Introduction

This was my GCSE Electronics project which I built way back in 1997! The original report, with diagrams, is shown here and a PDF version of the report is available for download.

Download GCSE Electronics Project - VU Meter.pdf

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1. Problem Statement

I have a portable CD player, radio, cassette player, a couple of microphones and PC sound card. I am feeding these into the inputs of an audio mixer the output of which is attached to a pair of active loudspeakers. I want to be able to monitor the signal levels going in and coming out of my audio mixer to make sure that I am not overloading the next stage. This could be a tape recorder if I were to record a CD for example.

The audio mixer has four inputs each of which have a volume control. There is also a master volume control on the output amplifier. The reason for using an audio mixer is so that I do not have to swap wires around just to listen to the output from another device. I can have the output of four devices each driving an input of my mixer. I can then adjust the volume of each of the inputs so I can only hear, say, my CD player instead of my radio.

1.1. Problem Analysis

Table 1 shows some of the audio devices that I have found in the house. I have listed the name of the device, the maximum output voltage and the impedance it expects to drive. From this I can decide how sensitive hoe sensitive the meter needs to be and what the input impedance needs to be.

Figure 1 below shows the three main stages that the meter consist of:

Figure 1: Three stages of the meter

The input consists of a buffer that may provide any gain or attenuation needed. It also provides the required input impedance for the source. The rectifier will turn the audio input signal into a d.c. voltage to feed into the display function.

1.2. Input Impedence

The meter will need to be A.C. coupled so that in the event of there being any D.C. on the output of the source, then the meter will not detect it.

The meter will be connected in parallel with the load impedance that is being driven by the source. This means that the total impedance that the source will see will be lower than the load impedance of the device that is being driven. If the total impedance is too low then distortion can occur because the source is trying to drive too much current. Even if the impedance of the meter is similar to that of the load then the total impedance will be reduced by half. Ideally, by making the input impedance of the meter ten times greater than the highest load then any distortion on the signal being measured will be minimised and a higher signal level can be achieved.

Figure 2: Typical setup

The highest load impedance for the devices listed in Table 1 is 100 kΩ. This is for the Reel to Reel Tape Deck which I rarely use, so the next biggest load impedance is 50 kΩ. If I multiply this by ten to find the input impedance for the meter, it would need to be 500 kΩ. With this in parallel with a 50 kΩ, then the impedance that the source will be driving is 45 kΩ. An impedance of 50kΩ would result in an effective load of 25kΩ. The lowest tolerable load for the sources is 10kΩ but this is not recommended. This means that a design impedance of 50kΩ or greater will provide an adequate margin.

1.3. Input Sensitivity

Looking at Table 1, the output levels for the different devices are quite different, so there can not be a single reference level. I have decided that I want two selectable reference levels, these will be:

• 0.150v - This would typically drive a pre-amp to full output
• 0.775v - This would typically drive a power amplifier to full output

Another requirement is the range of frequencies that the meter will respond to. For this meter the frequency response needs to be the Hi-Fi range, which is 20Hz-20kHz. The range of the meter will be -20dB to +13dB. A level of -20dB (one tenth of the 0dB voltage) will provide an acceptable lower threshold, whereas an upper (overload) level of about 12dB will give a good margin of headroom for peaks.

1.4. Meter Characteristics

One way of measuring audio frequency signals is to use an audio millivoltmeter. I have looked up a circuit for a millivolt (mV) meter, and it has the following characteristics:

• Three ranges; 10mV, 100mV and 1v RMS
• Input impedance of 110 kΩ.
• Frequency range of 20Hz 20kHz

The meter uses a moving coil meter which means that it will not be quite so responsive as the needle has to move back and forth rapidly. Moving coil meters that are purpose-designed for audio use are expensive and difficult to obtain. The circuit mainly consists of a three step attenuator and a fullwave rectifier driving a moving coil meter. A key disadvantage of this meter for my application is that it has a linear scale whereas decibels are based on a logarithmic scale.
Source: "Operational Amplifier User's Handbook - R.A. Penfold", page 109

There are two types of meter specifically designed for audio use. A VU (Volume Unit) meter, commonly found on domestic audio recorders, displays the average volume level of an audio signal. A PPM (Peak Program Meter), used in professional audio equipment, displays the peak volume level of an audio signal. For a steady sine wave, the difference between the average level (VU) and the peak level (PPM) is about 3dB. But for a complex audio signal, such as speech or music, the difference between the average and peak can be between 10 and 12dB. This means that peaks my be four times larger than the average level. The VU meter and PPM also have different acceleration/deceleration rates. If a 1kHz steady tone is fed into a VU meter, it takes 300 milliseconds for the meter to stabilise. However, the PPM stabilises within 10 milliseconds. The VU meter is slower because it reads the average, and so it must "sample" the audio signal over a longer period of time than the PPM.

The VU meter closely corresponds to the characteristics of the human ear (The human ear does not respond to signals in a linear fashion). This means that it provides a useful indication of the subjective loudness of different programs and is very useful when matching levels between programs. But the VU meter does not give an accurate indication of the peak signals as it is only measuring the average signal level which could cause distortion or overload a device during a recording.

The VU meter also has the following characteristics:

• Logarithmic scale, normally calibrated around -20dB to +6dB
• The 0dB point corresponds to 0.775v rms

Source: "VU and PPM Audio Meters: An Elementary Explanation - Shure Technical Bulletin"
http://www.shure.com/app-meter.html

I have decided that my meter will be a peak program meter (PPM) to enable me to see the peaks of the signal that might cause distortion on a recording.

1.5. Power Supply

For maximum flexibility and ease of use, the meter will be built as a stand-alone unit. The easiest ways of powering the meter are by using batteries or mains power adapters. The meter will handle a maximum signal of +13dB with respect to 0dB (0dB is equal to 0.775v RMS in this example). +13dB is equal to a voltage of 3.5v RMS which is about 9.9v peak to peak. This shows that a supply greater than 9V would be needed. 12V would be suitable.

A single 9V battery, such as a PP3, has a nominal voltage of 9V, but this will decrease throughout its life. Rechargeable PP3 batteries have a voltage just under 9V (around 8.2V) and so would not be able to drive the meter. Two series connected rechargeable PP3s would provide a voltage supply of about 16.4V. A 12V regulator chip could then be used to ensure a stable supply rail. Mains power adaptors (unregulated and regulated) are available with 15V output which could also feed through the regulator chip. A suitable mains adaptor is available from Maplin.

1.6. Housing

As the meter will have its own power supply it will be built in a separate box from the audio mixer. The box should to be black so as to match with the colour of the other equipment. Suitable cases are available from Maplin. I have chosen the ABS Plastic instrument case H2502 as it a suitable size and is the same as that of my audio mixer.

1.7. Specification

Looking at the information on the previous pages I have drawn up a specification for my meter.

• The input impedance for the meter will ideally be 500 kΩ, but any value above 50 kΩ will be acceptable
• The meter will have a selectable 0dB point of 0.15v and 0.775v RMS
• The meter will measure signals up to +13dB and down to -20dB
• The frequency response will be that of the Hi-Fi range, which is 20Hz 20kHz.
• The power supply will be either two Rechargeable PP3s giving 16.4V or a 15V mains adaptor. This will be regulated down to 12V.
• The meter will be housed in a suitable black plastic box.
• The meter should be portable so it should be of a suitable size.
• The meter will be a Peak Program Meter (PPM).