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1kW+ Class-D Part 1 by Allan Linton-Smith Image source: https://unsplash.com/photos/SP9HcRASMPE Mono Amplifier This mighty monoblock amplifier uses a prebuilt module and some relatively inexpensive switchmode supplies to deliver well over 1kW into 2Ω loads and substantial power into 3-8Ω loads. It can be built for around $1000 (that’s more than 1W per dollar) and fits into a metal toolbox, so it’s even portable! T his potent monoblock amplifier uses a module designed by International Rectifier based on the IRS2092S Class-D controller and four IRFB4227 Mosfets – see Photo 1. This module is available from DigiKey ready-built for around $510. With some caveats, it can deliver up to 1700W RMS into 2W! That’s about the maximum audio output you could get from a single-phase mains 230V AC 10A supply. You don’t get super hifi performance at this dizzy level, but you will get very acceptable distortion (below 0.1% THD+N) at around 1000W. That’s very useful for large banks of PA speakers or music instrument reinforcement. Such monstrous power levels from domestic power supplies require a Class-D amplifier because of its high efficiency; in this case, it is 97% at 1700W. The module requires a very heavy-duty dual power supply at ±75V/18A, which will also be described in this article. The module is sold as an ‘evaluation board’ and has a few functions you can play with (eg, the ability to change the carrier frequency). It has very modest dimensions at just 192 × 149 × 56mm and only weighs 540g. It has a remarkably small heatsink, sufficient for ‘modest’ loads, but it can easily be enhanced, as we shall see. Not only does this amp put out enormous power, but it also has many essential protection features built in, like: • Output over-current protection (OCP), high side and low side, to handle clipping and accidental short circuits. • Supply over-voltage protection (OVP) over 82V. • Supply under-voltage protection (UVP) under 38V. • Output DC-offset protection (DCP) to prevent speaker damage in case of a fault. ◀ Photo 1: the pre-assembled IRAUDAMP9 mono Class-D amplifier module, wired up. Despite the relative complexity of the circuitry, using it is actually pretty easy. A thermal image of the amplifier module when delivering 400W (short term) is shown at left. The heatsink has only reached 44°C. At the same time, in the image at right, the 8W 800W dummy load dissipating 400W could boil water! 28 Silicon Chip Australia's electronics magazine siliconchip.com.au configuration). It does not apply to this mono amplifier. Amplifier Module Specifications » THD+N: typically <0.1% up to 1kW into 2Ω, 500W into 4Ω, 270W into 8Ω » Output power, 1% THD+N: 1.2kW into 2Ω, 575W into 4Ω, 315W into 8Ω » Load impedance: 2-8Ω » Dynamic range: 99.4dB » Residual noise, 20Hz-20kHz: 290μV » Damping factor: 81.9 (1kHz, 2Ω load) » Frequency response: ±1.25dB, 20Hz-20kHz (1W, 2Ω load) » Self-oscillating frequency: 300kHz (adjustable) » Gain: 33dB » Input sensitivity: 1V RMS input for 1kW into 2Ω » Modulation: second-order delta-sigma, self-oscillating » Power supply: ±48V to ±80V DC » Idle supply current: +67mA, -105mA » Idle power <at> ±72V: 13.2W » Efficiency: 74% <at> 100W, 94% <at> 1000W, 97% <at> 1700W » Heatsink temperature (unmodified): 56°C <at> idle, 104°C <at> 125W, 118°C <at> 1.2kW (shuts down after 130s) • Over-temperature protection (OTP) for a heatsink temperature over 100°C. The IRAUDAMP9 does not use a series relay to disconnect the speaker to prevent switch-on and switch-off thumps. Instead, it uses the IRS2092S’s on-chip noise reduction circuit which suppresses these transient events to levels below those generated by relays. Many copies of this module are available online, based on the same ICs. So while we recommend you purchase the known-good manufacturer version from a supplier like DigiKey, there are alternatives should it no longer be available. On the reference design, a lit red LED signifies a fault/shutdown condition. It also has a green LED that lights when conditions are normal. There are three switches on the reference design. S1 is a three-position switch that can select self-oscillation (middle position – “SELF”), internal (“INT”) or external (“EXT”) clock synchronisation. A BNC fitting is provided for the external clock, but no data is given for the amplitude, so we haven’t tried it. The purpose of S2 is not explained but it appears to control synchronisation between the clocks for two modules (eg, in a stereo or bridged Photo 2: the switchmode banks fit