Timothy Cose, Author at Headphonesty

Want to better understand the tech-speak you find in marketing materials and equipment reviews? Here’s your crash-course in amplifier terminology and how it should impact your purchasing decisions. Have you ever read an amplifier review and been overwhelmed by technical jargon? How about ad copy that packed in esoteric terminology like the barker at a sideshow? A string of words like discrete direct-coupled single-ended Class A without feedback might seem calculated to dazzle average consumers into a state of shopping compliance, but there’s real engineering behind it. Besides, this is Headphonesty and we’re above average here, aren’t we? This Amplifier Architecture Primer is written to give you the basic conceptual understanding you need to follow the technical discussion that often seeps into reviews and amplifier marketing materials. Each section highlights a common technical term or family of terms, and includes a light working-knowledge explanation, some general ways that it may impact the sound signature or other aspect of a design, and a specific product example illustrating the concept. Amplifiers should be understood as a strategic set of trade-offs and not just the sum of the devices or specifications. Structural Circuits. Amplifiers are divided at the most fundamental level into two groups: Single-ended amplifiers. Push-pull (AKA differential) amplifiers. These terms describe how a signal flows through and is amplified by a circuit. In a single-ended amplifier, the signal is amplified as a whole from input to output. A push-pull amplifier, on the other hand, amplifies the signal as two mirror images and combines them at the output. Single-ended. Single-ended amplifiers can be extremely simple, potentially using just a single tube or transistor as an amplifying device. For this reason, single-ended amplification seems to attract designers who follow a ‘less is more’ philosophy. While a single-ended amplifier will tend to produce more harmonic distortion than a push-pull amplifier, it is often of the monotonically decaying harmonic flavor. This is a fancy way of saying it is inoffensive or even pleasing to the ear. In a single-ended amplifier, the devices must each be able to handle the entire signal as a whole. This leads to larger voltages and more current per device for the same output power. In other words, single ended amplifiers are less efficient than push-pull amplifiers. The theoretical limit is 25% efficiency. Because of this, you don’t often find true single-ended amplification in a portable, battery-powered format. The well-known Bottlehead Crack, a DIY darling, is a perfect example of single-ended amplification in a desktop format. Sold in a kit that anyone with a soldering iron can build, the Crack epitomizes the simplicity of this style amp. Many users describe a sweet and organic sound, perhaps a result of the monotonically decaying harmonics common to single-ended amplifier design. Push-pull. Where more power or efficiency is required, push-pull amplifiers use pairs of devices to amplify mirror images of a signal and recombine them at the output. When compared to holistic single-ended amplification, splitting a signal and stitching it back together sounds like a destructive and potentially lossy process. Reality, however, is closer to the difference between one lumberjack and a saw and two lumberjacks on each end of a saw working together. The doubling of devices in a push-pull amplifier increases the power and efficiency over single-ended operation at the cost of extra complexity. The added device(s) will also each contribute some amount of noise and output impedance—when you double the lumberjacks you double the potential for human error—but managing other aspects of the design mitigates these undesirables. In terms of harmonic distortion, the push-pull amplifier structure intrinsically cancels even-harmonic distortion, leaving just the odd-harmonics (which some label as subjectively ‘cold’ or ‘clinical’ in tone). Overall total harmonic distortion (THD) is generally lower for push-pull than for single-ended, though the composition of the distortion harmonics differs. The Audio-gd NFB-1AMP (as well as several other Audio-gd amplifiers) uses a variation of a popular push-pull output stage known as the diamond buffer. This circuit is common to many transistor-based headphone amplifiers because of its low noise and distortion contributions and its low output impedance. You will also find diamond buffer output stages in amplifiers like the Lehmann Cube and the innards of integrated circuits (ICs) like the LME49600 used in the Neurochrome HP-1. Amplifier Classes. An amplifier’s class does not tell us whether it serves hors d’oeuvres or appetizers at get togethers. Amplifier classes are not night school for aspirational little transistors and tubes (as adorable as that mental picture may be). The class an amplifier falls into tells us a lot about how much power the amplifier consumes under