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A LTHOUGH the power amplifiersdescribed last month have arespectable amount of gain, some

signals may be too weak to produce anadequate loudspeaker output without addi-tional amplification. They can also be fur-ther weakened by an excessive mismatchbetween signal source and amplifier. Tonecontrols are usually required when musicis being reproduced, and restricting thebandwidth will clarify speech signals,especially under noisy conditions.

These three issues: preamplification,impedance matching and tailoring thefrequency response, are covered in thisarticle.

Impedances

The impedances presented by the inputand output ports of transistor amplifierstages are extremely variable. Load andbias resistors exert a major influence, as dothe gain of the transistor and its emittercurrent. Negative feedback can either raiseor lower impedance and, to further confusethe issue, the load connected across oneport influences the impedance presentedby the other.

The impedance figures quoted are,therefore, intended as no more than aguide when selecting the best circuit for aparticular application.

BiasingTransistor amplifier stages are usually

biased so that the output (collector or emit-ter; drain or source) rests at half the supplyvoltage under no-signal conditions. Thisenables the stage to deliver the greatestpossible signal swing; i.e. the highest out-put, before the onset of clipping.

Transistor gain (hfe), and supply voltage,affect the biasing. However, over a widerange of hfe values (at least 200 to 600),and supply voltages from +9V to +12V, thecircuits described here will deliver a lowdistortion output that is more than

sufficient to fully drive the power ampli-fiers described last month.

Experimenters who require the stages tohave the highest possible signal-handlingcapability for a given supply voltage mayhave to adjust the bias resistors. Guidanceon this is given later.

CascadingThe various preamplifiers, tone controls

and filters can be combined to suit individ-ual requirements. Blocking capacitorshave been provided at the inputs and out-puts, and the units can be used safely withany equipment.

Cascading makes one of these capaci-tors redundant. Similarly, when they areconnected to the power amp described lastmonth, the output blocking capacitor canbe omitted (C1 on the power amplifierp.c.b. duplicates this component).

DecouplingAll of the preamplifier circuits are

decoupled from the power supply by aresistor and capacitor. Failure to includethese components will almost certainlyresult in motor boating (low frequencyinstability).

The main cause of this instability is thewide swing in power amplifier currentdrain: even with small units this can rangefrom 10mA to 150mA. These signal-induced current swings cause variations inthe voltage of dry batteries or badly regu-lated mains power supplies. When highgain preamplifiers share the same supplyrail, the resulting feedback causes low-fre-quency oscillation.

If problems are encountered, increase thevalue of the decoupling resistor, or capacitor,or both, by a factor of ten. A capacitor of2000F or more, connected across a dry bat-tery power supply, will also help to eliminateinstability at high volume levels.

R.F. InterferenceThe single transistor preamplifiers

described here have an extended high

418 Everyday Practical Electronics, June 2002

Part 2 – Preamplifiers, Tone Controls and Filters

Four single-transistor preamplifiers (left-to-right). Low Impedance MediumImpedance High Impedance F.E.T. High Impedance.

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frequency response, and problems with r.f.interference may be encountered.Connecting a low value ceramic capacitorbetween the input (emitter or base) and the0V rail will cure the problem, and theaccompanying printed circuit board(p.c.b.) makes provision for this.

In many cases, all that is required is theadditional gain and/or impedance match-ing afforded by a single transistor stage.Four circuits will now be considered.

It is convenient, with simple intercomunits, to make the speaker double up as amicrophone. Voice coil impedance andoutput are very low: a few ohms and lessthan 1mV at a close speaking distance.Transformers are often used to increase theimpedance and voltage of this signal

The printed circuit board component

layout, wiring details and full-size copperfoil master pattern are shown in Fig.2. Thisboard is available from the EPE PCBService, code 349 (Single Trans.).

Before commencing assembly, checkthe component, construction and intercon-nection notes at the end of the article.

Readers wishing to operate the stage

from lower supply voltages should checkthe voltage on the collector (c) of transis-tor TR1 under no-signal conditions. If it ismuch more than half the supply voltage,reduce the value of resistor R3 to increasethe bias current. With 3V on the supplyrail, R3 will need reducing to around 6·8kilohms and, with a 6V supply, its valuewill be in the region of 12k.

Because of its very low input impedance,the circuit of Fig.1 is not prone to capacita-tive hum pick up, and the input lead can be

Everyday Practical Electronics, June 2002 419

source, but a transistor can be made to dothe job just as well.

The “grounded base’’ stage illustratedin Fig.1 has an input impedance ofaround 50 ohms, an output impedanceroughly equal to the collector load resis-tance (R2) of 10 kilohms, and a voltagegain of around 100. Although more com-monly encountered at the front-end of aradio receiver, this configuration is suit-able for matching low source impedancesto the power amplifier and, at the sametime, providing a useful amount ofvoltage gain.

In the circuit diagram for the Low InputImpedance Preamplifier shown in Fig.1,C1 is a d.c. blocking capacitor, R1 and R2are the input and output load resistors, andresistors R3 and R4 bias the transistor. Thebase (b) is grounded at audio frequenciesby capacitor C3.

Supply line decoupling is effected byC4 and R5, and C2 is the output couplingand d.c. blocking capacitor.

