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Class-A Single Ended TX-gain Headphone Amplifier

Date post: 01-Nov-2015
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Class-A Single Ended TX-gain Headphone Amplifier

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  • And now for something completely different...

    A class-A headphone amplifier...

    But not like the ones you've seen before.The voltage amplification parts differ from what is normally used in amplifiers which in this case is a transformer.

    Most 'normal' amplifiers use transistors, FET's, IC's or tubes as amplifying device(s) with overall feedback or local feedback in strategic places to get it to perform well. IC's (operational amplifiers or for short op-amps) often have around 100,000x to 1,000,000x amplification (open loop gain). Most discrete transistor or FET amps have open loop gains between 10,000x to 100,000x. Feedback is needed to the reduce gain to the desired levels, increase bandwidth and lower distortion. Feedback means the input signal is constantly compared to the, lowered by a fixed amount, output signal. When this divided output signal is not equal to the input it is (almost) immediately compensated by the high gain of the circuit ensuring the output signal is a (near) perfect copy of the original signal but bigger in amplitude.

    Also quite popular are amplifiers that use local feedback instead of overall feedback which eliminates certain effects but add other and are generally considered to be more musical.Tube amplifiers, that is those with 2 or more tubes, usually have an open loop gain between 100x and a 1,000x and also use overall and/or local feedback with or without an output transformer.Tube amps with 10x to 20x open-loop gain use the amplification of a single tube and no overall nor local feedback is used. The output of these amplifiers must be buffered with another tube or semiconductors to provide output current.

    Transformers are generally used in input stages of amplifiers to perform level and/or impedance matching (step-up transformers), galvanic separation, balance or unbalance signal lines.Transformers are also found in tube amplifiers in the output stage to perform the needed impedance matching, galvanic separation and lowering of the high voltage swing in tube amplifiers to lower them to levels needed by loudspeakers or headphones.In some tube amplifier designs transformers can also be found in other places then the output stage to create phase shift within amplifiers needed for push-pull designs. This can also be done with an additional tube and is more common then the use of transformers. Transformers can also be found in electrostatic speakers, Air motion transformers and certain ribbon speakers/tweeters. All these speakers have an excellent reputation as far as sound quality is concerned.

    Transformers have their quirks/flaws too of course and one in particular. The magnetic material in a transformer has a certain hysteresis that introduces harmonic distortion. This happens when a signal goes from positive to negative and vice versa. Once the magnetic field is inverted the linearity is very good again. The reason for this behaviour is the fact that the used metals in the core have certain magnetic properties. When a current in the primary coil is present a magnetic field is generated. When the electrical signal is gone there will always remain a small magnetic 'charge'. This is a property of this material and causes this hysteresis which is audible in the form of harmonic distortion.Further more they are sensitive to external magnetic fields by transformers of other equipment and magnetic fields from large magnets which causes saturation effects and even microphonic behaviour.

    Why would anyone be using transformers if they are not that ideal ? Transformers are good in performing certain tasks (in the audio domain) and in some cases are not easily replaced by other parts such as in loudspeakers for instance or galvanic separation. These loudspeakers all have very analytic and extremely detailed and excellent sounding qualities so they can't be all bad and those that have used step-up transformers with MC cartridges to be able to use MM inputs of pre-amps also know these transformers do not affect the sound quality much.

    Transformers actually do have some positive properties too. The total harmonic distortion of transformers may be higher then those found in most solid state amplifier designs and the harmonics are of the uneven kind (3 rd, 5th and up) but also getting smaller in amplitude the higher the harmonics are. Also they do not have even harmonics (2nd and 4th and up) Transistors, IC's, tubes etc. usually have lower harmonic distortion levels when in used in designs with overall feedback. Uneven harmonics (3rd, 5th etc.) are considered less pleasant to the hearing. Distortion levels are also dependent on the frequency. The lower frequencies and higher ones have higher amounts of distortion than the (more important) mids.

