altifalantes ativos

PC&AUDIO

34 Elektor Electronics 1/2003

Active LoudspeakerSystem (1)for multimedia applications

Design by T. Giesberts

The 2-way active loudspeakers described here are primarily intended for usewith 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’renot tied to using the speakers and enclosure suggested here. Otherwoofer/tweeter combinations may also be used. An accompanying activesubwoofer is currently under design and will be published in the near future.

PC&AUDIO

351/2003 Elektor Electronics

have a poor response at the lower end of thefrequency range. For this reason we haveinclude a corrective filter that can be acti-vated with a jumper and which boosts thefrequencies between 1000 Hz and 100 Hz upto a maximum of 6 dB at 100 Hz. Our proto-type seemed to benefit from this addition andthis correction will probably also have a pos-itive effect with other speaker combinationshaving similar bass/midrange units.

The electronics designNow that the introduction is out of the waywe can start to look at the circuit diagram.This is shown in Figure 1 and consists ofthree distinct sections:

Input buffer and supplyWhen we look at the circuit diagram in theusual way, from left to right, we first see a ter-minating resistor, isolating capacitor and trimpot P1. This feeds the signal into an inputbuffer built around IC1a, which forms part ofquad rail-to-rail opamp TS924IN. This type isdistinguished by its relatively large outputcurrent of up to 80 mA.

R2 and C2 form a low-pass filter that sup-presses any high frequency interference. Thenetwork C3/R3/R4 provides the previouslymentioned correction at low frequencies;closing JP1 enables this filter.

The starting point for this designwas the irritation with the averageto mediocre quality of the majority ofPC loudspeakers. We thought thatwas something that could beimproved upon without expendingtoo much effort or money. Of courseyou don’t need the ultimate HiFiquality for the PC, but it would benice to have a set of loudspeakersthat gave a decent reproduction. Inthat way you could start to enjoy themusic from a CD or DVD.

To begin with, we looked for acompact woofer/tweeter combina-tion that gave a good performance ata reasonable cost. Another require-ment was that they should be mag-netically shielded, because the loud-speakers are likely to be placed closeto a monitor or TV, which would beaffected by stray magnetic fields. Forour prototype we finally decided touse a 25 mm dome tweeter and a 13cm bass/midrange unit made byVisaton (the SC10N and SC13). Othermanufacturers of magneticallyshielded speakers worth looking atare Vifa and Monacor (Monarch). Forthis reason we would like to make itclear that other speaker combina-tions with associated enclosures

(recommended by the manufacturer)can also be used in this design.

The electronics required to makethe loudspeaker active have beenkept as simple and yet as adaptableas possible. A two-way crossover fil-ter was designed round a quadopamp, with a choice of slopes, char-acteristics and crossover frequen-cies. For use with the speakers rec-ommended by us, the filter was setup as a 3rd order Butterworth typewith a crossover frequency of 4 kHz.

For the power amplifiers we usedan integrated dual bridge amplifier,which requires very few externalcomponents. At a supply voltage of16 V it delivers 2 × 19 W into 4 Ω or2 × 12 W into 8 Ω. Compared tousual HiFi standards this may seem abit skimpy, but in combination withspeakers with an average efficiency asound pressure level of about 100dB can be attained — and that reallyis very loud!

What else can be added to thissummary? One of the advantages isusing an ‘active’ design is that itallows us to overcome a disadvan-tage found with many small loud-speaker enclosures. Most enclosureswith a volume of a few litres tend to

