11THE COMPlETE PREAMPllFIER (1)

by T. Giffard

After the several 'spartan' preamplifiers we have published overthe past few years, we feeI the time has co me to ofter our readersa more sophisticated design, one with more controls than just a

selector switch and a volume control. Although the designpresented has quite a few useful features, and ofters very good

signal handling qualities, we have been ableto keep its price to a reasonable level.

LOOKlNG around in hi-fi dealers' andrerailers' shops (and tal king to the man-

ager or owner), it appears that a majoriry of(prospectivc) preamplifier buyers are in-tertested in at least some of the followingfearures. First cn the lisr is a copying facil-ity thar is independent ofthe selector switch;second, the possibility of recording fromone recorder 10 an ot her; third, a tone controlwirh variable cut-off frequencies; fourrh, aheadphone outpur (wh ich rneans that themain amplifier can be swuched off if listen-ing only via the headphones is required); fifth(and surprising in rhese days of the CornpacrDisc), adynamie pickup input. And,ofcourse,the prearnplifier musr not degrade the signalin any way.

With these requiremenrs, and that of lowcost, in mind, we have come up with a unitIhm meets them handsomely and still offersthe possibiiity of using ICs of different prieeclasses.

The design lends itself ro fairly easy COIl-

strucrion. The source and record selecrorswitches are housed on a busboard, togetherwith the input and output connecrcrs. whilethe remainder of the electronics is fitred Oll

a motherboard. The controls are connectedto the circuitry via a number of rerminalsthat are siruated at the front edge of the 1110th-erboard.

Overview

The block diagram of the prerunplifier-seeFig. I, shows that there are six inputs, of wh ichone is rhe dynamic pick-up input. If thatinput is not needed, it may be transformedro a normalline input by asimple wire bridge,The CD input has provision for adding aboard (for instance, a digual-ro-analogue COI1-

verter) ar a later date.The input signals are fed to two ratary

switches: one for selecting the input mode(i.e., the signal timt will be processed, andthe other for selecting a signal to be recorded.In that way, it is possible to record from aCD and continue listening tothe tuner. Standardrotary switches are used since these are mucheheaper than the 12 relays that would other-wise be necessary. The switches are located

immediately adjacent LO the the inputs at thereal' of rhe enclosure, ::;0 that long signalpaths are avoided.

Either switeh is followed by a buffer.which in turn is followed by a stereo/monoswitch which uses a summing opamp. Thisobviates the level differences that frequentlyoccur in stereo/mono systerns.

The rone contral is a low-high designwith two cut-off points ateither siele. The con-trol range has been kepr fairly lirnired toavoid overdriving of subsequent stages: itis, however, more than adequare for n01111alusage and offers smooth operarion. For thosewho da not want tone courrot in any cir-cumstances, it may be raken out of circuitby a simple 'tone defeat ' switch.

The balance and volume control are fol-lowed bythc ourput amplifier, which providesthe only aruplification of the line signals,The oparnp chosen for this srage is able 10drive loads of 600 Q and higher.

A relay providing a delay at switch-on ispravided at the output: this gives the unir afew seconds after the supply is switched onto stabilize during which period 110 signal isapplied to the output.

The outpur arnplifier also supplies part

.'C}-J.-------'

•of the signal to a special headphone ampli-fiel' that consisrs of an opamp and discreteoutput stage. It provides enough power fordriving low-resistance headphones.

The circuits

The blocks of diagram Fig. I are easily rec-ognized in the circuits of Fig, 2. and Fig. 3.The circuit has been split into two to corre-spond more closely 10 the drawings of theassociated printed-circuit boards.

Figure 2 shows the record and sourceswitches SI and S2 with associated buffers,JC I and IC2, and all inputs and outputs withthe exceprion ofthat for the headphones. Eachinpur is shunted by a potential divider, forinstance, R ,-R2 for the lefthand CD chan-nel. These dividers largely deterrnine theinput imedance of 47 kQ. It is advisable rouse the dividers only if really necessary,since they may adversely affect thecross-talkberween the left- und righr-hand channels,as weil as between the inputs. If dividers arenot used. resisrors R" R3, Rs, and so onshould be replaced by wire bridges.

The CD input has some additional facili-ties. Norrnally. the analogue CD signal is

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Fig. 1. Block diagram 01the preamplilier.

