Contents
- Why Use a Preamp?
- Sound Quality Considerations
-
Volume Controls
3.1. Digital vs Analogue Volume Controls
3.2. Noise Floors
3.3. Output Voltages - Do I Need a Preamplifier?
Preamplifiers (preamps) are a popular feature in hi-fi systems, but there are some common misconceptions around their use and benefits – particularly as it relates to dCS DACs. Here, we explain why listeners might wish to use a pre-amp in their system, and discuss some of the factors that should be considered before deciding whether one is needed.
1. Why Use a Preamp?
There are several reasons why someone might choose to use a preamp in an audio system:
- To select different sources.
- To connect an analogue source – such as a turntable / phono amplifier – to the system.
- To level match a source device with a line level output to a power amplifier.
- To set the listening volume.
In systems with a DAC and power amplifier, the preamplifier would sit between the DAC and power amplifier in the signal chain. The intention is that the preamplifier will amplify the signal being output from the DAC to a level that better drives the power amplifier than the output of the DAC would otherwise do if connected directly to the power amplifier. Whether this is the case or not depends on the DAC being used, and the characteristics of its output stage (the circuitry that drives connected cables and amplifiers)
Even in systems where the DAC has an integrated volume control and a suitable output voltage to drive a power amplifier directly, there are potential benefits to the addition of a preamp:
- Interfacing between a source which has only an unbalanced output to an amplifier that only has a balanced input or vice versa.
- To allow a “difficult” amplifier input and/or cable to be driven. This is more likely to be an issue if the cable is long and, of course, different preamps will have different drive characteristics.
- The preamp has tonal characteristics the listener likes.
The output drive issue is not a problem with a dCS DAC. Our balanced output stage has very low output impedance (less than 1 Ohm) and very high current delivery. It’s really very unlikely that any preamp would out-perform in these regards and any input/cable combination that caused it difficulty would be questionable in itself.
2. Sound Quality Considerations
It is often said that the addition of a preamplifier will increase the sound quality produced by a hi-fi system. An important distinction to make is that it can only increase an individual listener's subjective preference for a particular sound signature - it cannot objectively improve the performance of a system.
A preamplifier cannot reveal more information (be it frequency content, dynamic resolution, transient response) from the signal than would be revealed when a DAC is connected directly to a power amplifier – it simply adds in noise and frequency response deviations that aren't in the original recording. To some listeners, this euphonic distortion might be pleasing, but from a purely technical perspective, the addition of a preamp cannot improve the signal, only alter it.
dCS DACs are designed to preserve audio signals, and deliver source music to amplifiers / transducers, without altering the sound or adding a sonic signature. If a dCS DAC is connected directly to a power amplifier and the dCS DAC's volume control is used, then crosstalk, noise, distortion and frequency response errors will be less than if a pre-amplifier is added to the system.
3. Volume Controls
3.1. Digital vs Analogue Volume Controls
Some listeners might prefer the analogue volume controls offered with preamps over the volume controls implemented inside a DAC. Whilst this is a matter of personal preference, there are some common misconceptions around volume controls that are worth considering when comparing a DAC's volume control and a preamp.
The main misconception is that an analogue volume control is preferable to a digital volume control on the basis that a digital volume control must work by simply reducing the number of available bits to express a given amplitude - effectively reducing the dynamic resolution of the system when the volume control is turned down. This may be the case with other DACs where fixed width interfaces are used to send digital audio signals between ICs. However, as all DSP inside a dCS DAC, including volume control, is carried out within the FPGA, we can operate with signals that have as many bits as are needed to accurately represent any possible combination of signal amplitude and volume control, with absolutely no loss in dynamic resolution - it is mathematically transparent.
To clarify this point, the assumption that digital volume controls discard bits and decreasing dynamic resolution is not correct. The idea that a digital volume control truncates the signal and throws away bits to achieve a volume reduction would only be true if the volume control was to take place before the filtering stage in the DAC, which is not the case – at least with a dCS DAC. Considering how a digital filter operates, there are hundreds, thousands or potentially even millions of multiples/accumulates throughout the signal path. The volume control is effectively just the very last one in this chain. If it is carried out with enough bit depth, this process is transparent and does not lose any dynamic resolution.
As an example, consider a 16 bit sample from a CD, along with a 16 bit volume control. All possible permutations of volume and sample are guaranteed to fit within 32 bits. Even if the signal was upped to 24 bit, there is still a digital noise floor of -192dB, with zero distortion or quantisation artefacts, assuming it is dithered properly. This is way below any real-world analogue noise floors. This is important, as it likely means that the noise floor created by the DAC at any volume level will be far below the noise floor of the amplifier (pre- or power) that it is connected to.
