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By John Clarke Amplifier Clipping Indicator Driving an amplifier into clipping can cause some unpleasant sounds and even damage expensive speakers. So it is best to find out right away if you’re about to run into clipping. This easy-to-build Amplifier Clipping Indicator is ideal for that; its LED shows even the briefest of clipping events. A clipping indicator is a valuable accessory for any audio amplifier. It indicates when the amplifier has reached its limit and is clipping the peaks of the audio signal. In practice, quite a lot of clipping can occur before you notice it and even brief clipping events can cause tweeter damage. That’s because when an amplifier is clipping, it ‘squares up’ the waveform; the result contains lots of higher-­ frequency harmonics, which can easily damage the tweeters in loudspeaker systems. Our Amplifier Clipping Indicator flashes its LED whenever clipping is detected. That’s considered to be any time the amplifier output is within about 4V of the positive or negative supply rails. Most amplifiers will clip within about 3V of the supply rail, although some can require slightly more headroom; choosing 4V gives a small safety margin. There can be a significant ripple on the amplifier supply rails when it’s delivering a lot of power (when clipping is most likely to happen). So a proper Clipping Indicator like this one will compare the output waveform to the instantaneous supply voltages to compensate for that. Its thresholds adapt as power supply voltages fluctuate. A timer is included to extend the duration of the LED lighting up, to ensure even momentary clipping is visible. The indicator LED is mounted on the Clipping Indicator PCB, but 68 Silicon Chip it also provides a connection for an external LED mounted on the amplifier’s front panel. This clipping indicator is presented as a bare PCB designed to be housed within an existing amplifier. You can build a single unit for a mono amplifier or two for monitoring two channels in a stereo amplifier. Power for the circuit is derived from the amplifier’s supply; it only draws a few tens of milliamps, so it won’t affect the amplifier’s maximum output power to any significant degree. When building a stereo version, you could use a single, common LED to indicate clipping from either channel or a separate LED indicator for each channel. The second PCB does not require the full complement of components for the common LED version. Circuit The clipping detector circuit is shown in Fig.1. A few components don’t have values: resistors R1a, R1b, R2 and R3 and zener diodes ZD4 and ZD5. Their values depend on your amplifier’s power supply voltage. Table 1 shows the component values required to suit amplifiers with supplies ranging from ±10V to ±80V. The clipping detector generates positive and negative reference voltages. For the positive reference, zener diode ZD1 generates a voltage about 4.7V below the V+ positive rail. Resistors R1a and R1b limit the current through ZD1 to approximately 10mA; together, Australia's electronics magazine they connect across the V+ and 0V amplifier supply rails. The generated voltage is shown on the circuit as V+ − 4.7V at Q1’s emitter. The 4.7V between this rail and V+ is also used to power timer IC1. We allow 5mA for IC1’s supply and 5mA to bias ZD1. More on IC1’s operation later. Zener diode ZD2 and resistor R2 between the V− supply and 0V generate the negative reference; R2 limits the current through ZD2 to about 5mA. Detecting positive clipping The positive reference voltage (V+ − 4.7V) is connected to the emitter of NPN transistor Q1. Its base goes to the amplifier’s output via a 100kW current-­ limiting resistor, while diode D2 stops Q1’s base-emitter junction from being reverse-biased. Just before clipping, the amplifier output voltage will rise above the V+ − 4.7V reference plus Q1’s base-emitter on-voltage of about 0.7V. Q1 switches on when the amplifier output voltage is within 4V of the positive supply. It then sinks current via diode D1, the 100W resistor and zener diode ZD3. The anode of ZD3 connects to the pin 2 trigger input of IC1, and as this voltage drops, timer IC1 starts running. This means that IC1’s pin 3 output goes high, switching on Q4 and the indicator LED (LED1) via a 1kW current-­ limiting resistor. A second external LED will also be lit if connected to the external LED connections. IC1 is a CMOS version of the 555 siliconchip.com.au Fig.1: the Clipping Indicator monitors the amplifier’s output and lights LED1 whenever it comes within about 4V of either supply rail. NPN transistor Q1 detects positive signal excursions, while PNP transistor Q2 detects when the signal approaches the negative rail. IC1 lights the LED for at least 110ms each time clipping is detected. timer and is set up to operate as a monostable timer. Timing is initiated when the pin 2 trigger input goes below a third of its supply voltage. With a 4.7V supply, the trigger