64 Elektor Electronics 4/96

In Part 1 of this article, the benefit wasexplained that a subwoofer may havefor realistic reproduction of hi-fisound, particularly in audio-visual sys-tems with surround sound. So, what isthe value added of thisactive version, it may beasked. Is a cut-off fre-quency of around 40 Hznot sufficient for good(bass) sound reproduc-tion?

The answer is yes andno: it depends what youwant. For most music re-production 40 Hz is agood figure: it corre-sponds roughly with thelowest tone of a doublebass. Loudspeakers thatcan reproduce this fre-quency with good soundpressure are few and farbetween. Nevertheless,there are a.f. signalswhere 40 Hz is not suffi-cient. This is the case, forinstance, when the canonfire in Tchaikovsky’s‘1812’ is to be reproduced

faithfully, or when thunder claps areto sound realistic. Also, the soundtracks of films like Jurassic Park and TopGun gain in reality if the audio rangegoes down well below 40 Hz.

This active version ofthe subwoofer

described in lastmonth’s instalment is a

plus for virtually any hi-fisystem. Where the lowcut-off frequency of the

passive version isaround 40 Hz, it is down

to about 20 Hz in theactive subwoofer. With

its integral 240 W ampli-fier, it is the answer for

those seeking a realisticbass foundation for their

system. The more so,since its building cost is

very reasonable.

surround-sound

subwoofer Part 2surround-sound

subwoofer Part 2

Design by T. Giesberts

Technical data Drive unit 300 mm (8 in),

e.g. Monacor (SPH-300TC),KEF, Radio Shack (40-1024);

Parts Express (295-240) Type of enclosure Bass reflex Box dimensions 660×406×420mm (incl.legs)

26×16×169⁄16 in Volume of box 65 l Frequency range 20 Hz to 40 Hz, 50 Hz,

60 Hz or 70 Hz (as selected) Cross-over frequency 40 Hz, 50 Hz,

60 Hz or 70 Hz (as selected) Power output 245 W into 4 Ω (thd = 0.1%)

130 W into 8 Ω (thd = 0.1%) THD + N at 100 Hz at 1 W into 8 Ω: 0.0046%

at 50 W into 8 Ω: 0.001%at 1 W into 4 Ω: 0.007%

at 100 W into 4 Ω: 0.0016% Signal to noise ratio 90 dB linear (93 dBA)

at 1 W into 8 Ω Damping factor >400 (with 4 Ω load)

Although the question may beasked how far down to go, to whichthe answer is ‘the further the better’, asensible, practical limit appears to beabout 20 Hz. This is because thethreshold of human hearing is ataround that figure. Lower frequenciesare ‘felt’ rather than heard (to hearthem would require a battery of loud-speakers that could not be accommo-dated in the average home. Moreover,even if it could, the possibility of dam-age to the building at the required vol-ume is not imaginary).

If the cut-off frequency is set at 20Hz, very good low-frequency repro-duction is possible, while the requiredair displacement can beachieved with normalmeans. However, witha passive system, thiswould required an en-closure of a couple ofhundred litres, andthat again would beunacceptable in the average home.Therefore, what is required is an …

A C T I V E D E S I G N

The most notable difference betweenan active and a passive loudspeaker isthe amplifier in the former. In a mul-tiple system, two or more would beneeded, but fortunately only one in asubwoofer. The fact that an active de-sign has its own amplifier makes it eas-ily brought into line with the loud-speakers in the system into which it isbeing introduced.

Another beneficial aspect of an ac-tive design is that the necessary filter-ing can take place before the poweramplifier. This filtering is carried outelectronically, which has the advan-tage of offering virtually limitless op-portunities for correcting or manipu-lating the frequency response of thedrive unit.

In the present design, these oppor-tunities are taken gratefully, since theyallow a relatively small enclosure to re-produce frequencies down to 20 Hz.This is done by measuring the re-sponse of the drive unit in its (toosmall) enclosure and creating a filterwith a mirror image of that response.This results in the filter compensatingthe irregular response of the box untila straight response curve is obtained.