With analogue volume controls, there are other factors at play. Potentiometers, typically used for analogue volume controls, have a range of operation where there is a tight level match between the left and right cannels. Outside of this area of operation (towards the top or bottom end of operation), level mismatching can occur. Depending on the implementation of the potentiometer, it may also be directly in the signal path, meaning the component has the possibility of impacting the overall tone & sound quality output from the preamplifier. Neither of these are considerations with digital systems.
3.2. Noise Floors
All electronics will have a fixed analogue noise floor. This is true of DACs, preamplifiers, power amplifiers and integrated amplifiers. In general, the noise floor of a DAC is going to be far lower than the noise floor of a connected amplifier, simply by virtue of the type of circuitry that each product is driving. They are achieving different goals, so this is not in any way a criticism of amplifier design - it simply comes with the territory.
There are users who will opt for using a preamplifier and an analogue volume control because they do not like the idea of having a digital volume control running at a lower level, primarily as they are concerned it will be reducing the signal to noise ratio and harming sound quality.
When reducing the volume control in a DAC, the level of the signal is reduced but the fixed analogue noise floor of the circuitry remains the same. The same is true with an analogue volume control in an amplifier, but here the signal is being reduced against a higher noise floor than in the DAC - noise will become audible and deteriorate quality much quicker. The noise floor of a dCS DAC is low enough that the volume control can happily be run down to below -40dB with no audible difference in dynamics.
It should be noted that this does not apply to other DAC architectures, most notably ladder DACs. Here, the distortion products of the D/A conversion structure are essentially fixed. They become proportionally bigger as the signal gets smaller (i.e. when the volume is turned down), so poor low-level linearity is an issue. This is not the case for the Ring DAC, as the distortion goes down faster as the signal gets smaller (An example of this can be seen in the measurements of the Lina DAC taken by Paul Miller for HiFi News here).
3.3. Output Voltages
dCS DACs have multiple output voltages to aid correct setup. The main settings are for a maximum output voltage of 2V or 6V, which normally correspond to connecting the DAC to a preamplifier or power amplifier respectively (though there are some amplifiers of each type that work better with different voltages - 2V and 6V are the most common). The 2V and 6V settings are both generated independently with different output circuitry for each - this is why you will hear relays click in a dCS DAC when changing the Output Voltage setting, as different circuitry is being used. We would recommend that if a volume level of -30dB or less is being used as the normal listening level, the Output Voltage setting should be lowered. This means the volume control can be kept towards the top of the range irrespective of the sensitivity of the connected amplifier, without the need for introducing a preamplifier with an analogue volume control.
The purpose of a dCS product is not to make something which simply performs well on a specifications sheet. The aim is to make products that are at the forefront of what is possible for sound quality, which offer the utmost clarity and resolution, and the most engaging musical experience. For example, if digital purity was the ultimate goal, even at the expense of the “end product”, one could simply connect the DAC outputs to the output jacks of the product and let whatever amplifier is connected deal with issues like load matching. Adding a line output stage to the DAC will add in the “downsides” an analogue stage will naturally bring (distortion, noise etc.), but will likely allow cables and (unknown) input stages down the chain to be driven much better than would be possible simply by connecting the DAC output directly.
Adding a line output stage would therefore be seen as a compromise in unit design - but it is one that in the real world results in an improvement in system performance. This is the reason that all dCS DACs have such high performing output stages, designed to drive any connected amplifier to its full potential.
This is what, from a spec sheet, would be seen as a compromise in the unit design – but it is one that in the real world results in an improvement in system performance.
4. Do I Need a Preamplifier?
To summarise the information in this article, you may want to use a preamplifier if:
- You need to interface between a source which has only an unbalanced output to an amplifier that only has a balanced input or vice versa.
- To allow a “difficult” amplifier input and/or cable, such as one with an impedance that places a high demand on the connected device's output stage, to be driven.
- To allow an analogue source, such as a turntable, to be connected to the system.
- The preamp has tonal characteristics you like, and these tonal characteristics are sufficiently pleasing that the reduction in noise and distortion performance are worthwhile
In all other instances, connecting a dCS DAC directly to a power amplifier will result in less crosstalk, noise, distortion and gain & frequency response errors, and will provide the cleanest and most transparent sound quality.