mPHASE CHECK FOR AUDIO

SYSTEMS

While setting up and connecting audio equipment it is important tohave all the units — microphone, loudspeakers and everything in

between — 'in phase', that is, interconnected with the right polarity.The low-cost instrument described here is particularly handy for

checking out the phase of almost any audio system, whetherinstalled in a living room, in a car, in a studio, or on a stage.

K. Orlowski

REVERSED phase connections in anaudio equipment system give strange

and unpredictable effects such as the un-wanted attenuation or boosting of a particu-lar frequency range, jet-plane effects,whistling noises, or amplifier output powerwhich does not seem to produce any usablesound level. To avoid these problems, usethe simple instrument described here. Basedon a transmitter and a receiver with a simplegood/fault indication, the instrument willcheck out the system from the input (micro-phone or line input) right through to the out-put (loudspeaker or line output).

The transmitter supplies positive or neg-ative needle pulses, which are fed either elec-trically to an equipment input, via theline-cinch output socket, or acoustically to amicrophone, via the built-in loudspeaker.Accordingly, the receiver has an electrical(line) input and an acoustic (microphone)input.

The drawings in Fig. 1 illustrate twoways of using the transmitter and the re-ceiver for phase tests on audio equipment.

Figure la shows the set-up used to check the the right way around. The LEDs on the re-polarity of a microphone, and Fig. lb that ceiver provide a quick indication whether orused to ensure a loudspeaker is connected not the received pulses have the same polar-

a

b

JV^^»ioL !̂ \r

^,̂I

MIC LS|iLs«nr

IN ^OUT1 LINE '

Amplifier

MIC LSINK.OUT

IN ""OUT' LINE (

~V®

10 — °i i A®T ^_

Receiver

j_J A®

900114-11

Fig. 1. Application examples of the phase-check system.

ELEKTOR ELECTRONICS USA DECEMBER 199(1 ELEKTOR ELECTRONICS DECEMBER 1990

PHASE CHECK FOR A U D I O SYSTEMS

ity as the transmitted pulses. If the receiverindicates the opposite polarity of the trans-mitter, the chances are pretty high that there

a reversed signal connection somewhereTh the system.

The pulse transmitterThe needle pulses are generated by oscillatorIClA (see Fig. 2), which is built from aNAND gate with two Schmitt-trigger inpu ts.After applying the supply voltage, these in-puts take on complementary logic levels, i.e.,one is high, the other is low. Consequently,the output of the gate is logic high. CapacitorC2is charged via resistor R i , until the voltageon it reaches the high threshold voltage ofabout 5.5 V. Next, the output of the Schmitt-trigger toggles to 0, so that C2 is dischargedvia Di and R2, until the low threshold volt-age of about 3 V is reached. The NAND gatetoggles, and the charging of C2 starts again.

The above process is cyclical and resultsin a self-oscillating circuit. Since R2 is much

laller than R i , the discharge time of C2 is-much shorter than the charge time. As a re-sult, the on-off (mark-space) ratio of the out-put signal is about 2 ms/1 s, or 0.002. Mindyou, 'off means 'logic high' here since weare dealing with a NAND gate.

The oscillator output signal is fed to twosub-circuits. One is a small loudspeakerdriver based on emitter follower 12. Theloudspeaker connections can be swapped byswitch contacts SK and Sid. When an oscillo-scope is connected to the loudspeaker, it in-dicates negative-going needle pulses withthe switch set to the centre position, and pos-itive-going pulses with the switch set to theupper position. Likewise, in the other signalbranch, the polarity is changed by switchingtransistor Tl from a common-emitter circuit(Sib at centre position) to a common-collec-tor circuit (Sib at centre position). Couplingcapacitor O takes the test signal to an atten-

itor that supplies output levels of 1 Vpp-„, dBV), -20 clBV and -40 dBV.

The receiverThe circuit diagram of the receiver (Fig. 3)shows that two almost identical detectors areused. The test signal is supplied to the twovoltage amplifiers T.1-T2 and T4-T5 either bythe electret microphone, or by the signalsource connected to Ki. In the latter case, thesignal is taken through a high-pass filter, R i -C3, before it arrives at a voltage limiter, Di -D2. The input source, microphone or line, isselected with switch Si. The voltage amplifi-ers are complementary circuits: Ti-T2amplifies the negative pulses, T4-T? the pos-itive pulses.

