Audio Test Equipment from Audio Millivoltmeter

When performing any tests on an audio system, some form of measuring device is essential. Digital multimeters are not useful, since they will not give the true picture of what is happening, and most have a fairly limited frequency range. An oscilloscope equipment is the ideal tool for audio test, but not all hobbyists can afford the outlay for a scope, and would find justifying the not inconsiderable cost a tad difficult.

An AC millivoltmeter - calibrated in dB - with a range of 30V down to 3mV full scale (80dB range) would be extremely useful. Attach a microphone (electret mic capsules are quite good), and you have a relative audio or sound level meter, even better if you have some way of calibration. The meter presented here has a very wide frequency range, and uses a switched attenuator for audio range adjustment. The attenuator uses the 30-10-3 sequence, which provides 10dB steps between ranges. The standard attenuator provides an input impedance of over 2M Ohms, but is a nuisance because with such high impedances stray capacitance causes havoc with the calibration, so a parallel capacitive attenuator is also needed. If you expect to work with valve amplifiers, you will want the high impedance, but otherwise the low impedance attenuator should do nicely.

The AttenuatorsThe two attenuator networks are shown in Figures 1 and 2, and as you can see the Hi-Z version requires all those capacitors. They must be accurate, too. Otherwise high frequency performance will be all over the place, so you need a capacitance meter or a source of close tolerance caps. The resistors are standard E24 series 1% metal film types, and the caps (if used) should ideally be polystyrene or polyester, but if ceramic is all you can get, then ceramics are what you use. If you do have to use ceramic caps, make sure that they have low thermal drift - NPO or similar.

AmplifierThe amplifier(s) used in such a audio test project are critical - we need wide bandwidth and low noise, coupled with low current drain, since we want to be able to run the meter on a 9V battery. The meter amplifier also requires high input impedance - especially for the high impedance attenuator version.

Complete Meter AmplifierThe entire audio test circuit can be built easily on a piece of perforated board (Veroboard or similar is good for this type of circuit), and a printed circuit board is quite unnecessary. Lay the physical circuit out following the schematic layout as closely as possible. This nearly always works well with discrete circuits, and makes it easy to follow 10 years later when you need to fix it. (I have had mine for nearly 20 years, and have not had to fix it yet.)

Test and CalibrationThe initial test involves connecting the meter amp to the attenuator, and applying power. All wiring must be carefully checked before you do this - the 9V batteries can supply enough current to damage the transistors, but batteries are more expensive than the transistors, and a wiring mistake may place a heavy discharge on the batteries rendering them dead before their time. Normally, batteries should last for quite a while, since the current drain is only about 4.5mA.