(Almost) everything you always wanted to know about bass amplification.
- The bass loudspeaker: construction and size
- The power handling of bass loudspeakers: fables and facts
- Loudspeaker magnets
- The importance of the right cabinet dimensions: standing waves
- The superiority of the vented-box loudspeaker system
- The positioning of bass cabinets
- Bass amplification: tube or transistor?
- Instrument cable
- Loudspeaker cable
- Power and power handling
- Signal amplification and distortion
- Efficiency and sound pressure
- Class-D amplifiers
1 The bass loudspeaker: construction and size
Without any doubt the bass loudspeaker has the greatest influence on your bass sound. Of course there are (cheap) bass amps that colour your sound without your permission, but bass loudspeakers always have a major influence on your sound, the extend of that influence depending on speaker brand and type. Sometimes , it results in a characteristic sound that is typical for a certain particular brand, but most bass players want loudspeakers to reinforce their natural tone of their bass guitar or double bass without any additonal colouration. The figure shows the construction of a bass loudspeaker.
Bass loudspeakers come in many different sizes. The most common sizes of loudspeaker for bass players vary from being between 10″ to and 15″, although some companies use other sizes too as well. In general small loudspeakers and cabinets have difficulties with reproducing sub-bass tones, but are good in reproducing mid-high to high tones. For large loudspeakers the reverse is often common. That is due to the fact that large loudspeakers have large cones that have a large moving mass, this being the weight of the cone plus voice coil.
The cone is usually typically made of paper due to paper’s superior properties. There are many alternative modern cone materials in the marketplace such as carbon, kevlar, and polypropylene, however paper has a trusted and proven track record.. Paper has great advantages because of a few superior properties.
The cone must be rather stiff and therefor should not be too thin, otherwise cone breakup spoils your bass sound. The magnet also plays a very important part in the control of the cone movements. Not only is the magnet the motor of the cone movements, but it also controls these movements so that these movemenmts won’t become too extreme. This control and drive function can only be guaranteed with a heavy magnet structure.
Smaller loudspeakers have lighter cones which can move faster and follow the signal easier. Bass players that slap a lot are usually fond of 10″ loudspeakers because of that quick response. But if you want massive sound pressure levels, you have to use large loudspeakers. Small and large loudspeakers are supplementary to each other. That is why many bass stacks consist of a cabinet with small loudspeakers (for instance 2×10″) an d a cabinet with a large one (usually a 15″). For that matter, differences between loudspeakers of the same size are just as common as between speakers of different size, depending on their electro-mechanical parameters. Hevos 10″ loudspeaker handles a low B as easy as many 15″ of other brands do.
A good bass loudspeaker has a stiff structure, which prevent unwanted movements. In the first place that concerns the chassis or frame, which are nowadays often made of aluminium in stead of the old fashioned steel sheet chassis of years ago. Suspension need to be stiff as well, because bass signals are very impulsive and should move the cone too extreme.
2 The power handling of bass loudspeakers
Bassists ask often how come that Hevos bass cabinets sound so well, even compared to famous foreign brands. Are other producers of bass equipment not able to do what Hevos does? It is not that simpel. Companies like Glockenklang, Eden and SWR often have large R&D departments, much larger than Hevos with its 2 companions!
As far as bass loudspeakers are concerned, there are two types of power handling that are important:
The first is the thermal power handling, in AES or RMS Watts. That is about heat dissipations, because about 96% of the electrical signal is transformed into heat. The diameter of the voice coil is important for that reason.
The second type of power handling has to do with the maximum peak lineair excursion of the voice coil in the magnet structure. Because of the heavy magnet structures of the loudspeakers that we use in our cabinets, it was possible to use long voice coils in the loudspeakers. The result is a high sensibility combined with a large Xmax. ( the maximum or peak linear excursion of the loudspeaker). It is usually defined as the distance the voice coil can travel in one direction and still maintain a constant number of turns in the gap of the magnet. This definition yields the maximum excursion before distortion occurs.
The reason why not every company uses long voice coils, is because they only work with enough sesitivity (98-100 dB/W/m) in very large magnet structures. And magnets are very expensive! So the Xmax is a very important loudspeaker parameter for bassists, because a signal from a bass guitar or double bass is very irregular. A bass signal will hardly ever reach the thermal edges of the loudspeaker, but can easily bring the loudspeaker to and even over its maximum excursion. That is why many bass loudspeakers sound lousy when you play your bass a bit louder than normal. It is just a matter of money!
Hevos uses only loudspeakers with heavy magnet structures. That is why Hevos bass cabinets sound clean even when they are pushed to the edge of their thermal power handling. Hevos uses loudspeakers made after Hevos standards.
