A computer is very suitable for making (audio) measurements thanks to the sound card that is usually built in. Unfortunately, the audio input on laptops is usually too sensitive to measure somewhat larger AC voltages. A small amplifier/attenuator circuit then comes in very handy.
DIY Audio Equipment: Multimeter vs. Oscilloscope
When working on audio equipment, an oscilloscope isn’t always necessary. Basic direct current or voltage measurements can be accomplished using a multimeter. If you possess a more advanced multimeter capable of measuring small AC voltages, you can expand your testing capabilities. For intricate tasks like analyzing frequency response or distortion, utilizing a computer’s sound card in combination with specialized software proves highly advantageous. Using a laptop or notebook without an AC outlet can prevent issues like earth loops and hum during measurements, although compensating for the oversensitivity of the input often requires creating voltage dividers for accurate measurements.
Designing a Specialized Measurement Amplifier
This measurement amplifier is tailored for situations where precise audio measurements are crucial. It features an adjustable input attenuation and a 1 MΩ input impedance, allowing the use of standard scope probes with built-in attenuators for larger AC voltages. The input signal is initially attenuated and then amplified to achieve the desired transfer function. DC coupling is maintained in the input. The signal undergoes initial attenuation by a factor of at least 10, and logarithmic potentiometers P1 and P2 enable further attenuation.
Capacitors C1 and C2 offer DC decoupling after the input attenuators, preventing large time-constants caused by high-impedance probes. The amplification stage, built around IC1.A and IC1.B, features potentiometers P3 and P4 to adjust the gain between 1x and 100x. The choice of opamp is critical; the TS922, with a Gain Bandwidth Product (GBP) of 4 MHz, proved optimal for handling the entire audio bandwidth at a gain of 100x, as demonstrated in the Elektor Labs prototype.
Power Supply and Protection Measures
A 9 V battery powers the amplifier, its voltage divided into negative and positive components with a Ground in between. LED2 serves as a LowBattery indicator, lighting up when the battery voltage is above 7 V, signaling the need for replacement or recharge. Diodes protect the opamp inputs against high voltages or electrostatic discharges commonly found in (switched off) audio circuits, especially those using valves. The project includes a dedicated printed circuit board, accommodating all components, connectors, and potentiometers. Standard through-hole components simplify construction, with potentiometers mounted through the solder side of the board and secured into place before soldering. Calibrating logarithmic input potentiometers post-mounting ensures precision, allowing for accurate measurements during the calibration process.