ELEKTOR ELECTRONtCS JANUARY 1991

THE COMPLETE PREAMPLIFIER (I) m

•

K1 (Record selec!) (Source seiet!)CO (o-l--E:::h

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Fig. 2. Cireui! diagram 01!he busboard

ELEKTOR ELECTRONICS JANUARY 1991

A UDIO & HI-FI

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rctc 15V

ELEKTOR ELECTRONICS JANUARY 1991

THE COMPLETE PREAMPLIFIER (I)

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890169 -1- 13

ELEKTOR ELECTRONICS JANUARY 1991

AUDIO & HI-FI

applied direct to rhe CD input. A wire bridgeberween A and B (E and F) feeds the signalLa the two rotary switches. Terminals C, Dand ground (G, Hand ground) enable a dig-ital input, coupled to CI separate digiral-to-ana-lague amplifier, to be provided at a laterstage.

The dynamic pick-up input also has ad-ditional facilities. Normally, the associatedpreamplifier on the motherboard is con-nected between K and J (Ieft-hand channel)and between M and L (right-hand channel).If a dynamic pick-up input is no longer re-quired, the input rnay be used as microphoneinput, for wh ich the preamplifier is easilyadapred, or as line input, in which case theprearnplifier is not used and J-K and M-Lmust be Iinked by wire bridges.

The opamps shown in the circuir diagramare not necessarily the cheapest types, butIhey are the best for the parricular purpeseas far as our prototypes showed, This aspectwill be touched upon again in Pan 2 underConstruction. Für the rnomeru, as an exarn-pie, buffer ICI is a Type 5532, an inexpen-sive yet cxcellent double opamp, whereasbuffer IC2", a Type AD712, is a much dearertype. This type was found necessary ro ob-viate audible switching noises when S2 isturned. These noises result from the changein total resistance ar the input of the opamp:if, for instance, the CD input is used, Rl8 isshunted by R2. Ir an opamp with rransistorinputs, such as the NE5532, were used, rhechange in resistance would cause a corre-sponding change in the output of the opamp.An opamp with FETinputs, such as theAD712.bardly reacts to the resistance change. lfnotwithstanding this a Type5532 is used, R 16and R36 should be reduced to 220 kQ. Theinput impedance will then reduce to about39 kQ, and this means that rhe ratio of thepotential divider will change slightly.

Apart from the inpurs, the busboard alsocontains all the outputs: rwo tape outputs withan output impedance of 100 n, dererminedby R13•R14, R31, and R32; anel two line out-puts, also with an outpur impedance of about100 Q, derermined by R 15, R 16, R33, andR34, and R55 and Rso Oll the motherboard.

The circuit contained on the motherboardisshown in Fig. 3. Since the design ofthe left-and right-hand channels is identical, onlythe left-hand one will be referred to in whatfollows.

The preamplifier for the dynamic pick-up is shown somewhat away from the maincircuit to ernphasize its short connections toterminals J-K and L-M on the busboard. Theopamp chosen for this stage is a very-low-noise Type LTI 028. lf thar is found too ex-pensive, the eheaper OP27 or 5534 may beused. The RlAA (LEe) correction is providedby RsI-RS3 and CSS-C61 _Network R 12-C62-C63 forms a high-pass filter with a cut-offfrequency of 20 Hz to conform to the rsc re-quirement. If that frequency is lhought un-necessary, the values of C62 and C63 may beincreased.

The input of the preamplifier for the dy-narnic pick-up is entirely nc-coupled 10 ob-viate any degradarion of the signal by cou-

pling capacitors. The input capacitance isdetennined primarily by C72, whose valuedepends on what is required by the pick-up,If that is not known, 47 pF rnay be assumed,Because of the direct coupling, the offsei ofthe opamp depends on the internal resis-tance of the pick-up and P6 is therefore pro-vided to cancel the offset as appropriate.

The supply lines to this sensitive stage havebeen provided with additional filters (aroundT 5 and T 6) 10 even out any ripple on the±15 V supply. Ir should be noted in this con-text that the use oF an LTI028 is justifiedonly in conjunction wirh low-impedancesignal-sources «400 Q). If a standard high-impedance dynamic pick-up is used, an OP27will be perfectly all right, since the totalnoise then consists primarily of the thermalnoise emanated by the pick-up.

The mono-stereo switch, S5' is precededby a sumrning amplifier, JC), that providestrue addition ofthe lefr- and righr-hand chan-nels wirheut any attenuation when stereosignals are ro be processed as mono olles.Passive circuits always introduce sorne at-tenuation.