3 Loudspeaker magnets
The “motor” of every loudspeaker is its magnet. The function of the magnet is to move its cone when a current is moving through the coil. A “heavy” (=strong) magnet is preferable to a light one. A strong magnet is not only capable of moving the cone in a proper way, but it also takes care of the cone following the signal in an accurate way. After all, there is a tendency that the cone moves through a bit when moving inward and outward. A strong magnet keeps these unwanted movements better under control than a light one. Furthermore, strong magnets make it possible to use long coils, preventing distortion in this way.
The magnets that are used for loudspeakers are all alloys of different metals. Well known magnet types are Aluminium-Nickel-Cobalt (Alnico), Strontium-Iron (Ferrite or ceramic magnets), Neodymium-Iron-Boron (Neodymium magnets), and Samarium-Cobalt. The last two magnet types are also known as “rare earth” magnets. Although guitar players often swear by Alnico magnets, this type is not used by bass players. Alnico gets quickly sarurated and distort the signal, which is often wanted by guitarists but for bassists unwanted.
Most popular magnet types these days are Ferrite and Neodymium. The Neodymium has the advantage of a much smaller size at a given magnetic strength. Volume of a Ferrite magnet is almost 10x as big as a Neodymium type, consequently resulting in the fact that a Ferrite magnet is much heavier than a Neodymium one. Especcially when saving weight is essential, which is often the case with bass cabinets, Neodymium can be interesting.
But every advantage has its disadvantage. Not only the price of Neodymium is higher than of ferrite, but it is also more sensible to higher temperatures. Under normal circumstances magnet will last a lifetime. A loss of magnetisme of about 1% is normal. But above a certain critical teperature the magnetism of a loudspeaker decreases rapidly. As far as Ferrite magnets are concerned that critical temperature is 300 °C, but the critical temerature of Neodymium is only 140 °C! When temperature exceeds the critical temperature of a magnet, magnetism will decrease rapidly and so does the usefulness of the loudspeaker.
The temperatures mentioned above seem abnormal high for a loudspeaker, but they are definitively not abnormal. Bare in mind that a common loudspeaker has an efficiency of maximal 4%. Which means that about 96% of the supplied energy is transformed into heat. Two bass loudspeakers, each fed with 500 Watts deliver the heat of an average electrical heater!
So cooling is a must for a loudspeaker. For Neodymium loudspeakers cooling is even absolutely essential to survive, given the critical temperature of 140 °C. Occasionally there are brands that do not worry about sufficient cooling. Afterwards the results are for their customers. Hevos uses also Neodymium magnets in some of its products, where weight is critical. All loudspeakers that Hevos uses are very well designed and do not suffer from inefficient cooling. In products where total weight is not critical, Hevos uses Ferrite magnets. Ferrite loudspeakers are heavier indeed, but this extra weight is compensated by using them in light-weighted cabinets.
4 Standing waves and the cabinet dimensions
Most loudspeaker cabinets have 2 or more parallel sides, which cause standing waves in between. These waves are present whenever the distance betrween 2 sides is as large or a multiple of half the wavelength of a tone. Very deep bass tones have large wavelengths and will not be able to produce standing waves in a small cabinet. But that changes with the hight of tones because of de decrease of the wavelength.
Now standing waves at themselves should not be seen as bad. However, they can disturb the sound of a cabinet when there exist clusters of standing waves at certain frequencies. Next graphics shows such clusters near 435 Hz, 545 Hz en 710 Hz. These clusters make this cabinet sounds bad. As you can see in the graphics, the number of standing waves increases with the rise of the frequencies, which is predictable. Above about 1000 Hz these clusters can easily be eliminated by means of a good damping of the cabinet, but for frequencies lower than 1000 Hz this is hardly possible.
How to prevent these clusters of standing waves in the lower registers? This should already be done in the phase of designing the cabinet, by applying the right dimensions. In the graphics below you can see the result: in the same cabinet als above we have made the distance between 2 opposite sides of the cabinet about 4 cm shorter, and the result is much better. The standing waves are more equally spread and there are no disturbances left.
There should be no doubt about the dimensions of Hevos cabinets. Each cabinet is designed in the most acccurate way. Beside that, we prevent standing waves in our bass reflex tunnels by giving them a conic form. The inner side of each bass reflex tunnel is one side of the cabinet and that alos prevents standing waves inside the cabinet. The result is that Hevos cabinets always sound clean and open. And with these optimal dimensions comes the advantage that the internal damping can be minimal, which increases its bass reflex qualities.
5 Bass reflex cabinets
All Hevos cabinets are vented box types. A vented-box is a cabinet with a so called vent, usually placed at the front side of the cabinet. This vent connects the interiour of the box with the outside. Here you see a schematic drawing of a Hevos cabinet. The vent is at the bottom and is an integrated part of the cabinet.
Wrongly, many people think that the purpose of this vent is to lead the air inside the cabinet to the outside. Reality is a bit more complicated. The air mass inside the vent is triggered at a specific frequency by the moving air energy from the back of the loudspeaker frame and is going into resonance. The whole secret is to make this air mass move at the right time and at the right frequency. Crucial in this matter are the position and the dimensions of the vent. Within certain limits the cabinet designer can choose at which frequency the vent is tuned.