point is 1.56V above the V+ − 4.7V rail or 3.13V below the V+ supply rail. Pin 2 is usually held at V+ by a 100kW pull-up resistor. However, when current flows through ZD3, D1 and Q1, the voltage at pin 2 goes low enough to trigger the timer. Once the pin 3 output goes high, the 1μF capacitor at pins 6 and 7 of IC1 begins to charge from the V+ supply through a 100kW resistor. When the capacitor reaches two thirds of the supply (3.13V above the V+ − 4.7V reference), the pin 3 output goes low, and this capacitor discharges into pin 7. This sequence of events occurs when the trigger voltage at pin 2 is only low for a very short period. If the trigger voltage is low for longer than the timing period, the pin 3 output will stay high until pin 2 goes high again. The timing period is about 110ms, as set by the 100kW resistor and 1μF capacitor values. IC1 acts as a pulse extender for brief detection of amplifier clipping. It ensures that clipping is shown on the LED for at least 110ms (ie, a bit more than 1/10th of a second). siliconchip.com.au Detecting negative excursions ZD2, PNP transistor Q2 and diode D3 work to detect negative excursions from the amplifier. When the amplifier output swings low, within 4V of the negative supply, transistor Q2 switches on and, in turn, switches on transistor Q3. This then pulls the pin 2 trigger input of IC1 low via two series zener diodes (ZD4 and ZD5) and resistor R3. Transistor Q3 is rated for a maximum collector-emitter voltage of 80V. Without the two zener diodes, the transistor could be subject to the total of the V+ and V− supply rails and so would only be suitable for use with a maximum of ±40V supply rails. By including the zener diodes, the voltage at the collector is reduced to a maximum of around 66V. While we could have used a transistor with a higher voltage rating, they are not as readily available as the BC546. Table 1 shows the required values for resistors R2, R3 and zener diodes ZD4 and ZD5 for various amplifier supply voltages. Resistor R3 is included to limit current in zener diode ZD3 when transistor Q3 conducts. Australia's electronics magazine While this is not the simplest clipping detector circuit, it has the advantage of presenting an almost entirely linear load to the amplifier output, to minimise the possibility of any distortion due to loading. Note that if you want to monitor clipping in a stereo amplifier and use a single indicator LED, you can dispense with the components in the blue shaded areas for the second channel. Interconnection is made between the two PCBs at the top end of R3. This way, a clip event at either input will trigger IC1 on the board where it is fitted. Alternatively, you can build two complete copies of the circuit for independent channel clipping indication. The boards are small and can 69 Figs.2 & 3: the board is not difficult to assemble; the components are fitted as shown at left. The diodes, LED, IC & electrolytic capacitors are polarised. If you’re building a mono version or a stereo version to drive two independent LEDs, build the fully populated version. For a stereo version with a single clip indicator LED, build one of each version and join the indicated pad between the two boards (not present on the prototype PCB pictured). be stacked to take up relatively little room. Construction The Amplifier Clipping Indicator is constructed on a double-sided, plated-­ through PCB coded 01112211 that measures 54 x 60mm. There are two overlay diagrams shown. Fig.2 is the version used for a mono amplifier, or for the left channel in a stereo amplifier (or both channels if you want independent clip indication). If the second channel is built as shown in Fig.3, clipping in either channel will be indicated with a single LED. Begin by fitting the resistors. First, refer to Table 1 to select the resistor value and power ratings for R1a, R1b, R2 and R3. The parts list contains a resistor colour code table, but you should ideally also check each resistor using a digital multimeter (DMM) before installing it. Once these parts are in place, follow with diodes D1, D2 and D3, orientating them correctly. The zener diodes can be mounted next. ZD1 and ZD2 are 4.7V types, while ZD3 is rated at 3.9V. The ZD4 and ZD5 voltages are as per Table 1, or replaced with a wire link if indicated. Transistors Q1, Q2, Q3 and Q4 can be mounted next. There are three different types (although Q1, Q3 & Q4 can all be BC546s if desired), so take care to install each in its correct place. The screw terminal blocks making up CON1 need to be joined together first by fitting each side-by-side by sliding the dovetail mouldings together. Solder them in place with the wire entry side of the terminals facing the nearest edge of the PCB. Now fit LED1 with its longer lead inserted into the anode hole. Mount it so that the top is about the same level as the adjacent screw terminal. IC1 can be soldered directly onto the PCB, making sure its pin 1 is facing as shown. Finally, install