How to Get The Best From Your Kit Part 8
Give me Bass
Having discussed the mechanics of PA speakers and amplifiers in parts 4 & 5 this article looks at how to upgrade a PA system by adding bass bins. A concept that every mobile DJ will no doubt consider at some time, all opinions expressed are in no way linked to the editor of Mobile DJ magazine.
Do I really need more Bass?
Oh be serious! What kind of sad DJ doesn't want more bass? How else would you deal with a reluctant audience if you couldn't bounce them on to the dance floor? One point to consider before you increase your credit card limit - again: take into account the type and size of the functions you perform at, the size of the room and on average how many people attend. Does your sound system as it stands do an adequate job for the majority of functions? If it does then for the odd function where you decide you might want a bit more 'Welley' in your sound you can opt for hiring the extra equipment as required.
What adding bass bins to your sound
a). You get more bass (Duh !).
b). The volume of your sound system increases, however, you can also achieve this just by purchasing a bigger amplifier for your current speakers - taking into consideration what's been discussed in the previous three articles in this series. But, increasing the volume over the whole frequency spectrum will eventually become painful on the hearing as the higher frequencies start to pierce the eardrums.
c). Adding bass bins achieves more than just an overall increase in volume, by concentrating the increase of volume in purely the lower frequencies the overall "presence" of your sound increases. The difference is between sounding loud and sounding big, a powerful bass sound will reverberate around the room and into your chest (but we aren't after de-fibrillation).
d). You certainly get more flexibility in the size of functions you can adequately cater for. A standard disco PA system consisting of a pair of full range speakers and an amplifier at about 1000 to 1600 Watts will cater for audiences up to 200 or 300 people. For larger venues and audiences you will find enhancing your sound system with a pair of bass bins will be beneficial.
Bass bins - purpose built for the job
So how do you increase the volume of the bass frequencies? It all depends on moving large volumes of air. There are two ways a DJ can do this; the first is to get a descent set of bass bins the second is to go on a diet of baked beans. One method will achieve a more practical, precise and less unpleasant result than the other. Bass bins are purpose built for the job of producing bass frequencies and will do so more efficiently than standard full range loudspeakers but this also means they are big and, unfortunately, heavy. Good bass bins will have at least a single 15 inch loudspeaker driver, a larger 18 inch driver will be able to produce even lower sub-bass frequencies down to as low as 30 Hz. Bass bins with two 12, 15 or 18 inch speakers will produce a more powerful bass sound but will need a larger amplifier to drive them (not to mention the fork lift truck to get them in and out of a venue). The greater the number of bass drivers hence the larger the bass bins - the greater the volume of air you can shift so the louder the bass sound. There are plenty of models to chose from including; Peavey Hi-sys 115XT, Hi-sys 118XT and Hi-sys 215XT, RCF Event ESW1018, JBL SR4715A, SR4718A, SR4719A and MR905.
Upgrading your sound system - two approaches
Lets assume you are choosing to upgrade your current sound system and to do this you'll keep your original full range speakers to use as mid / high cabinets and possibly keep your original amplifier as well. O. K. so you rich kids who are going to buy a brand new JBL concert system all in one go, just turn the air conditioning up in your Porche and chill out. The two methods of upgrading I'm about to describe can make a good upgrade path to a large Bi-amplified sound system for us mere mortals. The difference between the two methods depends on using a passive (post-amplifier) or an active (electronic pre-amplifier) crossover.
What's a crossover?
The ideal speaker would be a point source capable of producing the full audio frequency range. Allegedly one did exist several billion years ago just before the big bang. In the real world of compromises when a single speaker attempts to generate both high and low frequencies at the same time intermodulation distortion occurs. Visualise the back and forward movement of a speaker cone. Low frequency waves are greater in amplitude than high frequencies and so will move the speaker cone a greater distance at a slower oscillation rate. As the cone moves forward at low frequency the high frequencies produced by the same cone added on top of the low frequency oscillation get compressed and so are raised in pitch. As the cone recedes the high frequency signal is lowered in pitch. This change in pitch is known as the Doppler effect, it's the same effect used to describe the change in pitch of a train whistle as it passes you. The interference with the high frequency wave production due to the low frequency oscillations of the speaker cone is known as intermodulation distortion. The solution is to use multiway speaker systems splitting the frequency band between different purpose built drivers for the appropriate frequency ranges. A crossover is used to separate the audio frequencies and route them to the correct speakers, the bass frequencies go to the bass bins, the mid range frequencies to the mid range drivers and the high frequencies to the tweeters or compression drivers. However, as Mike Taylor interjected in a previous article that mentioned this subject, using crossovers introduces the problem of crossover distortion, cheers Mike! This means at the crossover frequency there will be an element of distortion introduced by the crossover process itself. Every time you process or alter an audio signal in some way you distort it, it's difficult to win. However, crossover distortion is easier to handle than intermodulation distortion and with careful engineering crossover distortion can be kept to a minimum. Multiway speaker systems will usually come with a built in crossover, for this article we need only concern ourselves with the low frequency crossovers required for use with bass bins.
