Why your amp displays a negative figure

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As a hi-fi or home theater enthusiast, you have probably already noticed that on the vast majority of amplifiers, the volume is indicated by a negative figure in decibels. Why does your amp display a negative figure? Why does the volume increase as the value nears 0 dB? How do you change the volume scale? We will answer all of these questions in this article.

Why does your amp display a negative figure? Why does the volume increase as the value nears 0 dB? How do you change the volume scale? We will answer all of these questions in this article.

Why is the volume expressed as a negative figure?

Although it may be disconcerting that the volume is expressed as a negative figure, it is actually very precise and easy to understand. On an amplifier, 0dB is the absolute value and therefore the maximum power that the amp can deliver. For example, an amp with an RMS output power of 2 x 100 watts into 8 ohms, like the Yamaha MusicCast R-N803D, will deliver this power at 0dB. Consequently, if you connect a CD player to the amplifier with the volume set at 0dB, it will amplify the signal to its maximum power, which will be much too loud in most cases. This is why amplifiers have a volume control system. Without this, they would always play at full power, like a power amplifier. The volume controller can be somewhat compared to an attenuator which limits the power of the output signal. For example, if you set the volume to -20dB, that means that the volume sent to the speakers will be 20dB quieter than the maximum volume allowed by the amp. As a result, when you turn the volume up on an amp, you actually decrease the amount of restriction placed the amp, which as a result plays louder.

On an amplifier, 0dB is the absolute value and therefore the maximum power that the amp can deliver without generating distortion.

Note that the decrease in volume expressed in decibels is measured using a logarithmic scale and consequently is not uniform. For example, a 100W amp will have an output power of 50W at – 3dB (regardless of the input signal level). This will only be 25W at – 6dB and 10W at -10dB. Broadly speaking, you only have to remember that each time you increase the volume by 3dB, the power will be double the previous value. However, the ear cannot perceive a change in volume every 3dB, but every 6dB. For example, going from -12dB to -6dB makes the sound twice as loud. On the other hand, going from -6dB to -12dB makes the sound half as loud.

Logarithmic scale of the power of a 100W amp per decibel.

How can the volume be positive?

As we have just seen, hi-fi amps and A/V receivers deliver their full output power at 0dB. So how do some amplifiers go over the 0dB reference point, with figures like +6 or +12dB? It is simply because 0dB is the maximum power that the amp can deliver without generating distortion. To reuse our previous example, the Yamaha MusicCast R-N803D amp provides 2 x 100 watts of power at 0dB with a very low distortion rate of 0.019%. It can go up to +16.5dB to provide a maximum power output of 2 x 145 watts into 8 ohms, but with a distortion rate of 10%. This is why it is not advisable to go over the 0dB reference point if you wish to retain the amp’s musicality.

The Yamaha MusicCast R-N803D amp provides 2 x 100 watts of power at 0dB with a very low distortion rate of 0.019% and 2 x 145 watts of power at +16.5dB with a distortion rate of 10%.

How do you change the volume scale?

If you find that displaying the volume in negative dB is a little too confusing, some amplifiers allow you to change the scale to display a range from 0 to 50 or 0 to 100, for example. This scale can be selected via the amplifier or receiver’s settings menu. For example, with the Marantz NR-1609 A/V receiver you simply have to go into the settings, then to scale, then select “0-98” for the volume to be displayed from 0 (minimum) to 98 (maximum).

Note that displaying the volume in this manner means that you will no longer know the maximum volume that the amp can deliver without generating distortion.

The Marantz NR-1609 A/V receiver can display a volume scale from – 79.5dB to +18dB or 0 to 98.

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