (record monitor) loop that provides the same interconnection for the inputs and outputs. Initially, set trimpots VR1, VR2, VR4-VR7, VR9 & VR10 fully anti-clockwise. Set VR3 and VR8 fully clockwise. Connect LED1 and LED3 to CON7 and CON8 (temporarily if necessary). You will need a source of 20Hz, 1kHz and 20kHz tones at around 1V (RMS) AC. These can be obtained from a computer, smartphone app or signal generator. There are also CDs that have audio tones for test purposes. Computer programs such as Audacity can produce audio tones. The quality of the output, especially at 20Hz and 20kHz, will depend on the sound card/DAC within the phone or computer. Connect the signal source to the Clipping Indicator inputs (CON1 & CON2). Connect one channel, such as the left, first. CON3 (CON4) goes to the left (right) channel power amplifier input, while the power amplifier output goes to CON5 (CON6), using the non-inverted input for most amplifiers, or the inverted amp input for amplifiers that invert. For BTL amplifiers, only connect one of the outputs at a time, setting up each output individually first before changing the 10kW resistors to 20kW. With everything powered up and a 1kHz signal applied, adjust the power amplifier so there is a normal listening volume level, ensuring it is not clipping. The clipping LED will be lit because the trimpots haven’t been adjusted yet. Now adjust the relevant trimpot, VR4 (VR9) or VR5 (VR10), carefully until the clipping LED extinguishes. Try to find the middle of the pot range that allows the clipping LED to remain off. Now set the audio signal to 20kHz and adjust VR2 (VR7) so the clipping LED goes out. Again, find the middle of the suitable range if you can. In the unlikely event that you can’t adjust the trimpot so the LED goes out, the 22pF capacitor for the left (right) channel may need changing. Use a smaller capacitor if VR2 (VR7) is wound fully clockwise. Next, set the audio oscillator to 20Hz and adjust VR3 (VR8) so the clipping LED goes out. If you can’t get the clipping LED to go out when VR3 (VR8) is fully clockwise, the 1μF capacitor for the left (right) channel will need to be larger. This is a non-polarised (NP) capacitor. It’s unlikely that you will need to change this value, though. Now repeat all the same adjustments for the other channel. Adjusting the automatic attenuator (using VR1 for the left channel and VR6 for the right channel) is best done with the loudspeakers disconnected. Apply a normal music signal and wind up the volume until it starts clipping, as indicated by the LEDs. Adjust VR1 and VR6 so that the signal attenuates just enough to stop clipping, except for occasional momentary flashes from the LEDs. After that, if you want, you can test with the loudspeakers connected and make adjustments for the best clipping reduction. Take care that you don’t damage your ears while doing this – wear ear protection. If you’re using high-efficiency loudspeakers and a high-power amplifier, you may need SC to skip this step! Fig.7: the drilling diagram for the sides of the UB2 jiffy box (197 × 112 × 63mm). This diagram is printed at 60% of actual size and all dimensions are in millimetres. 38 Silicon Chip Australia's electronics magazine siliconchip.com.au