nicely into the aluminium toolbox; the kilowatt amplifier module occupies a small area on top mounted on plastic insulators. The small module on the right provides 12V from one of the 24V supplies to power the fan and VU Meter light (Photo 4). Note the large stainless bolts used to secure the switchmode power banks. siliconchip.com.au Australia's electronics magazine Operating principles and uses The IRAUDAMP9 reference design is a single-channel 1.7kW (into 2W) half-bridge Class-D audio power amplifier. At its heart is the IRS2092S Class-D audio controller that uses sigma-­delta PWM (pulse width modulation) to produce an audio signal with relatively low distortion and noise. An external gate buffer is also used to provide various protection modes, with the final power output coming from four IRFB4227 Mosfets. This module provides all the necessary housekeeping power supplies from the main ±75V for ease of use. The internally-generated power supplies include ±5V for analog signal processing (preamp etc) and a +12V supply (Vcc), referenced to –B, to supply the Class-D gate-driver stage. Above 1kW, it’s a good idea to use a larger heatsink than the one supplied (<2°C/W), especially for long-term use at high power levels. This is not a project for domestic use. Suitable applications include: • professional audio amplifiers and powered speakers; • active PA subwoofers; • other professional PA systems; • musical instrument amplifiers. Its manufacturer specifications are shown above, and we have produced three THD+N vs power level plots for standard load impedances in Fig.1. Those curves demonstrate it can easily deliver 1kW into a 2W load. Distortion Photo 3: six of these switchmode supplies give us an output of over 1kW into a 2W 2W load; three for the positive side and three for the negative side. They each have their own cooling fan and overload protection and are efficient and costeffective. Their output voltages are also adjustable. October 2023  29 Fig.1 (left): plots showing our measured THD+N vs power output for our complete prototype amplifier into three typical load impedances. 0.1% distortion at a massive 1000W is not bad! Fig.2 (right): this THD+N vs frequency plot into 8W reveals that distortion rises from around 300Hz. That is a little earlier than a good linear amplifier but is not unusual for a switching amplifier operating at a few hundred kilohertz. Typical program material has a lot of signal content below 1kHz, where the distortion level is pretty reasonable. rises quite a bit above 1kW, so if you want it to sound good, you can consider it a 1kW amplifier (that’s still a lot!). This module has a high PSRR (power supply rejection ratio), so you don’t need super smooth DC rails. It will reject 80dB of a 200mV peak ripple thanks to the balanced bipolar power supply. We used a ±80V 5A lab supply for some initial tests, then increased the power available to the module to 2,880W from six 24V DC 20A switchmode power supplies connected in series (see Photos 2 & 3), with additional capacitors for slightly improved performance. Those were two 10,000μF 100V chassis-­ mount electrolytic capacitors (Jaycar RU6712). converts the signal to lower resolution values with error diffusion/correction so that the final result, after filtering, reconstructs the desired signal accurately. In the case of a Class-D amplifier, the output only has two states (high or low), so it is effectively a 1-bit DAC, usually running at several hundred kilohertz. The delta-sigma modulator and filtering allow this to produce a signal in the audio range with an effective resolution of around 16 bits. Power output The quoted power output is 1700W RMS into 2W and we measured over 450W RMS into 8W. At these colossal power figures, you won’t get low distortion (in fact, the amp is already well into clipping), but at lower output levels like 1250W (2W) or 350W (8W), the distortion is not gross; see Fig.1. 