Giancarlo Valletta
It has always been a firm belief of mine that passive multi-amplification is a harbinger of excellent performance, succeeding in greatly improving performance even in mid- and low-level systems. And at the end of the day, its adoption is not that difficult. In the following article I will try to explain in a simple way why I favor it and what are the advantages and prerequisites needed to adopt it.
It can be done, Dr. Frankenstein said.
The current market moment sees the massive presence of stereo power amplifiers of various shapes and calibers of not exorbitant cost, with performance hardly comparable to counterpart models even of a few years ago, so much so that some enthusiasts are turning to passive multi-amplification. Obviously in concept-although practicable-is much more difficult to implement with an integrated amplifier because of the volume control essentially, but with a few tricks it is still practicable, although the best thing is to use pre or DAC and two power amps. However, this opportunity-the harbinger of superior performance-is too little used and understood compared to its true potential. But let us take a step back. To be employed, it is indeed necessary for speakers to be bi-wiring-ready, essentially giving the possibility of separating the “careers” between woofer and mid/tweeter in the case of 3-way or tweeter in the case of two.
Multi-amplifying passively in fact means dedicating one amplification section for bass only and one for mid-range or treble. But why this? The acoustic speaker has a rather low system efficiency. Do not think that all the energy we supply to it through the amplifier is transformed into membrane movement: far from it. The crossover filter first of all, and the loudspeakers second of all, are not only difficult, but extremely dynamic loads: they change electrical characteristics continuously as the membrane moves and as the frequency goes; and if this property is less influential for the tweeter and the midrange, which are called upon to reproduce high frequencies the former and medium frequencies the latter (and therefore with a high number of vibrations and small excursions), it is very much felt for the woofer, which is called upon to make slow, but always generous cone movements.
The crossover filter, for its part, in order to divide the frequencies and sort them to the various loudspeakers “wastes” an additional portion of energy, and the greater the number of components with which it is equipped, the worse the situation is with respect to the power supplied by the amplifier and dissipated for its operation. The 6 dB/oct filters from this point of view are the best, because they are simpler, although they need very good speakers with excellent out-of-band behavior to be adopted. If the damping factor-the dimensionless number expressing the driving capacity of an amplifier-has value 100 (for example), at the loudspeaker, adopting a 12 dB/oct filter, this factor becomes 3; with 6 dB/oct it rises to 20! In fact, we have to think about the presence of multiple paths and a more complex network that precipitates this value.
Passive multi-amplification
With this type of connection, we will then use two power sections for each speaker, specializing one amplifier to the bass only, and one to the mid-high or treble. To do this, it will obviously be necessary to double the speaker cables, and we will see below how to also perform an operation on the input PINs, which become two (L&R) for each channel, while the cable coming from the preamp or DAC is only one. The advantages are innumerable. To those who trivially ask if the power is doubled of course I say no, but it will greatly increase the driving capacity, and therefore the musical signal will appear extremely clearer and more detailed. This is because the amplifier dedicated to bass alone will not also have to contend with speakers and filter for the mid-high (or high) section-and vice versa-and therefore its final efficiency will go up considerably, so much so that it will be able to make much better use of the watts delivered by the amplifier, which in this way will “seem” many more. In these terms-and this is always true-passive multi-amplification increases perceived power precisely because it greatly improves the efficiency of the entire system. Most obvious consequence will be a real damping factor-that is, the one applied to the loudspeakers and mentioned earlier-extremely higher than it would be with normal driving. It goes without saying that before trying your hand at it, it is absolutely necessary to check whether your speakers have bi-wiring capability, but at present most models have it.
