Volume Control Technologies

By Elias Gwinn and John Siau 

With the introduction of Benchmark’s HDR-VC™ (High Dynamic Range Volume Control), many audio enthusiasts are taking another look at volume control methods.

• What are the differences between the different volume control topologies?
• Is one sonically superior, and why?  
• What is HDR-VC™?

Volume controls can impact the following performance characteristics:

• Signal-to-noise ratio (SNR), also known as dynamic range
• Total harmonic distortion + Noise (THD+N)
• Frequency response
• Inter-channel gain matching

There are four common systems for controlling volume:

• Digital attenuator - digital signal processing (DSP)
• Integrated analog volume circuit - (IC)
• Passive attenuator - resistor network, or passive potentiometer
• Active gain circuit - amplifier and variable resistance

Benchmark's HDR-VC™ combines these two systems:

• Passive attenuation - low-impedance passive output pads
• Active gain circuit - controlled by front-panel potentiometer

Qualities Affected by Volume Controls

Dynamic Range and Singal to Noise Ratio

‘Dynamic range’ and ‘signal-to-noise ratio’ (SNR) specifications describe the same quality. They both indicate the ratio of the highest possible signal level to the level of the noise floor produced by the component. This noise may be dominated by thermally produced white noise, or it may be dominated by discrete frequencies such as those that that are related to hum and buzz produced by AC power.

SNR measurements are conducted by measuring the noise floor without any signal present. However, this method proved to be inadequate for devices that contained auto-mute technology. These auto-mute circuits would engage when no signal was present, and the ‘noise floor’ measurements would be deceivingly low. Dynamic-range measurements were introduced as an alternative to SNR measurements. Dynamic-range measurements are conducted by measuring the noise floor with a low-level test signal that prevents auto-mute from engaging. For the purpose of this article, ‘dynamic range’ will refer to the noise floor of an audio device relative to the maximum undistorted output level.

A volume control’s effect on dynamic range can be explained via the concept of ‘gain-staging’. Gain-staging is the practice of optimizing head-room and dynamic range by properly aligning the levels of each active gain stage and/or attenuation stage (active and/or passive). A simplified description of proper gain-staging is: coordinating all devices to reach maximum input/output at the same time, operating in the upper regions of those input/output levels, and, when needed, attenuating near the end of the signal chain.

To Maximise Dynamic Range You Must Use all of Your Headroom

A usable analogy to dynamic range is the height of a room, floor-to-ceiling, where the ‘floor’ represents the noise floor of an electronic device, the ‘ceiling’ represents the highest signal amplitude possible, and the ‘height’ represents the dynamic range. A tall person use more of the available headroom than a short person. An active audio device with analog signal paths will have an intrinsic noise floor due to thermal noise and other electronic factors. The ‘height’ between the ‘floor’ and ‘ceiling’ of a signal stage will remain consistent, but this ‘floor-to-ceiling height’ will only be utilized if the signal path has the proper gain-staging and SNR. This analogy will be used later, when discussing specific volume-control implementations.

Total Harmonic Distortion + Noise (THD+N)

All audio devices add errors to a signal. These errors are categorized as ‘noise’ and ‘distortion’. A device’s THD+N measurement quantifies the total errors added by the device.

A THD+N measurement will detect many different phenomena: harmonic distortion, intermodulation distortion, jitter-induced distortion, crosstalk, random white noise, power supply hum, external noise, etc. For a given THD+N ratio, these errors may have very different thresholds of audibility.

Frequency Response

A device’s frequency response is a measurement of deviation from linear response. A perfectly linear device would reproduce all frequencies equally. The amount of gain or attenuation would be the same for all frequencies of an input signal in a device with linear frequency response.

In reality, every device has a limited bandwidth. In addition, the accuracy of the frequency response within the pass-band varies among components. An example of an intentional non-linear response is a device with a ‘bass boost’. In that case, the low frequencies will have more gain than other frequencies within the signal.

However, many components have unintentional irregularities in their frequency response. For example, some volume control circuits can change the frequency response as the volume is changed.

Inter-Channel Gain Matching

‘Inter-channel gain matching’ describes a device’s ability to achieve precise matching of separate channel output levels (i.e. between left and right in a stereo device).

Volume Control System Details

Digital Attenuation (DSP)

DSP-based volume control of digital audio can be implemented via hardware (DSP in a chip) or software (DAW, media player). It performs multiplications on the digital audio data before the data is delivered to the D/A converter.

In most cases, a DSP-based volume control will limit the dynamic range of the playback system. This is because it doesn’t utilize the entire dynamic range of the D/A converter. Going back to the ‘height of a room’ analogy, the maximum level that a D/A converter can produce is the ‘ceiling’ (or ‘headroom’). The converter reaches the ceiling when a ‘full-scale’ (the highest possible digital amplitude) signal is present at the input.

