e031034.pdf

PC &AUDIO

Active Loudspeaker

System (1)

for multimedia applications

Design by T. Giesberts

The 2-way active loudspeakers described here are primarily intended for use

with a PC, but could in principle be used in any other ‘medium-ﬁ’ application.

Since the universal crossover ﬁlter can be adjusted to your own liking, you’re

not tied to using the speakers and enclosure suggested here. Other

woofer/tweeter combinations may also be used. An accompanying active

subwoofer is currently under design and will be published in the near future.

Elektor Electronics

1/2003

PC &AUDIO

+8V

* see text

zie tekst

* siehe Text

R14

2k00

R20

* voir texte

+16V

IC1.B

3n9

R15

470

IC1.D

R21

7k68

9k53

8k25

C13

+8V

63V

R13

R19

C15

SC13 8 Ω

LS1

8n2

3n3

10k

VCC

1N4148

3n3

120n

C14

IN1

OUT1

IC1.A

470

IC2

470n

IN1

OUT1

IC1 = TS924IN

C17

IN2

OUT2

TDA7374

LS2

470n

IN2

OUT2

JP1

R17

2k00

10k

1N4148

R11

STBY

SVR

CLIPDET

PWGND

SC10N 8 Ω

C16

R18

470

SGND

+16V

+8V

IC1.C

C10

C11

C12

IC3

+16V

7808

* see text

R23

zie tekst

4n7

* voir texte

R10

R12

R16

* siehe Text

C18

C19

C20

C21

R24

C24

C22

C23

63V

25V

100n

10k

IC1

3n3

4700

25V

100n

63V

R22

POWER

020054 - 11

Figure 1. The circuit consists of an input buffer, crossover ﬁlter and an integrated dual power ampliﬁer.

The starting point for this design

was the irritation with the average

to mediocre quality of the majority of

PC loudspeakers. We thought that

was something that could be

improved upon without expending

too much effort or money. Of course

you don’t need the ultimate HiFi

quality for the PC, but it would be

nice to have a set of loudspeakers

that gave a decent reproduction. In

that way you could start to enjoy the

music from a CD or DVD.

To begin with, we looked for a

compact woofer/tweeter combina-

tion that gave a good performance at

a reasonable cost. Another require-

ment was that they should be mag-

netically shielded, because the loud-

speakers are likely to be placed close

to a monitor or TV, which would be

affected by stray magnetic ﬁelds. For

our prototype we finally decided to

use a 25 mm dome tweeter and a 13

cm bass/midrange unit made by

Visaton (the SC10N and SC13). Other

manufacturers of magnetically

shielded speakers worth looking at

are Vifa and Monacor (Monarch). For

this reason we would like to make it

clear that other speaker combina-

tions with associated enclosures

(recommended by the manufacturer)

can also be used in this design.

The electronics required to make

the loudspeaker active have been

kept as simple and yet as adaptable

as possible. A two-way crossover ﬁl-

ter was designed round a quad

opamp, with a choice of slopes, char-

acteristics and crossover frequen-

cies. For use with the speakers rec-

ommended by us, the filter was set

up as a 3 rd order Butterworth type

with a crossover frequency of 4 kHz.

Fo r the power ampliﬁers we used

an integrated dual bridge ampliﬁer,

which requires very few external

components. At a supply voltage of

16 V it delivers 2

have a poor response at the lower end of the

frequency range. For this reason we have

include a corrective filter that can be acti-

vated with a jumper and which boosts the

frequencies between 1000 Hz and 100 Hz up

to a maximum of 6 dB at 100 Hz. Our proto-

type seemed to beneﬁt from this addition and

this correction will probably also have a pos-

itive effect with other speaker combinations

having similar bass/midrange units.

The electronics design

Now that the introduction is out of the way

we can start to look at the circuit diagram.

This is shown in Figure 1 and consists of

three distinct sections:

2 × 12 W into 8 Ω . Compared to

usual HiFi standards this may seem a

bit skimpy, but in combination with

speakers with an average efficiency a

sound pressure level of about 100

dB can be attained — and that really

is very loud!

What else can be added to this

summary? One of the advantages is

using an ‘active’ design is that it

allows us to overcome a disadvan-

tage found with many small loud-

speaker enclosures. Most enclosures

with a volume of a few litres tend to

19 W into 4

Input buffer and supply

When we look at the circuit diagram in the

usual way, from left to right, we ﬁrst see a ter-

minating resistor, isolating capacitor and trim

pot P1. This feeds the signal into an input

buffer built around IC1a, which forms part of

quad rail-to-rail opamp TS924IN. This type is

distinguished by its relatively large output

current of up to 80 mA.

R2 and C2 form a low-pass ﬁlter that sup-

presses any high frequency interference. The

network C3/R3/R4 provides the previously

mentioned correction at low frequencies;

closing JP1 enables this ﬁlter.

1/2003

Elektor Electronics

PC &AUDIO

In order to avoid the need for a symmetrical

power supply, but still obtain the optimum

signal processing by the buffer and ﬁlters, we

have used IC1b to create a stable virtual

ground. C13 decouples the output of poten-

tial divider R20/R21, providing a virtual

ground at exactly half the supply voltage. The

large output current capability of the TS924IN

is obviously a great advantage in the circuit

round IC1b.

