by Charles Kitchin [Analog Devices Norwood, MA ]
For the owner of a modest stereo system utilizing one speaker per channel, phasing the speakers is a relatively easy task. By correctly connecting the speakers by their polarity dots or by switching the leads to one speaker and listening for the most solid bass response and best stereo centering, proper phasing is accomplished.
However, if more than one amplifier is used such as in biamp and triamp systems, speaker phasing can become a horror show. Unless the amplifiers are identical (i.e., the same phase shift in each), wiring up the speakers by polarity will not work. The problem is compounded by the electronic crossover which may, or may not, have the bass, midrange, and/or treble outputs in phase with one another. Also, microphones and audio lines are difficult to phase correctly.
Device Description
A simple and inexpensive solution to this problem is to send a coded signal through the system. It is then picked up on a portable receiver which can listen to each speaker individually and indicate its phase relative to the other speakers in the system. Such a device is the audio phase detector presented here. The coded signal is a sawtooth waveform generated by an integrated circuit voltage-to-frequency converter (a voltage-controlled oscillator), the Analog Devices AD537. The sawtooth waveform is ideal for the code because the sloping portion of the waveform, the ramp, A, is low-frequency energy for a relatively long period of time, while the rise, B, is very fast, high frequency energy for a short time (Fig. 1a). This sawtooth waveform is connected to the line input jack of an amplifier, amplified, and the speakers will reproduce the waveform into a sound wave picked up by a microphone on the receiver. The air mass between speaker and receiver acts like a capacitor and differentiates the waveform, (Fig. 2). The long, slow ramp (A) becomes a long duration, very low signal, while the fast falling portion (8) becomes a high amplitude spike. So in practice, the high amplitude spike can be detected, and its polarity will be due to the relative phasing of that particular speaker.
Fig. 2--Block diagram of the detection system.
Fig. 3--The transmitter schematic.
Circuit Operation
The heart of the transmitter is the Analog Devices AD537, a monolithic voltage-to-frequency converter. The AD537 was chosen because it is very stable over temperature variations and contains its own voltage reference which is used as the voltage input to pin #5. The AD537s are closely matched to one another, because of this the user does not need an oscilloscope or frequency meter, and no adjustments are required. Frequency of operation is determined by capacitor C, and resistor R1, (Fig. 3). The output of the AD537, pin 14, is normally a square wave, however, for this application a sawtooth waveform is desired. This is accomplished by an RC time constant of 10 mS at the output, using resistor R, and capacitor C1. The time constant is long enough, compared to the operating frequency, that the ramp of the sawtooth wave is linear. The output of the 537 is then reduced to an amplifier line input level of approximately 1 1/2 volts by resistors R, & R1. Resistor R1 also minimizes loading of the output, keeping the output waveform undistorted. Power is supplied by a 9-volt transistor radio battery.
Receiver
The receiver uses an inexpensive portable cassette recorder microphone; any dynamic microphone with a high-impedance output will also work. The microphone output is connected to J, and amplified by IC1, a 741 operational amplifier with a gain of 200 (Fig. 4). The signal then runs to a dual peak detector D1& D2, is filtered by C1, & C2, and the two d.c. signals are mixed by R, & R6. The two d.c. signals are the detected ramp and the detected spike of the sawtooth waveform that was transmitted through the speaker. The spike will be much greater in amplitude and override the smaller ramp signal.
The resistors R, & R6 also serve as the gain setting resistors for IC2 in conjunction with resistor R,. The resulting d.c. level is amplified by IC2 which has a gain of 10 and runs to voltage divider R1 & R1. Resistor R. is an overall gain adjustment to make up for the output level variations in microphones. The meter is a miniature 50-0-50 d.c.microammeter with a 1 3/4 inch square outside diameter.
Power for the receiver is also a 9-volt transistor radio battery. Resistors R, & R10 convert the 9-volt battery output into ± 4.5 volts to ground for powering the operational amplifiers. Capacitors C, & C. assure a low a.c. impedance to ground. Jack J2 is a differentiator input for inputs from audio lines, amplifier outputs, etc. Capacitor C6 differentiates the input signal, instead of using an air mass.
Construction
Both the transmitter and receiver were built on small breadboards and placed in 26/6 x 4 inch miniboxes. The best boxes to use are the Model 3301 miniboxes made by Pomona Electronics Co., which can stand up to heavy duty and look professional. The power switch for the transmitter may be located at any convenient point on the minibox. On the receiver, the meter should be located near the front of the top side of the minibox. The microphone is attached to the bottom side, its front extending out about an inch from the front of the minibox.
