Rose

The Feasibility of Class D Amplifiers for Active Loudspeaker Applications

THE FEASIBILITY OF CLASS D AMPLIFIERS FOR ACTIVE LOUDSPEAKER APPLICATIONSDARREN ROSEGenelec Oy, Iisalmi, Finland
Darren.rose@genelec.com

In 2002 the author made a comparison of the audio quality, audio quantity and cost of some commonly available power
amplifier modules. This paper will investigate the hypothesis that today the best value can obtained using Class D amplifiers. This will be studied by comparing four topologies. The amplifiers have been measured in the same controlled
conditions representing an application in an active loudspeaker. To make a fair cost comparison the amplifiers havebeen assembled on PCBs from components in-house rather than using complete ready power amplifier modules. In addition to the audio and cost criteria, energy efficiency will be considered.
INTRODUCTION
This study compares amplifiers in a controlled, objective manner simulating normal use with music material.
The measurements are concerned with the frequencies
below 3 kHz which typically limit the headroom of aloudspeaker and are reproduced by a woofer/midrange
driver representing a more demanding load on the amplifier than a tweeter.
Since the author’s previous work [1], there has beenmuch research work to improve Class D performancee.g. [2] and other comparisons of Class AB and Class D
amplifiers [3].This study can be seen as a continuationof the author’s previous work but with a slightly shiftedfocus and simpler methodology.

1.2 Thick Film Hybrid (TFH) Class AB design
A non-inverting amplifier with an almost datasheet configuration. It is very simple to use if kept within voltage
and load impedance specifications, but the minimum
gain is too high in many applications. The hybrid is astereo device, so only half its cost is included in the costcalculations. It is assembled on a two layer PCB with 23
components, of which 3 are through hole mounted. The
availability and longevity of TFH amplifiers is not as
good as that of many transistors and ICs that are used inaudio amplifiers. In the author’s previous study [1] the
TFH offered the best value of the compared amplifier
types.

1 AMPLIFIER DESCRIPTIONS
The four amplifiers in the study have configurations andperformance typical of commercial designs. For a simple comparison, they have been assembled with thesame feedback/input resistors, setting the gain to 29.2 ±0.2dB, the minimum gain recommended for the thickfilm hybrid. This is not an ideal situation for the otheramplifiers –the integrated circuit amplifier can operate
at 26 dB gain, while the discrete Class AB and Class D
amplifiers could operate at much lower gains.

1.3 Integrated Circuit (IC) class AB design
A non-inverting amplifier with an almost datasheet configuration. In this study two ICs are paralleled, runningat the limit of their voltage handling. The circuit andlayout are extremely easy to implement, the ICs protect
themselves for high reliability, but thermal design is achallenge because of the ICs’ small surface area. It is
tested here on a four layer PCB, using just 16 components, of which the two ICs are the only through holemounted parts. The most common Class AB audio power amplifier ICs have been in production for many
years, suggesting that they are fairly future proof.

1.1 Discrete Class AB design
An inverting design that can easily be tailored for a certain gain, distortion, noise and voltage and current handling. It can be configured with a bandwidth of over 200
kHz and operated with gain down to unity which can
result in very low noise. It is sensitive to the layout of
the input traces and it needs its DC offset and bias current adjusted. Heat sinking is simple to arrange, though
the bias tracking is not very fast. It is a robust design on
a two layer circuit board. It has 86 components, ofwhich 7 are through hole mounted. The design is quite
future proof because obsolete transistors can be replacedwith pin compatible parts.

1.4 Class D design
This contains ICs and a discrete output stage for a totalcomponent count of 69, of which four parts are throughhole mounted – the output coil, output capacitor and two
bypass capacitors. It is assembled on a four layer PCB.
It needs no heatsink. It is an inverting, analogue, self
oscillating half-bridge design with feedback taken before the output LC filter. It does not represent the intellectual property minefield of the best performing Class
D amplifiers. It has a strong similarity to datasheet designs, but is not trivial to implement because of potential EMC issues. A common mode choke is necessary

