28 Elektor Electronics 5/96

The circuits shown in Figures 10and 11 (Part 2), including thepower supply for the cross-over fil-ter in Figure 10, are intended to bebuilt on the three printed-circuitboards shown in Figure 12. Thethree sections shown in the illus-tration are easily separated fromone another by cutting or snappingoff along the indicated lines. Thepower supply for the power ampli-fier will be described later in thisinstalment.

Building the filter board isstraightforward, as one would ex-pect with only three ICs, a handfulof resistors, and some capacitors ona good-sized board. Make sure tofit the six wire bridges first. Rotaryswitch S1 may be mounted directlyon to the board. Connect the twoleds and switch S2 to the board viashort lengths of stranded circuitwire (7/029).

The ±15 V power supply for thefilter does not take much morespace than a box of matches. Sincethe filter draws only a modest cur-

rent, the mains transformer can bekept small (1.5 VA). The voltage reg-ulators need not be cooled. Con-nect terminals ‘+’, ‘0’ and ‘–’ to thecorresponding terminals on the fil-ter board via short lengths ofstranded circuit wire. The mainsvoltage from the central mainsentry is connected to the supplyvia terminal block K1.

Populating the power amplifier

Part 3 (final)After the detailed

descriptions of the pas-sive and active versionsof the subwoofer in the

previous two instalments,this third and final part

deals with the completeconstruction. If the enclo-

sure has already beenbuilt on the basis of

Figure 5, only the print-ed-circuit boards need to

be completed and builtinto the enclosure togeth-

er with the drive unit.

surround-soundsubwoofer

Parts list:FILTER

Resistors:R1, R2 = 470 kΩR3, R4 = 22 kΩR5 = 18 kΩR6, R11 = 15 kΩR7 = 820 ΩR8 = 1 kΩR9, R37 = 2.7 kΩR10 = 8.2 kΩR12, R38 = 6.8 kΩR13, R17, R21 = 6.65 kΩ, 1%R14, R18, R22 = 5.36 kΩ, 1%R15, R19, R23 = 4.42 kΩ

surround-soundsubwoofer

board is straightfor-ward, but a few aspectsneed to be watched.Firstly, transistorsT1–T3 must be fitted atthe track side of theboard. These devices,together with T4–T7,must be screwed to oneand the same heat sink,in each and every casewith the aid of insulat-ing washers andbushes. To ensure max-imum heat conduction,apply heat conductingpaste to both sides ofthe washers for theIGBTs.

Secondly, as de-scribed in the previousinstalment, the collec-tors of T4–T7 providethe output current inunison. To ensure thatthe transfer resistancesare kept small and atthe same time that theboard does not becomeunduly warm, the con-nection between thecollectors and the out-put relay is rather un-usual. Immediately ad-jacent to the collectorterminal is an addi-tional hole on theboard into which a sol-der pin is to be inserted(at the track side). Thefour pins must be inter-connected by heavy-duty single-strand cop-per wire (≥ 1.5 mm2) or

29Elektor Electronics 5/96

960049-1

960049-1

B1

C1

C1

C2

C2

C3

C3

C4

C4

C5

C5

C6

C6

C7

C7

C8

C8

C9

C9

C10

C10

C11

C11

C12

C13

C14

C15

C16

C17

C18

C19

C20

C21

C22 C23

C24

D1

D1

D2

D2

D3

D4

D5

D6

D7

HOLE35E

HO

LE

35

EC

2

HO

LE

35

EC

3

HO

LE

35

EC

4H

OL

E3

5E

C7

IC1

IC1

IC2

IC3

IC4

IC5

JP1

K1

OUT

P1

P1

R1

R1

R2

R2

R3

R3

R4

R4

R5

R5

R6

R6

R7

R7

R8

R8

R9

R9

R10

R10

R11

R11

R12

R12

R13

R13

R14

R14

R15

R15

R16

R16

R17

R17

R18

R18

R19

R19

R20

R20

R21

R21

R22

R22

R23

R23

R24

R24

R25

R25

R26

R26 R27

R27

R28

R28

R29

R29

R30

R30

R31

R31

R32

R32

R33

R34

R35

R36

R37

R38

RE1

S1

S2

T1

T4

T5

T6

T7

T8

TR1

0

0

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+ -

TT

-+

A

B

LS

-

-+

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T

LS+

0

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96

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49

-1

96

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T1

T2

T3

96

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49

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96

00

49

-1

12

Figure 12. Because ofthe relative simplicity ofthe circuits, the ampli-fier and filter can be ac-commodated on oneboard; it is, however,advisable to separatethe boards.