Although the design of the rone-controlstage, IC4• appears conventional, it has someinteresting aspects. Usually, the connecrions10 the ends of the carbon tracks 01' the 'high'potentiometers are via capacitors that deter-mine the onser of the control, If the onsetfrequency is to be altered. the value of bothcapacitors needs 1O be changed. In the pre-sent design, only Olle component per chan-nel needs to be altered. Switches S3 and S4enable rwo different onset frequencies ro beselected. The circuit is consequently some-what simpleras regards wiringand switching,

A I Mn resistor, R4S, provides a feed-back loop rhar preverus the outpur of theopamp froru rising to unwanted heights ifrhe wiper of the 'Iow' potenriometer occa-sionally losescontact with tbe carbon track.Capacitors C[4 und CIS have been addedto ensure that the offser ofthe input buffersis not amplified in the tone control stage.This parallel network ofa bipolarand a Filmcapacitor guarantees faithful signal pro-cessing even at high frequencies. The COIl-

trol range für various Oll sei frequencies isshown in Fig. 4 (published in our ne xtissue). If required, the entire tone contro!may be taken out of circuit by S6.

It is recornmended to use the very bestqualiry porentiorneters for the balance andvolume controls, for insrance, Alps types.These are not cheap, bUI they give consis-tently good performance over a long life.In some locations, a real balance controlmay be difficult or even impossible to ob-tain: the right type has half-silvered rracksso that 00 attenuarion occurs in the centreposition, If so, a standard linear stereo po-tentiometer may be used wirh a 2k2 resis-tors soldered between its wiper and the poleof S6a: the attenuation in the centre posi-tion is then minimal.

The line amplifier.Kjj , is aType NE5534,which has the advantage of being able todeliver more than 8 V r.m.s. into a 600 Qload. Its amplification has been set at x6.6

to give a sensitivity at the line inputs of150 IIIV r.m.s. for a nominal output of I Vr.m.s. (if a 'real' balance control is used).

Immediately at the output of the opampare two parallel-connected 10 J.lF film ca-pacitors, C24 arid C25• that isolate any off-ser in the preamplifier from the power am-plifier in use (remember that all oparnpsin the signal path are oe coupled when thetone control is not in circuit). The layoutof the printed-circuir board allows the useof cither MKT or MKP types.

The output contains a relay that providesadelayatswitch-on ro suppress any switch-ing noises in the power amplifier. Thisrelay may be switched off if listening viaheadphones only is required. Thedelay cir-euit is based on T9. When the supply isswitched on, capaciror Csg is charged slowlyvia resistor R9), so that ir takes a !ittle timebe fore the base-emirter juncrion voltagehasreachecl the value at which the transistorswirches on. When the supply voltage is re-moved, C78 discharges fairly rapidly via D,and the relay is deenergized virrually in-stantly. Thecircuit has its own rectifier net-work, R9S-R96-C91-DIO-DII, that makesrapid switch-off possible.

The headphone arnplifier consists of aType 5532, ICS", and a discrete cornple-mentary outpur stage, T) and T4. Threediodes, D I-D), ensure correct class-A op-erurion. The amplifier deli vers sufficientpower for driving low-impedance and iJ1-sensitive headphones (with the exceptionof elecrro-static olles that often require areal power arnplifier). The 100 Q resistorin the outpur limits the rnaxirnum current,which can rise to 100 mA with an outpurimpedance of 8 n. The amplification ofthe stage has been set to a value at whichclipping of the line outpur voltage of I Vjust does not occur.

The supply of the outpur stages, lCs,T 3. T4, T7, and T8, has its own decoupl.ing,provided by R97, R9S, ClOO, and CIOI> toprevent any feedback to the other opampsin the amplifier when the outpur current ishigh.

The power supply of the preamplifieris simplicity itself: a bridge rectifier, BI,decoupled by capacitors C79-CS2, buffercapacitors C84 and C88, and two regulatorICs, lC9 and ICIo. Note ihar each IC inthe preamplifier has additional supply de-coupling in the form of an electrolytic anda cerarnic capacitor,

(10 be continuedi

ELEKTOR ELECTRONICS JANUARY 1991

THE COMPLETE PREAMPLIFIER (2) by T. Giffard

ollowing the circuit description in our E anuary 1991 issue, we will now focus our attention on the practical construction of the preamplifier. The busboard is shown in Fig. 5 and 6 and the motherboard in Fig. 7 and U.