TDA7374

CLIPDET

PWGND

IC2

OUT1

STBY

OUT1

OUT2

OUT2

SGND

IN1

IN1

IN2

IN2

SVR

VCC VCC

12

11

13

15

14

10

1

3

4

6

5

7

9 8

2

C2

1n

C5

*

C7

3n9

C6

8n2

C8

3n3

C15

3n3

C18

3n3

C21

100nC23

100n

C1

2µ2

C3

120n

C4

2µ2

C16

2µ2

C9

*

C10

4n7

C11

4n7

C14

470n

C17

470n

C13

10µ63V

C20

47µ25V

C12

4n7

C19

10µ63V

C24

4700µ 25V

C22

4µ763V

R1

1M

R3

6k

8

R44k

7

R20

15

k

R21

15

k

R19

10

k

R9

*R11

9k

76

R10

4k

99

R12

12

k7

R162

k0

0

R13

2k

00

R24

2k

7

R2

470Ω

R5

0 Ω* R6

7k68

R7

9k53

R8

8k25

R14

2k00

R15

470Ω

R18

470Ω

R23

0Ω1

R22

0Ω1

R17

2k00

D1

1N4148

D2

1N4148

13

12

14IC1.D

9

10

8IC1.C

6

5

7IC1.B

2

3

1IC1.A

10k

P1

10k

P2

10k

P3

+16V

IC111

4

7808

IC3

D3

POWER

+8V

+8V

JP1

SC13 8ΩLS1

LS2

SC10N 8Ω

+8V

IC1 = TS924IN

zie tekst*see text*siehe Text*voir texte*

+16V

020054 - 11

+16V

*

zie tekst*see text*voir texte*siehe Text*

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

In order to avoid the need for a symmetricalpower supply, but still obtain the optimumsignal processing by the buffer and filters, wehave used IC1b to create a stable virtualground. C13 decouples the output of poten-tial divider R20/R21, providing a virtualground at exactly half the supply voltage. Thelarge output current capability of the TS924INis obviously a great advantage in the circuitround IC1b.

To avoid supply-borne interference fromaffecting the input buffer and filters, opampIC1 has been equipped with its own voltageregulator (IC3). For proper operation of this 8V regulator, the supply voltage to the activesystem should be at least 11 V. Resistor R22has been added to separate the signal groundand supply ground for cases where a singlepower supply is used for two or more chan-nels. When each channel has its own powersupply then R22 can be replaced with a wirelink.

FilterNext comes the crossover filter.

As can be seen in the circuit diagram, theoutput of the input buffer is fed to two filtersections. These are built round the tworemaining opamps of IC1. The low-pass filteris built round IC1d and the high-pass filter isfound round IC1c. For the design of the filterwe started out with a 4th 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 forseveral variants. Table 1 shows the compo-nent values for a 3rd order Butterworth filterand a 4th order Linkwitz-Riley filter withcrossover frequencies at 1, 2.5 and 4 kHz.With the Visaton loudspeakers used in ourprototype we found that a 3rd order Butter-worth filter with a crossover frequency of 4kHz gave the best result. The component val-ues given in the circuit diagram are thereforefor this type of filter. When we used a 4th

order Linkwitz-Riley filter at 4 kHz the mea-surements showed that there were problemswith the radiation pattern due to the largephase shifts this filter produces. The 3rd orderButterworth variant noticeably suffered lessfrom this. The Linkwitz-Riley values are there-fore mainly included for experimentation pur-poses.

Take care with the connection of thespeakers when using the Linkwitz-Riley fil-ter, because the connections to the tweeterare then reversed. On the PCB the polarityindicated is for use with 3rd order Butter-worth filters. (In this case the tweeter istherefore ‘out of phase’ compared to thewoofer!) And as a final point of interest: the

crossover point of Butterworth filtersis at -3 dB, for Linkwitz-Riley filtersit is at -6 dB.

Power amplifierThe output signals of the filters arefed to the power amplifiers via pre-sets P2 and P3. The potentiometerscompensate for the different effi-ciencies of the woofer and tweeter.Many dome tweeters are about 3 dBlouder than small bass/midrangespeakers at the same input level.However, the loudspeakers chosenfor this design do have similar effi-ciencies, so that in practice both P2and P3 can be turned to their maxi-mum level.

For the power amplifier we’vechosen a TDA7374B double inte-grated amplifier. These are primarilyintended for automotive use, but areobviously also very suitable in appli-

cations such as these. This ICrequires surprisingly few externalcomponents (no Boucherot network,nor output capacitors) and also con-tains adequate internal protectioncircuits against overheating andshort circuits. Due to all these fea-tures our dual power amplifier is ashining example of compactness.This is clearly seen in the circuit dia-gram.

We have already mentioned theoutput power. With a load of 8 Ωloudspeakers IC2 can be cooledusing a small heatsink with a ther-mal resistance of 3 K/W. R19 andC19 ensure that virtually no plopsare heard when the amplifier isswitched on (there is always a smalloffset voltage at the output of theamplifier). RC networks R15/C15 andR18/C18 restrict the bandwidth ofthe power amplifiers in order to min-

PC&AUDIO


(C) ELEKTOR

020054-1C1 C2

C3

C4 C5

C6

C7

C8C9

C10

C11

C12

C13

C14C15

C16

C17 C18

C19

C20

C21

C22

C23

C24

D1

D2

D3

H1 H2

H3 H4

IC1

IC2

IC3

JP1

LS1 LS2

P1

P2 P3

R1

R2R3 R4

R5

R6 R7

R8

R9

R10

R11

R12

R13

R14

R15

R16

R17

R18

R19

R20

R21

R22

R23

R24

+-

+

0

T

-

+

020054-1

(C) ELEKTOR

020054-1

Figure 2. It is amazing that this compact PCB contains a crossover filter as well as a2 x 20 W power amplifier!

imise the effect of any HF interference. Inprinciple it would have been better to placethese networks after the potentiometers, butthen the bandwidth will vary noticeably. C20decouples the internal potential divider,which supplies several stages with half thesupply voltage and which is also responsiblefor supply ripple suppression, which is about50 dB at 100 Hz.