R16, R20, R24 = 3.83 kΩR25 = 10 MΩR26, R27 = 2.2 kΩR28 = 10 kΩR29 = 4.7 kΩR30 = 1 MΩR31, R35 = 100 ΩR32, R34 = 14.0 kΩ, 1%R33 = 2.00 kΩ, 1%R36 = 15 ΩP1 = 47 kΩ preset potentiometer

Capacitors:C1, C3, C6 = 2.2 µF, metallized

polyester, pitch 5 mmC2, C9–C14 = 100 nF

C4 = 470 nF, metallized polyester, pitch 5 mm

C5 = 820 nF, pitch ≤ 7.5 mmC7 = 120 nFC8 = 680 nFC15 = 47 µF, 25 V, radialC16 = 470 µF, 25 V, radialC17, C18 = 10 µF, 63 V, radialC19, C20 = 220 µF, 25 V, radialC21–C24 = 47 nF, ceramic

Semiconductors:D1, D2 = LED, low currentB1 = B80C1500T1 = BC640

Integrated circuits:IC1 = NE5532IC2 = TL084IC3 = LM319IC4 = 7815IC5 = 7915

Miscellaneous:K1 = 2-way terminal block, pitch

7.5 mmS1 = rotary switch, 3-pole, 4-position,

for board mountingS2 = single-pole change-over switchTr1 = mains transformer, 2×15 V,

1.5 VA

PCB Order no. 960049

a narrow strip of thin copper plate.It hardly needs emphasizing thatthorough soldering of these paral-lel links is a prime requirement.

Furthermore, in order to ensurethat the copper tracks for the sup-ply lines and loudspeaker connec-tions are not longer than strictlynecessary, they are not taken to theedge of the board as is usual. In-stead, the relevant solder pads areat the centre of the board and havebeen designed to allow flatcable(car-type) sockets to be screwed tothem, preferably at the track side.Solder tags may be used, but theseare nowhere near as robust as carconnectors, which must, therefore,be preferred.Three final remarks. (1) When pop-ulating the amplifier board, do not(yet) fit resistors R3 and R4; why

will be discussed shortly.(2) Wire bridge JP1 provides thenecessary link between the nega-tive power supply line and earth;it should, however, not be fitted ifsuch a link is already present inthe power supply itself. (3) Turnpreset potentiometer P1 fully anti-

clockwise.Figures 13 and 14 show the

completed prototype boards.Moreover, Figure 15 shows the un-derside of the power amplifiermodule, which gives a good viewof T1, T2 and T3, the flatcable con-nectors for the supply lines and theloudspeaker connections, and thevoltage rail that links the collectorsof T4–T7. Note that this rail in theprototype consists of a folded stripof copper plate.

AM P L I F I E RP O W E RS U P P LY A N DCA L I B RAT I O NThe power amplifier

needs a symmetrical supply of±49 V. The design of the supplymay be simple, but it should en-sure the provision of sufficient cur-rent. The design shown in Figure16 is advised, since a transformerof 2×35 V at 300 VA, a 35-A bridgerectifier and four 10 000 µF, 63 V

30 Elektor Electronics 5/96

Parts list:AMPLIFIER

Resistors:R1, R5, R13 = 1 kΩR2, R6 = 31.6 kΩ, 1%R3, R4 = 8.2 MΩ (see text)R7 = 100 kΩR8, R9 = 3.3 kΩ, 0.5 WR10, R11 = 22 kΩR12 = 3.3 kΩR14 = 68 ΩR15, R19 = 220 ΩR16, R17, R20, R21 = 100 ΩR18, R22 = 22 ΩR23 = 220 Ω, 5 WR24 = 680 Ω, 5 WR25–R28 = 0.22 Ω, 5 WR29 = 390 ΩR30, R31 = 47 kΩR32 = 5.6 kΩ

P1= 2.5 kΩ preset potentiometer

Capacitors:C1 = 270 nFC2 = 3.3 nFC3 = 22 pF, 160 V, polyesterC4 = 1 nFC5, C6 = 220 µF, 63 V, radialC7 = 2.2 µF, metallized polyester,

pitch 5 mmC8 = 47 µF, 50 V, radialC9 = 100 µF, 40 V, radialC10, C11 = 100 nF

Semiconductors:D1, D2 = 18 V, 1.3 W zenerD3, D4 = 10 V, 1.3 W zenerD5–D7 = 1N4004T1 = BD139T2 = MJE15030 (Motorola)T3 = MJE15031 (Motorola)

T4, T5 = GT20D201 (Toshiba)T6, T7 = GT20D101 (Toshiba)T8 = BC640

Integrated circuits:IC1 = AD847JN (Analog Devices)

Miscellaneous:JP1 = 2-way contact row and jumper,

or wire bridgeRe1 = relay, 16 A, 24 V, 875 Ω

(e.g., Siemens V23056-A0105-A101)LS1 = drive unit, see text5 off flatcable (car-type) connectors

with screw fitting 1 off heat sink for T1–T7, <0.55

K W–1, e.g. SK47/100 SA(Fischer, available from Dau Ltd01243 553 031)

Insulating kits (washers; bushes) for T1–T7PCB as for filter (see text)

13

Figure 13. Completedprototype of the filterand its power supply.