To avoid supply-borne interference from

affecting the input buffer and filters, opamp

IC1 has been equipped with its own voltage

regulator (IC3). For proper operation of this 8

V regulator, the supply voltage to the active

system should be at least 11 V. Resistor R22

has been added to separate the signal ground

and supply ground for cases where a single

power supply is used for two or more chan-

nels. When each channel has its own power

supply then R22 can be replaced with a wire

link.

LS1

LS2

C24

C20

IC2

R19

C21

C16

C15

C14

C17

C11

R11

R22

C10

IC1

C22

JP1

R20

020054-1

Filter

Next comes the crossover ﬁlter.

As can be seen in the circuit diagram, the

output of the input buffer is fed to two filter

sections. These are built round the two

remaining opamps of IC1. The low-pass ﬁlter

is built round IC1d and the high-pass ﬁlter is

found round IC1c. For the design of the ﬁlter

we started out with a 4 th order conﬁguration,

so that the same PCB could be used with sim-

pler filters just by leaving out some compo-

nents.

We have already calculated the values for

several variants. Table 1 shows the compo-

nent values for a 3 rd order Butterworth ﬁlter

and a 4 th order Linkwitz-Riley filter with

crossover frequencies at 1, 2.5 and 4 kHz.

With the Visaton loudspeakers used in our

prototype we found that a 3 rd order Butter-

worth filter with a crossover frequency of 4

kHz gave the best result. The component val-

ues given in the circuit diagram are therefore

for this type of filter. When we used a 4 th

order Linkwitz-Riley ﬁlter at 4 kHz the mea-

surements showed that there were problems

with the radiation pattern due to the large

phase shifts this ﬁlter produces. The 3 rd order

Butterworth variant noticeably suffered less

from this. The Linkwitz-Riley values are there-

fore mainly included for experimentation pur-

poses.

Take care with the connection of the

speakers when using the Linkwitz-Riley fil-

ter, because the connections to the tweeter

are then reversed. On the PCB the polarity

indicated is for use with 3 rd order Butter-

worth filters. (In this case the tweeter is

therefore ‘out of phase’ compared to the

woofer!) And as a final point of interest: the

Figure 2. It is amazing that this compact PCB contains a crossover ﬁlter as well as a

2x20 W power ampliﬁer!

crossover point of Butterworth ﬁlters

is at -3 dB, for Linkwitz-Riley ﬁlters

it is at -6 dB.

cations such as these. This IC

requires surprisingly few external

components (no Boucherot network,

nor output capacitors) and also con-

tains adequate internal protection

circuits against overheating and

short circuits. Due to all these fea-

tures our dual power amplifier is a

shining example of compactness.

This is clearly seen in the circuit dia-

gram.

We have already mentioned the

output power. With a load of 8

Power ampliﬁer

The output signals of the filters are

fed to the power amplifiers via pre-

sets P2 and P3. The potentiometers

compensate for the different effi-

ciencies of the woofer and tweeter.

Many dome tweeters are about 3 dB

louder than small bass/midrange

speakers at the same input level.

However, the loudspeakers chosen

for this design do have similar effi-

ciencies, so that in practice both P2

and P3 can be turned to their maxi-

mum level.

For the power amplifier we’ve

chosen a TDA7374B double inte-

grated ampliﬁer. These are primarily

intended for automotive use, but are

obviously also very suitable in appli-

Ω

loudspeakers IC2 can be cooled

using a small heatsink with a ther-

mal resistance of 3 K/W. R19 and

C19 ensure that virtually no plops

are heard when the amplifier is

switched on (there is always a small

offset voltage at the output of the

ampliﬁer). RC networks R15/C15 and

R18/C18 restrict the bandwidth of

the power ampliﬁers in order to min-

Elektor Electronics

1/2003

PC &AUDIO

imise the effect of any HF interference. In

principle it would have been better to place

these networks after the potentiometers, but

then the bandwidth will vary noticeably. C20

decouples the internal potential divider,

which supplies several stages with half the

supply voltage and which is also responsible

for supply ripple suppression, which is about

50 dB at 100 Hz.

COMPONENTS LIST

C10,C11,C12 = 4nF7, lead pitch 5

C13,C19 = 10 µ F 63V radial

C14,C17 = 470nF

C20 = 47 µ F 25V radial

C21,C23 = 100nF, lead pitch 5mm

C22 = 4 µ F7 63V radial

C24 = 4700 µ F 25V radial, lead pitch

7.5mm, diameter 17mm max.

Resistors:

R1 = 1M

Ω

R2,R15,R18 = 470 Ω

R3 = 6k Ω

R4 = 4k

R5 = 0 Ω

R6 = 7k

R7 = 9k Ω 53

R8 = 8k Ω 25

R9 = open *

R10 = 4k Ω 99

R11 = 9k

Semiconductors:

D1,D2 = 1N4148

D3 = LED, green, high-efficiency

IC1 = TS924IN (ST, from Farnell)

IC2 = TDA7374B (ST, from C-I

Electronics, www.dil.nl

IC3 = 7808

The PCB

Figure 2 shows the PCB that was designed

to hold the electronics for the active 2-way

loudspeaker.