AES 51st International Conference, Helsinki, Finland, 2013 August 22–24

1

Rose

The Feasibility of Class D Amplifiers for Active Loudspeaker Applications

on the mains connection with this amplifier. This cost is
included in the calculations.
Unlike the class AB amplifiers, the frequency response
peaks so that at 20 kHz it has risen about 1 dB.
FETs, coils and other parts can be replaced if they are
discontinued. FET replacement appears likely given therate of FET development. An applicable integrated circuit for a half bridge FET driver will probably always
be available in some package.
2 AMPLIFIER COSTS
Costs (Table 1) have been normalized to the cost of theClass D amplifier. The first row concerns only the partsand circuit board. The second row includes the cost of aflat sheet metal heatsink and mounting hardware. The
same heatsink and hardware has been specified for all
Class AB amplifiers. The class D amplifier needs noheatsink. The heatsink is large enough for normal music
listening with and active loudspeaker, but not for sine
waves at continuous full output. Both lines are included
since there are times when it is fair to ignore theheatsink cost if it is anyway no more than just the necessary amplifier enclosure.

PARTIAL: Parts,
PCB
FULL: Parts, PCB,
mounting, heatsink

Disc.
AB
1.23

TFH
AB
0.73

IC AB
0.86

Class
D
1.00

2.25

1.75

1.88

1.00

Table 1: Normalized amplifier costs.
3 GENERAL TEST CONDITIONS
The amplifiers are powered by a 130VA toroidal transformer, a 6 A bridge rectifier and 10,000 µF capacitance
per rail, at an idle voltage of ± 52 V.
Signals used are sinusoidal or multitone. The multitone
signal, used to represent wideband spectrum music, contains 31 tones at equal amplitudes from 20 Hz to 3 kHz.
The frequency range is sufficient for the woofer rangeof typical two way loudspeaker.
The loads used were a 7.7 Ω resistive load or an 8 inch
woofer loudspeaker in a bass reflex enclosure with anaverage impedance of 7.7 Ω from 20 Hz to 3 kHz. With
this power supply and loads, the amplifiers can be considered nominally 100 – 120 W amplifiers. A common
approximation of the normal use power level for audio
amplifiers is 1/8 of continuous output power [4, 5],
equating to 12.5 W in this case.
All measurements were made using Audio Precision
SYS-2722 with the measurement bandwidth set at < 10
Hz – 20 kHz using the AES17 brick wall filter. A 20 Ω,
unbalanced, floating input signal connection was used.

4

TEST MEHODS AND RESULTS

4.1 Noise
The idle noise of an audio system is subjectively very
important, more so than even the signal to noise ratio,
since potential buyers will often place their ear rightnext to a loudspeaker to listen for noise. The amplifiers
were measured with no load because the extra load cabling can pick up low frequency noise in a laboratory
situation. The input cable was connected but the signal
was turned off. Amplifier output voltage was measured.
Swept bandpass filter measurement curves for the TFH
and IC amplifiers were very similar with peaks at 50
Hz, then almost straight increase from 100 Hz to 20
kHz. The discrete Class AB amplifier had extra peaks at
100 Hz and 200 Hz, but was otherwise the same level.
The Class D amplifier was similar to the TFH and IC
amplifiers below 100 Hz but its increasing noise levelwas 2 – 3 times higher.
The total unweighted noise in the audio bandwidth (Table 2) is highest for the Class D amplifier, even whencompensated by the full cost of the amplifiers. This is aserious concern, but the amplifier is responsible for only
part of the total noise in any system. The input/feedback
resistor values are best suited to the TFH amplifier andleast well suited to the Class D amplifier. In practice theClass D amplifier can still be better optimised for noise
performance whilst the TFH amplifier cannot.

Noise (µV)
Noise × normalized
full cost

Disc.
AB
55
124

TFH
AB
44
77

IC AB
58
109

Class
D
127
127

Table 2: Total unweighted audio band noise.
4.2 Distortion
There is no widely accepted single figure for measuring
distortion to predict subjective quality [6, 7], but
THD+N comes closest [7]. For the purpose of simple
comparison, only a few frequency points at normal listening level, 12.5 W, are presented. These are 20 Hz and
3 kHz, the highest and lowest frequencies in this study,and 1 kHz, the most common single frequency used forshowing audio amplifier performance in datasheets.

20 Hz
1 kHz
3 kHz
20 Hz × normalized
full cost
1 kHz × normalized
full cost
3 kHz × normalized
full cost

Disc.
AB
0.030
0.010
0.011
0.068

TFH
AB
0.00170.00110.00180.0030IC AB
0.00440.00200.00280.0083Class
D
0.00180.00320.012
0.00180.023

0.00190.00380.00320.025

0.00320.0053

0.012

Table 3: THD+N percentage at normal listening level.