SIGNALINPUT

SIGNALOUTPUT

SCREEN SCREEN

bc

e

++

+

+

TR1BC549C

R5100Ω

C1100µ

C347µ

C24 7µ

C4100µ

R210k

R11k

R318k

R42k2

0V

++9V

TO 12V

VOLTAGE GAIN 100 OVER AN hfe SPREAD OF 110 TO 600.CURRENT DRAIN AT 9V SUPPLY 0·75mA.

Fig.1. Circuit diagram for the single-transistor Low InputImpedance Preamplifier.

+

+

+ +

SCREENED SIGNALINPUT LEAD, OR USETIGHTLY TWISTED PAIR.(SEE TEXT) SCREENED LEAD TO

POWER AMPLIFIER

+ +9V TO 12V

TO COMMON 0V POINTON POWER SUPPLY P.C.B.

R1R

4

R3 R2 R5

C4C2C1

C3

TR1e

c

b

INPUTOUTPUT

1.7I

N (

43. 2

mm

)

2.35IN (59.7mm)

349

Fig.2. Printedcircuit boardcomponent layout,wiring and full-sizecopper foil masterpattern for the LowInput ImpedancePreamplifier.

!

Approx. CostGuidance Only ££77

LOW INPUT IMPEDANCE

ResistorsR1 1kR2 10kR3 18k (see

text)R4 2k2R5 100

All 0·25W 5% carbon film

CapacitorsC1, C4 100 radial elect. 25V

(2 off)C2 47 radial elect. 25VC3 47 radial elect. 25V

SemiconductorsTR1 BC549C npn transistor

(or similar – see text)

MiscellaneousPrinted circuit board available from

the EPE PCB Service, code 349 (SingleTrans); audio screened cable; multi-strand connecting wire; input and outputsockets, type to choice; solder pins;solder etc.

SeeSSHHOOPPTTAALLKKppaaggee

Low Input Impedance Preamplifier componentsmounted on the “single’’ p.c.b.

"" !!!!##

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tightly twisted flex rather than screenedcable. If r.f. interference problems areencountered, connect a 100nF capacitorbetween the emitter (e) of TR1 and the 0Vrail: provision is made for this on the p.c.b.

Combining this low impedance circuit(Fig.1) with the LM386N-1 or theTBA820M power amplifiers (fully describedin Part 1, last month) will produce a decentintercom unit, but more amplification isneeded for surveillance purposes. Cascadingthe grounded base stage with the mediumimpedance preamplifier described next(Fig.3) is one possible answer.

The input impedance of the singletransistor, common emitter preamplifier

illustrated in Fig.3 is approximately 1500ohms (1·5k), and the output impedanceroughly equal to the value of the load resis-tor, R2; i.e. 4700 ohms (4·7k).

Base bias resistor R1 is connected totransistor TR1 collector (c) rather than thesupply rail. The resulting d.c. negativefeedback makes the biasing more immuneto transistor gain spreads and variations insupply voltage.

Preset potentiometer VR1 acts as theemitter bias resistor. Connecting capaci-tor C2 to the slider (moving contact)enables part of it to be left un-bypassed.This introduces varying levels of negativefeedback and, with the specified transis-tor, the gain of the stage can be setbetween 10 and 160 times to suit differentapplications.

Comment has already been made aboutsupply rail decouplers, R3 and C4, andblocking capacitors, C1 and C3.

The printed circuit board component

layout, wiring details and full-size cop-per foil master pattern are shown inFig.4. This board is available from theEPE PCB Service, code 349 (SingleTrans.).

Before undertaking assembly work, seethe component, construction and inter-connection details at the end of thearticle.

Provision is made for connecting an r.f.bypass capacitor across the input. A 1nF or10nF ceramic component should be ade-quate if problems arise.

420 Everyday Practical Electronics, June 2002

SCREEN SCREEN

SIGNALINPUT

SIGNALOUTPUT

bc

e

+

+

+

+

R11M

R24k7

TR1BC549C

C247µ

C14 7µ

C34 7µ

C4100µ

0V

++9V

TO 12V

VR1470Ω

R3100Ω

Approx. CostGuidance Only ££77

MEDIUM INPUT IMPEDANCE

ResistorsR1 1MR2 4k7R3 100

All 0·25W 5% carbon film

PotentiometersVR1 470 enclosed carbon

preset

CapacitorsC1, C3 47 radial elect. 25V

(2 off)C2 47 radial elect. 25VC4 100 radial elect. 25V

SemiconductorsTR1 BC549C npn transistor

(or similar – see text)

MiscellaneousPrinted circuit board available from

the EPE PCB Service, code 349 (SingleTrans); audio screened cable; multi-strand connecting wire; input and outputsockets, type and size to choice; solderpins; solder etc.

SeeSSHHOOPPTTAALLKKppaaggee

VOLTAGE GAIN WITH VR1 SLIDER AT 0V RAIL, 8 TO 10 OVER AN hfeSPREAD OF 110 TO 600.VOLTAGE GAIN WITH SLIDER AT TR1 EMITTER, 80 TO 600 OVER AN hfeSPREAD OF 110 TO 600.CURRENT DRAIN AT 9V SUPPLY: 1·25mA.

Fig.3. Circuit diagram for the Medium Input ImpedancePreamplifier.

