Amplifiers

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published: Mar-9-2013

Amplifiers with different purposes exist to drive different … urrmm things that need to be driven with a certain amount of power.

Most portable equipment has built in ‘headphone amplifiers’ but these often lack in several ways. They lack when called upon to deliver more than just a little bit of power (> 30mW) or when higher output VOLTAGES are needed.
Due to the way the circuits are made, higher amounts of distortion or roll-of in the lows can be a problem. This is a common problem when driving low impedance headphones.  ‘Low impedance’ headphones are those with an impedance below 60 Ohms. More detailed info on this can be found in the article headphone power and amplifiers also found in the explanations section.

The amplifiers discussed on this web-site are all amplifiers intended for usage with headphones.

Amplifiers have different gains, gain is the amount of amplification and is usually specified in x-‘times’ or x-dB. Also the output resistances differ (even considerably). Output resistances are, mostly not specified but CAN have a significant influence on the sound with some (most) headphones. Also amplifiers can have different input resistances, different frequency ranges (bandwidths)  and different maximum output power levels (voltage levels). Because of the latter (output voltage/power levels) some amplifiers are better suited for higher or lower impedance headphones.. see  resistance, impedance and other issues article also found in the explanations section. Note that when you start on a search for headphone amplifiers you will end up with hundreds of different amplifiers in a wide range of prices varying between € 14.- and € 100,000.- roughly. Surely pride of ownership and having spend a lot of cash to impress or because we can ‘hear’ the differences can warrant spending a lot of money.

This website is not about the bottom part of that price range as the cheapest amplifiers around sometimes lack what it takes to be good enough.  Also most definitely not about the exuberant part of the price range. Spending more than € 100.- on a portable amp is not really needed as there are already good ones out there. Even simple single op-amp designs running of a single 9V battery CAN (when properly designed) give excellent results already. Desktop amplifiers are more expensive as they also require a mains power supply and more components. Spending more than € 250.- is not really needed for most headphones. A few specialistic headphones are not included such as electrostatic headphones or ortho-dynamics that have a low efficiency.

DIY can bring the price down but it can be hard to compete with some existing amplifiers out there, certainly when a few modifications are done (mostly in the power supply or output stage section) to them.

To determine which amplifier suits your headphone(s) best you will need to know / decide on a few things.

• portable or stationary (mains fed)
• Form factor (big or small)
• impedance of the used headphone
• how LOUD you want it to play
• efficiency of the headphone
• sources connected (CD/DVD player, PC audio, portable equipment)
• price range
• looks
• ease of operation

Some of these answers are hard to give by the occasional listener who isn’t really interested in the technical sides of these questions. Instead most will rely on reviews or testimonials. But here is the snag… so many people, so many opinions … what’s good for the goose may not be good for the gander. On a lot of forums and websites sometimes completely wrong advice is given by people who feel are experienced listeners or even people who never compared anything and just say they like what they own and how good it is. It is really quite difficult to recommend people to buy this or that because of ‘beliefs’ or things they have come to accept as ‘real’ based on observations they made. This last part is where MOST of the confusion and prejudices come from and are only based on what is perceived disregarding the circumstances.

Amplifiers can be made with different components but in the end all try to achieve the same goal. Amplify the incoming signal in a way the headphone can properly be driven.

Amplifiers can have different topologies and have different types of parts. We will divide them in 3 groups but in these groups various different ways of achieving the end goal (amplification and adaptation to the load, headphone in this case)

• All tube
• Hybrids (tube combined with solid state)
• Solid State

simplified theory of operation

The operation principle of almost all amplifiers is basically the same:

In general an amplifier consists of 3 basic blocks which combined determine the capabilities of the amplifier. The importance of each block depends on the properties of the other blocks.  The 3 basic blocks are power supply, voltage amplification stage and current amplification stage.