AES 51st International Conference, Helsinki, Finland, 2013 August 22–24

2

Rose

The Feasibility of Class D Amplifiers for Active Loudspeaker Applications

At 20 Hz the THD+N of the Class D amplifier is as
good as the best of the Class AB amplifiers in the test.
By 3 kHz it is at the level of the worst of the Class AB
amplifiers. When cost-compensated, the Class D amplifier is only advantageous up to about 400 Hz. The best
absolute THD+N performance of these amplifiers isgiven by the TFH amplifier.
If the harmonic distortion of a good active loudspeaker[8] is less than 0.5 %, an amplifier can be consideredgood enough if its distortion is insignificant in comparison to that, say 0.05 %. All of the amplifiers in thisstudy are more than good enough even up to 7 kHz,beyond which third harmonic distortion is inaudible.
4.3 Output power
Maximum continuous sinewave output power wasmeasured during the THD+N vs output power tests. The
results at 1 % THD+N are presented (Table 4) for 100
Hz sinewaves. The Class AB amplifiers gave the same
output at 1kHz and 6.67 kHz. The Class D amplifier
showed less output power at higher frequencies, but thisis because the clipping point is less well defined with amore rounded knee. For music material in an active
loudspeaker, the 100 Hz value is more significant.

Power to 7.7 Ω (W)Power to 7.7 Ω / normalized full cost

Disc.
AB
83
37

TFH
AB
92
52

IC AB
101
54

Class
D
113
113

Table 4: Sine wave output power comparison.
The Class D amplifier gives more output power than theClass AB amplifiers. Per unit cost the Class D amplifier
is around three times better value for output power thanthe discrete Class AB amplifier and about twice the value of the TFH and IC amplifiers.
Since sine waves and THD+N measurements are not arealistic representation of music signal quality, a different approach was required. The multitone signal was fed
to the amplifier and the multitone distortion was measured. This is drawn as a function of frequency, not asingle number, so for comparison the signal level wasincreased until the amplifier output compressed by 0.5
dB. At this level the higher frequency part of the plot
was at about -45 dB relative to the full output signal.

Power to 7.7 Ω (W)Power to 7.7 Ω / normalized full cost
Power to loudspeaker(W)
Power to loudspeaker/ normalized full cost

Disc.
AB
53
24

TFH
AB
57
33

IC AB
62
33

Class
D
73
73

53

57

62

73

24

33

33

73

The first thing to notice is that all the amplifiers give thesame output to the loudspeaker load as to the resistive
load. This information makes future power testing withmultitone or other music-like signals much simpler because a resistive load can be used. As expected, theClass D amplifier delivers considerably more power tothe load than the Class AB amplifiers in both absoluteterms and per unit cost.
4.4 Efficiency
The mains power of the complete power supply andamplifier system was measured (Table 6) at different
audio power output levels from 0 W to 100 W into bothresistive and loudspeaker loads. The power supply efficiency is included in the measurements, but it is thesame power supply, so the measurements can be fairly
compared with each other. The TFH amplifier has twoidle channels included in the measurement.

Idle consumption (W)Idle consumption ×
normalized full cost

Disc.
AB
8

TFH
AB
7

IC AB
7

Class
D
6

18

12

13

6

Table 6: Idle power consumption.
The Class D amplifier has the lowest idle consumption
both in absolute terms and per unit cost.
The multitone signal was applied to the amplifier. Below 40 W output power, the power consumption was thesame for both the resistive and loudspeaker loads. It was
not possible to measure fast enough to avoid heating theloudspeaker voice coil, so that above 40 W, the loud theamplifier seemed more efficient in to the loudspeakerload than it actually is. For this reason the efficiency
plot below was measured with the resistive load. Efficiency is given as the output power divided by the power consumption and is expressed as a percentage.

Figure 1: Efficiency at different power levels.

Table 5: Multitone output power comparison.