The microphone can be attached to the bottom of the box by stick-on tie wrap anchors and tin wraps. The breadboarded circuitry for the transmitter and receiver can be placed in the miniboxes on standoffs. The cost is under $30.00.
Calibration and Use
Connect the output of the transmitter to the line input jack of the left channel of the stereo amplifier in the system to be phased. If more than one amplifier is used in the system, connect the output of the transmitter to the high level input of the preamplifier. Set the tone controls to Flat position, and turn off the loudness contour control. Turn on the transmitter, and adjust the output of the speakers to a fairly loud but comfortable level. Turn on the receiver and hold it so that its microphone is 1/2 to 3 inches from the speaker you wish to phase first. Adjust the trimpot, R. in the receiver so that the meter registers a strong indication to either the left or right of center. The direction the meter travels will depend on the relative phase of that particular speaker. Next, check all the other speakers in the left channel. They should all indicate the same direction. If any are different, reverse their leads until all the speakers in that channel cause the meter to deflect in the same direction.
After the left channel has been phased, connect the output of the transmitter to the right channel and phase all the speakers in that channel so that they deflect the receiver's meter in the same direction as the left channel. When you are finished, all speakers in the system will be in phase.
Fig. 4--The receiver schematic.
It must be pointed out that proper operation of the phase detector is dependent on the sawtooth waveform from the transmitter arriving undistorted at the speaker terminals.
Tone controls and other frequency shaping devices must be set to their Flat position. Loudness controls should be turned off. Electronic crossover networks should be set to flat or bypassed. If the preamplifier system can not be set for flat response or if it has no unequalized inputs, phasing should be performed by feeding the transmitter output directly into the power amplifiers.
To phase microphones, simply unplug the microphone from J, and substitute the microphone to be phased. They can be plugged in one at a time and held 1/2 to 3 inches from the speaker. They should all deflect the meter in the same direction if correctly phased. The second jack, J,, is for phasing amplifiers, lines, and various other audio gear. The jack accepts a line level of from 0.5 to 1.5 volts.
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PARTS LIST
Transmitter
C1-10,000 pF capacitor, mica or polystyrene
C2-1 µF,12 V tantalum capacitor
C3-0.1 µF paper capacitor
IC-AD537J VHF converter, manufactured by Analog Devices
J1-RCA phono jack
R1-91 kilohm, 5% carbon resistor
R2-10 kilohm, 5%, 1/4 watt carbon resistor
R3-100 kilohm, % watt 5% carbon resistor
R4-20 kilohm potentiometer 1-14-pin dip socket 1-9V transistor radio battery 1-Battery clip 1-2% x4x1 V2 inch minibox, Pomona Electronics, Model 3301 1-Single-pole, single-throw On/Off switch, Microswitch 8C1021 4-Small rubber feet
Receiver
C1, C2, C3, & C4-1 µF, 10V tantalum capacitor
C5-100µF tantalum capacitor
C6-20 pF mica capacitor
D,, D,IN4148 diodes
IC, IC,-AD741 J operational amplifier
J1, J,-RCA phono jack
R,-100 ohm, 5%, 1/4 watt carbon resistor
R2-20 kilohm, 5%, 1/4 watt carbon resistor .
R3, R6, R,-1 kilohm, 5%, 1/4 watt carbon resistor
R4-10 kilohm, 5%, 1/4 watt carbon resistor R5-50 kilohm, 5%, 1/4 watt carbon resistor R6-10 kilohm trimpot R9, R,0-5 kilohm 5%, 1/4 watt carbon resistor 1-50-0-50 d.c. microammeter, International Model 163 2-8-pin TO-99 sockets 1-9V transistor radio battery 1 Battery clip 1-2%x4x 1 1/2 inch minibox-Pomona Electronics Model 3301 1-Single pole, single throw On/off switch, Microswitch 8C1021 1 Inexpensive dynamic microphone Tie wraps and tie wrap anchors
Note:
The three ICs may be purchased by ordering: One AD537JD @ $13.00; two AD741CH @ $3.00; postage and handling, $1.00, for a total of $17.00. Send prepaid by check or money order to: Order Processing, Analog Devices, Inc., Rt. 1 Industrial Park, Norwood, MA 02062.
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Also see:
TDS (time delay spectrometry) in Sound Measurements (Jan. 1978)
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