2W loads are increasingly becoming the norm for modern big subwoofer drivers that demand this sort of power level. If using 4W or 8W drivers, you could parallel multiple to achieve 2W so that this amplifier can drive them at full power. Series/parallel sets with an overall impedance of 2W could be used to run many drivers from a single amp. Amplifier power output specifications Amplifier manufacturers (and their Delta-sigma modulation Delta-sigma (or sigma-delta) modulators (DSMs) are a class of oversampling digital-to-­ analog converters (DACs) that perform ‘quantisation noise shaping’ to achieve a high signal-to-noise ratio (SNR). They are an efficient solution for resolutions above approximately 12 bits. DSMs are extensively used in analog and RF applications. Effectively, a DSM involves using a low-resolution, highly oversampling DAC to reconstruct a signal with a much higher resolution but a lower frequency. The intended signal passes through a filter (usually digital) that 30 Silicon Chip Fig.3: the main distortion component is the third harmonic at -64.8dBv (0.05%), while the second harmonic is lower at -99.7dBv (0.001%). The delta-sigma design provides significant distortion cancellation. Australia's electronics magazine siliconchip.com.au Photo 4: this optional VU Meter gives you an idea of the current output level. designers) always want to find a way to publish the most impressive power specifications. Remember the ridiculous “PMPO – peak momentary power output” ratings where a small boombox was rated at over 1000W? Luckily, that isn’t the case here, as the >1kW ratings are real RMS power ratings, although you need a 2W load to achieve them. However, they are still a little cheeky in how they measure these power levels. You can get an inflated RMS power rating if you don’t care how much you distort the signal. Suppose you crank the gain or input signal level until the amplifier delivers an almost square wave into the load. In that case, you will get a rating about Photo 5: here, you can see the internal wiring of the speaker outputs with the 75μH inductor. The IEC mains input socket is under the black Jiffy box and is secured via screws and nuts on the base of the chassis to provide insulation and separation from the lower-voltage wiring above. 50% higher than you would with a more reasonable distortion level. The manufacturer states this is a 1.7kW amplifier, but that is at 10% distortion. We think it’s more realistic to rate it closer to 1kW (0.1% distortion). For PA use, you might be willing to accept a higher distortion level, so we’ve also given specifications at 1% THD+N (for example, 1.2kW into 2W). That’s approximately the point above which the output will start to sound lousy. Distortion As well as the plot of distortion vs power (Fig.1), we’ve also produced a plot of THD vs frequency for an 8W load, shown in Fig.2. As you’d expect Fig.4: the frequency response is pretty flat for 2W, 3W & 4W loads. For 8W loads, we recommend a 75μH series inductor to avoid that big spike at 25kHz, which could cause tweeter damage. siliconchip.com.au Australia's electronics magazine from a Class-D amplifier with a self-­ oscillation frequency of only around 300kHz, distortion rises significantly above 1kHz. Still, we already know this is not a hifi amplifier... Fig.3 shows the distortion spectrum for a 1kHz output at 1W. The first harmonic is -99.7dB <at> 2kHz (0.001% distortion), with the more critical third harmonic being -64.8dB <at> 3kHz (0.05% distortion). Frequency response The quoted frequency response by the supplier is ±1dB from 20Hz to 20kHz for a 2W load, but they didn’t give specifications for 4W or 8W loads. We made the plots shown in Fig.4, which reveal that with an 8W load, Photo 6: the rear panel has connections for the mains input (IEC), signal input (RCA) and binding posts for the speaker outputs. The top binding posts are for 2W & 4W loads, while the bottom posts provide frequency compensation for 8W loads. October 2023  31 there is a 7.25dB lift at 25kHz, at low power levels. The huge blip around 23kHz could easily destroy tweeters, especially at high power levels. Generally speaking, 2W, 3W or 4W loads are preferred for this board, and judging from the results, the IRAUDAMP9 was deliberately designed with lower load impedances in mind. We connected a 75μH 5A RF choke in series with the load and got the much more reasonable curve shown in orange. Therefore, our final amplifier design has a separate output for 8W loads fed via such a choke. Signal frequencies around 20kHz may cause LC resonance in the output low-pass filter, causing a large reactive current flow through the switching stage, especially if the amplifier is not connected to any load. This can activate over-current protection. Therefore, filtering out frequencies above 20kHz before feeding the signal to the amplifier is a good idea. That explains the 7.25dB spike we measured at around 20kHz with an 8W dummy load. Adding the extra choke fixed this, but it should only be used for 6-8W (nominal) loads. Listening tests After making all the measurements, we hooked up the amplifier to various speakers that presented 2W, 4W and 8W nominal loads. We were