The bushoard contains all thecomponents shown in Fig. 2 and the SOURCE SELECT and RECORD SELECT switches. These switches are fitted with extension spindles toenable them to be controlled from the front panel.

The motherboard contains the compo- nents shown in Fig. 3 with the exception of the controls, which are fitted on the front ~ane1 .B~ virtueofitsdouble-sidedand thmugh- plated design, this board will be fairly easy to populate.

Most of the work lies in preparing the en- closure and connecting the controls to the motherboard.

Choice of components ~ i g . 4. General view of the preamplifier.

The quality of any preamplifier depends to a large degree on the opamps used and the type of capacitors in the signal paths.

In the present design, there is only one capacitor in the signal path: at the output. v

The board has space for two large parallel- connected types (C24-C25and C56-C57).These ' -

should preferably beMKP types, but at these values they are fairly expensive. Second choice is MKT types.

There are also some capacitors at the out- Fig. 5. Populated busboard.

Finally, the tone control circuit also con- tains a couple of capacitors which is virtu- ally unavoidable. Since this circuit in any case degrades the quality to some degree, the quality of the capacitors used in it is hardly of any consequence.

Thereis a widechoiceof suitableopamps. The quality of the preamplifier is, however, not necessarily indirect proportion to the price of the opamps chosen. I t is rather a matter of choosing the opamps on their merits and to individual requirements. We shall look at each of them in turn.

IC1. In this position a low-noise type with smalloffsetvoltageispreferred.The Signetics 5532 is a good and economical choice. A slightly faster version is the 5535, while the SM2132iseven faster. NationalSemimnductor's LM833 is, as far as its specification is con- cerned, almost identical to the 5532. More expensive,butfast,isAnalogDevices'AD712 (which has the added advantage of FET in- puts).Then thereis theOP270(a doubleOf27) or the OP249, which has a high slew rate and FET inputs. In the Burr-Brown family there are theOPA2107andOPA2604, both with FET

Sensitivity dynamic PU 2.4 mV r.m.s. line inputs 150 mV r.m.s.

Input impedance 47 kn Output voltage

nominal 1 V r.m.s. maximum 9.5 V r.m.:

Output impedance < l 0 Q Bandwidth

dynamic PU 20 Hz-20kHz53dB line inputs 5 Hz-I MHzM.5 dB (load = 10 k n )

Signallnoise ratio dynamic PU >X0 dB (linear) line inputs >l00 dB (linear) (inputs shorted)

Channel separation >l00 dB (1 kHz) >70 dB (20 kHz)

Harmonic distortion 4.003% (line in, U, = 1 V) (20 Hz-20 kHz)

Intermod. distortion 4 . 0 5 % (line in, U, = 1 V) (2.50 Hz14 kHz. 4:l)

Resistors: Ri,R~,R~,R7,Rq,Rli.Ri~1Rzi1Rz3.Rz5

1 R,,.R, = see tevt (Pan I) R ~ , R ~ , R ~ , R ~ . R ~ o ~ R ~ z ~ ~ z o ~ ~ ~ ~ . ~ z ~ . R x . R ~ ~ , R ~ o = 47 kS1 Ri3,Ri4,R3i,R32 = 100 R Ris1Ri6.R33.R34 = 47 Q

Ri7.Ria,R35.R36 = 1 MQ

Capacttors: C1.C3.C5,C, = 47 nF, ceramic

4.~6,;Ca = 47 )IF, 25 V l C r C Semiconductors: IC1= NE5532 ICz = AD172 (NE5532 - see text)

Miscellaneous: Sl.S2 = 2-pole. 6-position rote

switch for PCB mounting K,-KZo = 3.5 mm audio chassis

socket PCB Type 890169-1

THE COMPLETE PREAMPLIFIER l21 m

Flg. 6. Printedcircuit busboard. Note that these drawings are reproduced here at 71% of true slze.

ELEKTOR ELECTRONICS MARCH 1991

p,.:?. -. . - /L a I * M= L U

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O "-a Fig. 7. Prlntedclrcult motherboard. Note that thesedrawings are reproduced here at 71% of bue size.

ELEKTOR ELECTRONICS MARCH 1991

THE COMPLETE PREAMPLIFIER 121 m

Fig. 8. Populated motherboard.