If the digital signal is attenuated before it reaches the D/A, its peak amplitude will be further removed from full-scale. In other words, the peak output from the D/A converter will be further below the ‘ceiling’, but the ‘floor’ (inherent noise) will remain. In our ‘room analogy’, digital attenuation added a drop-ceiling to our converter, and the new ‘height of the room’ (dynamic range) has been reduced.

Also, when digital attenuation occurs, dither (noise) should be applied to avoid quantization distortion (distortion from quantization errors is beyond the scope of this article, but worth investigating). This dither can lower the SNR of the audio signal because new noise is added to a lowered signal. This is a serious noise contribution because, if the output of the digital volume control is 16 bits, the dither noise is at -96 dBFS (96 dB below full scale). However, if the output is 24 bits, the dither noise is at -144 dBFS. At 144 dB below full scale, this will not add significant noise to the system.

These dynamic-range limitations become less of a problem in D/A converters with significantly high dynamic range, since a playback system will only be as quiet as its noisiest component. In other words, if a system has a power amp with a 100-dB dynamic range, it won’t matter if the dynamic range of the D/A is reduced from 125 dB to 110 dB. The dynamic range of that system will still only be 100 dB. Generally, the higher the dynamic range of the D/A, the more digital attenuation can be applied without affecting the dynamic range of the entire playback system.

In addition to dynamic-range limitations, one should also be concerned about distortion induced by an inferior DSP algorithm. If the designer does not implement proper dithering, severe non-harmonic distortion will occur. Many computer playback systems lack dither. 16-bit systems have noticeable distortion when dither is omitted. 24-bit and 32-bit systems are much more forgiving when dither is omitted.

Other errors in DSP implementation can also result in distortion. However, properly-designed digital volume controls are becoming more common. A well-designed digital volume control will add virtually no distortion to the audio.

DSP-based volume control can work well provided that the following conditions are met:

1. The digital volume control must be properly designed. The DSP calculations must internally use word-lengths of at least 24-bits.  If the output word length is less than 24-bits it must be dithered.  either be dithered or long word-lengths (24 or more bits ) must be delivered to the D/A converter. Many media players, computers, and digital devices use 16-bit undithered volume controls. In most cases, these 16-bit volume controls can bypassed by setting the volume to maximum. When this is done, an external D/A converter can be used to control the playback level.
2. The D/A converter must have a spectacular dynamic range. It is not uncommon to require 20-30 dB of attenuation for normal listening levels. In those cases, the dynamic range of the D/A will be reduced by 20-30 dB. If your D/A converter only has a 110-dB dynamic range, the output will have a dynamic range of 80-90 dB – a dynamic range that is less than that of a 16-bit CD.
3. The peak level of the D/A converter’s output should match the maximum input level of the next device in the signal path (amplifier, pre-amplifier, etc). This is a fundamental part of proper gain-staging, as it fully utilizes the headroom of both devices. This is the reason that professional audio facilities will standardize the operating signal level between audio devices (usually at +4 dBu at -20 dBFS). With this type of configuration, the dynamic range of the amplifier will usually be the dominate noise factor.

Analog Integrated Circuit (IC)

Many products use an integrated circuit (IC) to manipulate volume in the analog domain. These analog volume-control IC’s suffer from noise and distortion.  These defects are due, in part, to the complications of implementing many precision elements within a single IC package. Specifically, the size, materials, proximity, and voltage limitations of these elements (within the IC) severely limit the overall quality of this solution.

The performance of analog volume-control IC’s are determined by these factors:

1. Crosstalk between electronic elements within the IC chip: When small-form electronic elements are packed into a single wafer (chip), they are very susceptible to crosstalk. Crosstalk is a phenomenon that occurs via substrate coupling (voltage coupling from one node to another through the substrate). The amount of crosstalk is proportional to the proximity of the nodes. Therefore, small IC packages are extremely susceptible due to the components proximity to each other.
2. Power consumption and noise: A trade-off relationship exists between power consumption and noise. Typically, an IC will be designed to minimize power consumption. This requires high-impedance circuitry. However, ‘Johnson noise’ increases with increased impedance. In other words, a high-impedance circuit causes more noise. In contrast, a low-impedance circuit will cause more power consumption, which will result in more heat. The elements are so small that significant heating and cooling of individual elements can occur at audio frequencies. This heating and cooling alters the resistance and capacitance of these elements. As a result, the response of the components becomes non-linear, which causes distortion.
3. Non-linear behavior of elements within the IC chip: Non-linearity can be caused by a variety of issues. The issue of non-linearity due to thermal variation was discussed above. Also, IC’s have parasitic leakage paths between circuit elements. Leakage currents and DC-offsets cause non-linear behavior. Non-linear behavior results in distortion.