1.7I

N (

43. 2

mm

)

2.35IN (59.7mm)

349

Fig.4. Medium Input Impedance Preamplifier printed circuit boardcomponent layout, wiring and full-size copper foil master.

Medium Input Impedance preamplifier componentsmounted on the “single’’ p.c.b.

+

+

+

+

SCREENED SIGNALINPUT LEAD SCREENED LEAD TO

POWER AMPLIFIER

+ +9V TO 12V

TO COMMON 0V POINTON POWER SUPPLY P.C.B.

R1

R2

C4

C2

C1

C3

TR1e

c

b

R3

VR1

INPUTOUTPUTwww.ep

emag

.com

Crystal microphones and ceramicgramophone pick-ups (there are still a fewin use) require an amplifier with a highinput impedance, and a stage of this kind isuseful when the damping on a signalsource has to be kept low.

Configuring a bipolar transistor in the emit-ter-follower (common collector) mode resultsin a high input and low output impedance, anda typical High Input Impedance Preamplifiercircuit diagram is shown in Fig.5. The inputimpedance is roughly equal to the gain of thetransistor (hfe) multiplied by the value of theemitter load resistor R2.

This is, however, limited by the biasresistor R1, and the output load, whichshunts the emitter resistor. Nevertheless, ahigh gain transistor will still produce aninput impedance of about 100 kilohms.

Often the low output impedance is thesought after feature, either for matchingpurposes or for avoiding high-frequencylosses and hum pick-up when long screenedcables have to be used. Output impedance is

directly related to the impedance presentedby the signal source, and is usually in theregion of 1000 ohms. The voltage gain ofthe circuit is a little less than unity.

The printed circuit board component lay-

out, wiring details and full-size copper foilmaster pattern for the High Input ImpedancePreamplifier are shown in Fig.6. This boardis the same one used for all the single tran-sistor preamplifiers, and is available from theEPE PCB Service, code 349 (Single Trans.).See the component, construction and inter-connection notes at the end of the article.

High input impedance makes the stagevery vulnerable to hum pick up. Carefulattention must, therefore, be paid to screeningthe input leads and, possibly, the entire unit.

It is possible to obtain higher input

impedances with a bipolar transistor byapplying positive feedback from the emit-ter to the base bias network. This involvesan extra pair of resistors and a capacitor,and an alternative solution, if very high

input impedances are required, is to use afield effect transistor (f.e.t.); a devicewhich tends to introduce less noise ataudio frequencies.

!"!!A circuit diagram for a F.E.T. High Input

Impedance Preamplifier is given in Fig.7.The gate resistor R1 is tapped down to thesource resistors R2/R3 in order to improvebiasing and, hence, signal handling. By thismeans the f.e.t. develops its gate biasacross R2, and R3 drops an additional 3Vor so to fix the voltage on the source ataround half the supply voltage.

Connecting the gate resistor R1 in thisway applies a proportion of the in-phase out-put signal to its lower end, and the resultingpositive feedback, or “bootstrapping’’,increases its effective resistance, and theinput impedance of the circuit, to around6 megohms (6M).

Output impedance is independent of sig-nal source impedance. It is governed by thetransconductance (gain) of the device, andis usually of the order of 500 ohms.

Everyday Practical Electronics, June 2002 421

#"

Approx. CostGuidance Only ££77

HIGH INPUT IMPEDANCE

ResistorsR1 1MR2 4k7R3 100

All 0·25W 5% carbon film

CapacitorsC1 100n polyesterC2 10 radial elect. 25VC3 100 radial elect. 25V

SemiconductorsTR1 BC549C npn transistor

(or similar – see text)

MiscellaneousPrinted circuit board available from

the EPE PCB Service, code 349 (SingleTrans); audio screened cable; multi-strand connecting wire; input and outputsockets, type to choice; solder pins;solder etc.

SeeSSHHOOPPTTAALLKKppaaggee

##""""""##$$ ""

SIGNALINPUT SIGNAL

OUTPUT

bc

e

+

+

SCREEN SCREEN

R11M

C1100n

R24k7

TR1BC549C

C210µ

R3100Ω

C3100µ

0V

++9V

TO 12V

VOLTAGE GAIN: UNITYCURRENT DRAIN AT 9V SUPPLY: 1·25mA.

Fig.5. High Input Impedance Preampli-fier circuit diagram.

+

+

SCREENED SIGNALINPUT LEAD SCREENED LEAD TO

POWER AMPLIFIER

+ +9V TO 12V

TO COMMON 0V POINTON POWER SUPPLY P.C.B.

R1

C2

C1

C3

TR1e

c

b

R2

R3

INPUTOUTPUT

Fig.6. Printed circuit board component layout, wiring and full-sizecopper foil master for the High Input Impedance Preamplifier.

High Input Impedance Preamplfier circuit board.1.

7IN

(43

. 2m

m)

2.35IN (59.7mm)

349

www.epem

ag.co

m

This is the circuit of choice when a highimpedance source has to be connected to along screened cable; e.g., a capacitor orcrystal microphone. However, f.e.t. charac-teristics vary widely, and readers wishingto use the circuit of Fig.7 should be pre-pared to adjust the value of resistor R3,over the range of 1500 to 4700 ohms, espe-cially when low supply voltages are used,in order to optimise signal handlingcapability.