the capacitors. The 1μF capacitor must be oriented correctly, with its longer + lead into the pad shown. You could use a non-polarised 1μF plastic film capacitor, but it will be substantially larger Table 1 – component values that vary with amplifier supply rail voltages Supply R1a R1b R2 R3 (½W) ZD4 ZD5 ±80V 15kW 1W 15kW 1W 15kW 1W 33kW 75V (1N4761) 18V (1N4746) ±75V 15kW 1W 15kW 1W 15kW 1W 33kW 75V (1N4761) 9.1V (1N4739) ±70V 12kW 1W 12kW 1W 12kW 1W 33kW 75V (1N4761) wire link ±65V 12kW 1W 12kW 1W 12kW 1W 33kW 33V (1N4752) 33V (1N4752) ±60V 12kW 1W 12kW 1W 12kW 1W 33kW 27V (1N4750) 27V (1N4750) ±55V 10kW ½W 10kW ½W 10kW ½W 33kW 22V (1N4748) 22V (1N4748) ±50V 9.1kW ½W 9.1kW ½W 9.1kW ½W 33kW 16V (1N4745) 18V (1N4746) ±45V 8.2kW ½W 8.2kW ½W 8.2kW ½W 33kW 12V (1N4742) 12V (1N4742) ±40V 7.5kW ½W 7.5kW ½W 7.5kW ½W 30kW 15V (1N4474) wire link ±35V 6.2kW ½W 6.2kW ½W 6.2kW ½W 30kW 3.9V (1N4730) wire link ±30V 5.1kW ½W 5.1kW ½W 5.1kW ½W 27kW wire link wire link ±25V 3.9kW ½W 3.9kW ½W 3.9kW ½W 22kW wire link wire link ±20V 3kW ½W 3kW ½W 3kW ½W 18kW wire link wire link ±15V 2kW ½W 2kW ½W 2kW ½W 13kW wire link wire link ±10V 1kW ½W 1kW ½W 1kW ½W 8.2kW wire link wire link 70 Silicon Chip Australia's electronics magazine siliconchip.com.au and probably more expensive than the electrolytic. If you’re building the two-channel version to light a single clip indicator LED, build a second board as per Fig.3 and solder a ~20mm length of solid-core ‘Bell wire’ to the top of that board, into the pad between ZD3 and Q4. It makes sense for the more sparsely populated board to be at the bottom of the stack as it lacks the LED, and you’ll want to be able to see the LED on the other board. Alternatively, if you’re building a two-channel version with separate LED indicators, make a second identical board and don’t fit the vertical wire. Mounting it & wiring it up Parts List – Clipping Indicator (per channel) 1 double-sided, plated-through PCB coded 01112211, 54 x 60mm 2 3-way screw terminals with 5.08mm spacing (CON1) OR 2 2-way screw terminals with 5.08mm spacing (CON1; for minimised second channel) Semiconductors 1 7555 CMOS timer, DIP-8 (IC1●) 2 BC547 or BC546 NPN transistors (Q1, Q4●) 1 BC557 PNP transistor (Q2) 1 BC546 NPN transistor (Q3) 1 yellow, amber or red 3mm or 5mm LED (LED1●) 1 yellow, amber or red LED (optional; external LED●) 2 4.7V 1W (1N4732) zener diodes (ZD1, ZD2) 1 3.9V 1W (1N4730) zener diode (ZD3●) 3 1N4148 small-signal diodes (D1-D3) 2 zener diodes or wire link (ZD4▲, ZD5▲) 1 LED bezels (optional; for chassis-mounting external LED) 4 M3 x 6mm tapped Nylon spacers (or 15mm spacers for the upper board in the stack) 8 M3 x 6mm machine screws (or 4 M3 x 15mm machine screws for the upper board in the stack) 1 20mm+ length of solid-core hookup wire (optional; to join stacked stereo version) various differently-coloured hookup wires, rated for amplifier supply voltage Capacitors 1 1μF 16V PC electrolytic● 1 100nF 63V or 100V MKT polyester● 1 10nF 63V or 100V MKT polyester● Resistors (¼W, 1% axial metal film) 6 100kW (● 4 required for minimised version) 1 10kW● 2 1kW● 1 100W 4 other resistors, values as per Table 1▲ ● not required for the minimised second channel ▲ see Table 1 for values and power ratings Use the board to mark out four holes in a convenient location within the amplifier chassis, ideally, between the amplifier modules and speaker terminals, or at least near the terminals. If it’s a stereo amplifier, you can stack the two boards by feeding longer machine screws up through the spacers on which the lower board is mounted, then screwing some ~16mm tapped spacers on top of the threads once the first module is in place. If you have space, you could mount the two modules separately, eg, side-by-side. Connect the Clipping Indicator(s) to the amplifier’s V+, V− and 0V supply rails and the amplifier speaker + output(s) to AMP OUT input(s) on the Clipping Indicator module(s). Make sure the wiring is suitably voltage-­ rated, especially when the supply rails are at high voltages from Earth. The external LED connects to the A and K terminals on the board. If you are building the minimised stereo version with IC1 and associated components missing, feed the wire you soldered earlier to the bottom board up through the matching pad on the top board. Solder it on top and mount the upper board using longer tapped spacers and short M3 machine screws. In all cases, when using a second Clipping Indicator module, all three supply connections must be made to both boards, along with the AMP OUT terminal. The only terminals that aren’t needed on the board with components missing are the LED A & K. The external LED or LEDs can be attached to the amplifier chassis using suitable LED bezels, or (less ideally) glued into tight-fitting holes using This shows the Clipping Indicator installed inside our upcoming 500W neutral-­cure silicone sealant. SC Amplifier chassis. siliconchip.com.au Australia's electronics magazine March 2022  71