AES 51st International Conference, Helsinki, Finland, 2013 August 22–24

3

Rose

Normal level (%)
0.5dB compression
level (%)
Nominal maximum
level (%)
Eff. normal / normalized full cost
Eff. 0.5 / normalized
full cost
Eff. max / normalized
full cost

The Feasibility of Class D Amplifiers for Active Loudspeaker Applications

Disc.
AB
20
48

TFH
AB
20
48

IC AB
20
48

Class
D
60
78

60

60

60

78

9

11

11

78

21

27

26

78

27

34

32

78

Table 7: Efficiency percentage at different levels.
The Class AB amplifiers (Figure 1, Table 7) are almost
indistinguishable in their efficiencies, being only 20 %efficient at the normal 12.5 W output, and increasing toaround 48 % at the 60 W level at which their output
compresses 0.5 dB. The Class D amplifier has efficiencies of 60 % and 78 % at the same output levels. Therefore in normal music listening conditions the Class D
amplifier would be three times more efficient than theClass AB amplifiers.
Table 7 describes the data in Figure 1 and also includes
cost compensated data. The Class D amplifier is verymuch more efficient at all output levels and when compared per unit cost the difference between the Class D
amplifier and a Class AB amplifier can be over eighttimes.
5 CONCLUSIONS
Four nominal 100W audio power amplifiers with basic
design were assembled and evaluated to test the hypothesis that the Class D amplifier offers the best value today.
The thick film hybrid and integrated circuit amplifiers
are the simplest to design if heatsinking can be arranged. The Class D amplifier is the most complex to
implement because it must not radiate or conduct radio
frequency energy above internationally accepted limits.
The cheapest to implement in a practical situation with
sufficient cooling is a Class D amplifier. The Class D
amplifier offers higher output power and efficiency withboth resistive and reactive loads. Its efficiency can bethree times greater than that of the Class AB amplifiers.
This has particular relevance in today’s world where
consumers make purchasing choices based also on environmental reasons.
Below 400 Hz the Class D amplifier offers THD+N
performance similar to the best Class AB amplifiers. Up
to 3 kHz Class D amplifier still fits in with the range of
THD+N performance offered by the Class AB amplifiers, being easily good enough for a high quality active
loudspeaker.

The noise performance of the Class D amplifier was theworst of the amplifiers on test, .even when compared to
the full cost of the amplifiers. However, the gain settingused in the tests was better suited to the Class AB amplifiers, particularly the thick film hybrid and integrated
circuit amplifiers. The noise of an audio system issomewhat easier to control than distortion, and can be
distributed throughout the whole system design by careful gain structure design, so this is not a bad result for
the Class D amplifier.
The output power test results suggest that it would havebeen fair to use a smaller, cheaper power supply transformer for the tests and include the cost of the power
supply in the comparison, which would have improved
the per unit cost for the noise and distortion performance.
An interesting result was that the amplifier efficiency
was the same using both resistive and reactive loads
when a wideband signal was used. The expectation wasthat the amplifiers would be less efficient when loaded
by a reactive driver load.
This was not an exhaustive test. The amplifiers used do
not represent the absolute best possible audio quality
available. No listening tests are presented. There is noattempt to measure momentary output power or intermodulation distortion.
Though simple, the study does indeed prove the hypothesis that Class D amplifiers can offer the best value today.
REFERENCES[1]
Darren Rose, “A Comparison if Modular Stateof-the-Art Switch Mode and Linear Power Amplifiers”, proc. AES 112th Convention, May 2002
[2]
Ploug et al., “Minimizing Crosstalk in Self Oscillating Switch Mode Audio Power Amplifiers,proc. AES 48th Intl. Conference, Sept. 2012.
[3]
Tounier, “Class AB Versus D for Multimedia
Applications in Portable Electronic Devices”,
proc. AES 29th Intl. Conference, Sept. 2006.
[4]
IEC, “International Standard IEC 60065 Seventh
edition”, Section 4.2.4 a), p 55.
[5]IEC, “CISPR 13 Edition 4.1”, Section 5.3.5, p 31
[6]
Eugene Czerwinski et al., “Multitone Testing if
Sound System Components ... Part 1”, JAES,
vol. 49, pp 1011-1048, Nov. 2001.
[7]
Bob Metzler, “Audio measurement handbook”,
Audio Precision, Beaverton, p 32 (1993)[8]
Genelec 8040B 8050B operating manual,
http://www.genelec.com/product-documentation/

AES 51st International Conference, Helsinki, Finland, 2013 August 22–24

4