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.
If you build or repair audio equipment yourself, you don’t always need an oscilloscope. Any direct current or voltage can be measured with a multimeter. You can do a lot more if you happen to have a better model that can also measure (small) AC voltages. For more advanced measurements such as the frequency response or the distortion, it is very handy to use the sound card in a computer combined with some software. A laptop or notebook can also function without an AC outlet, which means you’ll avoid earth loops and hum during measurements. However, a laptop often has just an oversensitive (microphone) input, so that you need to make a range of voltage dividers for your measurements.
This measurement amplifier has been designed especially for these situations. It has an adjustable input attenuation and an input impedance of 1 MΩ, so that standard scope probes with built-in attenuators can be used for even larger AC voltages. The input signal is first attenuated and then amplified to get the required transfer function. The input is DC coupled. The input signal is attenuated by at least a factor of 10; with the help of logarithmic potentiometers P1 and P2 the signal can be attenuated further. C1 and C2 provide DC decoupling after the input attenuators to prevent irritatingly large time-constants caused by high-impedance probes. This is followed by an amplification stage (built round IC1.A and IC1.B). Potentiometers P3 and P4 are used to vary the gain of this stage between 1x en 100x. Bear in mind what the value of the GBP (Gain Bandwidth Product) is for the opamp used. The author first tried an LM258 and a TS912, which should have a GBP of about 1 MHz (typical). In practice, a bandwidth of 15 kHz was measured with the gain set to 30x and a 9 V supply voltage. This means that the GBP was 450 kHz, although that can be compensated for by the measurement software. The best opamp is a TS922 (GBP of 4 MHz), which managed the complete audio bandwidth at a gain of 100x. This is also the type that was used in the prototype built at Elektor Labs.
The power is supplied by a 9 V battery that has its voltage split into a negative and positive component with a Ground in between. LED2 functions as a LowBattery LED (it has to be a type that lights up at 2.5 V); the addition of R15 makes it light up when the battery voltage is above 7 V, which is high time to replace or recharge the battery! The inputs of the opamps are protected by diodes against very high input voltages or electrostatic discharges since you can never be sure what voltages you’ll find in (switched off) audio circuits, especially when valves are used! A printed circuit board has been designed for this project, which has room for all components, including the connectors and potentiometers. The layout can be downloaded free from the usual place . Standard through-hole components have been used throughout, which makes the construction very easy. The potentiometers are put through the solder side of the board and screwed into place, after which the solder tags are bent onto the pads on the board and subsequently soldered.
Since logarithmic input potentiometers can deviate by as much as ±20% it is best to calibrate them after they have been mounted in the box. The calibration should be carried out in steps of 10 dB (= a factor of 3.1623) using the measurement software. First mark out the calibration points on a piece of paper placed over the potentiometer, then scan this into the computer and use a drawing package to create a professionally looking scale. During the calibration, P3 and P4 should be set to a gain of 1x, which is normally only used with very small input signals that still need some amplification.