ULTRA AUDIO -- Archived Article
 

December 15, 2008

The High-End Solid-State Power Amplifier

Investment

So you’ve won the lottery, or inherited some money. More likely, you’ve been saving for a while. You’ve decided to invest in a high-resolution stereo system, but to make an informed decision, you’d like to learn more about the subject. Keep on reading.

Most salespeople prefer working with customers who have some idea of what they want. This encourages a more meaningful conversation with a greater likelihood of eventually leading to mutual satisfaction. The buyer should view this as a good strategy that helps insulate him or her from predators who sell products only to collect their sales commission without having properly evaluated the customer’s needs.

This article proceeds in alphabetical order, listing and defining many of the terms you’ll need to know to have a clearer understanding of power amplifiers. (Much of what follows is also applicable to preamplifiers and integrated amplifiers.)

Aesthetics and packaging

One of the most intriguing facets of modern power amplifiers is their appearance. Sales of a great deal of equipment is based on first impressions: not hearing, but seeing. Thick, anodized faceplates of brushed aluminum with proverbial belly-button On/Off switches, gold-plated machined terminals, sexy blue pilot lights -- while all of these can look impressive, they are also necessary functional attributes.

The elegant, stealthy behemoths of high-end power amplifiers are usually finished in black, gold, or silver. Complemented by rounded edges and extruded heatsink fins, many can deliver over 50 amperes of brute-force current into most any loudspeaker load and, with equal panache, gracefully exude the subtle nuances of a pianissimo.

Amplifiers are normally packaged very securely, often in inner and outer crates lined with molded polystyrene. Imagine being presented with an array of these, from which you must make an investment decision. I’ve often thought that modern hi-fi trade fairs have much in common with beauty pageants and fashion shows.

Architecture and internal layout

The internal layouts of high-resolution amplifiers are often joys to behold. Many designers use toroidal transformers in their power supplies, for many sound technical reasons. Banks of paralleled filter capacitors easily provide in excess of 100,000 microfarads of capacitive energy storage. These are mounted alongside other components on single- or double-sided printed circuit boards of glass epoxy made to military specifications. Most designers pay careful attention to ease of maintenance of their products by service personnel.

The location of the power supplies is optimized to reduce hum and background noise. More important, transformers should be quiet and free from mechanical vibration. Circuit layouts are often optimized through use of computer-aided design (CAD) software. To minimize losses and enhance safety by reducing current leakage, high-end manufacturers commonly use three-conductor detachable power cords, the DC resistance of which can be as low as a few milliohms.

Once, at a Consumer Electronics Show, a very-well-known company removed the casings of its products and replaced them with Plexiglas. Just looking at the architecture and layout of the circuit boards was enough. Ever since, I’ve always tried to get a peek inside as part of my overall assessment of a component. No one wants to know that, inside, their equipment looks like the proverbial rat’s nest.

Circuit topology

Many in the Who’s Who of the upper echelon of the high end design balanced circuits. In this topology, the positive and negative halves of the signal waveforms are amplified separately, then joined at the zero crossing. Improper connection of the halves can lead to an unpleasant phenomenon known as crossover distortion.

The complementary symmetry of a balanced circuit offers several design advantages. Probably the most important for you is the fact that balanced interconnect cables can carry audio signals hum-free over long distances, with negligible signal loss. This becomes important when you place your power amplifier between the speakers while your preamplifier sits on a rack far away.

Climatic and microclimatic operating environments

Although modern hi-fi gear is designed to operate reliably in a wide variety of climates, heat, moisture, and dust remain the nemeses of all electronics. To prolong the life of their gear, consumers must be aware of the actual microclimatic environment in which they expect their systems to operate. This is especially relevant in regions of climatic extremes.