a bit nervous as such a huge power delivery would mean that, if anything went wrong, our speakers would immediately be toast! However, the switch-on was a letdown, as the module was silent except for the click of the switch and the quiet whirring of the cooling fans. The mute function from the IR2092S keeps the red LED on and the output muted for about three seconds. After that, the green LED switches on to indicate that the amplifier is functional. The amplifier mutes everything again at switch-off time after the DC supply voltage drops below ±38V. Switch-on and switch-off are absolutely silent; if it didn’t perform this way, speaker cones would probably pop out of their surrounds! Despite the compromised THD+N typical of Class-D amplifiers, the output sounds much better than expected, and the bass is undoubtedly effortless with all that available power. After playing several CDs, a quick check of the heatsink showed that it was merely warm and measured just 38°C with an infrared thermometer. Fig.5: this simplified circuit shows the overall configuration of the Class-D amplifier module, including the power Mosfets that drive the load and the bipolar transistor buffers that drive their gates. 32 Silicon Chip Australia's electronics magazine siliconchip.com.au The fan was able to cool everything, including the power supplies, which have their own internal fans. There are seven fans all up. With this sort of Class-D amplifier, efficiency improves as power increases, so there is likely no need for additional heatsinking. Class-D operation A simplified circuit diagram of the module, redrawn from the one provided in the data sheet, is shown in Fig.5. Capacitors C2int & C1int and resistor Rfreq form a second-order front-end integrator. This receives a rectangular feedback signal from the Class-D switching stage and produces a quadratic oscillatory waveform as a carrier signal. To create the modulated PWM signal, the input signal shifts the average value of this quadratic waveform (through the gain relationship between RFB, RFBfilt and Rin) so that the duty cycle varies according to the instantaneous value of the analog input signal. The IRS2092S input comparator processes the signal to create the required PWM signal, which is internally level shifted down to the negative supply rail where it is split into two signals, with opposite polarity and added dead time, to drive the high-side and low-side Mosfet gates, respectively. The IRS2092S drives two pairs of IRFB4227 TO-220 Mosfets in the power stage with PWM gate signals to drive the load. The amplified analog output is recreated by demodulating the PWM signal with an LC low-pass filter (LPF) formed by Lout and Cout, which filter out the switching carrier signal. Driving these pairs of Mosfets requires a peak of more than ±1A to drive the gates to rapidly charge and discharge their gate capacitance. To do this, a bipolar transistor emitter-­ follower buffer stage is used, comprising NPN & PNP transistors in totempole configuration, as shown in Fig.6. One pair is used for the low-side Mosfets and one for the high-side Mosfets. This buffering is necessary to achieve fast enough switching of the Mosfets to avoid exceeding the over-current protection voltage monitoring time. For over-current protection, the IC measures the voltage between the drain and source of the siliconchip.com.au Adjusting the Class-D switching frequency The total delay time inside the control loop determines the self-oscillating frequency. That includes delays from the logic circuits, the Mosfet gate driver, the external buffer, the IRFB4227 switching speed, the front-end integrator’s time constant, and variations in the supply voltages. Under normal conditions, the switching frequency is around 300kHz with no audio input signal and a ±75V supply. The PWM switching frequency greatly impacts the audio performance. Generally, distortion due to switching time becomes significant for higher frequencies, while at lower frequencies, the amplifier’s bandwidth suffers. Higher switching frequencies also result in higher switching loss in the power stage, so the thermal performance degrades. Another consideration when determining the switching frequency is to avoid it or one of the most significant harmonics causing interference in the AM broadcast band (531-1602kHz). If the switching frequency is 300kHz, its third harmonic at 900kHz could be a problem as it’s usually only 40dB below the switching frequency – see the diagram below. Adjustments are made by varying potentiometer P1 on the amplifier