Class D amplifier module

A class-D amplifier or switching amplifier is an electronic amplifier where all power devices (usually MOSFETs) are operated as binary switches. They are either fully on or fully off. The term “class D” is sometimes misunderstood as meaning a “digital” amplifier. While some class D amps may indeed be controlled by digital circuits, the class D amplifier control circuit can be also implemented with analogue component. I would rather call class-D amplifier as a switching amplifier rather than “digital amplifier”.

Class D amplifiers work by generating a square wave of which the low-frequency portion of the spectrum is essentially the wanted output signal. In a simplest form he generated signal can be just pulse-width-modulated (PWM) square wave, but in many practical class D amplifiers the output signal is slightly more complicated. After the output driver stage a passive low-pass filter removes the unwanted high-frequency components, i.e., smoothes the pulses out and recovers the desired low-frequency signal. To maintain high efficiency, the filter is made with purely reactive components (inductors and capacitors). The switching frequency is typically chosen to be ten or more times the highest frequency of interest in the input signal. This eases the requirements placed on the output filter. In cost sensitive applications the output filter is sometimes omitted.

I bought some time ago Y148 Audio Amplifier Module so learn more than just the theory of the class D amplifiers. Hands on on some new technology gives better insight than just reading the theory. Here is the picture of the amplifier module ready to use on my desk.

classd1

To make connecting the wires easier I have added screw terminals to Y148 Audio Amplifier Module (the module came only with soldering holes for power and speaker connections). Speaker and power connections have large well spaced holes. I also added pin strip (L_in, GND, R_in) for audio input and audio cable to connect the audio source.

Here is some data of the amplifier:
- Model: Y148
- PCB board
- Adopts YDA148 high-efficiency digital audio power amplifier IC
- DC input voltage: 9~15V
- Current: 2~4.5A
- Power output at DC 15V input: 15W x 2 (8 ohm), 30W x 2 (4 ohm)
- Power output at DC 12V input: 10W x 2 (8 ohm), 20W x 2 (4 ohm)
- Needs heat sink at 4 ohm, doesn’t need heat sink at 8 ohm
- Frequency response: 10Hz~20KHz(+/-0.2dB @1KHz)
- Load speaker: 4 / 6 / 8 ohm
- SNR: at least 90dB
- Harmonic distortion: THD+NC no more than 0.1%P=0.1W

classd2

Y148 Audio Amplifier Module is cheap super small amplifier. It works great, and plays all right. No noticeable noise/distortion at reasonable audio levels. It runs nicely on 12V power. Great product, especially the price is really attractive.

The specifications are pretty good for such small device. The module drives 8 ohms speakers from 12V power source directly. The tiny IC in the center of the circuit board can handle 2x10W without any heatsink! If you try to run 4 ohm speakers or higher power the chip gets quite hot (you should be able to solve the problem with a suitable small heatsink added on top of that that little chip).

You can use it even on battery powered applications, because you can get quite a bit of audio output with about 100mA from the battery! Not the full power but useable volume levels. The high efficiency is the reason that class D amplifiers are popular in many battery powered electronics devices.

classd3

As I told earlier Class D amplifiers work by generating a square wave of which the low-frequency portion of the spectrum is essentially the wanted output signal. YDA148 high-efficiency digital audio power amplifier IC operates around 500 kHz frequency according to datasheet (I measured 508 kHz on my module). The IC generates pulses (starting from 0V and going up to +12V in level) to both + and – side of speaker output line at that frequency. The pulse length varies depending on the signal that needs to be sent to speaker. When there is no output signal, both pulses are short. When the amplifier needs to send positive voltage to speaker, the pulse at + line gets longer and longer depending how high voltage need to be set to speaker. The – side pulses stay at the same short length. In case of negative voltage needs to be sent to speaker, the – side pulses get longer and + side stays short. The picture below show the pulses that can be seen on speaker + output on IC output.

classd_scope

YDA148 high-efficiency digital audio power amplifier IC datasheet says that a low pass filter at corner frequency of 50 kHz normally needed (digital amplifier operates around 500 kHz).

yamaha_filter

The amplifier board has this kind of filter in it. Ther filter works pretty well in attenuating the high frequency signal. I measured around 50 mV (RMS) 508 kHz triangle wave on the speaker oputput after the filter. So the switching feququency is pretty well attenuated, and in taking account the fact that speaker elements can’t play back those high fequencies and will not hear them, I don’t think it could hurt the the sound quality at all.

classd4

The size of components is one that might in some applications drive to try to avoid the filters. As you can see on the board I have the filter components take much more board space than the actual IC. In the picture below you see the amplifier IC on the top and filter components for one channel on the bottom.