Most prototypes undergo rigorous tests of their environmental and mechanical robustness: tests under conditions of dry heat, damp heat, cycling between the two, as well as dust, mold, bumping, dropping, and vibration. These are performed in controlled environments with varying degrees of severity. Failures are analyzed, and corrective action taken to ensure reliability. Bear in mind that most audiophile preamplifiers are designed to be left on all the time; hence, they have no On/Off switch. This alone justifies the elaborate precautions taken to reduce premature and/or catastrophic failure in the field.

Subsequent manufacturing processes and subassemblies also adhere to very stringent quality-control procedures. Components are chosen and properly derated to ensure that production models perform satisfactorily even under adverse climatic and operating conditions, including misuse and abuse. The likelihood of early failure is therefore drastically reduced.

Heatsinks are designed with low thermal impedance to efficiently dissipate energy through the surrounding air by means of conduction and radiation. You should therefore provide adequate natural convection, or movement of air, to ensure that the fins don’t run excessively hot. If they do, the thermal-overload protection circuits will be triggered to shut down your amplifier as a precautionary measure.

Always examine the warranties that cover the equipment you intend to purchase. Warranties of five or ten years -- or even for the life of the product -- are not uncommon in the high end, and serve as useful guides for assessing the expected useful lifetime of equipment. But be sure that you understand the subtle differences between warranties and guaranties. Don’t wait until you have a problem before filling out the registration form and returning it to your dealer.

Crosstalk

When the signal from one channel of an amplifier leaks into the other channel, whether due to inductive, capacitive, or conductive coupling, the phenomenon is known as crosstalk. Amplifier specifications refer to channel separation, expressed in decibels (dB), as a measure of this type of interference. A weighting curve is generally used in crosstalk specifications to compensate for the varying audibility of different frequencies to the human ear.

Stereo power amplifiers that use a common power supply for both channels are most susceptible to crosstalk. To overcome this problem, designers often use dual-mono configurations with independent power supplies on the same chassis. Better still are monoblock amplifiers -- a separately housed monaural amplifier for each channel -- which eliminates the problem altogether.

Distortion, slew rate, and damping factor

A power amplifier multiplies the amplitudes of incoming signals of all frequencies by the same factor and delivers the result to the loudspeakers. Departures from this linearity are called distortion. The clipping of waveform peaks results in harmonic distortion. Intermodulation distortion occurs when different frequencies in the signal interact to produce sum-and-difference components that were not present in the source signal.

Transient distortion results when an amplifier can’t respond quickly enough to the attack of rapid signal changes -- e.g., the roll of timpani, repeated cymbal crashes, or flamenco dancing. There is also transient intermodulation distortion (TIM).

The slew rate of an amplifier is a measure of how quickly its output follows the input, and is expressed in volts per microsecond (V/Ás). Amplifiers with high slew rates typically have low TIM characteristics, coupled with lightning-fast attack and decay times.

The damping factor is a measure of how quickly an amplifier can stop a transient without overhang. A function of the amplifier’s output impedance and the attendant ability of its power supply to quickly store and deliver large reserves of energy, damping factor is expressed as the ratio of the nominal impedance of the loudspeaker (typically 8 ohms) to the output impedance of the amplifier.

Dynamic range and signal/noise ratio

The dynamic range of a musical instrument, recording medium, or amplifier is the ratio, usually expressed in decibels (dB), of the difference between the softest and loudest levels that can be produced, captured, or reproduced, respectively. For an amplifier, the term signal/noise ratio (usually expressed in dB) is the difference between the noise floor and a predetermined reference level, usually full power or 1 watt, into a specified load. A signal/noise ratio in excess of 100dB is desirable.

Frequency response and power bandwidth

Continuous power defines a power amplifier’s ability to sustain its full output into a given load. Commonly called RMS power, it is usually specified at a frequency of 1kHz into a resistive load of 8 ohms. Power is measured in watts. This is one of the most important specifications of an amplifier, so be sure that you thoroughly understand its implications.

In the real world, two things happen. First, amplifiers drive reactive loads known as loudspeakers, whose impedance varies with frequency. Second, amplifiers boost complex signals that occupy the entire range of the audible spectrum, before delivering an output signal to loudspeakers. We need to study the behavior of amplifiers under these circumstances to determine whether or not they will perform efficiently at all times.