Resistors: R40.Ral.R43.R4a.R63.R5.~.Ri~.R5g.R~8~R8g =

6k8 Ro2 = 3k3 R45.R6o.Rfii,Rio.R~5,9ss = 12 k R ~ B . R J ~ . R S ~ . R ~ I . R , ~ , R ~ ~ = 1 M R48;R49.R73.R:d = 2kZ R50.R775 = 18 k R52.R77 = 1 k, 1% R5?,R78 =5k62. 1% Rsa.Rig = l 0 k RS5,RRo = 47 S1 R56,R81 = 88k7, 146 RS7,Ra2 = 7k5, 1% R5B,R83 = 154 51, 1% RS9,RR4 = 47k5. 1% R~z.R8i = 1 k R65,R66,R9o,Rg> = 27 R R S ~ . R ~ Z , R ~ ~ , R ~ ~ . R ~ O O = 100 0 Rg3 = 47 k Rg5 = 68 Cl, '12 W Rg6 = 2k7 Rg7.Rg8 = 4R7 P, = 10 k log. stereo potenliometer P2 = 10 k balance control PS = 22 k (or 25 k) potentiometer P4 = 10 k linear potentiometer P5.P6 = 10 k preset potenliomeler

Capacitors: Cia~C12~C20~C22.C33C37.C52.c5454c555c699

C79,C~0,C81,C82,C96-Cgg = 47 nF ceramic C17 .Cl3.C21.C%.C32.C36.C53,C55,C6464c688

C9&gs = 47 pF. 25 V. radial C14.C46 = l 0 pF, 40 V, bipolar, radial ci~~c16~ci7~c35~c39~c43.c477c48Bcd9~c67~

C71.C75.Ce3,Css.Cs7.C~ = l00 nF C18,C50 = In8 Clg.Cs, = ln2 C~a,C25.Cs6.C57 = 10 pF. MKP or MKT C26.C27,C58.C59 = 18 nF, 1%. MKT C28,C6C = 5n6, 1%. MKT

ELEKTOR ELECTRONICS MARCH 1991

inputs. An inexpensive but good choice is theTL072, which has FETinputs butaratlier- higher noise figure.

IC2. The same considerations as for IC1 apply, but in this position it is even more im- portant to have a type with a low offset volt- age, becauseall deviationsbetween input and output are amplified. In view of the switch- ingclicks and ievel changes that occur when a different inout channel is selected.ooamos

. L 8

with FET inputs are preferred. Both condi- tionsaremet by theAD712.OP249.OP.42107 and OP.42604. The least expensive choice is the TL072 (hut remember its noise figure).

IC3. In this, the mono count, position a low-noiseopampisessential: theOP27,55.UA and SSM21.U are suitable. The last two tvpes are not unity-gain stable, however, which makes it necessary to fit a 22 pF capacitor between pins 5 and 8 at the undersideof the motherboard. FET inputs are not necessary. Again, a TL072 may be used where econ-

CdO.Cj:, = 47 pF, polystyrene Cal.Cx = 100 pF polystyrene C42.C74 = 100 !IF l0 V, radial C44.Cd5,C76.C:7 = 150 nF C78,C91 = 220 (IF. 25 V. radial CR4,CR8 = 2200 }[F, 40 V, radial Cloo.Clot = l00 (rF. 25 V. radial C102.Ci03 = 150 PF, POly!

Semiconductors: D1-D4.D6-Dg = 1 N4148 DS.D~~.D1, = IN4001

IC3.1C4,1C5 = OP27 IC6.1C7 = LTlOZBCNB

ICli.IC12 = NE5534

Miscellaneous: S3,S4 = DPST miniature toggle switch S5.S6 = DPCO miniature toggle switch S7 = SPST miniature toggle switch Rel,Re2 = 12 V relay, 1 CO contact 1 off 6 mm stereo audio socket 2 off heat sink for ICg and lCio 1 off 3-way PCB-type terminal block 1 off DPST mains press button switch 1 off fuse holder with 315 mA slow fuse 1 off strain reilef sleeve

omy is a must. IC4 and IC5. The same considerations as

for IC3 applv: remember the 22 pF capaci- tors if a 5534A or SSm2134 is used.

IC6and 1C7. Becauseverylownoisein the dymnucpick-upcircuit isimperative,a Linear Technology Type LT1028 would he ideally suitable in these positions, were it not for a tendency to become unstable (for reasons we have not been able to ascertain) and, therefo~, thesamemanufach1rrrs'TypeLTlll5 is preferred. This type hasvirtually thesame specificationas the 1028,butdoesnotbecome unstable. A lessexpensivechoiceis theOP27, while the 5534A and SSM2134 would also be suitable. These last two types require, apart from the22pFcapacitormentioned ear- lier, a change of value to 100 kR for presets P5 and P6.