    So distortion of transformers may be relatively high in amplitude on the test bench but are not perceived as such.Another (BIG) advantage of transformers is they do not exhibit Transient Inter Modulation Distortion (TIM) like other amplifier designs do. TIM distortion is caused by clipping of the internal loop of an amplifier, that uses feedback, when very fast signals (much higher than the audible range) are applied to the amplifier and the design doesn't have an appropriate input filter. These high frequencies are not common but can be found in NOS DACs with little to no analog filtering behind it.Intermodulation distortion is more noticeable then harmonic distortion and audible amounts can degrade the sound.This kind of distortion happens when more than 1 signal is amplified by non linear devices, such as transistors, FET, tubes, transformers etc. These additional signals are generated and consist of products or subtractions of these signals which have no harmonic relation to the original signal. This distortion is is also present in live acoustic live performances by the way. Another positive behaviour of a transformer is the roll off behaviour at the low end of the frequency spectrum (from 1Hz to 20Hz) over the usage of input or output capacitors which is less 'steep' and the phase shift is not as bad and in a different direction.

  • Transformers are, as explained above, used at the front (step up transformers) or at the end (line drivers or matching/step down transformers) of circuit designs but not used in amplifier designs as the amplifying device itself. These transformers are rarely driven and/or loaded (terminated) in an ideal way. If they were they would perform even better. This led me to design a different amplifier where the (voltage) amplification is done by a transformer and the very important impedance matching, in order to get transformers to perform at their best, by solid state components. A bonus of this topology is it needs NO feedback at all. More meant as an exercise then a serious audiophile attempt to create an ultimate amplifier.

    Having experimented with several kinds of (class-A) output stages with transistors, MOSFETs and other output devices that just 'follow' the input signal without any voltage amplification led me to the conclusion that a good yet simple 'follower' in this particular, feedback less, application is a MOSFET with a Constant Current Source (CCS) in it's Source.

    A valid question would be why this topology/design (transformer used as a voltage amplification device) is not used in other amplifiers before... I actually have no clue why nobody came up with the idea or why it's not common practice, other then the general conviction that transformers are only used as a necessary 'burden' if other parts can't be used. I have never seen circuits using this idea/topology, but it is very well possible that if one would search intensively for these kind of schematics they will probably turn up. I haven't found one myself and the only way to find out if it works and, more important, how well it works is simply by building one.

    Design criteria were:

    No usage of electrolytic capacitors in the signal path (preferably no capacitors at all).Use as few as possible parts.Preferably no expensive or exotic parts (except the audio transformers) are to be used .NO overall and NO local feedback.Wide enough frequency range (minimal 20Hz to 20kHz within 0.5dB).Able to drive all headphones above 32 (Ohm) to a voltage of 10Veff (30Vpp)A gain ranging between 3x to 15xHaving various output impedances to match certain headphones better.Full class-A input and output stage (needed because a simple non feedback follower design is used).Having no active amplifying parts for voltage amplification in the audio path at all.Good sound quality and at least decent specifications.Must have a good square wave representation (phase relation) without ringing or overshoot.Headphones must be protected when the amp becomes defective.No DC voltage on the output and NO output coupling capacitors.Input impedance between 10 k & 100 k.A decent PSRR (power supply rejection ratio) meaning none of these unwanted signals enter the audio path.Amp should be reasonably small in size.Easy to build with readily available parts.If needed, must be easy to adjust.Must be stable under various types of loads.Stable over a reasonable temperature range.

  • This list of demands led to the following (basic) design of the amplifier:Only the relevant parts in the signal path are shown in the schematics below.

    The difficulty with the input design of this amplifier was the DC offset that is present at the Source of the input MOSFET as the Gate is referenced to ground and must be eliminated as the transformer may not receive DC voltage.Using a capacitor in this path leads to resonance points combined with the inductance of the primary coil and it must be a large value to 'conduct' the lowest frequencies which is not beneficial to the overall sound quality.This was solved by using a similar circuit as the input stage but referencing this to ground.Added bonus... this stage can also be used to create a symmetrical input if desired.Another big advantage is the signal (having a positive and negative half of the sine wave) is always flowing through a similar circuit (MOSFET + CCS) and not only the upper part of the sine wave through the MOSFET and the bottom half through the CCS to ground.More benefits are any unwanted signals on either, or both, power supply rails will be present on both input stages in similar phase and amplitude resulting in 0V differential voltage over the transformer input coils. This means none of these unwanted signals enter the transformer i.e. signal path.The input stages must be able to drive the transformer without clipping down to the lowest frequencies and preferably be in class-A. A transformer starts to behave like a resistance in the lower frequency region and this causes the impedance to drop to the low resistance values of the wires. To eliminate the small DC offset between the 2 input stages due to spread (tolerances) between the input MOSFETs, the reference stage has a current source that can be set to a value where the GS voltage of both stages are EXACTLY the same and thus no capacitor or other measures have to be used to prevent the transformer from receiving DC voltage on its input which would saturate the core, cause distortion and microphony of the transformer.