The response of the passive loud-speaker described in Part 1 (using theMonacor drive unit) is shown in Fig-ure 6. It will be recalled that the vol-ume of the enclosure is 65 l. The cut-off frequency is about 45 Hz, but aclose look at the curve shows that theresponse begins to roll off at around 85

Hz already. The curvebecomes slightly steep-er at about 60 Hz andeven more so at 30 Hz.The latter is a direct re-

sult of the bass reflex vent: above thevent frequency, the roll off occurs at 12dB/octave (second order) and below it,at 18 dB/octave (third order).

The active design has a basic fre-quency range of 20–70 Hz. To straight-en the response curve, the electronicfilter must have a response as shownin Figure 7. This curve peaks at 20 Hz(note that it begins to straighten outbetween 30 Hz and 20 Hz). There arefour curves in the figure, because thefilter is designed with four switched(upper) cut-off frequencies. Thismakes it easier for the loudspeaker tobe combined with ex-isting systems. Actual-ly, we have jumpedahead slightly, becauseFigure 7 shows the re-sponses of the activepart of the subwoofer.

The acoustic end result of the de-sign is shown in Figure 8, whichshows that the response is virtuallystraight between 20 Hz and 70 Hz.The solid curve is that of the activelycorrected subwoofer and is obtainedwith a standard microphone andspectrum analyser. Comparing thiscurve with that of Figure 6 shows im-mediately the enhancement providedby the added electronics. The dottedcurve is obtained when the (upper)cut-off frequency is set to its lowestvalue of 40 Hz.

D E S I G NC O N S I D E R A T I O N S

The active subwoofer is based onthe 30 cm drive unit specified in Part

1, and is housed in thesame bass reflex boxdescribed in that in-stalment. The tradi-tional cross-over filteris not used in the ac-tive design: it is re-

65Elektor Electronics 4/96

6

Figure 6. Frequencycharacteristic of the300 mm drive unit inits base reflex box,without filter and with-out correction.

Elektor DEFAULT vs

-20.00-18.00-16.00-14.00-12.00-10.00-8.000-6.000-4.000-2.0000.02.00004.00006.00008.000010.00012.00014.00016.00018.00020.000

AMPL(dBr)

10 100

FREQ(Hz)

960049 - 12

7

Figure 7. Response ofthe combined correc-tion filter and cross-over filter. The fourcurves refer to(upper) roll-off fre-quencies of 40 Hz, 50Hz, 60 Hz and 70 Hz.

placed by an electron-ic filter and a poweramplifier.

The electronic filteris a combination of acorrection filter and across-over filter. Itstraightens the re-sponse curve of thedrive unit and can be switched to giveone of four different (upper) roll-offfrequencies.

Since the filter correction is no lessthan 10 dB at 20 Hz, the amplifier

must provide a reason-able output power: inthe present design, 240W. The amplifier drivesboth voice coils of thedrive unit, which areconnected in parallel.

Since the electronicfilter has line inputs as

well as high-level inputs, the activesubwoofer may be driven by a pream-plifier (or via the pre-out terminals ofan integrated amplifier)or via the loudspeaker

terminals—see Figure 9.The existing a.f. amplifier and the

subwoofers must be linked byscreened audio cable, not by loud-speaker cable.

The filter, output amplifier and thenecessary power supply are housedin a common enclosure that is placedclose to the loudspeaker or even fas-tened on to it.

T H E F I L T E R

The circuit of the filter is shown in thediagram in Figure 10. It consists of fourdistinct parts: correction filter IC2d,IC2c; cross-over filter IC2b, IC2a; drivelevel indicator IC3, T1; and symmetri-cal power supply IC4, IC5.

Operational amplifier IC1a func-tions as an up-counter for the left-hand and right-hand channels. Its am-plification is varied with P1. High-value resistors R1 and R2 ensure thatloudspeaker signals can be processedwithout any difficulty.

The op amp is followed by thecorrection filter. This is a second-order low-pass type based on IC2d. Itsoutput is added to the unfiltered sig-nal in IC2c. Capacitors C3 and C4 limitthe bandwidth (as does capacitor C1at the input of the amplifier—see Fig-ure 11). The correction is enhanced

by output buffer R28-R29-C8, which enablesthe response of theloudspeaker to changegradually from secondorder to third order.