Because of the the resonance of the air mass in the vent extra sound energy is freed. Here are some graphics that show how things work:
In the upper illustration frequency range of a loudspeaker in a closed box is shown. The so called roll offof the sound pressure levels at the left of the illustration (at lower frequencies) is 12 dB/octave for closed cabinets. This roll offstarts at the blue arrow.
In the second illustration you see the frequency range of the same loudspeaker , but now mounted in a vented-box. As you can see the vent delivers within a small range sound energy, with a maximum at the F-vent, the blue arrow. This is the frequency at which the vent is tuned. The total sound pressure can be seen at the next illustration:
The dotted line is the sum of the direct sound pressure at the front of the loudspeaker and the sound pressure coming from the vent. As one can see the sound pressure is straight from the F-vent upward, and only below the frequency at which the tunnel is tuned there is a steep roll offof 24 dB/octave. Such a tuning is normal for cabinets used in hifi sound reinforcement. At other tuned vents the sum line gets a totally different and irregular character, as you can see at the illustration below. This is sometimes applied in cabinets to create a pseudo-bass. Extra sound energy at frequencies between 70-100 Hz make non-critical listeners believe there is a good bass performance. Such a tuning is often used to hide a bad loudspeaker performence at low frequencies.
The tuning of the vent depends strictly on the use of the cabinet. For a cabinet that will be used for sound reinforcement of bass guitars and double basses a straight course of the sound pressure from the F-vent upward is less necessairy as it is with hifi cabinets. For bass reinforcement a more important feature is the power handling at lower frequencies, especially for basses with a low B string. And it is this specific tuning that makes the ‘secret’ of a well tuned vented-box.
The advantages of the vented box loudspeaker system are:
- The cone of the loudspeaker makes a smaller excursion at the same sound pressure, causing a smaller modulation distorsion and a better power handling.
- The cabinet has an extended frequency range at the bass side compared to a closed box.
- A higher output (theoretically 3 dB)
- Steeper roll offof 24 dB/octave.
Quoting a well known Dutch ex-soccerplayer: every advantage has its disadvantage. This is also true for vented-box systems. The disadvantages and the Hevos solutions are:
- Below the tuning frequency there is a very fast decrease of power handling. That is why Hevos tunes its vents at such a low frequency that Hevos cabinets can handle even low B strings with ease.
- Vented boxes allow only sparse damping. No problem for Hevos cabinets because the optimal cabinet dimensions are calculated with a special Hevos computer application. Internal damping is therefore hardly necessary.
- Standing waves in the vent. Vents in Hevos cabinets have a wedge or conic form and therefore disable standing waves.
- Possible moving air noise in the vent. Hevos vents have large dimensions. The edges of the vents are round shaped to prevent moving air noise.
In brief, with the right dimensions and other precautions, the vented box loudspeaker system is superiour for the sound reinforcement of both bass guitar and double bass.
Moreover, many manufacturers use a plastic port, of the type as shown below. This is done in order to save expenses. One of the disadvantages is that the cabinet cannot be tuned properly.
6 Positioning of bass cabinets
Positioning of bass cabinets strongly influences the performance of lower tones. When a loudspeaker radiates sound, the waves tend to focus more or less. Especially tweeters focus the high tones in a small bundle. This effect is very well observable: at the back side of a cabinet the higher frequencies are almost completely absent. Tones with high frequencies radiate in a bundle when the wavelength is smaller than or equals the diameter of the loudspeaker.
The lower the tones however, the more the tones are spread out. At the lowest frequencies of a bass there is no focussing at all: the loudspeaker radiates towards all sides with the same energy. That is why the positioning of a sub woofer is far less critical than a mid- or high loudspeaker. That is…… as far as the sound direction is concerned! A 15″ bass loudspeaker has a diameter of less than 40 cms, while a low E has a wave lenght of 340/42 = over 8 meters! Of course the dimensions of the cabinets are also a factor, but even the largest Hevos cabinet is small compared to this hugh wavelenght.
So, the positioning of a bass loudspeaker is not critical in relationship to the direction of the source. But as far as the radiated energy is concerned, the positioning is very important! Low tones radiate spherically. However, this spherical radiation is only possible up to a certain frequency, depending on the diameter of the loudspeaker. A 15″ bass loudspeaker has a spherical radiation up to about 900 Hz.
Putting a bass loudspeaker cabinet on the floor means that radiation is only possible in half a spherical way (halfspace). Sound energy that is radiated from the loudspeaker remains unchanged, resulting in a redoubling of the radiated energy in halfspace, compared to spherical radiation. So this results in a gain of 3 dB, so the energy of the lower tones double! The same effect occurs when the cabinet is placed near a wall. Positioning a cabinet both on the floor and close to a wall results in a quarter space, and the radiated energy increases with another 3 dB. Not much imagination is needed to understand what the result is of positioning a cabinet in a corner!