board with no input signal. The default amplifier switching frequency is 310kHz. The second harmonic is 60dB lower, but the third is just 40dB lower and could interfere with local AM radio stations. The carrier frequency is adjustable in case the interference causes problems with your local AM frequencies. Fig.6: this section shows just the output drivers and buffers. The bipolar transistors are needed as the IC can’t sink or source enough current to rapidly switch the relatively high-capacitance power Mosfet gates. Australia's electronics magazine October 2023  33 Parts List – 1kW Class-D Mono Amplifier 1 IRAUDAMP9 Class-D amplifier module [DigiKey IRAUDAMP9-ND] 6 24V 15-20A switchmode supplies [Mouser 709-LRS350-24, DigiKey 1866-3346-ND, element14 3596594, Wagner LRS-350-24, eBay 292508020804] 1 24V to 12V 1A+ DC/DC buck converter module [eBay 204144932095] 1 120mm 12V or 24V DC low-noise fan [Jaycar YX2584] 1 120mm fan guard [Jaycar YX2554 or YX2515] 1 100μH 5A toroidal inductor [Jaycar LF1270] 1 10kW 24mm logarithmic single-gang potentiometer plus knob [Jaycar RP3610 + HK7788] 2 red binding posts [Jaycar PT0460] 2 black binding posts [Jaycar PT0461] 1 chassis-mount IEC mains input socket with integral fuse and switch [Jaycar PP4003] 1 IEC mains input cable 1 10A M205 fast-blow fuse 1 panel-mount RCA socket to RCA socket [Jaycar PS0442] 1 1m RCA-RCA cable 1 high-efficiency fan heatsink (optional) [Jaycar HH8573] 1 small tube of thermal adhesive (optional, above heatsink) [Jaycar NM2014] 2 10,000μF 100V chassis-mount capacitors (optional) [Jaycar RU6712] 1 panel-mount VU meter (optional) [Altronics Q0490] 1 120kW ¼W 5% axial resistor (for optional VU Meter) 1 1N4148 small signal diode (for optional VU Meter) 1 UB5 Jiffy box Hardware 1 aluminium toolbox, 575 × 245 × 220mm or larger [eBay 192790170418, Bunnings 6120223] 4 M10 × 150mm cup head bolts and nuts [Bunnings 2310405] 4 M10 flat washers [Bunnings 2430052] 1 100 × 75mm aluminium pressed wall vent [Bunnings 0810902] 1 800mm length of 25 × 3mm aluminium flat bar [Bunnings 1079373 (3m length)] 1 800mm length of 20 × 10 × 2mm aluminium rectangular tube [Bunnings 1130559 (1m length)] 16 M4 × 20mm panhead machine screws and nuts [Bunnings 0168397] 18 M4 × 15mm panhead machine screws and nuts [Bunnings 0168393] 20 M4 × 10mm panhead machine screws and nuts [Bunnings 0247265] 36 M4 flat washers [Bunnings 0130531 × 3] 1 M4 shakeproof (toothed) washer 18 M3 × 20mm panhead machine screws and nuts [Bunnings 0247264] 20 M3 × 15mm panhead machine screws and nuts [Bunnings 0168388] 20 M3 × 10mm panhead machine screws and nuts [Bunnings 0247262] 6 M3 × 6mm panhead machine screws 4 M3 × 6mm countersunk head machine screws 4 M3 x 9mm tapped Nylon spacers (for mounting the amplifier module) 2 M3 hex nuts (for securing the Jiffy box) [Bunnings 2310899] 48 M3 flat washers [Bunnings 0257725 × 4] 2 M3.5 right-angle brackets [Jaycar HP0872] Wiring etc 7 6.4mm insulated female spade crimp lugs to suit 10A-rated mains wire 4 5.3mm eye crimp terminals to suit heavy duty hookup wire 4 5.3mm eye crimp terminals to suit heavy duty speaker wire 32 3.7mm forked spade crimp lugs to suit heavy duty wire 1 2m length of 10A mains-rated Earth (green/yellow striped) wire 1 2m length of 10A mains-rated light blue (Neutral) wire 1 2m length of 10A mains-rated brown (Active) wire 1 short length of heavy-duty figure-8 speaker cable 3 2m lengths of 15A heavy-duty hookup wire (red, black & blue) Cable ties (as required) [Jaycar HP1244] 34 Silicon Chip Australia's electronics magazine Mosfets, as they have a more-or-less fixed channel resistance, so that voltage is proportional to the load current. The IC starts monitoring this voltage as soon as the HO/LO outputs go high after a short leading-edge blanking time. The self-oscillating PWM modulator results in the lowest component count and highest performance. It represents an analog version of a second-­ order sigma-delta modulator, with the Class-D switching stage inside the feedback loop. Compared to carrier-signal-based modulation, the benefit of sigma-delta modulation is that all the error in the audible frequency range is shifted to the inaudible ultrasonic range. With sigma-delta modulation, we can apply sufficient error correction for low noise and distortion. The IRAUDAMP9 modulator incorporates: • a front-end integrator; • a pulse width modulator and level shifters; • gate driver and buffer; • power Mosfets; • output LPF. Input and output signals The input signal can be up to 2V RMS. Given that the IRAUDAMP9 module is a single-ended