Direct connection from IC to speaker element is also possible according to YDA148 datasheet when speaker has element is suitable (inductance 20 microhenries or more), wires are short and speaker is present always. I think that direct connection method would be feasible only in active speaker applications where amplifier and speaker elements are inside same cabinet. In all other applications that filter is needed, because EMC reasons (feeding a strong 500 kHz signal to unshielded speaker cable generates easily lots of interference).

YDA148 datasheet also says that this IC supports differential input. Using differential interfaces is a good idea in many audio applications. Unfortunately this amplifier module board does not support using differential input. The module I have has only traditional unbalanced line level input option. That is useable, but I would have preferred to have differential interface as an option.

Y148 Audio Amplifier Module seems to be a good small audio amplifier. It was cheap, performed well and provided a good platform to lears about class D amplifiers. Class D amplifier technilogy is nowdays mature and ready to be taken into use.

16 Comments

  1. Tomi Engdahl says:

    Tips & Tricks: Avoid Harmonic-Balance and SPICE software flaws for time-domain simulation
    http://www.eetimes.com/design/communications-design/4392093/Tips—Tricks–Avoid-harmonic-balance-and-SPICE-software-flaws-for-time-domain-simulation?Ecosystem=communications-design

    There are severe flaws within the Harmonic-Balance and SPICE programs now widely used. Mentioned as far back as within an abstract of Session WSO at the 2008 IEEE International Microwave Symposium

    Engineers using SPICE time-domain software to find the steady-state periodic response, also have difficulties: they run the program for 100 to 1000 periods, depending on the Q (Quality Factor) of the resonant circuit at the operating frequency. That takes a long computing time, and it suffers from increasing errors, the longer the running time

    In the case of switching-mode power amplifiers, now a very popular application, those problems can be avoided completely, by using, instead of Harmonic Balance or SPICE, a mathematical algorithm for a direct computation of an exact solution for the steady-state periodic response [2] and subsequently improved [3]. This “Liou-Sokal” algorithm works for two reasons: (a) the algorithm is an exact analysis, and (b) the computations are executed in double precision, which gives correct results, even in nearly unstable cases.

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  3. Tomi Engdahl says:

    This seems to be a lot like Y148 module packed to metal box:

    YDA148 Aluminum Alloy Digital Amplifier – Silver (DC 9~16V / 3A)
    http://www.dealextreme.com/p/yda148-aluminum-alloy-digital-amplifier-silver-dc-9-16v-3a-153343

    Reply
  4. Tomi Engdahl says:

    Digital-Input Class D amplifiers expand the benefits of traditional Class D and simplify system design
    http://www.edn.com/design/analog/4400490/Digital-Input-Class-D-amplifiers-expand-the-benefits-of-traditional-Class-D-and-simplify-system-design-?cid=EDNToday

    A new generation of digital-input Class D audio amplifiers achieves high PSRR performance that is comparable to traditional analog Class D amplifiers. More importantly, digital-input Class D amplifiers provide additional benefits of reduced power, complexity, noise, and system cost.

    Electronics vendors commonly use high-efficiency, filterless, analog-input Class D amplifiers to manage the power requirements of portable audio speakers found in cell phones, tablet computers, and personal navigation devices. These Class D amplifiers allow direct connection to a battery which minimizes losses and reduces component count. The amplifiers also achieve >70dB PSRR performance which is important to avoid audible buzzing with 217Hz demodulated GSM signals.