Details of the printed circuit board

component layout, wiring and copper foilmaster pattern are given in Fig.8. Theboard is the single transistor version andis available from the EPE PCB Service,code 349 (Single Trans).

Before assembly, check the component,construction and interconnection details atthe end of the article.

Amplifiers introduce unwanted noiseand, as gain increases, more care has to betaken to prevent the noise becoming toointrusive. The noise generated by a bipolartransistor can be reduced by operating it ata low collector current, typically between10A and 50A. This technique has beenadopted for the first stage of the directly-coupled, two transistor, Low-NoisePreamplifier shown in Fig.9.

Overall gain is stabilised by negative feed-back applied via preset VR2. With the valueshown, gain is approximately 300. If a 47kpotentiometer is used instead, gain will bereduced to around 150, and it can be takendown to 70 or so with a 22k component.

Rotating the slider (moving contact) ofpreset VR2 causes it to be progressivelybypassed by capacitor C6, increasing the

negative feedback, and reducing gain, athigh frequencies. This feature is useful forreducing noise and for correcting therecording characteristic of long playingrecords. It is usual to incorporate morecomplicated RC networks in the VR2 posi-tion for the latter purpose but, unless thelistener has a very refined ear, there will belittle or no discernible difference.

Operating conditions are stabilised by d.c.negative feedback applied via resistor R5.This, together with the high value collectorload, R3, fixes the collector current of transis-tor TR1 at around 50A with a 12V supply.

Input impedance is around 50k, but theoptimum signal source resistance for low-est noise is between 5k and 10k. This hasinfluenced the value of the input poten-tiometer, VR1.

The purpose of the remaining compo-nents will be evident from earlier circuitdescriptions. However, because of the

Approx. CostGuidance Only ££77

HIGH INPUT IMPEDANCE (F.E.T.)

ResistorsR1 2M2R2 1kR3 1k8 (see

text)R4 100

All 0·25W 5% carbon film

CapacitorsC1 100n polyesterC2 10 radial elect. 25VC3 100 radial elect. 25V

SemiconductorsTR1 2N3819 n-channel field

effect transistor (f.e.t.)

MiscellaneousPrinted circuit board available from

the EPE PCB Service, code 349 (SingleTrans); audio screened cable; multi-strand connecting wire; input and outputsockets, type to choice; solder pins;solder etc.

SeeSSHHOOPPTTAALLKKppaaggee

SIGNALINPUT

SIGNALOUTPUT

g

d

s

+

+

SCREENSCREEN

R12M2

C1100n

R31k8

R21k

TR12N3819

C210µ

R4100Ω

C3100µ

0V

++9V

TO 12V

VOLTAGE GAIN: UNITYCURRENT DRAIN AT 9V SUPPLY: 1·75mA.

Fig.7. Alternative circuit diagram for aHigh Input Impedance Preamplifierusing a field effect transistor (f.e.t.).

R4

R3

R2 +

+SCREENED SIGNALINPUT LEAD SCREENED LEAD TO

POWER AMPLIFIER

+ +9V TO 12V

TO COMMON 0V POINTON POWER SUPPLY P.C.B.

R1

C2

C1

C3

TR1s

g

d

INPUTOUTPUT

Fig.8. Printed circuit board component layout, wiring and full-size copperfoil master for the F.E.T. High Input Impedance Preamplifier.

F.E.T. High Input Impedance Preamplifier p.c.b.

422 Everyday Practical Electronics, June 2002

1.7I

N (

43. 2

mm

)

2.35IN (59.7mm)

349

www.epem

ag.co

m

higher gain, the supply line decouplingcapacitor C7 has been increased in value toensure stability.

The printed circuit board component

layout, wiring details and full-size copperfoil master pattern for the Low-NoisePreamplifier are shown in Fig.10. Thisboard is available from the EPE PCBService, code 350 (Dual Trans.).

See the general construction, componentand interconnection guide-lines on the lastpage.

Some readers may wish to use this

circuit with electret microphones whichcontain an internal line-powered f.e.t.amplifier. The load for this remote deviceis provided by resistor R1, and the supplyvoltage is reduced to around 4·5V, which isoptimum for most microphones of thiskind, by resistor R2. Decoupling is bymeans of capacitor C1.

These components (R1, R2 and C2)should only be fitted if an electret micro-phone is used, as the circuit maintains a

Approx. CostGuidance Only ££88

excluding microphone

LOW-NOISE PREAMPLIFIER

ResistorsR1* 1kR2* 10kR3, R5 220k (low-

noisemetal filmpreferred) (2 off)

R4 270R6 6k8R7 560R8 100

All 0·25W 5% carbon film, except R3 andR5.*Only required if electret mic. used

PotentiometersVR1 10k enclosed

carbon presetVR2 100k

enclosed carbonpreset

CapacitorsC1*, C4 100 radial elect. 25V

(2 off)C2 47 radial elect. 25VC3 100n polyesterC5, C8 10 radial elect. 25V

(2 off)C6 10n polyesterC7 1000 radial elect. 25V

*Only required if electret mic. used

SemiconductorsTR1, TR2 BC549C npn transistor

(or similar – see text)(2 off)

MiscellaneousPrinted circuit board available from the

EPE PCB Service, code 350 (Dual Trans);audio screened cable; multistrand connect-ing wire; input and output sockets, type tochoice; solder pins; solder etc.