Each of those stages can be made from various components that do the amplification (be it voltage or current) and in case of the power supply regulation.
These amplification devices (components) can be transistors, FET’s, transformers, tubes (valves), MOSFET’s or other exotic components.
Opamps are also often used and most consider them to be a single component but they are not. In fact opamps are complex circuits made out of MANY transistors, FET’s or combinations of those which can include ‘components’ that are not available as single component. An opamp itself also consists of a voltage and current amplifications stage.

The power supply needs to deliver a stable voltage, with as little as needed/possible ‘pollution’ on it. This pollution can be noise or hum. It is also important the power supply voltage remains CONSTANT even when varying currents are drawn. The lower the internal resistance of the power supply the better it performs on THIS aspect (having a constant voltage under varying load). One way to achieve this is by using a regulator. This can be made in numerous ways each circuit topology has pros and cons. How well stabilised the internal power supply voltage(s) need to be depends on the topology of the amplifier building blocks and how they react on their outputs to fluctuations in the power supply voltage. This is called PSSR (Power Supply Suppression Ratio) = PSRR (Power Supply Rejection Ratio). The higher the PSRR, the less sensitive the whole amplifier design is against power supply fluctuations and the less those power supply fluctuations are found in the output signal.

The voltage amplification block can consist of just one single amplification device up to a design with a plethora of components. These components can even be different types of amplification devices including audio transformers. It’s sole purpose is to amplify the input signal in amplitude (voltage) and this can be done WITH or WITHOUT feedback (see explanation below). The goal is to amplify the input signal without adding audible quantities of noise (ALL circuits add some noise) to be fast enough to follow the fastest possible input signals (slew-rate / frequency range) and to be as close as possible to the original input signal BUT amplified in AMPLITUDE (signal level) ONLY. The closer the resemblance the lower the distortion is.

The current amplification stage does NOT amplify the voltage but is needed to deliver the needed current that the load requires/needs. The input and output voltage of these current amplification stages must be the same in amplitude REGARDLESS of the output current. The bandwidth must be wide (which is easy to accomplish) and distortion must be low. The output current has an upper limit which is determined by the circuit itself, the resistance (impedance) of the load or the power supply itself (regulator). This limit can set by the designer with the aid of output resistors (which is the easy way out) or by current limiter(s) or by other aspects in the design.

Output stages can be in class-A or in class-AB. These 2 types of output stages are mostly used in headphone amplifiers.  For Loudspeaker amps Class-B, Class-D (Class-T is a subset) output stages also exist. Class-A output stages get very HOT as they must have a constant current through them that is at least as high as the maximum current the amplifier must be able to deliver. That current x power supply voltage = Wattage = HEAT. The current in Class-AB output stages is determined by the current the load requires and thus can be between very small and maximum values, depending on the output Voltage AND the load Impedance. For this reason the output stage only gets warm when driving loads to their maximum values.

Distortion can be linear and non-linear. Mostly all non-linear distortion is swepped on one heap and is then referred to as (Total) Harmonic Distortion (THD).
Linear distortion affects the frequency range in both amplitude and time/phase. A roll off in the subbass or treble (frequency range) as in case of transducers (headphones, speakers), can manifest as peaks and dips in the frequency response.
This type of ‘distortion’ is not included in the THD numbers nor is it expressed in %. It is audible as a colouration of the sound.
When something sounds ‘nasal, hollow, sibilant, brighter or darker or any different than the original signal in tonal balance this is linear distortion. When it is specified a clue is given in the frequency range as +/- x dB or visible in frequency plots.
Phase distortion isn’t specified by most manufacturers. When there is a LOT of phase shifts within the audible range this can affect how ‘real’ we perceive instruments and such.