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  6. Tomi Engdahl says:

    New approaches to switched-mode audio power amplifiers (Part 1)
    http://www.edn.com/design/consumer/4408451/New-approaches-to-switched-mode-audio-power-amplifiers–Part-1-

    The Class D amplifier has been the usual switched-mode answer to linear amplifiers for three decades. Class D amps have a clear efficiency advantage over their linear alternatives, which, over the years, has led to much effort being invested in improving the linearity of switched-mode designs.

    Even so, the harmonic distortion of switched-mode amps remains inferior to that of linear power amplifiers.

    The time ratio of the ON time of the two switches determines the average output voltage.

    There are three main sources of distortion in Class D Amps.

    First, the best FET switches, driven optimally, still spend a significant time in the linear region when switching. That means dead time must be inserted to avoid upper and lower FETs conducting at the same time (shoot-through). Dead time causes non-linearity in the output which is difficult to correct for. The brief period of linear switch operation is a source of asymmetry

    Second, Class D amplifiers do not provide power supply rejection. Audio frequency noise on the supplies will appear only slightly attenuated at the output.

    Third, the switched inductor in a Class D amp interacts with the speaker’s inductance (or capacitance) in non-linear fashion. Care must be taken to avoid resonances and beat frequencies that can be heard in the audio range.

    One answer is to use Sigma-Delta modulation instead of Pulse Width Modulation. With Sigma-Delta modulation, periods of ON or OFF are integer multiples of the clock period. Linearity can then be improved, but at the expense of a higher clock rate. Sigma-Delta systems generally need to run at least 64 times the maximum signal frequency. There are significant efficiency penalties associated with clocking that much faster.

    Even after taking careful measures to minimize the error sources cited above, negative feedback is required to obtain good performance. Negative feedback is problematic in a Class D amplifier because the output filter necessarily inserts a delay, which tends to destabilize any feedback loop.

    The Class D amp is essentially a buck converter. Buck converters need compensation to remain stable for the same reasons. In order for feedback from the output to be applied without inducing a tendency toward oscillation, the feedback must be processed.

    The Bridge Tied Load Variation on Class D amplifiers

    The BTL form is more flexible, and it exhibits less bus pumping.

    Filterless Variations
    In some cases, the reactance of the speaker itself is relied on to filter out the chopping frequency. That can be satisfactory, but it requires the speaker to dissipate extra power due to the high-frequency ripple current. Not all dynamic speakers are sufficiently inductive at those frequencies to be efficient as a filter. Equivalent Series Resistance, ESR, causes analogous losses in piezo speakers.

    Care must be taken to avoid damage to dynamic speakers in the filterless case. Above the audible range, voice coil movement is proportional to 1 / frequency2. If the modulation frequency is high enough, the movement is small, so the voice coil won’t hit the limit of travel and cause damage. As long as the speaker is sized to dissipate the extra energy, the filterless variation can be satisfactory.

    The awkward overall conclusion is that the efficiency advantages of Class D amplifiers stem from the digital nature of their output, while the performance disadvantages of Class D amplifiers stem from that same digital nature. What is really called for is a digital amplifier with an analog output. There is now an option in that category, enabled by improved FET switches and a control strategy called Predictive Energy Balancing (PEB).

    Reply
  7. Tomi Engdahl says:

    New approaches to switched-mode audio power amplifiers (Part 2)
    http://www.edn.com/design/consumer/4410605/New-approaches-to-switched-mode-audio-power-amplifiers–Part-2-

    Because PEB allows an output to be regulated precisely on a cycle-by-cycle basis, instead of on the average, PEB enables an entirely new form of switched-mode amplifier.

    The PEB calculations are done in real time by the circuitry

    These calculations can be done in analog or digital fashion.

    In operation, the amplifier uses flyback energy transfers to push the output away from zero. It uses reverse flyback transfers to draw energy back from the speaker to pull the output towards zero. The inductor in a PEB amplifier is active at the switching frequency, so a much smaller inductor filters the digital power switching to produce an analog output, when compared to the inductors needed to filter Class D amplifiers. PEB amplifiers can be fully bipolar, or can be offset to drive the output above and below the power supply voltage

    The speaker could just as well be connected between the output and ground with a coupling capacitor to remove the DC bias voltage.