The cross-over filteris based on IC2b. It is anactive third-order low-

pass Butterworth filter that can be set

66 Elektor Electronics 4/96

8

Figure 8. After correc-tion, the frequencyresponse curve of theactive subwoofer isstraight from 20 Hz to70 Hz. The dottedcurve is measuredwith a roll-off frequen-cy of 40 Hz.

L R

L R

LINE LEVEL

HIGH LEVELactive subwoofer

Afilter

amplifier

B

A

B

960049 - 14

*

*see text

9

Figure 9. The activesubwoofer is a combi-nation of loudspeaker,amplifier and filter. Itcan be driven via aline output or via theloudspeaker output.

to any one of four dif-ferent (upper) roll-offfrequencies with S1.With component valuesas specified on the cir-cuit diagram, these frequencies are: 40Hz, 50 Hz, 60 Hz, 70 Hz.

The filter is followed by inverterIC2a, so that it is possible to select (withS2) either the original signal or onethat is 180˚ out of phase with it. Thisis often an advantage with certainloudspeaker systems.

The filtered signal is applied to theoutput via buffer IC1b.

The drive level indicator, IC3 andT1, is intended as a protection for theloudspeaker: when the amplifier is dri-ven to half its maximum output, diodeD1 lights. This optical signal is a warn-ing to turn down the volume to some

extent.The indicator is

based on IC3a andIC3b, which form awindow comparator,

which is designed such that the ledlights when the output voltage of IC1bexceeds a level of 1 Vpeak. Since theoutput amplifier has an input sensi-tivity of 1 Vrms, its drive remainsabout 3 dB below maximum (provid-ed that the warning signal has beenresponded to).

The brightness of D1 is enhancedby the high charging current (1 A)through C15 delivered by T1. This alsoresults in a certain amount of after-glow once the peak has passed. Net-work R35-C16 decouples the powerline, so that charging pulses do notcause any interference in the filter.

67Elektor Electronics 4/96

2

3

1IC1a

9

10

8IC2c

13

12

14IC2d

6

5

7IC2b

2

3

1IC2a

6

5

7IC1b

5

4

12IC3a

10

9

7IC3b

7815

IC4

IC5

7915

B1

B80C1500

C24 C21

C23 C22 C19

220µ25V

C20

220µ25V

R38

6k8

D2

POWER2x 15V

Tr1

1VA5

C17

10µ63V

C18

10µ63V

K1

C11

100n

C12

100n

C9

100n

C10

100n

IC2

4

11

IC1

8

4

IC3

11

6

R1

470k

R2

470k

R3

22k

R4

22k

R7

820Ω

R5

18k

R6

15k

C1

2µ2

C2

100n

C13

100n

C14

100n

C5

820n

C6

2µ2

C7

120n

C8

680n

47k

P1

R81k

R12

6k8

R32

14k0

R34

14k0

R33

2k00

T1

BC640

R36

15Ω

R37

2k7

D1

– 3dB

R35

100Ω

S1a

13

12

3

4

R9

2k7

R11

15k

C32µ2

C4470n

R10

8k2

R14

5k36

R13

6k65

R15

4k42

R16

3k83

S1b

14

56

7

8

R18

5k36

R17

6k65

R19

4k42

R20

3k83

S1c

15

910

11

12

R22

5k36

R21

6k65

R23

4k42

R24

3k83R25

10M

R26

2k2

R27

2k2

S2

R28

10k

R31

100Ω

R29

4k7

R30

1M40Hz

50Hz

60Hz

70Hz

S1:

C16

470µ 25V

C15

47µ25V

LINELEVEL

LEVEL

IC1 = NE5532IC2 = TL084IC3 = LM319

15V

15V

X

Y

15V

15V

1

2

3

Y

X

960049 - 15

C21...C24 = 47n

3

8

HIGHA

B

0V

0V

+1V

- 1V

A

B

15V...13V

14V4...12V4

A

B

0V 0V

0V

0V

0V

0V

0V

0V0V

0V

0V

0V

19V

- 19V

10

Figure 10. The circuitof the active filter issimplicity itself. Thedrive level indicatorbased on IC3 is aboon.