Now what is the optimal position for a bass cabinet? If you are looking for a lot of “free” bass then put it on the floor and in a corner. But there is a chance that you will get excessive bass tones, or that the sound gets boomy and undefined. Much depend also on the acoustic characteristics of the room or hall in which you are playing. Some rooms are almost incapable of creating a proper bass sound, while other spaces give you an abundance of bass, sometimes assisted by a wooden floor. In short, there is no standard answer to the question what is the best position for a bass cabinet. Putting the cabinbet on a table or seat will decrease the low tones for shure!
Stacking cabinets has more or less the same effect as halfspace has. It is called acoustic coupling, and it causes an increase in sound pressure of about 3 dB when stacking 2 cabinets. The cause of this effect is the fact that the loudspeakers in both cabinets have the same synchrone cone movement, and stacking doubled the total loudspeaker surface. Since the radiated energy increases with the square of the loudspeaker surface, the profit is 6 dB in total. 3 dB of it is originating from the doubling of the power coming from the amplifier. Stacking 2 cabinets will result in 3 dB extra “low”, as compared to 2 cabinets that are not stacked. That is why sound reinforcement systems always use stacked loudspeaker cabinets.
The best way to positioning your bass cabinets is the following. When you use 2 bass cabinets (or more), stack them and put them on the floor. If possible, keep some distance to walls, especcially corners. Only when acoustics are so lousy that you can’t get a decent bass sound, place your cabinets closer to the wall.
7 Bass amplification: tube or transistor?
The signal coming from your bass has to be amplified to produce sufficient sound pressure. Inside a bass amplifier there are two components taking care of this task: the tube and/or the transistor. Properly speaking, semi-conductor is a better name for transistor, since there are other semi-conductors doing the same job as the transistor, such as MOSFET, FET and integrated semiconductors like CMOS-IC. To keep it simpel we will speak about transistors when we mean semi-conductors.. Both types, the tube and the transistor , have there own advantages and disadvantages.
The tube is far out the oldest of the two. This product of the 19th century sustained up till now, which says a lot about its qualities. The tube got its substitute in the sixties of the last century, being replaced by the transistor in radios, amps and television. But it never lost its place in the reinforcement of instruments. And in the last few years there has been a revival of the tube in high-end sound reinforcement, due to the fact that there have been some electronical improvements giving the tube a second life.
What makes the tube such a popular thing? In the first place the tube has a very high internal resistance, making the input of an amp a very small load for the bass elements. And that has a positive effect towards the quality of the bass signal. This high internal resistance also provides a far better processing of impulsive signals than transistors do. And signals coming from basses are often very impulsive! Even if the tube should clip anyway, it sounds more pleasant than a clipping transistor does. It is called ‘soft clipping’. An other aspect are the overtones, which can not be heard seperate from the basic tone, but are very important for the colour of the sound. Tubes produce so called even (2nd, 4th….) harmonics, which sound more pleasant to human ears than the predominantly odd harmonics of transistors.
There are some disadvantages as well concerning tubes, such as its mechanical vulnirability, the heavy weight of powerfull tubes amps and the sensitiveness for radiation. Furthermore, tubes have aging problems, which alter their characteristics unwanted and uncontrolled. None of these things bother the transistor. One can produce a very powerfull transistor amp with a moderate weight. And there is no aging process that effects transistors. Besides that, transistor power amps do not need output transformers. Output transformers are notorious because of their saturation problems, causing strong distortions. And output transformers cause another problem: their so called damping factor is much lower than the damping factor of transistor power amps. A high damping factor means that the power amp has adequate control over the loudspeakers. Tube power amps control their loudspeakers worse, resulting in distortion. There is a relatively large sound pressure, but there is no tight control over the loudspeaker movements.
There have been many discussions about the question wether the transistor is superiour or the tube. The answer to that problem depends strongly on the users target. For high fidelity reproduction of music the transitor is superiour, due to the fact that distortion is almost absent in a good electronic design. Guitar players have other wishes: to them distortion is often an essential part of their sound. Tubes are unbeatable then. Every musician knows the raw and bity sound of a distort Fender guitar combo!
But here is a main difference between guitarists and bass players. Many bassists want an ‘honest’ amplification in the first place, sometimes filled up with a teaspoon of tubesound. The hybrid bass amplifier is a solution, offered by almost every bass amp manufacturer. These hybrid amps link a tubes preamp to a transistor power amp, using the the advantages of both the tube and the transistor. The tube does a good job with its soft clipping and good handling of impulses, while the transistor power amp can deliver many controlled Watts out of a few kilograms. And that is very important, since bass players need to have often 400 Watts of bass power just to cope with the screaming 50 Watts of the guitarist and to be heard well in the band.