Figure 14. The top view of the completedamplifier board shows nothing special.The four output transistors are locatedtogether and well insulated from theheat sink.

14

electrolytic capacitors are fully upto the required task. The series re-sistors, in conjunction with theelectrolytic capacitors, effectivelydecouple the supply lines to theamplifier.

As shown in Figure 16, thepower-on delay circuit for theloudspeaker is supplied directlyfrom the secondary windings ofthe mains transformer. If desired, amains switch-on delay circuit, simi-lar to that described on page 19 ofour September 1995 issue, may beincluded at the primary side of themains transformer.

Thoroughly check the amplifierboard before commencing with thecalibration. Provisionally connectthe power supply and check thatthe potential across zener diodesD1 and D2 is about 18 V. If this isso, the setting of IC1 is correct andit may be assumed that the remain-der of the amplifier is also all right.

Reverting to R3 and R4: these re-sistors are intended to compensatethe bias current of IC1 to ensurethat there is no direct voltage atthe amplifier output. Check this bymeasuring the voltage, UC, at pin 3of IC1 with a high-impedance volt-meter (multimeter) set to a milli-volt range. The value of the resis-tors is calculated by

R3 = R4 = (18/UC)× 31.6 × 103. [Ω]

In the prototype, thebias current throughR2 is 2.2 µA, whichresults in a voltageat the non-invertinginput of IC1 of70 mV. Substitutingthis in the foregoing

formula gives a value of the com-pensating resistors of 8.2 MΩ asspecified in Figure 11. Note thatthe current levels may differ ap-preciably from one AD847 to an-other.

Set the quiescent current, whichshould be 100 mA through each ofthe output transistors. This is doneby connecting a voltmeter or multi-meter set to the 100 mV rangeacross one of resistors R25–R28 andgently turning P1 clockwise untilthe meter reads 22 mV. Leave theamplifier on for about an hour andmeasure the voltage again; adjustP1 as required. Finally, check thatthe voltage drop across the otherthree resistors is the same.

F I N A L L Y …

The construction of the woodenloudspeaker enclosure has alreadybeen described in Part 1. It is nowtime to select a metal case for thefilter, amplifier and amplifierpower supply. Note that it is im-portant that the heat sink of thepower amplifier remains on theoutside so that it is in cooling am-bient air.

It is best to mount the filter onspacers directly at the back of thefront panel of the case so that ro-

tary switch S1 is read-ily accessible. Phaseselector S2, overdriveindicator D1 andon/off indicator D2must also be mountedon the front panel.Drill a small hole inthe relevant positionto make the sub-woofer ’s sound level

control P1 accessible for adjust-ment. It is, of course, possible touse a standard potentiometer forP1, mount this on the front panel,and connect it to the board withtwo short lengths of stranded cir-cuit wire.

Construction of the amplifierpower supply should not presentundue difficulties, but it should berobust. The transformer may be

31Elektor Electronics 5/96

Differentdrive unit?Although in the previous instalments refer-ence has been made to a number of driveunits, the preferred one remains the Mona-cor, since it gives excellent performanceand offers very good cost-to-quality ratio.However, as mentioned in Part 2, even thisotherwise excellent unit suffers from a defi-ciency. Although this deficiency is not seri-ous, some readers may, none the less, beinterested to know that we have found yetanother drive unit that is suitable for thesubwoofer. Finding alternative drive unitsis not easy since their parameters tend todiffer to such an extent that changes to thedimensions of the enclosure and to thebass reflex tuning are often required.The alternative is the Type 30WD300 fromVifa. This is also a 300 mm woofer that fits

readily into the enclosure described inPart 1 and which can use the same filteras the other drive units. The characteris-tics in the illustration show that there aresome differences between it and theMonacor unit, but that these are of notmuch consequence. The solid curve refersto the Vifa unit, and the dashed one to theMonacor unit.

The 30WD300 produces no spurioussounds and has an excellent overall per-formance. Unfortunately, like the RadioShack and Parts Express units, it does nothave a double voice coil, which makes itunsuitable for use in the passive version ofthe subwoofer. Used in the active version,it has a small benefit in that, because of itshigher impedance, it requires a lower cur-rent from the power amplifier. This in turnmeans that the mains transformer may bea (less expensive) 225 VA type (and thefuse rating can be reduced to 1 A). On theother hand, the Vifa unit is dearer than theMonacor unit, so the choice is yours.

