I. "What is the sound of one transistor clapping?" There are two most essential principles to audio amplifier design. The first is simplicity. The second is linearity. Einstein said, "Everything should be made as simple as possible, but no simpler." Simplicity is a common element of the best and most… More...

I. "What is the sound of one transistor clapping?"

There are two most essential principles to audio amplifier design. The first is simplicity. The second is linearity.

Einstein said, "Everything should be made as simple as possible, but no simpler." Simplicity is a common element of the best and most subtle designs. It is preferred for purely aesthetic reasons, but also because fewer elements color the sound less, and lose less information. Many audiophiles, including myself, are willing to sacrifice other areas of performance to achieve the intimacy with the sound available through a simple circuit.

An amplifier should be simple, but it also must be linear. Some measure of distortion in an amplifier is unavoidable and forgivable if it is of a less offensive type, but still it is important that the measured distortion performance be reasonably low. The advantage of a simple circuit is lost if the sound is overlaid with an excess of false coloration.

Many complex topologies have been justified by high quality of measured performance. By objective criteria, this is a perfectly valid approach. There are many applications where the need for measured precision is important and subjective performance is unimportant. Any application where the performance is crucial to obtaining accurate numbers, such as in an MRI field amplifier, should be judged by objective means.

But this is not rocket science; our objective is to make listeners enjoy sound. If we justify this approach by calling it art instead of science, that is perfectly fine, even preferable.

Resolving the apparent conflict between simplicity and objective performance is our goal. Commercially available power amplifiers have as many of 7 gain stages in series. The simplest I know of still has 3 stages. This succession of gain stages is essential to build up excess gain that can be used for negative feedback. The feedback is used to correct the performance of the gain stages. Paradoxically, the extra gain is used to correct the extra distortion of the additional gain stages.

How simple can we make a circuit and still have it perform well? Obviously an amplifier with a single gain stage will be about as simple as we can topologically create, and we ask the question, "How much performance can we get out of a single gain device?"

II. Single Ended Class A

Only one approach is available for linear performance from such a simple circuit: Single-Ended Class A. It was the topology in the earliest use of gain devices (tubes, of course), but has not been widely employed in the output stages of solid state power amplifiers due to its

Push-pull circuits have higher efficiency, and they also have an advantage in being able to source current in excess of the idle, or bias, current, by dropping into a lower class of operation. A Push-Pull Class A amplifier idling at a 1 amp bias current can deliver 2 amp peaks before leaving Class A, and can deliver still higher currents considered as a Class AB amplifier, where one half of the amplifier experiences cutoff, and does not carry the signal for a portion of the waveform. By contrast, Single-Ended Class A amplifiers cannot linearly deliver current beyond their bias point, and they generally must dissipate at idle more than 4

times their rated output. Typical efficiency is about 20% maximum.

This tremendous inefficiency alone explains why Single-Ended Class A has received limited attention, although careful consideration of possible circuits reveals that efficiencies approaching 50% are possible. In addition, there are ways in which a Single-Ended Class A amplifier can be operated as a push-pull device beyond its bias point, the assumption being that push-pull performance is preferable to clipping. Pass Labs has received one patent and has an application for another reflecting new developments in this area.

Figure 1 shows a simple example of a Single-Ended Class A circuit. In this case the gain device is a FET, although the concept applies equally well for a tube for bipolar transistor. The input signal is applied at the gate, and the transistor provides current and voltage gain which appears at the drain. The gain stage is biased by some form of impedance which sources the bias current to the transistor.

This impedance might be a resistor, or it might be a constant current source, or it might be some other load, such as a loudspeaker. Because this element carries the DC bias current, it is unlikely that we would want to use a loudspeaker for this, and typically we would want to attach the loudspeaker in parallel with the bias element, in series with a blocking capacitor.

If the bias element is a resistor, we see a typical efficiency of about 4%. This means we idle the circuit at 100 watts and have a maximum output

energy inefficiency. Single-Ended Class A operation has received increased attention lately, primarily from tube enthusiasts, and recently a number of companies have introduced tube SingleEnded Class A amplifiers. They are characterized by limited power, high cost, and multiple gain stages.