Bi-wiring and Bi-amplification
At this point it is necessary to briefly point out the difference-substantial-that exists between bi-wiring and a bi-amplification. The former simply involves double wiring, one dedicated to the bass section and the other to the mid-high section; the procedure although similar to that required for multi-amplification, leads to very different results. Bi-wiring assumes that each electrical conductor has its own intrinsic characteristics, and since the absorptions of the mid-high and bass sections are equally different, the conductor that is best suited for one or the other use is used; typically of smaller cross-section and higher conductive quality (silver for example) for the highs and mids, and of larger cross-section but perhaps worse conductivity for the lows. The improvement we will get will come only from the different signal transport. On the amplifier side, the cables will start from the same terminals. Multi-amplification, on the other hand, will involve different amplifiers for the two sections, and the power signal may also be carried by the most suitable conductor, adding both bi-wiring and multi-amplification improvements to the system.
Connecting inputs and controlling levels
Suppose we want to implement passive amplification and thus equip ourselves with two final amplifiers, one per channel. In this case it will be necessary to split -somehow- the input signal as well. There are two ways to do this. One is to use a single cable from the preamplifier or DAC to the power amplifiers and splitter on the latter with a Y or a “pigtail” as it is called in the jargon, or to use two signal cables for each power amplifier and pigtail on the preamplifier or DAC unless one of them is equipped with dual output (DACs never are, preamplifiers rarely are).
The first case is worse from the point of view of quality, I’m going to “load” with a double user a single cable, the second is undoubtedly the best, but it also means having to use two cables for each power amp, with a consequent increase not only in cost, but also in overall dimensions and ease of installation. So great is the difference in sound using two power amplifiers, that I have never given much thought to the problem of loading a single two-channel cable, and indeed, I always prefer if possible to move the power amplifiers close to the speakers, use a single XLR signal connection cable (provided PRE or DAC have balanced outputs), and use very short lengths of speaker cables of even not particularly high quality, in this way I maximize the total damping factor, and save considerably on speaker cables. Better for level control to prefer obviously identical power amps that do NOT have adjustments, so you are absolutely sure to get the same output level, which is also why matching different power amps is suicide. In fact, it is absolutely mathematical that for the same input signal, unequal amplifiers will have different gains, and therefore it will not be enough simply to connect them, but measurements that normalize the output levels will also have to be made. If a power amplifier with 1V (for example) at the input delivers 10W, you can bet that the other one you have chosen, perhaps with the same volt, will deliver 15W or 9W, resulting in an obvious level mismatch (as well as time, but we will see that later). The only difference is that while for the level I can do a few things (by framing a control if not already provided with the device) for the delay there is nothing you can do.
Horizontal and vertical multi-amplification
There are two types of multiamplification, the horizontal one, that is, using one stereo amplifier for the high or mid/high sections and the other, also stereo, for the low sections, or the vertical one-which I always prefer and apply-that is, using a stereo power amplifier for each channel, and thus amplifying high/mid with one section and bass with the other. The first case has many drawbacks. The power amplifiers have to be in the middle, they cannot be close to the speakers, and the one that amplifies the mids/highs of both channels makes much less effort than the other one, which will instead be subjected to much more effort. In the case, on the other hand, of the vertical, the same power amp will amplify on one channel the mid/highs and on the other the bass, and will therefore be more balanced than in the other case, as well as being able in this way to bring the power amp very close to the speakers, and as I said before to be able to use very short speaker cables.
But do the endings have to be the same ?