A graph of amplifier output vs. frequency will display a characteristic known in the industry as the amp’s frequency response. There are audio amplifiers whose frequency response is flat from DC all the way up to 100kHz or above. The designer also must consider how the impedance of loudspeakers varies with frequency, and the effect this would have on overall amplifier performance. This term is defined as impedance bandwidth. Finally, power bandwidth, the ability of an amplifier to sustain its rated output over the entire audioband, or range of frequencies audible to the human ear (ideally, about 20Hz to 20kHz), must be studied.

It is well understood that loudspeakers present reactive loads to amplifiers. The problem is that these loads vary from manufacturer to manufacturer. Dissimilarities also exist in the characteristics of loudspeakers belonging to the same family. It is time that the audio industry adopted a standardized reactive load for testing amplifiers. The International Electrotechnical Commission (IEC) should take the lead in this regard.

Input and output impedance, sensitivity

Impedance is the quantitative measure of the overall opposition of an electrical circuit to current; i.e., how much the circuit impedes the flow of current. It is similar to resistance, but also takes into account the reactive effects of capacitance and inductance. Impedance varies with frequency. Its unit of measurement is the ohm.

Input impedance -- the impedance seen by any source connected across the input terminals of an amplifier -- should be as high as possible. Typically, it can exceed 50k ohms. An ideal amplifier would have infinite input impedance.

Output impedance is the impedance seen by any device connected across the output terminals of an amplifier, such as a loudspeaker. This should be as low as possible, and typically is less than 0.1 ohm. The ideal amplifier would have zero output impedance.

The sensitivity of an amplifier is defined as the signal voltage needed at the input to deliver the amplifier’s full rated output. For a preamplifier, each input source -- CD player, tuner, tape deck, phono stage, etc. -- would be specified separately. When matching pre- and power amplifiers, ensure that the former can deliver sufficient voltage to effectively drive the latter for all input sources.

User manuals and acclimatization

"Reading time: 32 minutes." Sometime in 1982, after an enlightening trip to Binghamton, New York, I acquired my first set of new McIntosh instruments. I hurriedly unpacked the crates and began to install the system. After a while I noticed a user manual for the MC2500 power amplifier (serial no. CS2544) that contained the above words on its front cover.

Taken aback, I stopped in my tracks, located a stopwatch, and began to read. "Various regulatory agencies require that we bring the following information to your attention. Please read it carefully."

The Table of Contents enunciated:

  • Service contract information
  • Installation
  • How to connect
  • Front panel information
  • Rear panel information
  • Performance limits
  • Performance charts
  • Technical description
  • Block diagram

I then went on to digest the contents of the manuals for the C33 preamplifier (SN CX1682) and MR 80 digital FM tuner (SN CK3114), with reading times of 43 and 32 minutes, respectively. Sometime thereafter I assembled the system, but being totally drained from the overwhelming exercise, retired to bed. The equipment was first energized more than 24 hours later.

The wealth of information contained in these booklets made me realize that an entire department of McIntosh was dedicated to technical writing. Of course, I had seen all of the McIntosh operations at their factory, but the significance of the manuals had not registered at that time. Read instruction manuals carefully before proceeding to take your investment for granted.

More often than not, products shipped from far away pass through several different climates before arriving at their final destination. It is thus common sense to resist the temptation of instant hookup. I have always reminded my own customers that new equipment should be given a period of acclimatization before being powered up. I don’t know that any of them have ever taken my advice. Afford your system the courtesy of 24 hours’ acclimatization. This will also allow you more than enough time to fill out warranty registration forms and return them to the manufacturer or dealer.

Finally, it is wise to retain the original packaging, in the unlikely event that you have to return equipment for service. More important, it augments the resale value of your components when you’re ready for that inevitable upgrade.

. . . Simeon Louis Sandiford
simeon@ultraaudio.com

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