Of course there are great advantages to the all transistor amplifier, especially when weight, dimensions (combos) and low distortion are important. Sophisticated electronic designs prevent ‘hard’ clipping in all transistor amps these days. On the other hand there are always bassists who produce their sound best with an all-tube amp, and take the heavy weight of such an amp into the bargain. There is no accounting for tasts, even in music.
8 Instrument cable
The instrument cable connects the bass to the input of the amplifier. For an optimal connection there should be several conditions to fulfil. In the first place the loss of signal due to the resistance of the cable should not be too great. Further more the all frequencies of the bass signal should be passed without any problem. In other words, what enters the cable should leave the cable without changes. And one should be able to step on the cable without damaging it. In addition, stepping on the cable should not cause a ‘crack’ in the bass signal. And finally, a good instrument cable should be flexible and stay flexible, even after long use.
Most cables meet the first requirement. Although the signal conductor has a very small diameter, resistance does not play an important part. The amplification rate of the amplifier is always so enormous that the very small losses in the cable are more than compensated.
But very important is the requirement that the cable should pass the bass sound spectre without reducing any frequencies. Many cables have dificulties with that. Of great concern in this matter is the capacity of the cable , that should be less than 80 pF/m, preferably.
The shield prevents interferences from outside entering the bass signal. A shield consists mostly of many thin copper wires, braided around the signal conductor. A shield of 100% is impossible, because the cable should stay flexible as well, but a shield effect of at least 90% is desirable. The degree of shielding depends e.g. of the type of braid.
When you step upon a cable, it causes small electrostatic currents inside the cable. These currents will cause noises at the signal and therefore have to be eliminated. Good cables have an electrostatic shield transporting these unwanted currents. Finally the outside jacket has to be strong and yet flexible, and coiling up the cable should be no problem even after intensive use. Rubber is o.k., but PVC is better.
Do cables exist that can cope with all these demands? Yes, and the Hevos instrument cable is one of them. Thanks to a capacity of only 70 pF/m the bass signal is passed without unlevelness. The shield does >95% and the electrostatic shield works perfect. The outer jacket is made of PVC, is thin and flexible (thanks to the special braid of the shield) and stays flexible, even after long term use. Each cable is mounted with Neutrik jacks.
Finally, some remarks about exotic cables, which are disposed for even more exotic prices. OFC (oxigen free copper) does not conduct any better than normal copper, but can be processed easier than ‘normal’ copper. So making play with OFC is talking hot air: It’s hard to find good non-OFC cables these days! Silver conducts the electric signal a little bit better than copper does. It is is hardly measurable, but some people seem to be able to hear the difference. Most ‘silver’ cables are actually copper cables with a thin silver coating on it. That can cause difficulties because of passage problems from copper to silver. This is so much important because of the so calles skin-effect.: electrical signals have a tendency to move along the outside of a conductor. Not recommended so.
9 Loudspeaker cables
In bass circles there is a lot of interest in instrument cables, as we have discussed above. Much less interest is there for the Ciderella of the bass equipment, the loudspeaker cable. Erroneously, as we shall see.
The loudspeaker cable forms the connection between the power amp and the loudspeaker cabinet. Just as all the other parts of bass gear it should have as less influence on your bass sound as possible. For loudspeaker cable it means that its capacity and self-induction should not be too high, and that resistance must be as low as possible. It is certainly not the intention to go too deeply into electronics, but some things should be mentioned though. Particularly a too high capacity can have a bad influence on upper tones; it can bring on even a shortcut for the upper registers. Luckily this is a mere theoretical problem with bass equipment, since this phenomenon occurs only at extreme cable lengths of a few dozens of meters.
The resistance of the loudspeaker cable is of much more interest. Thick cables are better than thin ones, but strangely enough this is not because of the loss of power that occurs in thin cables. Should that be the case, then a thin cable should be sufficient. The power cable of an electric heater of 2200 Watts is not extreme thick, after all, while no bass amplifier delivers 2200 Watts! But thin loudspeaker cables have a bad influence in another way: they decrease the damping factor of your amplifier.
A high damping factor makes your loudspeakers respond more accurate to a bass signal. Loudspeakers do have the tendency to still vibrate after a move. A high damping factor controls these unwanted movements of a loudspeaker. When connecting a loudspeaker cabinet to an amplifier by means of a too thin cable, the total damping factor will decrease and as a result, the loudspeaker will move uncontrolled, not capable of reproducing a tight bass.
For a good reproduction of the bass sound the total damping factor, so from amplifier plus cables, should be at least 50. Under 50 quality loss of the signal will occur quickly. Let’s do some maths… When one uses a loudspeaker cable of 2×0.75 mm² and a length of 1 m., and the damping factor of your amp is 1000 (which is very good), then the total damping will fall from 1000 to 117, assuming that one uses a 4 Ohm cabinet. Largely above the critical value of 50 still, but when you take in account the internal cables of the cabinet, the critical value will be reached in a glimp. A 2x 0,75 mm² cable in the cabinet itself will decrease de damping to 63! Sometimes bass cabinets, even of well known brands, use internal cables that can hardly be used for the lights on your bike! So thick cables are important! Using a 2×2,5 mm² cable increases the damping from 63 up to 217!