    The used transformer (OEP A262A3E or Vigortronix VTX-101-003) has 2 separate input windings and 2 separate output windings with an internal shield between the coils and an optional screening can around it to prevent influence by magnetic fields from outside the amplifier.It having separate windings makes various amplification factors possible such as 3.25x, 6.45x or 12.9x.Having to drive most headphones and considering DAPs must be able to drive this amplifier too 6.45x and 12.9x are pretty good values.

    The output stage, again, is a class-A stage and must be able to drive low Ohmic headphones to high levels. For this reason it must have a rather big current of minimal 0.3A which means a lot of power is dissipated and thus heat is generated. An output capacitor is not desired if it can be avoided.The only stable way to create 0V offset in this case is by using DC servo.In essence ta DC servo compares the average output voltage (below 1Hz, everything above it is discarded) to ground and adjusts the output stage to an average 0VDC continuously.Because a MOSFET has an input capacitance and a very low (leakage) current the DC servo does not need to deliver much current at all... only a DC output voltage to set the Gate bias to the proper voltage.

  • This translates in the following complete circuit diagram (only one channel and shared components drawn):

    Because the output stage may not receive voltages above a certain value, which transformers can easily deliver, a limiter is added. This is needed to prevent the output stage from clipping.This is done by Z1 and Z2 ensuring the output devices do not receive too much voltage.To make the clipping less 'violent' R9 is added which provides a form of soft-clipping which is much less 'harsh' as normal clipping because less harmonic levels are generated in case the limit level is reached.

    The DC servo is constructed with a simple op-amp that has low (enough) noise figures and low power consumption.As this part only needs to deliver a very small output current and already works on a 5V power supply, this can be made by using reference voltage IC's without the need of intricate power supply configurations for this specific part.

    When the amp is switched on the output must be muted for the short time it takes the DC servo to adjust the output voltage preventing a 'plop' or 'click' to be heard from the headphones or power amp if used as a pre-amp.When the amp is switched off the relay should mute the output immediately preventing unwanted signals at the output.Another important safety feature for this kind of DC coupled output stages is DC protection. This prevents the headphones from receiving DC voltages (above a certain value) should any output devices or one of the power supply rails fail. All of these functions come together in a very simple but effective protection part in this schematic.

    Some headphones prefer to be driven from 120 sources others may be driven better from low Ohmic (0 - 10 ).One could opt for switches with 2 or more selectable output resistances but to avoid more switches in the output path (the relay is a necessary one) 2 output sockets with a different output resistance are used in this amplifier. The 'LOW' output has 20 output resistance (but can be lowered to a few when R16, R17 and R18 are replaced by jumpers) and the 'HIGH' output is 110 . Both outputs can easily drive complex loads such as inductive, resistive or capacitive (up to 10nF tested) or combinations of this.

    This amp can be used as a preamp too so RCA outs (with an output impedance of 50 ) are available. It can drive all amplifier designs with input impedances between 50 Ohms and 1 M . Because of it's high output voltage capabilities care must be taken that amplifiers are not over driven or input stages are damaged. Additional resistors from the output RCA to ground can lower the max output voltage to much lower levels but R19 must be increased in value.

    The power supply can be very simple and consists of 4 x 1,000F with 10nF ceramic decoupling caps in parallel.2 x 1,000F per Channel situated near the MOSFET devices as the supply voltage is only needed for the class A input and output stages and not for voltage amplifying parts in the amplifier like most amplifier designs have when fed from a stabilised DC voltage (60 Watt switch Mode Power Supply in this case). The output- and input-stages have rather high voltages on them and relatively high currents running through them (class-A) and thus need to be cooled with decent heat-sinks that can dissipate the power (30W) and keep the amps temperature to acceptable levels by means of natural convection to avoid noisy fans. This amp is powered with a 60 Watt SMPS (switch Mode Power Supply).