Non-linear distortion in the form of harmonic distortion doesn’t have to be very audible, even in larger quantities, as harmonics are also present in instruments themselves and will only give some ‘colouration’ of the output signal.
Harmonic distortion is caused by non-linear behaviour in amplifiers where not all voltages are amplified in exactly the same amount depending on its amplitude.
Because of this harmonics are ‘added’. So a pure 440Hz tone may ‘generate’ small levels of harmonics. The lower harmonics usually aren’t that ‘offensive’ to the sound. I.e. 880Hz, 1320Hz, 1760Hz (multiples of 440Hz in THIS case).
Of course all frequencies in the music signals will be generating ‘extra’ frequencies.
As long as the amplitude remains below -80dB opposite the ‘fundamental’ they will be masked by natural occuring harmonics (which every instrument has) and be too low in level to become audible anyway.
Consider levels above 0.1% to be audible, especially when those harmonics are the ‘higher order’ ones (5th, 6th,7th .. etc).
Lower order harmonics (2nd, 3rd) may not even be that audible below 1%.
In some amplifier designs this is used to give the amplifier a ‘sound’.
This ‘sound’ is caused by the non-linear behaviour every amplifying ‘component/part’ has by nature.
Components like resistors do NOT exhibit non linear (nor linear in the audible and far above it, range) distortion and thus cannot change the sonic signature even though this is reported by many.
Capacitors (depending on type and application) MAY exhibit small amounts of non-linear behaviour in certain circumstances.

Another type of non-linear distortion is (IM = Inter Modulation for instance) and is considered to be the unpleasant one and is more easily detected as ‘distortion’.  This is also caused by the same non-linearities in amplifying components and is caused by frequencies interacting with each other and create harmonics that are NOT simply harmonics but ALSO create frequencies that are the sum and differences of those frequencies. Needless to say these NON linear distortion MUST be as low as possible as these will NOT be masked by the music itself that well.

The maximum output voltage swing is determined by the power supply voltage and in a small degree to the used circuit topology.

To give an example of non linear behaviour (transfer function) is as follows: An amplifier design is specified to amplify 5 times yet for low signal levels the amplification may be exact 5 times but the higher the amplitude of the input signal becomes the more it deviates from the EXPECTED output signal. When the component COMPRESSES the output signal will have a smaller than expected output amplitude (for example 4.9x the input voltage). When the component expands the the output signal is higher than expected (say 5.1 x the input voltage). The good news is this behaviour can be addressed by using feedback.

Negative Feedback is essential (for most designs) and is needed for low distortion, wide bandwidths and stability. That feedback can be local (on individual component base) or in a part of the entire circuit.

The blue square block above is local feedback in the voltage amplification stage, the orange block is local feedback in the current amplification stage.

Some amplifiers have NO feedback at all and solely depend on enough linearity of the used components. All components that do amplification have their own specific transfer function. This means the output voltage is NOT an exact (albeit amplified in current/voltage) copy of the input signal. This can be in the form of flattening or expansion of the signal or an affected frequency range or the time it takes to go from one voltage to another one (slew-rate or rise/fall time).

Feedback eliminates these alterations the amplification parts make. The higher the gain (voltage amplification) of the ‘block’ the higher the feedback will/must be and the better it performs. In other words more feedback means less distortion and wider frequency range. Because of feedback the difference between the input signal and the amplified output signal that would otherwise be prominent is lowered enormously.

Most amplifiers, however, use overall feedback as shown below where the output signal is compared (controlled) by the input voltage.

Feedback basically consists of an attenuator that internally lowers the output voltage to the exact same level of the input voltage of the amplifier. The reciprocal of this attenuation determines the gain (amplification) of the amplifier, and is given in X(times) or dB. These are simply 2 different ways of notation,  2x gain = 6dB, 4x gain =12dB, 8x gain = 18dB, 16x = 24dB. As you can see each doubling of gain equals an increase of 6dB. So if the feedback circuit has an attenuation of 4 times (12dB) this means the output signal of the feedback circuit is 4 times smaller than that on the output of the amp (= input of feedback circuit). The amplifier circuit simply compares the audio input signal to what is present on the output of the amplifier (via the attenuated feedback signal). This ensures the output signal is an as exact as possible, within the limits of the entire design, copy (but enlarged in amplitude and capable of delivering higher amounts of current) of the input signal.

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