It can easily be understood now that an amp that has a modest damping will fast reach the critical point. Notorious in this case are tube power amplifiers, that almost always have a dramatically low damping. You can easily hear that because the signal is not tight and clean as from a high damping amp. Your bass sounds woolly, and at the same time sound pressure increases. Therefore an tube power amp of 100 Watts often delivers more sound pressure than a transistor power amp of 400 Watts! When you were looking for that typical woolly sound, the tube amp is ideal for you….not considering the heavy weight of such an amp.
Now knowing things about damping, it is easilly understood that the use of good quality connectors is very important. A bad contact between cable and amplifier or cabinet produces often a high transitional resistance, with all the consequencies for the damping of the system. Actually there is only one good connector for loudspeaker cables, and that is Speakon. Speakon connectors have the advantage that they are mechanically very strong and airtight. Jack connectors and XLRs are unsuitable for loudspeaker connections, although they still are in general use. Furthermore, jacks and XLRs are not airtight, resulting in background noise as a result of air movements inside the cabinet.
Conclusions can be make quickly. Always use thick loudspeaker cables and Speakon connectors. Round cables are mechanically stronger than flat cables. Furthermore, just have a look inside your cabinet. Did your cabinet manufacturer use thick cables, or did he make drastic cutbacks? And finally: have a look at the damping when you buy a new amp….unless you like a woolly bass sound.
10 Power and power handling
When you want to buy bass gear, power is an important aspect. The word “power” is often used for both amplifier and cabinet, which seems logic but is not. An amplifier can give a certain power. The larger this power the more energy is lead to the loudspeakers. The loudspeakers transform this electrical energy into acoustic energy: current becomes moving air. When this acoustic energy reaches our ear it will be transformed into an electric signal, that is sent towards the brain and makes that one experiences music or noise or voices etc.
A common bass amplifier delivers a power of about 400 Watts, but there are smaller and larger bass amps on the market. Whether 100 or 800 Watts will be enough for you depends on a few external factors, such as style of music, composition of the band, size of the stage and music hall, whether there is a PA and so on.
But there are also some internal factors that matter as far as amp power is concerned. One is how long an amp is able to deliver a certain power without damage. This so called long term power is a good measure for the real power abilities of an amp. A bass amp of 400 Watts should be able to deliver 400 Watts continuously without problems. But one should keep in mind that a bass player never asks 400 Watts from his amp continuously, a bass signal is rather a sequence of pulses than a continuous signal. So long term power is not important? Oh yes it is! The heavier the power supply of the bass amp, the better this amp is able to ‘follow’ the dynamic bass signal. One can notice that when slapping at the low E or B string. A bass amp should be able to deliver pulses up to 2000 Watts then, which is only possible when the power supply is heavy enough. Notorious are bass amps that sound ‘thin’ at low registers!
Unfortunately power data are not standard, and 400 Watts at one brand is sometimes not the same as 400 Watts at another brand. And having a look at the transformer is not always a good advice, since modern amps have sophisticated power supplies, that demand different properties from transformer that conventional power supplies.
On the other hand, loudspeakers do not deliver power, they handle power. The continuous power handling of a loudspeaker is the power (coming from an amplifier) that can be handled for a longer period of time without damaging the loudspeaker. Power handling is often specified in Watts RMS. That is short for “root mean square” and can only be used with AC current. In practice it means that the manufacturer feeds the loudspeaker with a sinus signal, sometimes pink noise, which was that strong that the loudspeaker was not damaged in a certain period of time. This RMS voltage of that signal was then raised to the square and divided by the nominal impedance of the loudspeaker, resulting in “power RMS”. Problem with this method however is that it lacks standardisation, both with the type of signal and the duration of the test. So every manufacturer measures power handling on his own private way, and as a consequence these measurements can not be compared.
Therefore it is much better to use the AES (Audio Engineering Society) standard. All measurement conditions are specified. But many manufacturers remain using the non-standardized RMS power, probably as a consequence of their conservatism. However, that does not cause a disaster, since power RMS rarely deviate from the AES values more than 10%. Moreover, other loudspeaker data are more important, as has been described earlier.
To conclude: a 1000 Watts amlipifier “sounds” only twice as loud as an amp of 100 Watts. A loudspeaker cabinet with a sensitivity of 100dB/W/m delivers twice the sound pressure of a 97 dB/W/m cabinet. The combination of a 100 Watts amp and a 100dB/W/m loudspeaker cabinet delivers the same sound pressure as a 1000 Watts amp and a 97dB/W/m cabinet. For your financial situation it is therefore often better to invest in loudspeakers with high sensitivity than to spend much money on extra amplifier power.
