What is the difficulty?

In the 11/2019 issue of Audio, the ATC SCM7 was featured in a test of five bookshelf speakers. A very respectable brand known to music lovers, and even more so to professionals, as many recording studios are equipped with its speakers. It is worth taking a closer look - but this time we will not deal with its history and proposal, but using the SCM7 as an example, we will discuss a more general problem that audiophiles face.

One of the important parameters of acoustic systems is efficiency. It is a measure of energy efficiency - the degree to which a loudspeaker (electro-acoustic transducer) converts the supplied electricity (from the amplifier) ​​into sound.

Efficiency is expressed on the logarithmic decibel scale, where 3 dB difference means twice the level (or less), 6 dB difference means four times, and so on. 3 dB will play twice as loud.

It is worth adding that the efficiency of medium speakers is a few percent - most of the energy is converted into heat, so that this is not only “wasteful” from the point of view of the loudspeakers, but further worsens their working conditions - as the temperature of the loudspeaker coil increases, its resistance increases, and the temperature increase of the magnetic system is unfavorable, which can lead to non-linear distortions. However, low efficiency doesn't equate to low quality - there are many speakers with low efficiency and very good sound.

Difficulties with complex loads

An excellent example is ATC designs, whose low efficiency is rooted in special solutions used in the converters themselves, and which serve ... paradoxically - to reduce distortion. It's about the so-called short coil in a long gapCompared to the typical (used in the vast majority of electrodynamic converters) system of a long coil in a short gap, it is characterized by lower efficiency, but less distortion (due to the operation of the coil in a uniform magnetic field located in the gap).

In addition, the drive system is prepared for linear operation with large deflections (for this, the gap must be much longer than the coil), and in this situation, even the very large magnetic systems used by ATK do not provide high efficiency (most of the gap, regardless of the position coils, it is not filled with it).

However, at the moment we are more interested in something else. We state that SCM7, both due to its dimensions (a two-way system with a 15 cm midwoofer, in a case with a volume of less than 10 liters), and this particular technique, has a very low efficiency - according to measurements in the Audio laboratory, only 79 dB (we abstract from the data of the manufacturer promising a higher value, and from the reasons for such a discrepancy; we compare the efficiency of structures measured in “Audio” under the same conditions).

As we already know, this will force the SCM7 to play with the specified power. much quieter than most structures, even the same size. So in order for them to sound equally loud, they need to be put more power.

This situation leads many audiophiles to the simplistic conclusion that the SCM7 (and ATC designs in general) requires an amplifier that is not so much powerful as with some difficult to determine parameters, capable of “drive”, “pull”, control, “drive” as would be “heavy load” i.e. SCM7. However, the more ingrained meaning of “heavy load” refers to a completely different parameter (than efficiency) - namely impedance (speaker).

Both meanings of "complex load" (related to efficiency or impedance) require different measures to overcome this difficulty, so mixing them leads to serious misunderstandings not only on theoretical but also on practical grounds - precisely when choosing the appropriate amplifier.

Loudspeaker (loudspeaker, column, electro-acoustic transducer) is a receiver of electrical energy, which must have an impedance (load) to be converted into sound or even heat. Then power will be released on it (as we already know, unfortunately, mostly in the form of heat) according to the basic formulas known from physics.

High-end transistor amplifiers in the specified range of recommended load impedance behave approximately like DC voltage sources. This means that as the load impedance decreases at a fixed voltage, more current flows across the terminals (inversely proportional to the decrease in impedance).

And since the current in the power formula is quadratic, even as the impedance decreases, the power increases inversely as the impedance decreases. Most good amplifiers behave this way at impedances above 4 ohms (so at 4 ohms the power is almost twice as high as at 8 ohms), some from 2 ohms, and the most powerful ones from 1 ohm.

But a typical amplifier with an impedance below 4 ohms can have “difficulties” - the output voltage will drop, the current will no longer flow inversely as the impedance decreases, and the power will either increase slightly or even decrease. This will happen not only at a certain position of the regulator, but also when examining the maximum (nominal) power of the amplifier.

The actual loudspeaker impedance is not a constant resistance, but a variable frequency response (although the nominal impedance is determined by this characteristic and its minima), so it is difficult to accurately quantify the degree of complexity - it depends on the interaction with a given amplifier.

Some amplifiers do not like large impedance phase angles (associated with impedance variability), especially when they occur in ranges with low impedance modulus. This is a "heavy load" in the classical (and correct) sense, and to handle such a load, you need to look for a suitable amplifier that is resistant to low impedances.

In such cases, it is sometimes referred to as "current efficiency" because it actually takes more current (than low impedance) to achieve high power at low impedance. However, there is also a misunderstanding here that some “hardware advisers” completely separate power from current, believing that an amplifier can be low-power, as long as it has a mythical current.

However, it is enough to measure the power at low impedance to make sure that everything is in order - after all, we are talking about the power emitted by the speaker, and not the current flowing through the speaker itself.

ATX SCM7s are low-efficiency (they are therefore "complex" in this respect) and have a nominal impedance of 8 ohms (and for this more important reason they are "light"). However, many audiophiles will not differentiate between these cases and will conclude that this is a "heavy" load - simply because the SCM7 will play quietly.