The passive system
Some, but not all, bass bins come with a built in crossover. You connect the output from your amplifier to the bass bins and then connect a signal fed back out from the bass bins to your full range speakers as shown in fig. 1. The built in crossover in the bass bin takes the bass frequencies below about 150Hz to 250Hz and feeds it to the speaker driver(s) in the bass bin, it sends the remaining low-mid to high frequencies out to the full range speakers which are now used as mid / high speakers. An important point to be aware of is the additional loading the bass bins will add to your amplifier. To explain consider this example, suppose you currently own a pair of Peavey Hi-Sys 2XT full range loudspeakers rated at 350 Watts at 4 Ohm impedance. You decide to add a pair of Peavey Hi-Sys 115XT bass bins to your sound system using their internal passive crossover. These bass bins are also rated at 350 Watts at 4 Ohm impedance. The instructions supplied with the bass bins state that by connecting the bass bins to the Hi-Sys 2XT speakers using the bass bin's internal crossover will present an overall speaker impedance of 4 Ohm to the amplifier. This is governed by the design of the passive crossover, by connecting some speaker systems together the overall impedance load may be lower for example 2 Ohm. It's generally a rule of thumb that if you connect 2 speakers of the same impedance together in parallel the overall impedance is half that of the individual impedance value of the speakers. You need to know the overall impedance load so you can be sure your amplifier has the capability of driving that particular impedance load, some amplifiers can't drive less than 4 Ohm on each channel. Read the manufacturers instructions for both the amplifier and the speakers if you're unsure. Now you know the overall impedance you need to calculate the total speaker power. This is the sum of the power handling of the bass bins added to the power handling of the full range speakers. Be sure the power rating is measured the same way for both bass bins and full range speakers you need the RMS, or better still AES power ratings. In our example the power of the bass bins added to the power of the full range speakers is 700 Watts. So to prevent the risk of overloading the amplifier you need an amplifier that delivers at least 700 Watts per channel into a 4 Ohm load if you wish to minimise clipping the amplifier. (The subjects of speaker power handling and how amplifier's work were covered in the previous two articles). If you do try using too small an amplifier you risk damaging the compression drivers in the full range speakers since the bass bins and the main speaker drivers in the full range speakers are taking up all the power. So you have to consider the possibility of buying a larger amplifier if yours isn't up to the job. The main disadvantage with using passive crossovers is they are inefficient. Because they are employed after the power amplifier they have to be capable of withstanding the large power outputs involved, in our example the region of 700 Watts. The required high power handling limits the types of components that can be used in the crossover to the passive type components, hence 'passive crossover', typically capacitors, resistors and inductors. Because of the heat dissipation in these components they have to be quite large and by their nature this makes it harder to engineer precise crossover networks. In addition the heat generated uses up power output from the amplifier for something we don't want instead of sound production, which we do.
The active system
This time the crossover point is before the amplification stage as shown in fig. 2. The sound signal from the mixer is fed directly into an electronic active crossover, which you have to buy in addition to the bass bins. There are slight variations in the types of active crossover available we need a stereo 2 way crossover, examples include; Behringer Super X, DOD 835, Studiomaster C180 and the Peavey XD 3/4. These crossovers use more precise 'active' (hence the name) components such as op-amps and transistors etc. for the job of splitting the sound signal between the bass bins and the mid / high speakers. This enables the crossover process to be more tightly controlled and better engineered thus introducing less distortion at the crossover frequency. Because the signal is still at pre-amplification levels there's no need to worry about the components ability to handle the high power output, as is the case when using a passive crossover. However, because the crossover stage is before the amplification stage you now need two amplifiers.