The two monostables in ICi have differ-ent networks at their trigger inputs to enablethem to respond to negative pulse edgesrTCiA) or positive pulse edges (1C I B ) . To pre-

,nt the trailing edge of a pulse triggeringthe wrong monostable, IC lA and ICiB dis-able one another when one of them is actu-ated. The monostables thus allow the circuitto determine whether a pulse starts with a

9V(

Fig. 2. Circuit diagram of the pulse transmitter.

Fig. 3. Circuit diagram of the pulse receiver. The polarity of the measured signal isindicated by two LEDs, D3 and D4.

ELEKTOR ELECTRONICS DECEMBER 199(1 EEEkTOR ELE( TRONCS USA DECEMBER 1'WO

TEST AND MEASUREMENT

Fig. 4a. Single-sided printed circuit board for the pulse transmitter.

COMPONENTS LIST

TRANSMITTER:

Resistors:1 10MQ2 3kQ31 10kQ3 1kQ

1 10001 10fJ

Capacitors:1 lO^F 63V radial1 1fiF 63V radial1 4nF7 63V radial

Semiconductors:3 1N41482 BC5601 4093

Miscellaneous:1

R1R2;R4R3R5;R6;R8R7R9R10

C1C2C3

D1;D2;D3T1;T2IC1

S13-way 4-pole rotaryswitch for PCB mounting

1 12-way 1 -pole rotary S2switch for PCB mounting

1 8-Q loudspeaker, dia. 50 mm LS11 ABS enclosure, e.g., OKWA94091261 clip for 9-V battery1 phono socket1 printed-circuit board 900114-1

Fig. 5. A look inside the completed pulse transmitter.

ELEKTOR ELECTRONCS LSA DECEMBER 1990

RECEIVER:

Resistors:4 iokn1 2kQ22 1MQ51 330kQ2 100kQ5 1kft

2 2MQ21 270kQ2 100kQ preset H

Capacitors:3 33nF1 47nF1 1nF51 10n F 63V radial2 100pF2 330nF

Semiconductors:2 1N41481 red LED1 green LED4 BC550B2 BC560B1 4528

Miscellaneous:1 electret microphone Mic11 phono socket K11 miniature SPOT switch S11 miniature SPST switch S21 clip for 9V PP3 battery1 ABS enclosure, e.g., OKWA94091261 printed-circuit board 900114-2

R1;R3;R8;R16R2R4;R13R5R6;R15R7;R10;R11;R14;R18R9;R17R12P1;P2

C1;C5;C6C2C3C4C7;C9C8;C10

D1;D2D3D4T1 ;T2;T3;T6T4;T51C1

ELEKTOR ELECTRONICS DECEMBER 1990

PHASK CHECK FOR Al 'DIO SYSTEMS

,g. 4b. Single-sided printed-circuit board for the pulse receiver.

positive (rising) or a negative (falling) edge.The two LEDs, D3 and D4, indicate the re-specti\-e polarities. The monostable times areset at about 0.5 s with RL>-Cs and Ri7-Cio.This causes the active LED to flicker.

Building and testingThe receiver and the transmitter are bestbuilt on the printed-circuit boards shown inFig. 4. Be sure to fit all polarized components(electrolytic capacitors, ICs, transistors anddiodes) the right way around. Also makesure that the two rotary switches on thetransmitter PCB are fitted as shown on theoverlay (note the '!' mark, and the lettersthat indicate the poles). On completion of thetwo units, apply the self-adhesive foilsshown in Fig. 6 to the enclosure front panels.

Interconnect the transmitter and the re-ceiver via their line sockets, and check thatthe LED indication on the receiver is in ac-cordance with the polarity set on the trans-mitter. When the LEDs remain off, ICi in thereceiver may not have sufficient gain. In thatcase, adjust Pi and P2 until the receiver doestrigger correctly. •

o o

0°0°0

o o

900114-F2 900114-F1

Fig. 6. Design of the self-adhesive front panel foil for the transmitter (left) and the receiver (right).

ELEKTOR ELECTRONICS DECEMBER 1990 EEEKTOR El.ECTRONCS I SA DECEMBER 1990