SeeSSHHOOPPTTAALLKKppaaggee

Fig.10. Printed circuit board component layout, wiring and full-size copper foilmaster for the Low-Noise Two-Transistor Preamplifier.

R11k

C1100µ

R210k

C24 7µ

C3100n

VR110k

R4270Ω

R5220k

R3220k

R66k8

R7560Ω

C4100µ

C71000µ

C610n

VR2100k

C510µ

C810µ

R8100Ω

bc

e

bc

e

+ +

++

+

+

SCREEN

SCREEN

SIGNALINPUT

SIGNALOUTPUT

0V

++9V

TO 12V

TR1BC549C

TR2BC549C

**

*

*

SEE TEXT

VOLTAGE GAIN 300 OVER hfe SPREAD OF 450 TO 600. CURRENT DRAIN AT 9V SUPPLY: 1mA.

Fig.9. Circuit diagram for the Low-Noise Preamplifier. Components marked with anasterisk are only needed if an electret microphone is used. Increase the value ofR2 to 18k with 12V supplies.

Completed p.c.b. for the Low-Noise Preamplifier.

Everyday Practical Electronics, June 2002 423

SCREENED SIGNALINPUT LEAD

SCREENED LEAD TOPOWER AMPLIFIER

TO COMMON 0V POINTON POWER SUPPLY P.C.B.

350

3.4IN (87.5mm)

1.8I

N (

46. 5

mm

)

C2 C1

VR1

C3

R1

R2

R3

R6

R4

R5

R7C4

TR1

TR2

C5

C7

C6

C8VR2

R8

e

eb

bc

c

++

+

+

+

+

OUTPUT

+ +9V TO 12V

www.epem

ag.co

m

d.c. voltage on the input which could dis-turb the action of some signal sources.

This circuit, and variations of it, form thebasis of the front-ends of most high qualitypreamplifiers. With the component valuesshown, 3·3mV r.m.s. input will produce a 1Voutput before the onset of clipping.

The noise introduced by the amplifier isabout the same, or a little less, than thatgenerated by the single transistor amplifierset for a gain of 150. The noise level couldbe further reduced by using low-noise,metal film resistors for R3 and R5.

Although inductors are sometimes usedfor “tailoring’’ the frequency response, thekey components in networks which modifyaudio frequency response are normallycapacitors.

The resistance presented by a capacitor tothe flow of alternating current (a.c.) decreas-es as frequency rises. This frequency depen-dant resistance is known as reactance.

Capacitors combined with resistors formfrequency dependant potential dividerswhich can be used to tailor the response.

These RC networks can, of course, onlyattenuate signals. So called “bass boost’’ isobtained by reducing the response of thesystem to the higher audio frequencies.

Table 1 lists the reactances of a range ofstandard capacitor values, at spot frequen-cies, across the audio spectrum. Referringto it, an 0·1F (100nF) capacitor presents aresistance of 5300 ohms at a frequency of300Hz. This rises to 32000 ohms at 50Hzand falls to 320 ohms at 5kHz.

Fitting a blocking capacitor of this valueto an amplifier with an input impedance of5000 ohms will result in signal levels at300Hz being halved. (Capacitor and inputimpedance act as a potential divider). Thisattenuation will increase as the frequencyis lowered, and reduce as frequency israised, at a rate of 6dB per octave.

Fitting low value d.c. blocking capaci-tors to one or more stages will, therefore,roll-off the low frequency response.Capacitors connected from signal lines toground; e.g. across the tracks of volumecontrols, will progressively attenuate highfrequencies. Although simple, these mea-sures can make a significant improvementin clarity and signal-to-noise ratio.

Refer to Table 1 when selecting a capac-itor to give the desired roll-off with a par-ticular input impedance, then refine itsvalue by trial and error.

Capacitors are used to make gain-reduc-

ing negative feedback networks frequencydependant; for example, capacitor C6 inthe two-transistor Low-Noise Preamplifiershown in Fig.9.

Reducing the emitter bypass capacitorC2, in the single transistor preamplifiershown in Fig.3, to 4·7F, will progressive-ly increase feedback, and reduce gain, asfrequency lowers. This is another simple,but effective, way of securing low frequen-cy roll-off.

Some means of continuously varying the

frequency response is desirable whenmusic is being reproduced, and a suitableTone Control circuit diagram is given in

Fig.11. This is the medium impedancetransistor preamplifier illustrated in Fig.3with negative feedback applied, via a fre-quency dependant network, from transistorTR1 collector to base. First published by PJ Baxandall in 1952, the circuit has sincebeen used, with minor variations, in mosthigh quality preamplifiers.

Potentiometers VR1 (Bass), and VR2(Treble), control the impact of capacitors C1,C2 and C3 on the feedback network.Resistors R2 and R3 minimise interactionbetween the controls, and the circuit affords15dB of “boost’’ or cut at 100Hz and 10kHz.

The printed circuit board component

layout, wiring details and full-size copperfoil master pattern are shown in Fig.12.This board is available from the EPE PCBService, code 351 (Tone).

Before undertaking any assembly work,see the general component, construction andinterconnection notes at the end of the article.