11 Signal amplification and distortion
The electrical signal that comes from the bass is small, often not more than a few millivolts. Basses that have active electronics deliver often more output, but that signal is also very moderate compared to the signal that finally leaves the amp on its way to the loudspeakers. Voltages of more than 50 Volts are not rare at the loudspeaker terminal. So an amplifier does what its name already did presume: it amplifies the tiny signal at the input a few thousend times.
The bass preamp consists of at least two parts: the amplifier and the tone control sub-circuit. The amplification sub-circuit amplifies the bass signal up to the level that is able to feed the power amp. Of course most preamps have several other sub-circuits, such as a symmetrical line-out and an effect loop, while some have a variety of other electronic gadgets. For all these circuits goes that there is an upper limit for the signal level. Exceeding this level means that there are problems for semiconductors in the circuit. Nasty distortion occurs, with sometimes cracking and hissing noises. But a too low signal level is also unwanted, causing a bad signal/noise ratio. Top bass amps are made with parts that have a high signal/noise ratio anyway!
One can control the volume of the bass signal by means of the gain- or volume potmeter, which can be found next to the input. With this potmeter one controls the strength of the signal towards the pre-amp. You can do that by ear, carefully noting if there is any distortion. Yet many bass amplifiers have a signal led, which turns to red if there is any distortion. If that should occur, one should lower the input signal by means of the volume potmeter at the amp or the bass itself. Usually bass amps have separate inputs for passive and active basses. That is necessairy because basse with active electronics inside often give a higher signal that passive basses do. Connecting an active bass to the passive input, one can hardly open the volume without distortion. There is hardly any control then, so active inputs have a sensibility that is 1.5 to 3 times less than an passive input. Incidentally, the output signal of both active and passive basse can vary much, so a try out is the best thing to do. Some active basses give such a low signal that one can use the passive input!
It matters wether your bass pre-amp is a tube or a transistor type. As far as pre-amps, the tube is superiour to the transistor, due to it very high input impedance, by which the pre-amp is hardly a load to the elements at your bass guitar or double bass, offering a very good signal quality. Furthermore a tube delivers nice harmonics that are easy on the ear. Whenever a tube is overloaded, the distortion is warm , the so called soft clipping. Many guitarists are nuts about this tube property, and so do many bass players. What a difference to a transitor distortion: a horrible shrill sound falls to you!
The sensibility of the power amplifier is between 0.5- 1 Volt. In order to get the pre-amp give enough signal to the power amp, there is often a master potmeter to control that. Best thing to do is to open the master knob far enough (12 o’clock – 5 o’clock) and set the right volume with the gain- or volume knob. But some brands advise to open the volume knob wide and control the overall volume with the master knob, indicating that distorsion increases heavily at higher volume levels (bad amp!).
The task of the power amp is simple: amplify the incoming signal several times. If the incoming signal is too small, the power amp will not be able to give full power. If the signal is too large, distortion at the loudspeaker output is the consequence. This is called ‘clipping’ and can have disastrous results for your loudspeakers. As a matter of fact, the loudspeaker output wants to deliver a higher voltage than the power supply gives, which is impossible. Not only is distortion a result of clipping , but the signal also gets dc-like features, bringing your loudspeakers to an end! Good bass amplifiers have clip protection therefore.
To conclude, the signal that leaves your bass depends very much on the way you strike the strings. Some bass players often have a strong strike, resulting in a large output signal, other bass players have a soft strike. When your signal is too large (distortion!), simple reduce it by turning the volume knob on your bass or amp. You do not lose power by doing that!
12 Efficiency and sound pressure
Efficiency and sound pressure have been dealt with on several occasions, without it being explained properly. So, time to dedicate some words to these issues. Efficiency is the degree to which a loudspeaker resond to a signal. Generally, this efficiency lies between 1% and 4%. A loudspeaker with an efficiency of 4% (which is high!) transforms 4% of the supplied electricity into sound. The rest (96%) is transformed into heat. Because of that, the contruction of a loudspeaker must be such that heat will be tranported to its environment.
High efficiency loudspeakers produce more sound pressure than low efficiency loudspeakers do. The amount of sound produced, or sound pressure, is expressed in dB. Agreed is the standard of dB/W/m, which is the sound pressure produced by a loudspeaker that is fed with a certain signal of 1 Watt. Since sound pressure decreases with distance, it should be measured at 1m from the loudspeaker. Loudspeakers for electric bass and double bass have at least a 95 dB/W/m level.
Efficiency of a loudspeaker depends on 3 things: the magnetic intensity in the magnet gap, the structure of the voice coil, and the surface area of the cone. The figure shows the voice coil moving in a very small gap in the magnet. The smaller that gap, the more powerful the magnetic field is. For loudspeaker companies it is a challenge to make that gap as small as possible, and still give way to an undisturbed movement of the voice coil. That requires skill.