There is not much to be said about the

PCB. The layout is fairly well organised and

the various connections are clearly marked.

At the bottom-left are the input pins, diago-

nally opposite (top-right) are the connections

for the supply voltage and a bit below that

we find power LED D3. The connectors for

the woofer and tweeter (LS1 en LS2) are on

either side of IC2.

The wire links on the board deserve a

mention. There are two: one just next to R20

and another underneath (!) the pins of IC2.

The last could also be soldered to the under-

side of the PCB, but in either case an isolated

wire should be used for this link. IC2 has

been placed near the edge of the PCB, mak-

R12 = 12k

R13,R14,R16,R17 = 2k Ω

R19 = 10k

Ω

R20,R21 = 15k Ω

R22,R23 = 0 Ω

1 5W

Miscellaneous:

JP1 = 2-way pinheader w. jumper

LS1 = SC13 8

R24 = 2k

P1,P2,P3 = 10k Ω

preset

Visaton (Conrad

Electronics)

LS2 = SC10N 8 Ω

Capacitors:

C1,C4,C16 = 2

Visaton (Conrad

F2 MKT, lead pitch 5

Electronics)

Heatsink for IC2: 3 K/W

Wood: 12 mm MDF — see Figure 4

Wadding material (BAF)

PCB, order code 020054-1 (see

Readers Services pages)

or 7.5mm

C2 = 1nF, lead pitch 5mm

C3 = 120nF, lead pitch 5mm

C5 = open *

C6 = 8nF2, lead pitch 5mm

C7 = 3nF9, lead pitch 5mm

C8,C15,C18 = 3nF3, lead pitch 5mm

C9 = wire link *

* see text and Table 1

Figure 3. A few prototypes usually precede the ﬁnal PCB.

1/2003

Elektor Electronics

PC &AUDIO

ing it easy to mount a small heatsink (3 K/W)

to it; remember to use an isolating washer

between the IC and heatsink!

Once the PCB has been populated and

tested, there are several possibilities for com-

pleting the construction. It can be mounted

inside the loudspeaker enclosure, it can be

mounted in a separate box (probably as a

two-channel version) or it can be combined

with a subwoofer that will be described in a

forthcoming article. A separate box isn’t a

bad idea since we are thinking of adding a

tone control to this system. But at the end of

the day the choice is yours.

The supply can be provided by the usual

combination of a transformer, bridge rectifier

and smoothing capacitor. Each channel

requires a transformer of 12 V/15 VA and a

smoothing capacitor of 4700 µF/25 V. For a

stereo version these values should be dou-

bled. When a stabilised supply is used, the

voltage may be increased from 16 V to a

maximum of 18 V, which increases the out-

put power somewhat. At the moment we’re

designing a dedicated power supply for this

active loudspeaker system, so it may be

wise to wait a little before building the

power supply.

Table 1

3 rd order Butterworth

4 th order Linkwitz-Riley

1 kHz

2.5 kHz

4 kHz

1 kHz

2.5 kHz

4 kHz

link

6k34

6k98

8k06

6k65

7k68

13k3

9k31

10k7

8k25

8k45

9k53

6k49

7k68

8k25

6k81

8k06

8k25

9k31

7k50

8k45

open

22 n

10 n

5n6

33 n

15 n

8n2

39 n

18 n

10 n

18 n

6n8

3n9

18 n

6n8

3n9

15 n

5n6

3n3

8n2

3n9

2n2

open

7k50

6k49

7k15

R10

5k23

4k53

4k99

3k83

3k32

3k65

R11

10k2

8k87

9k76

11k0

9k53

10k5

R12

13k0

11k5

12k7

19k6

16k9

18k7

link

18 n

8n2

4n7

C10

18 n

8n2

4n7

18 n

8n2

4n7

C11

18 n

8n2

4n7

18 n

8n2

4n7

C12

18 n

8n2

4n7

18 n

8n2

4n7

Table 1. Component values for the ﬁlters at different frequencies. For the 3 rd

order Butterworth ﬁlter C5 and R9 aren’t mounted and R5 and C9 are replaced

by wire links.

Woodwork

The size and construction of the loudspeaker

enclosure depends primarily on the size of the

woofer that is used. The Visaton SC13 used

here is mounted in a closed box with a volume

of about 4 litres. This makes the construction

a fairly straightforward job, since a

closed box is little more than six pan-

els that are glued together. This may

seem a bit difficult to the inexperi-

enced carpenter, but once the wood

has been cut to size then the rest of

the job should be simple if you use

some clamps. In any case, it is an

advantage that no special baffles or

ports are required. The only part that

Dimensions in mm

Material: MDF 12 mm

Section A - A

Ø 85

Section B - B

126

Ø 113

151

175

150

020054 - 12

Figure 4. Design drawings for the enclosure, complete with all measurements. We’ve assumed that 12 mm thick MDF board is used.

Elektor Electronics

1/2003

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