I published a 20 watt bipolar Single-Ended Class A design in 1977 in Audio Magazine, and it had four gain stages. Pass Labs has been manufacturing the Aleph series of Single-Ended Class A amplifiers since 1992, and they have three gain stages. I am unaware of other solid state offerings in the US, although I expect that my hegemony will be shortlived, with the imminent appearance other single-ended transistor amplifiers.

Simplicity is not the only reason for the use of the single-ended topology. The characteristic of a single-ended gain stage is the most musically natural. Its asymmetry is similar to the compression / rarefaction characteristic of air, where for a given displacement slightly higher pressure is observed on a positive (compression) than on a negative (rarefaction). Air itself is observed to be a single-ended medium, where the pressure can become very high, but never go below 0. The harmonic distortion of such a medium is second harmonic, the least offensive variety.

It is occasionally misunderstood that single-ended amplifiers intentionally distort the signal with second harmonic in order to achieve a falsely euphonious character. This is not true. Low distortion is still an important goal, and it is my observation that deliberate injection of second harmonic into a musical signal does not improve the quality of sound.

Single-ended amplification is distinct from push-pull designs in that there is only one gain device for each gain stage, and it carries the full signal alone. Linear singleended designs operate only in Class A.

In contrast, push-pull designs share the signal between two opposing devices, one concentrating on the positive half, the other the negative half. This positive/negative half of an audio signal is an artifice imposed by the desire to efficiently handle an AC only signal, with no DC component. Most Push-pull Class A designs offer energy efficiency of twice that of most single-ended designs, and they also offer a measure of distortion cancellation.

A well matched push-pull pair of gain devices will have lower measured distortion due to cancellation, and will concentrate the harmonic content into third harmonic and other "odd" harmonics, reflecting the symmetry between the plus and minus halves of the waveform. Operation is possible in Class A, Class AB, and Class B modes. The most linear of these is Class A, in which the circuit will dissipate at idle more than twice its rated output.

of 4 watts. We can dramatically improve the efficiency if we separate bias current from signal current, so that the bias source handles purely DC, and of course our blocking capacitor insures that the speaker sees only the AC portion of the signal.

We can achieve this by biasing the circuit with a constant current source and the efficiency climbs to about 20%, or about 5 times better. The constant current source provides only DC current which does not vary with the signal. In addition to the improvement in efficiency, the constant current sources removes power supply noise from the bias and provides an absolutely constant load for the power supply. As a result of this absolutely constant power supply draw, it becomes relatively unimportant what resistance is seen in the power source circuit, and two channels can draw off of one supply without modulating each other's signal.

Clearly the use of a constant current source for the bias is justified by the performance over the use of a resistor, although not every singleended designer agrees.

III. Mosfets

We have to consider what type of gain device is suitable for this application. Actually the choice is simple: Bipolar devices have too low an input impedance to be useable, and tubes have too little gain to be used in a single stage power amplifier. The only usable device is the power Mosfet, which is a transconductance device like the tube, and has a high input impedance, but operates with enough gain at high enough currents to be driven directly by line-level signal.

As a coincidence, the Mosfet happens to be my gain device of choice in general. Even in more complex circuits Bipolar transistors do not have the transconductance characteristic I find desirable, and tubes require a coupling transformer, with its attendant degradation.

The use of transformers as a load for a single-ended circuit is very problematic, since the DC current passing through the transformer tends to saturate the core. The solution that has been employed is to use a core with an air gap, wish' the result that the primary and secondary coils become only loosely coupled, with performance suffering even more.

Mosfets have not been adequately appreciated for their characteristics in high end audio. I believe this is due to the relatively lackluster sound associated with the commercial offerings to date. This is not the fault of Mosfets, however. Examination of the amplifiers on the market reveal two serious flaws in how Mosfets are being used for amplification. First, nearly all designers simply dropped Mosfets into the same (complex) topologies that were developed for bipolar devices without regard for their special characteristics. This unimaginative use of a new gain device results in a sound that is marginally different than the original bipolar circuit, and not much of an improvement.

The Mosfet designs on the market are also Class AB designs. The transfer curve of Mosfets reveals serious nonlinearities at low bias currents, resulting in crossover nonlinearity in push-pull designs. This design flaw makes for a sonic signature that many have referred to as "Mosfet mist", where a loss of detail is apparent. To fully realize the

Copyright 1994 Nelson Pass

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