  • Measurements:

    The standard frequency graph, like the ones that are usually given by manufacturers, is pictured below.

    How the frequency range extends outside the, generally accepted, frequency range is seen in the graph below.Note that the lowest frequency is 1Hz and the highest 100kHz.Each horizontal line is 1dB.

    below are pictures of the square wave reproduction which says something about low-, medium- and high-frequency reproduction and how harmonics are related to the ground wave and if ringing or overshoot is present.

    100 Hz showing signs of limited LF 1 kHz midrange showing accurate 10kHz showing wide enough frequencyresponse below 10 Hz reproduction without ringing response to reproduce higher harmonics

    Below the workings of the normal clipping behaviour versus soft-clipping.

    input signal & undistorted output input signal & clipped output signal input & output signal soft-clipped signal at maximum output level. as found in most amplifiers. a much more pleasant behaviour.

  • As seen in the graphs above soft-clipping is applied in this amplifier.Above their maximum output voltage amps clip rather abruptly which produces a nasty sounding distortion with a high content of even and uneven harmonics. It has to be noted that before this clipping level is reached the output level is high enough to play at ear shattering levels on high and normal sensitivity (efficiency) headphones.In case insensitive (generally found hard to drive) headphones are used and played at loud levels these clipping levels could be reached.

    clipped 1kHz sine wave (upper trace) soft clipped sine wave (upper trace) frequency spectrum (lower trace) frequency spectrum (lower trace)

    The left plot shows a normal clipped sine wave with the limiter disabled and is similar to what's found in most amplifier designs. It clearly shows the harmonics are 10dB louder compared to the soft-clipped signal on the right.

    On the left a THD plot of this amplifier at near max. output power (9V) into 32 load. The even harmonics are caused by the MOSFET output stage, the odd harmonics are coming from the transformer. THD at 1kHz of 0.017% is below audible levels.It should be noted that 1kHz is an easy task for most amplifiers.

    The same amplifier but now at 10dB below clipping level (3V). As can be seen the MOSFET distortion (even harmonics) are much lower but the odd harmonics (caused by the transformer) are still there.THD at 1kHz is lowered to 0,0085%

    Here one can see the distortion levels at different frequencies. This plot shows transformers are having a hard time below 100Hz and above 5kHz. This plot is taken near clipping levels (9.2V) and shows 2nd and 3rd harmonics dominating the spectrum.Below 50Hz and above 5kHz the distortion levels reach 0.2% which is audible.Most headphones, however, will be playing ear shatteringly loud at this level though.

  • Intermodulation Distortion

    Finally a plot of the IMD (@-15dB) between a 500Hz tone and 5kHz tone.As can be seen 3rd and 5th harmonics are there and IMD products at the same levels.IM Distortion levels are 0.04% which is on the high side of things.

    Specifications:

    Frequency range: ...................................................................... 5Hz to 35 kHz (-3dB), 16Hz to 20 kHz (-0.5dB) Phase shift between 50 Hz to 5 kHz: ........................................... +6 o to -10 o

    Maximum output voltage (load > 1 k): ....................................... 9.4VRMS (27VPP)Maximum output power on LOW-Z output (20 ): ....................... 1 W (into 32 )Maximum output power on LOW-Z output (20 ): ................... 250 mW (into 300 )Maximum output power on HIGH-Z output (110 ): ................. 140 mW (into 32 )Maximum output power on HIGH-Z output (110 ): ................. 160 mW (into 300 )Suitable headphones impedances: ........................................... 32 to 1,000 Output resistance: ..................................................................... 20 and 110 headphone, 50 line-out.Level difference between Left and Right channel: .................... < 0.5 dBGain is selectable by jumper settings in the amp: ..................... 6x (15 dB) and 12x (21 dB)Signal to Noise ratio: ................................................................. > 104dBTotal harmonic Distortion (1kHz):................................................... @ 1V < 0.008%, @ 4V


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