The PCBFigure 2 shows the PCB that was designedto hold the electronics for the active 2-wayloudspeaker.

There is not much to be said about thePCB. The layout is fairly well organised andthe various connections are clearly marked.At the bottom-left are the input pins, diago-nally opposite (top-right) are the connectionsfor the supply voltage and a bit below thatwe find power LED D3. The connectors forthe woofer and tweeter (LS1 en LS2) are oneither side of IC2.

The wire links on the board deserve amention. There are two: one just next to R20and another underneath (!) the pins of IC2.The last could also be soldered to the under-side of the PCB, but in either case an isolatedwire should be used for this link. IC2 hasbeen placed near the edge of the PCB, mak-

PC&AUDIO


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

COMPONENTS LIST

Resistors:R1 = 1MΩR2,R15,R18 = 470ΩR3 = 6kΩ8R4 = 4kΩ7R5 = 0ΩR6 = 7kΩ68R7 = 9kΩ53R8 = 8kΩ25R9 = open *R10 = 4kΩ99R11 = 9kΩ76R12 = 12kΩ7R13,R14,R16,R17 = 2kΩ00R19 = 10kΩR20,R21 = 15kΩR22,R23 = 0Ω1 5WR24 = 2kΩ7P1,P2,P3 = 10kΩ preset

Capacitors:C1,C4,C16 = 2µF2 MKT, lead pitch 5

or 7.5mmC2 = 1nF, lead pitch 5mmC3 = 120nF, lead pitch 5mmC5 = open *C6 = 8nF2, lead pitch 5mmC7 = 3nF9, lead pitch 5mmC8,C15,C18 = 3nF3, lead pitch 5mmC9 = wire link *

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

C13,C19 = 10µF 63V radialC14,C17 = 470nFC20 = 47µF 25V radialC21,C23 = 100nF, lead pitch 5mmC22 = 4µF7 63V radialC24 = 4700µF 25V radial, lead pitch

7.5mm, diameter 17mm max.

Semiconductors:D1,D2 = 1N4148D3 = LED, green, high-efficiencyIC1 = TS924IN (ST, from Farnell)IC2 = TDA7374B (ST, from C-I

Electronics, www.dil.nlIC3 = 7808

Miscellaneous:JP1 = 2-way pinheader w. jumperLS1 = SC13 8Ω Visaton (Conrad

Electronics)LS2 = SC10N 8Ω Visaton (Conrad

Electronics)Heatsink for IC2: 3 K/WWood: 12 mm MDF — see Figure 4Wadding material (BAF)PCB, order code 020054-1 (see

Readers Services pages)

* see text and Table 1

ing it easy to mount a small heatsink (3 K/W)to it; remember to use an isolating washerbetween the IC and heatsink!

Once the PCB has been populated andtested, there are several possibilities for com-pleting the construction. It can be mountedinside the loudspeaker enclosure, it can bemounted in a separate box (probably as atwo-channel version) or it can be combinedwith a subwoofer that will be described in aforthcoming article. A separate box isn’t abad idea since we are thinking of adding atone control to this system. But at the end ofthe day the choice is yours.

The supply can be provided by the usualcombination of a transformer, bridge rectifierand smoothing capacitor. Each channelrequires a transformer of 12 V/15 VA and asmoothing capacitor of 4700 µF/25 V. For astereo version these values should be dou-bled. When a stabilised supply is used, thevoltage may be increased from 16 V to amaximum of 18 V, which increases the out-put power somewhat. At the moment we’redesigning a dedicated power supply for thisactive loudspeaker system, so it may bewise to wait a little before building thepower supply.

WoodworkThe size and construction of the loudspeakerenclosure depends primarily on the size of thewoofer that is used. The Visaton SC13 usedhere is mounted in a closed box with a volumeof about 4 litres. This makes the construction

a fairly straightforward job, since aclosed box is little more than six pan-els that are glued together. This mayseem a bit difficult to the inexperi-enced carpenter, but once the wood

has been cut to size then the rest ofthe job should be simple if you usesome clamps. In any case, it is anadvantage that no special baffles orports are required. The only part that

PC&AUDIO


ASection A - A

Dimensions in mmMaterial: MDF 12 mm

Section B - B

A

B B

Ø 113

Ø 85

118

231

255

126

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.