15

Figure 15. The under-side of the completedamplifier boardshows T1, T2 and T3,as well as the flatca-ble (car-type) connec-tors. The collectors ofT4–T7 are intercon-nected with the aid ofsolder pins and a nar-row strip of thin cop-per plate.

screwed to the bottom of the caseand the remainder on a small sheetof prototyping board. The connec-tions between the secondary wind-ings of the transformer, the bridgerectifier, the capacitors and resis-tors should be in heavy-duty, sin-gle-strand copper wire. The nega-tive supply line shown bold in Fig-ure 16 must be kept as short as pos-sible. It is best to take the capacitorterminals and the centre tap of thetransformer to a common point(star connection). The bridge recti-fier must be mounted on a smallheat sink or be screwed directly tothe bottom of the case.

As far as the interconnectionsare concerned, thosebetween the filtersupply board and thefilter board have al-ready been dis-cussed, as have theoperating controls ofthe filter. Connect thehigh-level and line-level input pins to a couple ofaudio sockets via single screenedaudio cable. The output of the filtershould be taken also via singlescreened audio cable to the inputof the power amplifier (if thelength of this cable is only a fewcentimetres, stranded circuit wiremay be used instead of screenedcable).

The connections that need mostattention are those between ampli-fier and amplifier power supply,and between amplifier and loud-speaker. The amplifier delivers anappreciable power output, so thatthe connections must be able tohandle fairly large currents, whilethe transfer resistances must bekept to a minimum. It is, therefore,recommended to use cable with across-sectional diameter of≥ 2.5 mm2. Connect the amplifierpower supply via three lengths ofthis cable, preferably terminatedinto car-type flat connectors to ter-minals ‘+’, ‘0’, and ‘–’ on the am-plifier board. Connect the loud-

speaker (with thetwo voice coils con-nected in parallel)via two lengths of

the same cable to terminals ‘+LS’and ‘–LS’ on the amplifier board.

Connect the primary of bothmains transformers via heavy-duty,well-insulated cable to the mainsentry at the rear of the case. Use agood-quality mains entry with in-tegral fuse holder and on/offswitch.

W H I C H C R O S S - O V E RF R E Q U E N C Y ?The setting of P1 and S1 can befound empirically only: there areno definite rules, since it is really aquestion of personal preference.Knowing the lower frequency limitof the existing loudspeaker system,either though personal measure-ment or from manufacturers’ data,is a help, however, since the cross-over frequency of the subwoofercan then be chosen close to thislimit.

The cross-over frequency cho-sen in the prototype is differentfrom that found in many popular

satellite or subwoofer systems. Thisis because such systems normallyuse small drive units that can justabout handle 100–150 Hz, whichmeans that the central bass unitmust take over at that frequency.The present subwoofer is meant re-ally as an addition to normal full-range loudspeaker systems, that is,compact to medium large loud-speaker enclosures that give a rea-sonable bass performance, but donot perform so well at the lowerbass range. Position ‘70 Hz’ willprove fine in use with most book-case-type loudspeakers, while withcompact loudspeakers ‘60 Hz’ willnormally the best value. Cross-over frequencies 40 Hz and 50 Hzare intended for combination withmedium to large loudspeaker en-closures. The two lower frequen-cies, 30 Hz and 20 Hz, are usuallybest for use with electrostatic loud-speakers to give the ‘thin’ bass ofthese units rather more ‘punch’ atthe lower end.

If your stereo loudspeakers arerather small, we can imagine thatyou will find the cross-over fre-quency chosen for the prototyperather low and would prefer to usethe standard frequency for satel-lite/subwoofer systems. This is pos-sible by altering the values of resis-tors R13–R24 as shown in Table 1for frequencies 80 Hz, 90 Hz,100 Hz and 100 Hz. These alter-ations do not affect the amplifier orthe loudspeaker. [960049]

33Elektor Electronics 5/96

100V/35A

4x 0Ω22/5W

35V 35V

4x 10mF/63V

49V

49V

960049 - 2 - 11

mainsswitch-on

delay

relay supply

to o

utp

ut a

mp

lifie

r

output amplifier board

2 t > 20

2

30WD300: Tr = 2x 35V/225VA: F = 1A T, ISPH-300TC: Tr = 2x 35V/300VA: F = 1A6 T, I t > 4016

Figure 16. The mainrequirement of theamplifier power sup-ply is the ability to de-liver sufficient cur-rent.

Alteration of range of S1

Cross-overfrequency

Values of resistors (kΩ)

R13,R17,R21 R14,R18,R22 R15,R19,R23 R16,R20,R24

80 Hz 3.32

90 Hz 2.94

100 Hz 2.67

110 Hz 2.43