design (with the – output connected to ground) and it can drive 2W loads, that means that, in theory, you could use two such modules to drive a 4W load in bridge mode and achieve more than 2kW output! We haven’t tried this and can’t imagine it would be necessary outside of stadium-level sound reinforcement applications. Power supply The power requirements are very heavy, as you might expect for a 1kW+ amplifier. For initial testing, we used a lab power supply based on a 500VA 55-0-55V toroidal transformer that delivered ±80V DC but only up to 4A. This limited total power output to less than 450W into 2W. This power supply caused the amplifier to occasionally go into protection mode, mainly at frequencies below 25Hz, because of ‘bus pumping’, as described in the data sheet. This occurs since the IRAUDAMP9 is a half-bridge configuration. In regular operation, during the first half of the cycle, energy flows from one supply through the load and into the siliconchip.com.au other supply, causing a voltage imbalance. In the second half of the cycle, this condition is reversed, resulting in bus pumping of the other supply rail. The following conditions worsen bus pumping: • Lower frequencies (bus pumping duration is longer per half-cycle). • Higher power output voltage and/ or lower load impedance (more energy transfer between the supplies). • Smaller bus capacitors (the same energy will cause a larger voltage increase). Rather than use several expensive toroidal transformers and bridge rectifiers, as mentioned earlier, we purchased six 24V 20A switchmode supplies. We used three in series for the positive side and the other three for the negative side. The total cost for these was only $347, including delivery. This arrangement provides ±72V DC at 20A, although each independent supply is adjustable up to 25V, giving the recommended ±75V. Each side is adjustable to within 0.1V of the other, so PSRR is improved, and distortion and hum are significantly cancelled. This worked well, and all the graphs here were made with that supply configuration. You can also add extra capacitance to slightly reduce the distortion level, although that makes the amplifier a bit more expensive. Next month That just about covers how the amplifier works. Next month, we’ll have the details on how it goes SC together. Alternative Class-D module After our initial evaluation, we noticed that many alternative modules supposedly using similar components were available – see the photo below. We purchased one from eBay seller “polestarmascot” (www.ebay.com.au/ itm/325534592503) for a brief evaluation. This alternative board requires a separate low-voltage input of ±12V or 6-12V AC but has the added advantage of being a dual/stereo amplifier with a switch for putting them in bridged mono mode. It was very cost-effective at just $187, including delivery from China. We performed a brief evaluation of THD+N and frequency response. Its distortion performance was OK, giving around 0.02% at 1W/1kHz and 0.7% at 100W/1kHz into 8Ω. It actually had a pretty flat frequency response into 8Ω – much better than the IRAUDAMP9 with its big spike around 20kHz. Note that as there are many similar unbranded units for sale online, the components and construction are not standardised and may vary considerably. So our cursory tests really only apply to the unit we obtained. In brief, if you don’t want to spend around $575 on the genuine board, this one is around one-third the cost and does work but probably won’t give quite as good performance, especially at very high power levels. Specifications (from supplier) Supply voltage: ±33-80V plus ±12V or 6-12V AC Stereo power (±80V supply, distortion <0.1%): 2 × 350W into 8Ω, 2 × 700W into 4Ω, 2 × 900W into 2Ω (±62V supply, fan-assisted cooling) Mono (bridge mode) power: 1200W into 8Ω, 2000W into 4Ω (±70V supply) Gain: -33 times Input sensitivity: 1.6V RMS Input impedance: 20kΩ Frequency response: 0-50kHz ±1dB Residual noise: 200μV Dynamic range: >100dB Thermal cutout: 85°C Overvoltage protection: ±81V Efficiency: >90% at 300W We only performed some basic tests on this alternative Class-D amplifier module, but it seems reasonably capable compared to the (considerably more expensive) IRAUDAMP9, which uses the same major components. Fig.7: connections are straightforward; besides three wires for the ±75V DC power supply, you just need to connect an RCA cable for the input signal and two heavy-duty wires from the CH1 Output terminal block to the external output terminals for the load. siliconchip.com.au October 2023  35