Drive capabilities are a core consideration when creating amplifiers for both headphone and loudspeaker listening. On a basic level, an amp must be able to meet the power requirements of whatever headphones or speakers it is paired with. These requirements vary from product to product: a small bookshelf speaker, for example, will have different needs to a large floor standing speaker, and amplifier creators must account for products that sit at both ends of the spectrum – those that are considered easy or difficult to drive – in their designs.
The same is true with headphones, but here, the spectrum is much wider. The difference in requirements between a pair of sensitive in ear monitors versus insensitive planar magnetic headphones is much larger than the aforementioned bookshelf and floor standing speakers, and headphone amplifier creators must ensure their unit can adequately drive both products, delivering enough power to allow the planar headphones to sing, but not overdriving the IEMs such that they have a higher noise floor and poor volume control.
Watts, Volts or SPL? Measuring an amplifier's performance
When designing the Lina Headphone Amplifier, we began with looking at the sensitivity and impedance of a large number of headphones available on the market. We used this information to come up with a power requirement that was based on driving these headphones to the correct sound pressure level (SPL). The amplifier’s design is optimised for 60 ohms (Ω) as that is where the headphones on the difficult to drive end of the spectrum sit.
The amplifier will drive the full 14Vrms (volts root mean square, the ‘average’ voltage output – the ‘peak’ output voltage will be considerably higher than this), down to 45 Ω, which was the objective to ensure headphones that require significant amounts of voltage are correctly driven. Because the impedance varies so much with headphones, the output voltage of an amplifier is a much more useful measure of the performance of an amplifier than the output power.
From a drive perspective, our benchmark was to create an amplifier that could drive these insensitive ~60 ohm headphones at a continuous 120dB SPL, and to do so cleanly and without any clipping. Clipping occurs when the required output voltage exceeds the maximum the amplifier can deliver, which is usually largely determined by its internal voltage rails. When this happens, the output is abruptly limited to this maximum voltage and high levels of harmonics are generated. (In extreme cases of clipping, the resulting output waveform approaches that of a squarewave.)
This severe distortion is readily audible and may even damage drive units. 120dB is more SPL than could ever be used in sensible listening – human hearing tends to stop working after around 105dB, where the ear itself starts misbehaving (hence the reason for needing musician’s earplugs at concerts with SPLs exceeding this for the music to still sound like music).
A, B, or AB? Comparing amplifier classes
There are several ‘classes’ of amplifier available on the market, and this classification relates to how the output devices (transistors) used to amplify the music signal are arranged and operated. With a Class A amplifier, the transistors remain powered at all times reproducing the entire waveform of the audio signal. What this means is that the device is ready to push current to the output immediately and can react very well to changes at the input. It can be free of distortion and exceptionally linear. The drawback with this method of running the output devices is that the amplifier is very inefficient – well under 50% of the power is actually turned into output signal, the rest is given off as heat.
This plays into the drive considerations, in so much as that the design may not be able to produce the necessary power to the desired headphone without dissipating excessive power and potentially causing the amplifier to overheat.
A good example of this would be the Bartók Headphone DAC - when using the built-in headphone amplifier on the Bartók, the amp primarily drives in Class A. However, at around 150mW into 33Ω, the stage transitions from Class A to Class AB operation, allowing the unit to drive up to 2.8W into a 66Ω load. The amplifier stage would not be able to drive the same amount of power into that load running in Class A. Therefore, the performance from an insensitive pair of planar headphones would be better when driving them in Class AB, even with the potentially better linearity of a Class A amplifier stage compared to the Class AB operation.
Class B amplifiers utilise a different approach, where each transistor is only powered during its half of the wave cycle. This means that far less energy is wasted as heat. As the transistor not being used is not powered, more energy is available for driving the active half of the wave cycle. The linearity of the amplifier is generally poorer compared to that of a Class A, particularly when the amplitude of the signal crosses 0 (when the signal passes from a positive voltage to a negative voltage, or vice versa), but the design can potentially provide more output power as a result of the more efficient design and usage of available power.
The Class AB topology sits somewhere between these two designs. Similar to a Class B design, the output devices are not fully powered when they are not in use, but unlike a Class B, they never fully switch off. A slight DC bias is applied to the transistor, which aids in the linearity of the design, whilst vastly improving the efficiency over Class A designs.
The Lina Headphone Amplifier employs a topology that is sometimes called Super AB or Class AA. This approach involves biasing the output transistors (only a low bias is applied with the Lina Headphone Amplifier), and crucially, doing so with a dedicated piece of circuitry which ensures that the output impedance of the output stage does not momentarily increase when both transistors are powered off – and to linearise the behaviour of the output stage. This allows the amplifier to drive an impressive maximum of 4.5W per channel continuous into 45Ω, while maintaining the excellent linearity typically associated with a Class A amplifier.
Looking beyond output power: design goals for the Lina Headphone Amplifier
It should be noted that the exact output power of the Lina Headphone Amplifier is somewhat arbitrary. As mentioned earlier, the amplifier’s design started from the standpoint of real world SPL driving some of the most demanding headphones on the market, and doing so with a sound which is linear and transparent. This is much more relevant in determining whether an amplifier will adequately drive a particular headphone than simply the output power into a given impedance. It is also worth noting that few amplifiers will actually achieve the level of drive capability that the Lina Headphone Amplifier can, irrespective of what their output power rating claims.
By contrast, there are a large number of headphones on the market which are sensitive. Having high levels of output power is fine on the amplifier when using an insensitive headphone, but could be problematic when driving a more sensitive headphone. To accommodate for this, the Lina Headphone Amplifier has a gain switch located underneath the volume control. This controls the gain going into the final output stage of the unit. This is beneficial as it aids in keeping the noise floor of the amplifier very low when using a sensitive headphone, and ensuring that the volume control can be kept in a sensible place, and not used right at the bottom end of the control (where any analogue volume control is less precise).
While drive capabilities are a core aspect of an amplifier’s job, they should not be looked at in isolation or as a marker of an amplifier’s real world ability. Creating an amplifier with lots of drive is relatively simple in practice - it is not what determines the sound quality, nor what drives the design philosophy of the amplifier - and as we’ll explain throughout this series, there are several other factors that must be considered during the design process in order to ensure an outstanding quality amplifier.
Testing the Lina Headphone Amplifier's drive capacity: reviews and feedback
‘I’ve had no issues driving the hardest to drive headphones I have…so I like that versatility.
With the gain control you can choose two different gain settings.
I’ve also found it’s quiet enough to use with sensitive in-ears as well.’
‘We’ve done an extensive test on just about every headphone we have here…whether its’s got a high sensitivity, low sensitivity, high impedance, low impedance, this headphone amp doesn’t flinch at anything.’
'The best solid-state amplifier I've heard with our headphones...I’ve used this with every one of our model headphones. It fits all of them like a glove.’
