Loudspeaker as a Microphone

This circuits allows you to use a cheap loudspeaker as a microphone. Sound waves reaching the speaker cone cause fluctuations in the voice coil. The voice coil moving in the speakers magnetic field will produce a small electrical signal.

The circuit is designed to be used with an operating voltage between 6 and 12 volts dc. The first transistor operates in common base mode. This has the advantage of matching the low input impedance of the speaker to the common base stage, and secondly has a high voltage gain. The second stage is direct coupled and operates in emitter follower. Voltage gain is slightly less than unity, but output impedance is low, and will drive long cables.

Speech quality is not as good compared to an ordinary or ECM microphone, but quite acceptable results can be obtained. Speaker cones with diameters of 1 inch to 3 inches may be used. Speaker impedance may be 4 ohm to 64 ohm. The 8.2 ohm resistor value may be changed to match the actual speakers own impedance.

Stereo balance tester

Description

The meter will show volume and tone control balance between left and right stereo amplifiers. For maximum convenience the meter is a zero-center type. Resistors are five percent or better and the diodes a matched pair. Optimum stereo level and phase balance occurs for matched speakers when the meter indicates zero. If the meter indicates either side of zero, the levels are not matched or the wires are incorrectly phased. Check phasing by making certain the meter leads are connected to the amplifier hot terminals and the common leads go to ground.

Circuit Diagram

Stereo balance tester  

Stereo Circuit Balance Meter

Description

When L R signals are equal, no output is present from Ul, and pin 6 is at a steady 4.5 V. Unbalanced audio causes the LEDs to vary in brightness,which causes a difference that corresponds to unbalance between channels. 

Circuit Diagram

Amplifier circuit with Tone Control

Description

A Motorola TCA5500 or TCA5550 can provide a stereo preamplifier system with tone controls. This circuit provides a gain of about 10X, a 14-dB tone-control range, a 75-dB volume control range, and it can operate from 8 to 18 Vdc. IC2 provides 15 V for ICl, and the input of IC2 can be supplied from the power amplifier`s power supply (+) rail. Dl and R5 should be used if over 30 V input will be used.

Circuit Diagram

Stereo balance meter 2

Description

    Outputs from each channel are fed to the two inputs of ICl connected as a differential amplifier. IC2 and 3 are driven by the output of ICl. Output of ICl is connected to the noninverting inputs of IC2 and 3. If the output of ICl approaches the supply rail, the outputs of ICs 2 and 3 will also go high, illuminating LED3. This would happen if the right channel were dominating. If the left channel was dominant, the outputs of ICs 2 and 3 would be low, illuminating LED1. If the two channels are equal in amplitude, the outputs of ICs 2 and 3 would be high and low respectively, lighting up LED2. 

Circuit Diagram

 Stereo balance meter 2

Stereo balance meter

Description

Play any stereo disc or tape and then set the amplifier to mono. Adjust left and right channel balance until meter Ml indicates zero; then the left and right output level are identical.

Circuit Diagram

Stereo balance meter

Guitar treble boost

Description

Ql is connected as an emitter follower in order to present a high input impedance to the guitar. C2, being a relatively low capacitance, cuts out most of the bass, and C3 with RV1 acts as a simple tone control to cut the treble, and hence the amount of treble boost can be altered.Q2 is a simple preamp to recover signal losses in C2, C3, and RV1.

Circuit Diagram

Guitar treble boost