Passive Attenuator

A passive attenuator is simply a resistor network or potentiometer creating a voltage divider in the signal path. The output of a voltage divider is a scaled version of the input signal. A passive attenuator uses only ‘passive’ components, which are components that do not require a power source.

Many people assume that passive attenuators are completely benign. The reality is that passive attenuators can degrade the audio quality.  Distortion will be produced when poor-quality resistors are used.  Noise will be produced if the impedance of the attenuator is too high.  Passive attenuators with high impedance (greater then 500 ohms) are particularly problematic. 

It is very difficult to build a high-quality continuously-variable passive attenuator.  Potentiometers can contribute distortion, and can cause impedance variations.  These variations can cause changes in the frequency response. 

In contrast, it is relatively easy to build fixed or stepped attenuators using fixed resistors.  High-precision metal-film resistors are readily available and these can be used to build high-quality fixed voltage dividers. 

The following characteristics dictate the performance of a passive attenuator:

1. High impedance (high-Z) will reduce common-mode rejection ratio. Common-mode rejection is the main purpose of a properly balanced system. In a perfectly balanced system, the differential input of the device will cancel any noise that is common on both transmission lines of a balanced connection (e.g. XLR). The result of this process is a signal that is free from artifacts that were not present at the output of the previous device. A device’s capability to achieve this is described by its ‘common-mode rejection ratio’ and varies based on several factors. In general, low source impedance and high load impedance will result in a higher common-mode rejection ratio. Therefore, for the sake of common-mode rejection, it is undesirable to increase the source impedance with a hi-Z passive attenuator.
2. Variable impedance will alter frequency response. The output impedance of passive attenuators often varies with volume setting, which causes a change in frequency response as the volume setting is changed. The output (source) impedance of a device, in conjunction with the capacitance and/or inductance of the load (the next device downstream), will create a filter. Different impedance values will cause different frequency responses. For example, a higher impedance and capacitance will result in a lower threshold frequency of a low-pass filter where attenuation occurs. This becomes a problem when the threshold frequency approaches and enters the audible range (below 20 kHz). If the cable and load capacitance approaches 500 pF, and the impedance of the passive attenuators is 15 kOhm, there will be 3 dB of attenuation at 20 kHz!
3. High impedance will increase channel gain differences. High source impedances will have more affect on the gain of the following device’s input. Any slight differences of source impedance between the left and right channels will result in differences in gain between the two channels.
4. High impedance will increase noise. All resistors produce thermal noise, also known as ‘Johnson noise’, or ‘Johnson-Nyquist’ noise. This noise is due to thermal excitation of electrons, causing random voltage fluctuations. When a resistor is in series with the signal path, thermal noise is added to the signal. The amount of noise added increases as the resistance increases. Therefore, high-impedance passive attenuators with will add more noise than low-impedance passive attenuators.  If the impedance is low enough, the thermal noise will be insignificant.
5. Low impedance (low-Z) can overload the output drivers. If the impedance of an attenuator is too low, it can overload the active stage that drives it. When this overload occurs, distortion will rapidly increase.
6. High-quality resistors must be used. Low-quality resistors will add additional noise and distortion to the audio signal. Resistors must be precision thin film or metal film type to maintain low noise and low distortion.

Active Gain Circuit

Active gain circuits are simply line-level amplifiers. They work by sending a signal into an active amplifying component (tube/transistor/opamp/etc), whose gain is often adjustable via a potentiometer.

While active gain circuits may suffer from noise and distortion, a properly designed active gain circuit will add very little noise or distortion to the audio.  Active gain controls can outperform other controls if the control range is limited to a reasonable range. High performance can be achieved over a +/- 10 dB control range.

Frequency response is constant at all gain settings. Output impedance is very low and is constant at all settings.

Benchmark's HDR-VC™ (High-Resolution Volume Control)

The HDR-VC™ system combines an active gain circuit with a low-impedance passive attenuator. The potentiometer in the active gain circuit is motor-driven by the remote-control system. This combination provides low noise and low distortion over a wide range of output levels.  Remote volume control can be achieved using DSP or an analog volume control IC, but at the expense of audio performance.  Benchmark has chosen to use a motor-driven pot in the DAC1 HDR in order to eliminate the compromises DSP and IC solutions.  The other DAC1 models use the same volume control structure, but without the motor drive.

In all DAC1 models, passive attenuators are located after all of the active devices. The SNR at the output of the passive attenuators is identical to the SNR at the input to the passive attenuators. The passive attenuators can be set in 10-dB increments from 0 dB to -30 dB. The passive output attenuators optimize the control range of the active volume control.

The HDR-VC™ system delivers very high performance over a wide range of signal levels while providing remote control.