Table 1

3rd order Butterworth 4th order Linkwitz-Riley

1 kHz 2.5 kHz 4 kHz 1 kHz 2.5 kHz 4 kHz

R5 link link link 6k34 6k34 6k98

R6 8k06 6k65 7k68 13k3 9k31 10k7

R7 8k25 8k45 9k53 6k49 7k68 8k25

R8 6k81 8k06 8k25 9k31 7k50 8k45

C5 open open open 22 n 10 n 5n6

C6 33 n 15 n 8n2 39 n 18 n 10 n

C7 18 n 6n8 3n9 18 n 6n8 3n9

C8 15 n 5n6 3n3 8n2 3n9 2n2

R9 open open 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

C9 link link 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 3rd

order Butterworth filter C5 and R9 aren’t mounted and R5 and C9 are replacedby wire links.

have edges with ‘difficult’ angles of 30º and73.9º! The front and base panels are the easi-est to make since they still have normal 90ºedges. We’ve used an equilateral triangle forthe base panel and the top panel slopes downfrom the front at an angle of 30º. Those of youwho are interested in this special design candownload the drawings from our website.

There are several options for finishing offthe enclosures. An ‘old fashioned’ veneer isof course a possibility, but they could alsobe covered with self-adhesive vinyl oranother material. Spraying with paint isanother popular option and for the perfectfinish you could ask somebody at a garageto do it for you.

(020054-1)

is a bit tricky is the cutting out of theholes for the speakers.

Due to the small dimensions ofthe box, there is no need to use thickwood (although you could if youwanted to). We have therefore basedthe drawing shown in Figure 4 onMDF board with a thickness of 12mm. This drawing shows all thedetails. The required acoustic damp-ing of the enclosure is done by fillingit loosely with polyester wadding.The connectors can be mounted onthe back panel. It may be unneces-sary to say this, but when the elec-tronics are mounted inside the enclo-sure it is important that the heatsink

is on the outside.We would like to make clear that

the design shown in Figure 4 doesnot have to be strictly adhered to. Adifferent style of enclosure is alsoacceptable, as long as its volume isnear the recommended 4 litres. Themain photo with this article showsthat we’ve also deviated from thestandard ‘shoe box’ design for theprototypes. We tried out some trian-gular designs for our enclosures,mainly for originality. An advantageof this is that there won’t be anystanding waves, but this comes atthe expense of added complexitysince three of the five panels now

PC&AUDIO


Specifications (using a 16 V supply voltage)

Input impedance 10 kΩSensitivity (12 W/8 Ω, JP1 open, P1/P2/P3 max.) 270 mVDistortion + noise (1 W/8 Ω, 1 kHz) 0.013 % (B = 80 kHz)Bandwidth woofer amplifier (P2 max., JP1 open) 32 Hz - 4 kHzBandwidth woofer amplifier (P2 half, JP1 open) 25 Hz - 4 kHzBandwidth tweeter-amplifier 4 kHz - 45 kHzOutput power per amplifier (THD+N = 0.5 %) 12 W (8 Ω)

19 W (4 Ω)Quiescent current (no load) 0.17 ABandwidth amplifier + box (-3 dB) 100 Hz - 18 kHz

Apart from this list of figures we’ve also provided three graphs. The first (A) is asimulation of the frequency response for the SC13 woofer. The absence ofprominent peaks or troughs make this graph look fairly neat, but it does show asteep drop at the lower end of the frequency range: at 100 Hz the amplitude isnearly 8 dB lower than at 2 kHz. A low-frequency booster therefore isn’t asuperfluous luxury.

The second graph (B) shows the measured response of the filters and thelow-frequency booster. It is noticeable that the crossover point is neitherexactly at -3 dB nor exactly at 4 kHz. This is mainly due to the tolerance of thecapacitors used in the filters. In practice these deviations can be completelyignored.

Graph C shows the measured frequency response of the loudspeakerswhen driven by their amplifiers. From this it is clear that the low frequencyboost really should have been set a bit higher. Keep in mind that when theloudspeaker is placedon a desk or near awall that the lower fre-quencies will beboosted more, makingthe curve somewhatflatter. The small peaknear 200 Hz is causedby the positioning ofthe loudspeaker in theroom where the mea-surements took placeand it varies dependingwhere the loudspeakeris positioned!

-27

+9

-24

-21

-18

-15

-12

-9

-6

-3

+0

+3

+6

dBr

20 40k50 100 200 500 1k 2k020054- 15

5k 10k 20kHz

A

B C

altifalantes ativos

Documents