The voice coil must stay in the gap at all times. Whenever the voice coil moves out of the gap, nasty distorsions occur. The voice coil consists of a number of coil turns of copper or aluminium, glued on a former. A single layer voice coil is often long and will therefore always stay in the magnet gap. So distorsion at this type of loudspeaker is low, but so is the efficiency. This type of loudspeaker is often used in hifi gear, and has an efficiency that is lower than 90 dB/W/m. Much too low for bass players gear!
Solution is to wind the voive coil in more layers. Output increases then, but the voice coil is shorter. At strong movement of the cone the voice coil can come out of the magnet, causing distortion. A lot of bass equipment (also of renowned marks!) use such short voice coils that sound well at lower sound levels, but will distort at larger volumes! The maximum movement for a voice coil and therefore for the cone without distortion is called the Xmax (see ” Power and power handling “). It is therefore better to make the voice coil not too short, but that requires a strong magnet, and magnet material is very precious.
The sound pressure also depend on the sensitivity of the oudspeaker and the surface area of the cone. A 15″ loudspeaker can move more air than a 10″ type. Hence an average 10 ” bass loudspeaker makes already a good turn with 96 dB/W/m, whereas its large brother of 15″ can often do 100 dB/W/m without much effort. Consider that an increase of 3 dB implies a doubling of the sound pressure! Tweeters generally have a much higher output than the ordinary dynamic loudspeakers: 105 up to 108 dB/W/m. For this reason combining a tweeter with loudspeakers needs a circuit reducing the tweeter output.
Combining loudspeakers means therefore considering the sensitivity of the used loudspeakers. Let us assume that we use a sensitivity 96 dB for 10 ” and 100 dB for 15″ loudspeakers. With a common combination of 2x 10 ” and 1x 15″ loudspeaker the sound pressure of both 10” working together doubles, and the sensitivity of both 10” speakers reaches then 99 dB/W/m. As a result, the total sound pressure of both 10” loudspeakers approaches the 15″ loudspeaker, and the combination of all loudspeakers will give a balanced sound. But if you combine only 1x 10” with 1x 15″ loudspeaker, then the largest speaker will dominate, often leading to an unpleasant sound. Stacking loudspeakers causes a particular phenomenon: the total sound pressure of the stack is larger than the sum of the sound pressure of the separate loudspeakers. This phenomenon is called acoustic coupling, and in the PA world this is frequently applied at festivals etc.. For bassisten stacking makes also sense!
Finally a few words about lower and middle class bass loudspeakers. The magnet is by far the most expensive part of a loudspeaker, chassis and cone are relatively cheap. A strong magnet makes the loudspeaker powerful, but also expensive. Hencemany manufacturers save on magnet material. To compensate the weaker magnet, they use short voice coils with multiple layers, increasing the efficency of the loudspeaker. As long as you play in a moderate style, these loudspeakers do often sounf well. But playing loud, or using slap technics, these speakers distort. For this reason a cheap loudspeaker is always a moderate or even bad loudspeaker.
13 Class-D amplifiers
Class-D power amplifiers have an efficiency of >90%, while conventional amplifiers never cross 80%. This explains why class-D amps hardly need any forced cooling of the power transistors and can be built very small. Anyway, the “D” does not stand for “digital”, but is simply the next character in amplifier classification. What class-D distinguishes itself for all other types of amps is that the power transistors are always open or closed, they behave as switches.
Class-D amps switch between 2 signal levels.The audio signal goes into one input of a comparator, while the other input is fed with a sawtooth signal of high frequency. As a result the output of the comparator knows only two stages: high and low. It is an example of so called Pulse Width Modulation. The power transistors, in the Hevos amps MOSFET’s, amplify this signal, that also has only two levels: high and low. As a result hardly any current is dissipated into heat, and the amp has a very high efficiency.
Of course it is necessairy to filter out the high frequence signal in order to get the clean bass signal out of the loudspeakers. A simple passive low-pass filter between power stage and output does the job. Additional advantage is that it also filters out the noise originated from the switching power MOSFET’s.
Every advantage has its disadvantage, that law is also in force for class-D amplifiers. First problem is that without measurements distortion is considerable as a result of the fluctuations of the power supply and timing failures of the switching MOSFET’s. Changes in the voltage as a result of pulling high currences have immediately influences on the amount of distortion at the loudspeaker output. And internal fluctuations of the power supply, like ripple, are hearable at the output. A very stable and technically advanced power supply is therefore a need. The second cause of distortion are timing faults as a result of the switching power MOSFET’s. A third problem is the frequency response at the output, which is load dependent.
Considering the above-mentioned, it is clear that a class-D amplifier itself does not guarantee a high fidelity sound reproduction. A well known American brand put a class-D bass amp at the market some years ago, and pulled it back soon afterwards because of severe problems. It may be clear that Hevos would never take the risk to do such a thing. The above-mentioned problems are solved recently by using a Philips technic, called UcD. It is no more and no less than a sophisticated analog feedback circuit, resulting in extreme good properties of the Hevos class-D amp. Result: a high end amplifier with only a few discrete components, as small as a packet of cigarettes.