At the same time, they will sound much quieter (at a certain position of the volume control) than other speakers, not only due to low efficiency, but also high impedance - most speakers on the market are 4-ohm. And as we already know, with a 4 ohm load, more current will flow from most amplifiers and more power will be generated.

Therefore, it is important to distinguish between efficiency and tenderness, however, mixing these parameters is also a common mistake of both manufacturers and users. Efficiency is defined as the sound pressure at a distance of 1 m from the loudspeaker when a power of 1 W is applied. Sensitivity - when applying a voltage of 2,83 V. Regardless of

load impedance. Where does this "strange" meaning come from? 2,83 V into 8 ohms is only 1 W; therefore, for such an impedance, the efficiency and sensitivity values ​​are the same. But most modern speakers are 4 ohms (and since manufacturers often and falsely portray them as 8 ohms, that's another matter).

A voltage of 2,83V then causes 2W to be delivered, which is twice the power, which is reflected in a 3dB increase in sound pressure. To measure the efficiency of a 4 ohm loudspeaker, the voltage needs to be reduced to 2V, but… no manufacturer does this, because the result given in the table, whatever it is called, will be 3 dB lower.

Precisely because the SCM7, like other 8 ohm loudspeakers, is a "light" impedance load, it seems to many users - who judge "difficulty" in a nutshell, ie. through the prism of the volume received in a certain position. regulator (and the voltage associated with it) is a “complex” load.

And they can sound quieter for two completely different reasons (or because of their merger) - a loudspeaker can have less efficiency, but also consume less energy. To understand what kind of situation we are dealing with, it is necessary to know the basic parameters, and not just compare the volume obtained from two different speakers connected to the same amplifier with the same control position.

What the amplifier sees

The user of the SCM7 hears the loudspeakers playing softly and intuitively understands that the amplifier must be "tired". In this case, the amplifier "sees" only the impedance response - in this case high, and therefore "light" - and does not get tired, and does not have trouble with the fact that the loudspeaker has changed most of the power to heat, not sound. This is a matter "between the loudspeaker and us"; the amplifier does not "know" anything about our impressions - whether it is quiet or loud.

Let's imagine that we connect a very powerful 8-ohm resistor to amplifiers with a power of several watts, several tens, several hundreds ... For everyone, this is a problem-free load, everyone will give as many watts as they can afford such resistance, having "no idea about how that all that power has been turned into heat, not sound.

The difference between the power that the resistor can take and the power that the amplifier can deliver is irrelevant to the latter, as is the fact that the power of the resistor is two, ten, or a hundred times greater. He can take so much, but he doesn't have to.

Will any of these amps have trouble "driving" that resistor? And what does its activation mean? Are you providing the maximum power it can draw? What does it mean to control a loudspeaker? Does it just put out maximum power or some lower value above which the speaker starts to sound good? What kind of power could this be?

If you consider the "threshold" above which the loudspeaker sounds already linear (in dynamics, not frequency response), then very low values, on the order of 1 W, come into play, even for inefficient loudspeakers. . It is worth knowing that the non-linear distortion introduced by the loudspeaker itself increases (as a percentage) with increasing power from low values, so the most “clean” sound appears when we play quietly.

However, when it comes to achieving the volume and dynamics that provide us with the right dose of musical emotion, the question becomes not only subjective, depending on personal preferences, but even for a certain listener is ambiguous.

It depends at least on the distance separating it from the speakers - after all, the sound pressure drops in proportion to the square of the distance. We will need different power to "drive" the speakers at 1 m, and another (sixteen times more) at 4 m, to our liking.

the question is, which amp will “do it”? Complicated advice... Everyone is waiting for simple advice: buy this amplifier, but don't buy this one, because "you won't succeed"...

Using the SCM7 as an example, it can be summarized as follows: they do not need to receive 100 watts in order to play beautifully and quietly. They have to make them play nice and loud. However, they will not accept more than 100 watts, because they are limited by their own power. The manufacturer gives the recommended power range of the amplifier (probably nominal, and not the power that should be supplied “normally”) within 75-300 watts.

It seems, however, that a 15cm midwoofer, even as high-end as the one used here, will not accept 300W... Today, manufacturers often give such high limits on the recommended power ranges of cooperating amplifiers, which also has different reasons - it assumes a large loudspeaker power, but does not oblige besides this... it's not the rated power that the loudspeaker is supposed to handle.

May the power supply be with you?

It can also be assumed that the amplifier should have power reserve (relative to loudspeaker power rating) so as not to be overloaded in any situation (with the risk of damaging the loudspeaker). This, however, has nothing to do with the "difficulty" of working with the speaker.

It makes no sense to distinguish between loudspeakers that "demand" this amount of headroom from the amplifier and those that don't. It seems to someone that the power reserve of the amplifier is somehow felt by the speaker, the speaker reciprocates this reserve, and it is easier for the amplifier to work ... Or that a “heavy” load, even associated with low speaker power, can be “mastered” with a lot of power in reserve or short bursts...

There is also the problem of the so-called damping factordepends on the output impedance of the amplifier. But more on that in the next issue.