When circuits are cascaded, the Tone

Control unit should always be the last inthe chain; i.e. the one connected to thepower amplifier. Most high quality pream-plifiers consist of the two transistor circuitillustrated in Fig.9 followed by this ToneControl circuit.

Reducing bandwidth to around 300Hz to

3kHz greatly improves the clarity of speechsignals, and the practice is adopted by tele-phone companiesaround the world.Limiting the frequencyresponse in this waysignificantly improvesthe signal-to-noiseratio. This is particular-ly desirable with sensi-tive radio equipmentand surveillance sys-tems, where the highlevel of amplificationneeded for the weakestsignals brings with it agood deal of back-ground and equipmentgenerated noise.

For best results,roll-off beyond thepass band should befairly steep: the 6dBper octave afforded bya single RC combina-tion is not sufficient.

The Bandpass Filter circuit diagram shownin Fig.13 cascades three high-pass (low fre-quency cut) sections between transistorsTR1 and TR2, and three low-pass (high fre-quency cut) sections between TR2 and TR3.By this means, a roll-off of 18dB per octaveis achieved above and below the desired fre-quency range.

Filter networks of this kind need to be fedfrom a comparatively low impedance, andfeed into a high impedance. The emitter fol-lower stages, TR2 and TR3, are thus emi-nently suitable, and amplifiers of this kindhave already been discussed. The input stage,transistor TR1, overcomes signal losses, or,with the slider of VR1 at TR1 emitter (e),ensures an overall circuit gain of around 25.

Emitter to base feedback around TR2and TR3, via the RC networks, improvesthe action of the filters. Component valueshave been selected to start the roll-off justwithin the pass band, and the response fallssteeply below 300Hz and above 3kHz.

Two capacitors have to be combined toproduce a difficult-to-obtain value. Toavoid confusion they are shown separatelyon the circuit diagram as C8 and C9.

Details of the printed circuit board com-

ponent layout, wiring and copper foil mas-ter are given in Fig.14. The Bandpass Filterboard is also available from the EPE PCBService, code 352 (Filter).

See component, construction and inter-connection notes before commencingbuilding.

424 Everyday Practical Electronics, June 2002

Table 1: Reactance, in Ohms, of standard value capacitorsat stated audio frequencies

Cap. 50 100 200 300 400 500 1 2 3 4 5 10 20F Hz Hz Hz Hz Hz Hz kHZ kHz kHz kHz kHz kHz kHz1000 32 16 08 05 – – – – – – – – –470 68 34 17 11 – – – – – – – – –100 32 16 8 5 4 32 16 – – – – – –47 68 34 17 11 85 68 34 17 11 – – – –10 320 160 80 53 40 32 16 8 53 4 32 16 –4·7 680 340 170 110 85 68 34 17 11 85 68 34 171 3k2 1k6 800 530 400 320 160 80 53 40 32 16 80·47 6k8 3k4 1k7 1k1 850 680 340 170 110 85 68 34 170·1 32k 16k 8k 5k3 4k 3k2 1k6 800 530 400 320 160 800·047 68k 34k 17k 11k 8k5 6k8 3k4 1k7 1k1 850 680 340 1700·01 320k 160k 80k 53k 40k 32k 16k 8k 5k3 4k 3k2 1k6 8000·0047 680k 340k 170k 110k 85k 68k 34k 17k 11k 8k5 6k8 3k4 1k7

Reactance values rounded off

Bandpass Filter (top) and Tone Control p.c.b.s.

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Everyday Practical Electronics, June 2002 425

Approx. CostGuidance Only ££99

excluding case

TONE CONTROL

ResistorsR1, R3,

R4, R6 4k7 (4 off)R2 27kR5 1MR7 470R8 100

All 0·25W 5% carbon film

PotentiometersVR1, VR2 100k min. rotary carbon,

linear (2 off)

CapacitorsC1, C3 2n2 polyester (2 off)C2 47n polyesterC4, C8 10 radial elect. 25V

(2 off)C5 1 radial elect. 25VC6 47 radial elect. 25VC7 100 radial elect. 25V

SemiconductorsTR1 BC549C npn transistor

(or similar – see text)

MiscellaneousPrinted circuit board available from

the EPE PCB Service, code 351 (Tone);metal case (optional), size and type tochoice – see text; audio screened cable;multistrand connecting wire; input andoutput sockets, type to choice; solderpins; solder etc.

SeeSSHHOOPPTTAALLKKppaaggee

351

C1

C1

C2

C3

C4

C5

C6C7

C8

ANCHOR PIN FORINPUT POTENTIALDIVIDER RESISTORS

2.8IN (71.0mm)

1.44

IN (

36. 5

mm

)

R1

RX

RY

R2

R3

R4

R5

R6

R7

R8

USE THIS RESISTORNETWORK TO ATTENUATEINPUT SIGNAL. (SEE TEXT)

SCREENED SIGNALINPUT LEAD

SCREENED SIGNALINPUT LEAD

SCREENED LEAD TOPOWER AMPLIFIER

TO COMMON 0V POINTON POWER SUPPLY P.C.B.