ICE. In this, the headphone amplifier, po- sition, very low noise is a prime require- ment, and this is met by the Signetics Type 5532 or 5535, an SSM2132, an LM833 or an OP27. If the preamplifier will be required to deliver regularly fairly large output volt- ages, i.e, larger than the nominal 1 V r.m.s., a type with FET inputs should be used in this position. The inputs of the 5532, for in- stance, are provided with protection diodes that may cause small d.c. voltages at the line output if the amplifier is overdriven.

ICl l and IC12.Theprimereason forusing a 5534 in this position is its facility to pro- vide sufficient current to low impedances (<h00 Q), but the SSM2134 is also suitable. MostopampswithFETinputsdonot perform well with such low impedances. If the load on thepreamplifierisneverlikely tobebelow 2 kR, an AD711 or TL071 may also be used.

Construction It is better not to use IC sockets: we believe that each extra point of contact, and this a p plies particularly to IC sockets, degrades the quality of any audio circuit. Our preference is to solder ICs on to the board, although we admit that sockets enable the quick and easy exchanging of ICs to compare their perfor- mance.

AUDIO 8 HI-FI

Fig. 9. Wiring diagram.

ELEKTOR ELECTRONICS MARCH 1991

THE COMPLETE PREAMPLlFlER [21 6

893159F

Fig. 10. Front panel foil (available through our Readers' services).

890169

Fig. 11. Tone control characteristics.

Populating the boards should not give any difficulties.Fitsolderpinsatallinputand output points: this will make the wiringlater on that much easier. On the motherboard solder a wire bridge between J and K and between L and M if you do not intend to use the dynamic pick-up amplifier. Wire bridges are also required between A and B and be- tween E and F.

Note that both voltage regulators need an individual heat sink.

Oncetheboards havebeencompleted,they can be fitted in the enclosure when this has been prepared. As shown in the wiring dia- gram in Fig.9, the motherboard fits into the left-hand side of the enclosure to enable the extension spindles of the rotary switches to just clear the heat sinks of the voltage regu- lators. At the right-hand side there is space for the mains transformers.

ThelayoutofthefrontpanelseeFig. l& determines the locationof the busboard at the back of the enclosure. Depending on the di- mensions of the enclosure, a mains input plug with integral fuse holder or a mains cable entry with strain relief may be used. In the last case, a fuse holder must be fitted within the enclosure, preferably near the

board (Fig. 6) may be used as templates for drilling the required holes in the front and rear panels respectively.

Once theenclosureisready, the boardsand the various controls can be fitted. If Bourns potentiometersareused,makesure that these haveh.3 mmspindlesand nottheusual6mm ones.

Finally, the inter-wiring can be carried out. Take good care to insulate all mains- voltagecarryingparts. Msignalwiresshould be~creened~buttheir diametershouldnotex- ceed 5 mm to prevent difficulties in solder- ing them to the motherboard where they are very close together.

The wiring between bus- and mother- board should be carried out first, followed by that between the boards and thecontrols on thefrontpaneland finally thepowerlines.

Although the screen of each and every signal line is connected to a separate solder pin,only thoseat thevolumeand balancecon- trols are actually connected to the earth line: those of all other screened cables are left un- connected .The enclosure earth is connected to the mains earth and this is the only real earth connection of the preamplifier.

The output socket of the headphone am- transformer. plifier must be well insulated from the en-

Photocopiesof the frontpanel foil (shown closure to avoid earth loops ; \' r

"m in Fig. 10 at a scale of 3:4) and of the bus-

ELEKTOR ELECTRONICS MARCH 1991

THE COMPLETE PREAMPLIFIER [2] Im

Fig. 6. Prlnted-clrcuit busboard. Note that these drawlngs are reproduced here at 71% of true slze.

ELEKTOR ELECTRONICS MARCH 1991

AUDIO & HI·FI

,r I ..'~~ v~r ~j

~-~~= -----'

I I n_lL -rc".--'-i~i, ,

Fig. 7. Prlnted-clrcuit motherboard. Note that these drawlngs are reproduced here at 71% of true slze.

ELEKTOR ELECTRONICS MARCH 1991