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Input Attenuator For The Digital Audio Millivoltmeter

If you'd like to be able to use our Digital Audio Millivoltmeter to measure AC voltages up to 140V RMS, this add-on project is the answer. It's a simple switched input divider which lets you add 40dB, 20dB or 0dB of attenuation ahead of the meter at the touch of a knob.

By Jim Rowe

The Digital Audio Millivolt­meter described in the March 2009 issue of SILICON CHIP can measure signals over a 79dB range, from about 160μV (-76dBV) up to 1.41V RMS (+3dBV). This is fine for low-level measurements but does make the meter unsuitable for measuring higher level signals.

Click for larger image

With the benefit of hindsight, wecould have built a switched input divider right into the meter itself. However, this would have involved a tight squeeze to fit the additional switch and components into the PC board and box and the front panel would have been very crowded as well.

Anyway, to increase the measured voltage range, we have designed this little “outboard” switched input attenuator. It’s designed to be connected ahead of the Digital Audio Millivoltmeter’s unbalanced input via a short BNC cable.

There’s no need for cable swapping to remove it when you are measuring small signals, either – because it incorporates a “straight through” (0dB) switch position as well. So once it’s built and connected to the input of the meter, its own input connector effectively becomes the meter’s unbalanced input.

Simple circuit

There’s very little in the attenuator, as you can see from the circuit schematic (Fig.1). A 2-pole switch is used to switch the input signal either straight through to the output (0dB) or via one of two taps on the resistive divider.

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