+

+

+

+

+

TR1e

c

b

VR1

VR2

BASS

TREBLE

+ +9V TO 12V

Fig.12. Tone Control printed circuit board component layout, interwiring and full-size copper foil master.The tape and CD player signal input attenuation resistors (see text) are shown in the inset diagram (left).

BASS

TREBLE

R14k7

C12n2

C32n2

R44k7

R227k

R34k7

C51µ

C410µ

R51M

R64k7

R7470Ω

C647µ

C810µ

C7100µ

R8100Ω

C247n

VR1100k

VR2100k

bc

e+

+

+

++

SCREEN

SCREEN

SIGNALINPUT

SIGNALOUTPUT

0V

++9V

TO 12V

TR1BC549C

= BOOST END OF POTENTIOMETERS (VR1, VR2) MOVING CONTACT (SLIDER).VOLTAGE GAIN UNITY WHEN VR1 AND VR2 SET AT MID TRAVEL.BOOST AND CUT ±15dB AT 100Hz AND 10kHz.CURRENT DRAIN AT 9V SUPPLY: 1·25mA.

Fig.11. Circuit diagram for the Tone Control (bass, treble boost and cut).

Tone Control printed circuit board.

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426 Everyday Practical Electronics, June 2002

BANDPASS FILTER

ResistorsR1, R5, R10 1M (3 off)R2, R6, R11 3k9 (3 off)R3 6k8R4 3k3R7 to R9 12k 1% metal film (3 off)R12 100

All 0·25W 5% carbon film, except R7 to R9

PotentiometersVR1 1k carbon preset

CapacitorsC1, C12 1 radial elect. 25V (2 off)C2 47 radial elect. 25VC3 to C5 10n polyester (5% or

better) (3 off)

C6 15n polyesterC7 22n polyesterC8* 1n polyesterC9* 470p ceramicC10 100n polyesterC11 100 radial elect. 25V

*Combined (parallel) to give 1n5

SemiconductorsTR1 to TR3 BC549C npn transistor

(or similar – see text) (3 off)

MiscellaneousPrinted circuit board available from the

EPE PCB Service, code 352 (Filter);audio screened cable; multistrand con-necting wire; input and output sockets,type to choice; solder pins; solder etc.

SeeSSHHOOPPTTAALLKKppaaggee

Approx. CostGuidance Only ££99

Bandpass Filter printed circuit board.

bc

e

bc

eb

c

e

+

+

++

SCREEN

SIGNALINPUT

SIGNALOUTPUT

0V

++9V

TO 12V

TR1BC549C

TR2BC549C

TR3BC549C

R11M

C11µ

C247µ

VR11k

R36k8

R23k9

C310n

C410n

C510n

R43k3

R51M

R712k

R63k9

C615n

C722n

R812k

R912k

C10100n

C81n

C9470p

R113k9

R101M

C121µ

C11100µ

R12100Ω

SCREEN

* *

*SEE TEXT

352

SCREENED SIGNALINPUT LEAD

SCREENED LEAD TOPOWER AMPLIFIER

+9V TO +12V

TO COMMON 0V POINTON POWER SUPPLY P.C.B.

3.84IN (97.5mm)

1.63

IN (

41. 5

mm

)

+

+

+

+

R1

C4

C2

C1 C3

TR1 TR2 TR3e e e

c c c

b b b

VR1

R2

C5

R3

R4

R6

R7

C6

C8 C9

C10

C7 C11

C12

R12

R10

R9

R8

R5

R11

INPUT OUTPUT

Fig.14. Printed circuitboard componentlayout, wiring and full-size copper foil masterfor the BandpassFilter.

VOLTAGE GAIN WITH PASSBAND, UNITY WITH VR1 SLIDER AT 0V RAIL; 25 WITH SLIDER AT TR1 EMITTER END.ROLL-OFF 18dB PER OCTAVE BELOW 30Hz AND ABOVE 3kHz. CURRENT DRAIN AT 9V SUPPLY: 4mA.

Fig.13. Circuit diagram for the Bandpass Filter for speech frequencies (300Hz - 3kHz).

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Operational amplifiers (op.amps) are

more commonly used in filters of this kindbut, when the need is simply for a unitygain buffer with a high input and low out-put impedance, the ubiquitous bipolar tran-sistor can be made to serve our purpose justas well.

Radio Receivers

The output from the detector or f.m. dis-criminator in a superhet radio receivershould fully load the power amplifiersdescribed last month. After the usual filter-ing, the signal can be fed directly to thepower amplifier, or via the Tone Controlunit shown in Fig.11 and Fig.12.

MicrophonesThe single transistor preamplifiers

shown in Fig.1 to Fig.8 will provide appro-priate matching and sufficient gain fordynamic (moving coil), electret and crystalmicrophones when they are used for inter-com purposes. (A circuit for line-poweringelectret microphones can be taken fromFig.9). The common emitter circuit givenin Fig.3 should be used with moving coilunits as these present an impedance ofaround 600 ohms.

When electret or dynamic microphonesare deployed for surveillance or “soundcapturing’’ purposes, the two transistor cir-cuit of Fig.9 will ensure a good degree ofsensitivity. Electret microphones have anextended low frequency response. If thisproves troublesome, reduce the value of thed.c. blocking capacitor C2. Try 47nF(0·047F) as a starting point.

