Sound Pressure Level Meter

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Suitable for home-cinema setup

9 Volt operated portable unit

Circuit diagram:

Parts:

R1______________10K  1/4W Resistor
R2,R3___________22K  1/4W Resistors
R4_____________100K  1/4W Resistor
R5_____________100R  1/4W Resistor

C1_______________1µF  63V Polyester or Electrolytic Capacitor
C2_____________100µF  25V Electrolytic Capacitor
C3_____________220µF  25V Electrolytic Capacitor

D1-D4_________BAT46  100V 150mA Schottky-barrier Diodes

IC1__________CA3140  Op-Amp IC

MIC___________Miniature electret microphone (See Notes)

J1,J2___________4mm  Output sockets

SW1____________SPST  Toggle or Slider Switch

B1______________9V   PP3 Battery

Clip for PP3 Battery

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Comments:

This project was requested by some correspondents wishing to setup their home-cinema set adjusting all the loudspeaker outputs to the same level when heard from the listening position.
In practice this device is a simple (though linear and precise) ac millivoltmeter, using an existing multimeter set to 50 or 100µA fsd with the probes connected to J1 and J2 to read the results.
The precision of the measure is entirely depending on the frequency response of the microphone used but, fortunately, for the main purpose of this circuit an absolutely flat response is not required. Therefore, a cheap miniature electret microphone can be used.

Use:

The amplifiers driving the loudspeakers must be fed, one at a time, with a sine wave in the 400Hz - 1KHz range, but different values can also be chosen. For this purpose you can use a simple signal generator circuit like one of those available on this site, namely: 1KHz Sine wave Generator or, better still, Spot-frequency Sine wave Generator.
As an alternative, the input sine wave can be provided by a CD test track, a cassette-tape or a personal computer.
Please be careful and set the volume control very low, to avoid loudspeakers' damage. Switch-on the Sound Pressure Level Meter and increase the volume of the amplifier in order to obtain an approximate center-scale reading. Repeat the same steps with all channels.

Notes:

• If external electret (battery powered) or dynamic microphone types are used, R1 must be omitted.
• Using a good quality microphone, this circuit can be very useful in setting-up parametric equalizers or tone controls in HiFi chains etc.