The symmetrical15 V power supply is atraditional design:mains transformer,bridge rectifier,smoothing capacitorsand two voltage regu-lators, IC4 and IC5.Diode D2 is the on/offindicator.

T H EP O W E R A M P L I F I E R

The output of the filter is coupled di-rectly to the power amplifier whosecircuit is shown in the diagram in Fig-ure 11. Considering its output power,the amplifier is fairly compact andstraightforward. The compactness is aconscious part of the design, while thesimplicity is brought about by the factthat the amplifier needs to performwell only up to about 100 Hz.

The amplifier is a combination ofan integrated voltage amplifier and adiscrete current amplifier. Since thevoltage amplifier needs to meet certainstrict requirements, it is based on avery fast op amp (IC1), the Type

AD847 from Analog De-vices. Its supply voltagehas been made as highas feasible (±18 V) withthe aid of zener diodesD1 and D2 to minimizethe risk of overdriving.

The current ampli-fier is formed by two‘darlington-like’ config-

urations, each consisting of a mediumpower driver, T3/T4, followed by twoparallel-connected Insulated GateBipolar Transistors (igbts), T4-T5 andT6-T7. Network R23-R24 ensures thatthe power stages not only provide cur-rent amplification, but also voltage am-plification of ×4. This is necessary be-cause IC1 works from a supply of only±18 V, whereas the output stages needto be driven to about ±45 V.

‘Zener’ transistor T1 enables thecorrect setting of the quiescent current.For good quiescent-current stability, itis necessary that T1 is fitted on to thesame heat sink as the drivers andpower transistors. The stage is de-signed so that it has a slightly negativetemperature coefficient. This meansthat when the heat sink warms up, the

quiescent current, set with P1, drops alittle so that the amplifier cools morequickly

Annoying and possibly damagingswitch-on plops are avoided by thetraditional relay, controlled by a delaycircuit, in series with the loudspeaker.Transistor T8 conducts only when C9has been charged to a certain level viaR31: that is, a few seconds after thesupply has been switched on.

The delay circuit is powered di-rectly by the secondary winding of themains transformer. This has the ad-vantage of the relay being deenergizedimmediately the supply is switched offand not after the reservoir capacitorsin the power supply have been dis-charged.

Next month’s instalment will dealwith the construction.

(960049)

69Elektor Electronics 4/96

AD847

IC1

2

3

6

7

41

8

T6

T7

T4

T5

T1

BD139

T2MJE15030

T3

MJE15031

R3

8M2

R4

8M2

R2

31k6

R12

3k3

R131k

R10

22k

R11

22k

R22

22Ω

R18

22Ω

R19

220Ω

R15

220Ω

R1

1k

R7

100k

R6

31k6

R9

3k32

R8

3k32

R14

68Ω

C3

22p

C2

3n3

C10

100n

C11

100n

R5

1k

C1

270n

2k5P1

C7

2µ2

R23

220Ω

R24

680Ω

C4

1n

R17

100Ω

R16

100Ω

R21

100Ω

R20

100Ω

R25

0Ω22

R26

0Ω22

R27

0Ω22

R28

0Ω22

Re1

LS1

D4

10V 1W3

D3

10V 1W3

D1

18V 1W3

D2

18V 1W3

D5

D6

D7T8

BC640R29

390Ω

R31

47k

R32

5k6

R30

47k

C6

220µ63V

C5

220µ63V

C8

47µ 50V

C9

100µ40V

JP1

C

B E

MJE15030 GT20D201

G C E

BD139

E

C

B

BC640

CBE

MJE15031 GT20D101

LS+

LS–

35V

35V

49V

49V

D5...D7 = 1N4004T4, T5 = GT20D201T6, T7 = GT20D101

960049 - 16

A A

AA

B B B

B B B

C

C

D

D D

D

E

F

G

DA 22mV

2V

200mV

B

C

G 1V6

+ 100%- 50%

+ 100%- 50%

E

F

0V

+ 18V

- 18V

D

11

Figure 11. Since theoutput amplifier doesnot have to processfrequencies aboveabout 100 Hz, itsdesign is spartan. Inspit of this, its perfor-mance is excellentand its power is suffi-cient to drive the sub-woofer to the very lim-its of its loadability.