So, in addition to buying your new bass bins and an active crossover you also have to buy a second amplifier to drive the bass bins and use your original amplifier to drive your full range speakers which are now used as mid / high cabinets. The obvious down side to this is expense, although the advantages are very beneficial, to start with you now have a serious amount of power to drive your sound system. The crossover sends all the sub-bass frequencies typically below 90 to 250 Hz to the amplifier driving the bass bins and the low-mid / high frequency ranges are sent to the second amplifier driving the full range speakers. What this process has done is to split the task of producing all the powerful bass frequencies between the two amplifiers. This means that where before your original amplifier's power was used up producing the full range sound to drive your full range speakers there is now more power available in this amplifier because it is producing just the mid and high frequencies. This extra available power means the top end will sound a lot clearer because of the extra available 'headroom' in your sound system. You will notice that your sound system is much louder at what were the lower volume levels on your mixing deck before you upgraded to using two amplifiers. You can also increase the volume on your mixer by a greater amount than before because your original amplifier will not start to clip so easily. Consequently at larger functions the volume is there when you need it. A point worth noting here is to check the power consumption of the two amplifiers if the total current required to drive then exceeds 13 Amps you will need to connect the second amplifier to a second wall socket, time to invest in another extension lead. Check that both power sockets are on the same phase or ring main.
2 is better than 1
Having two amplifiers removes the worry of 'what will I do if my amp blows?' Remember, the amplifier is the engine of your PA system, if you're only running with one amplifier and it breaks down your show's over if you haven't got a spare. Most other breakdowns in a PA system can be worked around, losing one CD player or record deck is inconvenient but not a disaster. If your running a bi-amplified PA system and one of your amplifiers breaks down, with some quick re-wiring you can be back up and running in 5 minutes. Bypass the active crossover by connecting your mixer output directly to one remaining good amplifier that in turn connects to your full range speakers. This will see you through the rest of the night, albeit at a reduced volume. I recently read an article aimed at mobile DJ's stating that professional DJ's never went to a gig with less than two amplifiers, this re-kindled the 'what makes a professional DJ debate'. This one's down to the individual, personally I have two amplifiers that I take out when I'm using my full PA system. I'll admit that if I'm not using my bass bins I usually go out with just one amplifier, normally because I'm in my car instead of a van. If you do use only one amplifier make sure it's a good one with a reliable track record and look after it.
What crossover frequency?
Another advantage of using an active crossover is you can control the actual crossover frequency thus enabling you to get the optimum performance out of both your bass bins and your mid / high speakers. When setting the crossover frequency always follow the advice given by the bass bin manufacturers, the operation manual for the active crossover should also contain some helpful advice. The actual process of getting it precisely correct involves going into the theory of phase and time alignment of multiway speaker systems. (Why? I don't know, try asking me one on 3rd century Byzantine Mosaics). It's very boring and not practical for a standard mobile set up anyway, it depends if you want to squeeze every last ounce out of your sound system or not. As a guide the crossover frequency for bass bins is between 90Hz to 250Hz, there should be little or preferably no vocals produced by the bass bins, that's the job of the mid / high speakers.
Passive or Active?
Having compared the two methods of adding bass bins to a PA system there are clear advantages in using an active crossover with a second amplifier compared to the passive choice. The only down side is the cost, the passive option being a lot cheaper as long as you don't have to buy a bigger amplifier. If you do then it's probably worth going for the active set up anyway. Also, not all bass bins come with built in crossovers, if you opt to buy some without then you have no choice but to use an active crossover system. If your budget won't stretch to the active bi-amplified system and you opt for bass bins with a built in crossover they usually have a way of bypassing the internal crossover so you can upgrade at a later date if you wish.
Tricks and Tips
1. Placing your bass bins together in an array, as in fig. 3, will increase efficiency.
Two bass bins side by side will provide +3dB more volume than the same two bass bins separated by a stage (this is the equivalent to doubling the power sent to your bass bins). Stack 2 more bass bins on top so you have four stacked in a square gives +6dB more volume. Why? Together they form a single source, apart they are discrete sources. As I mentioned in part 3 of this series, bass frequencies and high frequencies behave differently. Bass frequencies are non-directional and hard to localise, the spherical nature of bass sound waves means that having two sources some distance apart will introduce some phase cancellation in the middle. It's the same as dropping two stones into a pond together or a distance apart. Dropped together they cause larger waves in the water than when dropped apart when the waves will also collide with each other when they cross in the middle.
2. Run the bass bins in mono. As already mentioned bass frequencies are non-directional and hard to localise, hence they don't contribute to the stereo effect. If you have a bi-amplified system run the amplifier for the bass bins in mono, some amplifiers and electronic crossovers have a switch to enable you to do this. Otherwise just connect the left and right inputs from the crossover to the bass amplifier together. This eliminates possible loss of bass volume due to slight phase variations in bass frequencies between the left and right stereo channels. Keep the signal and the amplifier driving the mid / high cabinets in stereo to maintain the stereo sound where it is required in the upper frequencies.
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Copyright © 2000 Gerry Hayden.