operation. For an amplifier to produce power into headphones, it must deliver both voltage and current. We usually think of voltages in terms of potentials and currents in terms of flows. The current flows in an amplifier have a large impact on efficiency and distortion, as well as several other key audio parameters. Class A. The tubes or transistors in a Class A amplifier are always conducting current. This means that they must conduct enough current to accommodate the maximum power output at all signal levels, even at idle. As a result of this constant current flow, Class A amplification runs hot, leading to a larger chassis or heatsinks. Low power efficiency and a large size make Class A a difficult proposition in portable applications. So if Class A is so inferior in efficiency and size, why would anyone put up with it? Well, tubes’ and transistors’ amplification varies with the amount of current flowing through them. The variation of amplification with signal creates distortion. So, the higher and more consistent current flow in Class A generally leads to more linear (lower distortion) performance from the amplifying devices. Because there’s only one amplifying device, all single-ended amplification must be Class A amplifiers, but don’t confuse this with all Class A amplifiers being single-ended amplifiers. The Schitt Asgard is a transistor-based Class A desktop amplifier. Schitt claims a THD performance of 0.008% at 1Vrms output without the help of negative feedback (a topic covered later in this article). This is impressive performance for a purist single-ended Class A amplifier. Schitt dedicates no less than three mentions in its FAQ to the heat generated by Class A. If you want an amp that runs cold, it’s best to look elsewhere. -Schitt Audio Class B. Whereas the devices in a Class A circuit are always conducting at the maximum current needed for full output, the devices in a Class B circuit switch off where possible in order to increase efficiency. By nature, Class B circuits will use pairs of transistors or tubes in a push-pull arrangement. While one in the pair conducts, the other switches off. This is hugely more efficient than Class A, but you’ll probably never find a Class B audio amplifier. We rarely ever see high-fidelity Class B amplification because of something called crossover distortion. Remember how we said that devices’ amplification changes with current flow and that this creates distortion? Crossover distortion is created by the amplifying devices switching on and off as the output ‘crosses over’ from one device in a pair to the other. This is a significant source of distortion; luckily engineers have come up with a good compromise solution. Class AB. A Class AB amplifier always has some current flowing in it but that current is much less than what’s required by the maximum output. This makes Class AB amplification much more power efficient than Class A amplification and also allows it to be more compact because it generates less heat. Compact and portable amplifiers typically use Class AB amplification to maximize battery life and minimize the footprint. The Objective 2 made under various brands is an example of a Class AB battery-powered amplifier. The standard design advertises a 7-9 hour battery life from its pair of standard 9V cells. This improves to 20-30 hours in a low power version. Most versions of the amp include an DC (wall) power supply for charging and desktop operation. Unlike the on and off switching in Class B, a Class AB design maintains a minimum current flow in the amplifiers at all times. This greatly reduces crossover distortion created by devices switching on and off. Up to a certain power level, Class AB amplifiers operate completely in Class A with both output devices conducting equal current. Beyond this usually modest level, the amp will deliver higher power by operating in Class AB. The THX AAA series of amplifiers from manufacturers like Massdrop or Monoprice are desktop Class AB designs. While there’s some additional wizardry at work to reduce distortion, these amplifiers operate in Class AB as a compromise between efficiency and maximum output power. Class C-Z. There are many other classes of amplification, but in the grand scheme of things, headphones are pretty low-power devices. Exotic amplification strategies like the switching power rails of Class G or pulse width modulation of Class D are not generally employed or needed to power headphones. Output Coupling. How our circuit connects to our headphones also plays a role in the performance and overall sonic character of an amplifier. No, we’re not talking about what precious metal your cable is made of or how many rhodium crystals adorn the headphone jack. We’re referring to the coupling, or how the signal is interfaced, between the amp and headphone load. Direct. Direct coupling is exactly what it sounds like. The amplifier has nothing but wire between it and the headphones. While this is...

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