Gramophone Pick-upsThe low output of moving-coil pick-ups

necessitates the use of the two transistorpreamplifier detailed in Fig.9. Omit presetVR1 and feed the signal to the base of tran-sistor TR1 via capacitor C3. Low outputceramic pick-ups should be connected viaa 1M (megohm) or 2M2 series resistor topreserve low frequency response.

The F.E.T. Preamplifier circuit illustrat-ed in Fig.7 is more suitable for high outputceramic and crystal pick-ups.

Personal Tape and CD PlayersAn arrangement for extracting the signal

from personal cassette players and headphoneradios is given in Fig.15. The 47 ohm resistorssubstitute for the 32 ohm earpieces, and the470 ohm resistors attenuate the signal.

Preamplification isnot required, but read-ers may wish to usethe Tone Control unitto process the signal.Provision is accord-ingly made, on theTone Control p.c.b.illustrated in Fig.12,for a signal attenuat-ing network; resistorsRx and Ry.

The chosen system

must, of course, beduplicated if stereo operation is required.Tone and Volume controls are usuallyganged, and an additional potentiometer isprovided to balance the gain of the twochannels.

With the simple circuit arrangementshown in Fig.16, the Balance potentiome-ter is connected across the ganged Volumecontrols at the inputs to the two poweramplifiers (VR1 on the power amplifiercircuit diagrams).

All of the components, for this part of

the series, are readily available from a vari-ety of sources. Transistor types are not crit-ical and almost any small-signal npndevice will function in the circuits.

A low-noise, high gain transistor will,however, ensure the best performance, andthe base connections for some alternativetypes are given in Fig.17. With Europeantransistors, the suffix “C’’ indicates thehighest gain grouping.

If possible, use transistors with an hfe ofat least 450 for the input stage of the Low-Noise Preamplifier and for the variousemitter follower stages (where high inputimpedance depends on the use of a highgain device).

All the preamplifiers covered in this

part are assembled on printed circuitboards and construction is reasonablystraightforward. Solder pins, inserted atthe lead-out points, will simplify any off-board wiring. Remember to earth themetal bodies of rotary potentiometers andto use screened audio (mic.) cable for theleads to tone and volume controls to min-imise hum pick-up.

The single transistor preamplifiers alluse the same p.c.b. and wire links arerequired. If units are cascaded, and cou-pling capacitors deleted, remember toinstall wire links to maintain the signalpath.

It may help to start construction by firstplacing and soldering in position the vari-ous wire links on the chosen preamplifierp.c.b. This should be followed by the lead-off solder pins, and then the smallest com-ponents (resistors) working up to thelargest, electrolytic capacitors and presets.Finally, the lead-off wires (including thescreened cables) should be attached to thep.c.b.

On completion, check the orientation ofelectrolytic capacitors and transistors, andexamine the board for poor connectionsand bridged tracks, before connecting thepower supply. The approximate currentdrains are included with the circuitdiagrams.

Overall voltage gain can be in excess of

2000, and care must be taken to avoid humpick-up and instability.

Hum pick-up is of two kinds, capacita-tive and inductive. High impedance circuitsare prone to the former, and low impedanceto the latter. Housing the pre- and poweramplifiers in a metal case will do much tominimise these problems.

If hum increases when a finger isbrought near to the preamplifier, the pick-up is capacitative. It can usually be curedby providing an earthed metal screenaround the input wiring or even the entirepreamplifier board.

All mains and a.c. power leads withinthe metal case of the unit must be tightlytwisted to minimise external fields, and themains transformer should be sited at least150mm (6in) from the input circuitry.Tightly twist power amplifier output leads,and keep them as far away as possible frompreamplifier inputs. Keep all leads as shortas possible.

Run a separate negative power supplyconnection from each of the p.c.b.s to acommon 0V point on the power supplyboard, or to the negative battery terminal.Do not connect one circuit board viaanother to supply negative, or rely uponscreened cable braiding or a metal case toprovide this connection. Make only oneconnection to any metal case, close to thenegative terminal on the power supplyp.c.b.

If all of the above measures have beenadopted and hum problems still persist, trydisconnecting, one by one, the screens ofthe audio cables, at one end only.Reorientating the mains transformer canalso effect a cure.

Next Month: Mains power supplies,loudspeakers and signal filtering will bediscussed.

Everyday Practical Electronics, June 2002 427

47Ω

47Ω

470Ω

470Ω

TIP

TIP

RING

RING

SHANK

SHANK

LEFTINPUT

RIGHTINPUT

OV RAIL

PERSONALEARPHONE

JACK

LINK HEREIF MONO INPUT

REQUIRED

Fig.15. Method of connecting a“Walkman’’ tape or CD player.

BC549CBC239C

BC169C2N3711

BC109C

2N3819BF244A

BF245 MPF102

e e

e

b b

b

c c

c

d d dg g gs s s

UNDERSIDE VIEWS

Fig.17. Base connections for suitabletransistors and f.e.t.s.

LEFTCHANNEL

INPUT

RIGHTCHANNEL

INPUT

GANGEDVOLUME

CONTROLS

TO POWERAMP LEFT

TO POWERAMP RIGHT

BALANCECONTROL VR1a

10k

VR1b10k

22k

+

+

Fig.16. Circuit arrangement for a stereo Balance control.

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