Of course, my opinions are personal and entirely debatable -we would miss them-, but on this issue in particular I am VERY adamant, and I do not compromise. The answer is: YES, the endings must ABSOLUTELY be the same. I have read of many fanciful realizations, with tube power amplifiers for the mid/highs (so as to take advantage of the self-styled greater smoothness of tubes for vocals, for example) and instead with transistors for the low section, so as to better govern the woofers. I think that those who make these claims do not take into consideration not only the level adjustment that we have already addressed, but also, most importantly, time. Any amplifier on the face of the earth, for any application, has its own transfer function and a time in which it can resolve the input signal
Equal amplifiers, except for infinitesimal deviations due to tolerances between components, have the same “timing,” respond in exactly the same way. Imagine starting two identical Ferraris at the same time, with the same drivers and the same fuel load, they will arrive at the finish line together (barring accidents, of course !). Already if we use the same Ferraris with different tires the result will not be the same. This example dropped into the audio field means that only if I use two identical power amps do I have mathematical certainty that the paths I feed separately will have the exact same impulse response. And mind you, timing is a fundamental thing in music reproduction, because frequencies have different wavelengths and different arrival times, and if we use unequal power amps we alter the balance. That’s why we align the speakers, perhaps by placing the tweeter in a separate, set-back enclosure, or simply working on the emission planes. If you notice, the woofer has a different center than the mid-range and tweeter even in speakers without special sophistication, and a lot of work is also done at the cross-over level for time alignment. Using two different power amplifiers, even worse if with different technologies (tubes and transistors in particular) completely distorts this concept, introducing real distortions that were not at all foreseen in the design, arriving among other things at sound “conclusions” that are totally unexpected, and total fruit of the improvisation of this or that enthusiast. It’s as if we were virtually and arbitrarily “shifting” the center of emission of the speakers (not to mention the response that each power amp has to the load, but let’s not complicate life too much). Suicide, as I see it.
Active multi-amplification
It behooves me at this point to make another clarification. What is the difference between active and passive multi-amplification? Plenty, but not too much. Take, for example, the most iconic speaker in the world of Hi-fi that uses the active method, dear Laurence Dikie’s Nautilus, which are 4-way, i.e. woofer, mid-bass, mid-high, tweeter. Each speaker has its own amplifier, so it takes 4 stereo or 8 mono amplifiers all EQUAL, of course, to make the spikes sound (and even here I have seen absolute bestiality with monotriodes feeding the tweeters, classic tube for mid-highs, hybrid for mid-lows, and solid state on the woofers: what a perverse fantasy !). The amplifiers are connected directly to the loudspeakers, and the crossover function is performed by an external apparatus-supplied with the speakers-and that stands between the preamplifier or DAC and the power amplifiers.In this case, the efficiency of the system will be maximum, as there is no waste due to the crossover, which also induces errors, such as phase rotations or impedances that are not linear with the frequency trend
This is the absolute best solution, because it allows perfect interfacing between amplifier and speaker, with nothing standing in the way. Technology compared to Nautilus, which is from the 1990s, has made further big steps forward, and so the amplifiers have become so small and powerful that they can be placed inside the speakers, along with the crossover filters that are no longer analog, as in the case of Nautilus, but digital and equipped with DSP. This even makes it possible to be able to use different frequencies and crossover slopes according to the musical content, so that the speaker’s sound can be optimized to the fullest. We owe all this to the professional market, which in the search for greater efficiencies has always preferred the active method, and now the one with DSP, which allows you to best shape the output according to the environment in which the speakers are installed, as well as the content they are to play. Already, DSP also has the additional great advantage of being able to apply a convolution curve in real time that takes into account the problems of the environment.
Is it worth it ?
I am a strong proponent of passive multi-amping, especially in inexpensive or relatively budget systems. Today an even relatively powerful stereo power amp does not cost astronomical amounts of money, and it is often advisable to double it, rather than buy a single more powerful one, also because of the considerations made for speaker cables, which can be much shorter. The second-hand market can come of valuable help, but be careful NOT to match, I will never tire of saying this, different power amplifiers, even if of the same technology and perhaps the same manufacturer. They are not and never will be the same. You will see that on listening the difference will be abysmal, you will make a remarkable up-grade of the system, without then spending astronomical sums. Remember that by doing so you get much greater drive and control of the speakers, things that are normally the preserve of very expensive amplifiers. For my absolute delight, I built myself a class A power amp of “almost” 50W per channel that in the same enclosure has 4 separate sections, 2 per channel, and built exclusively to apply this principle. The power amps are all the same, each component is identical to the other, the cables are the same length, in short, for sure I will have no time delays or level adjustment problems. Yet -just to make you understand how then nothing is perfect- the power supply voltages, for example, are not exactly the same, nor are the powers, but we are talking about a few millivolts and consequently milliwatts… One can tolerate it, taking into account that the speakers are not perfect either, on the contrary, they are much more imperfect than the electronics. Good listening !
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