    Because of their differences from Class D amplifiers, there are a few cautions to observe when applying PEB amplifiers.

    Because the energy balance scaling is in proportion to the ratio of the switched inductance and the load capacitance, the PEB gain needs to be matched to the capacitance of the load.

    In the case of a dynamic speaker, which is inductive, not capacitive, a capacitor is added at the amplifier output to set the PEB scaling and the output ripple. Then, the resistance or inductance of the speaker has little effect on the dynamic performance. For piezo speakers, the PEB scaling must be set to match the chosen speaker’s capacitance.

    The theoretic maximum efficiency of Class D and PEB amplifiers is 100%. As a practical matter, both types of amplifiers can be expected to run in the region of 90% efficiency.

    Conclusions
    By rearranging the same circuit elements now employed in BTL Class D amplifiers and by adding predictive controls, real improvements in fidelity and efficiency of power amplifiers have been achieved. Since changing the control intelligence does not add to the cost of an integrated circuit, these benefits can be realized without an increase in cost. In fact, system costs can be reduced by eliminating filter inductors and by generating less waste heat. PEB amplifiers are particularly well suited for driving piezo loads.

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  11. Tomi Engdahl says:

    [Afrotech]‘s Guide To Class D Amplifiers
    http://hackaday.com/2014/06/09/afrotechs-guide-to-class-d-amplifiers/

    Hang around in any of the many guitar or audiophile forums or discussion boards for long enough, and eventually you’ll come across the arguments over amplifier topologies. One of the more interesting and useful of these classes of amplifier is class d – they’re extremely efficient and when well designed can sound pretty good. [Afrotech] is here to show you how they work, and how to build a 15 Watt amp using a $3 class d amplifier chip.

    To demo this, [Afrotech] used TI’s TPA3122 class d amplifier chip. It’s a pretty cheap chip for being a 15 Watt stereo amplifie

    Reply
  12. Tomi Engdahl says:

    I have taken this amplifier in use in Hifi system at seems to do it’s job well.

    Reply
  13. Tomi Engdahl says:

    Class D Amp with an H-Bridge
    http://hackaday.com/2014/12/28/class-d-amp-with-an-h-bridge/

    Class D amps are simple – just take an input, and use that to modulate a square wave with PWM. Send this PWM signal to a MOSFET or something, and you have the simplest class D amp in existence. They’re so simple, you can buy a class D amp chip for $3, but [George] thought that would be too easy. Instead, he built his own with an ATTiny and an H-bridge motor driver. No surprise, it works, but what’s interesting is what effect the code on the ATtiny can have on the quality of the audio coming out of the speaker.

    The microcontroller chosen for this project was the ATtiny 461
    The heavy lifting part of this build is an L298 chip found on eBay for a few dollars.

    Class D AVR
    http://www.georgegardner.info/electronics/class-d-avr.html

    Reply
  14. Tomi Engdahl says:

    Class D audio power amplifiers: Adding punch to your sound design
    http://www.edn.com/design/power-management/4443229/Class-D-audio-power-amplifiers–Adding-punch-to-your-sound-design?_mc=NL_EDN_EDT_EDN_analog_20170105&cid=NL_EDN_EDT_EDN_analog_20170105&elqTrackId=8a47a762bede4362a401be03200dacbd&elq=5f9b5fd786164463a212d94d88f5e3f2&elqaid=35405&elqat=1&elqCampaignId=30952

    Class D amplifiers were first conceived in 1958 and so much has been written about the different architectures that were cited just in 2016 which improve different aspects of their performance.

    I will share these recent techniques with you in this article.

    Such recent design improvement techniques are Power Supply Rejection Ratio (PSRR) improvement, lower distortion, Electro-magnetic Interference (EMI) reduction, Intermodulation (IM) distortion improvement, quiescent current reduction, Total Harmonic Distortion (THD) reduction, and driving capacitive transducers in electrostatic loudspeakers.

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