Setup Tips for Amps and Speakers
PLUG & PLAY
By Steve Rabe, SWR founder
© Miller Freeman Inc., reprinted by permission
Originally appeared in the July/August 1992 issue of Bass Player magazine
Before leaving for your next gig, make sure you've got the following: an AC-receptacle tester, spare fuses for your amplifier, and two 9-volt batteries. These items won't take up much space - in fact, they should all fit in your bass case. Why do you need them? Let's find out.
After arriving at the club and getting your gear onstage, the first thing you'll do is locate the nearest AC outlet. But wait—before you plug in your valuable amp, grab that receptacle tester and check the socket. These little gizmos are available at most hardware and electronics stores for less than five bucks, and they're worth their weight in gold. In less than five seconds, the best type will tell you the exact nature of any problem that exists. Armed with one, you will know beforehand why your amp won't go on, why it will blow a fuse, why it will hum like crazy—or why you will get shocked.
Okay, having found a good, well-grounded outlet with proper line voltage, you're ready to set up your system. First, check your cables. Every cable you use for your bass or for patching in effects units should be made with shielded audio cable. Make sure the cables you buy are specified for musical-instrument use, and avoid ones with molded ends, because they're next to impossible to repair. To avoid hum, all instrument and patching cables should be as short as possible.
When using an effect, it's generally best to use the effects loop found on most modern amplifiers. This should reduce the hiss coming from your speakers, because most effects loops are located after the initial preamp gain stages in the amp. Therefore, noise generated by the effect will be amplified less overall. However, some effects units on the market (even today) have gain loss—that is, the signal level out is less than the signal driving the effect. With these units, the volume with the effect bypassed is greater than when the effect is activated. This change in volume will be more prominent when the effects loop is used - so, if the effects loop does not have a level control to compensate for this, or if the change in level is too drastic, you may have no other choice than to place the unit between your instrument and amp. Before purchasing an effects unit, try it in the effects loop of your amplifier, preferable before your next gig.
When you're plugging in, never route instrument cables directly over or under your amplifier. Because of the high impedance (resistance) of most pickups and the high input impedance of your amp, hum can be injected into your cable from the internal power supply of the amp. This is especially critical in rack systems.
Onward to your speaker cabinets. Speaker cable should be made of 18-gauge, or heavier, wire. (The thicker the wire, the lower the gauge, so 18-gauge is heavier than 20-gauge and so on.) Do not use instrument cables to hook up your speakers. This can result in intermittent power loss, cause your power amp to oscillate, and damage itself and/or your speakers, and render the cables useless for any purpose.
If you're having a problem with your speakers and suspect one (or more) of your cables, one of those 9-volt batteries now becomes a cable-tester. Plug one end of the questionable cable into your speaker cabinet, and then touch the phone plug on the other end to the two terminals (+ and -) of the battery, contacting the tip and sleeve. When you connect the battery to the phone plug, a good cable will will pass the voltage to the speakers, which will be indicated by both an audible noise and the physical reaction of your speakers: the cones will move out. Disconnect the battery, and the cones will move back in. (Reverse the battery, and the speakers will move in when connected.) If you don't hear anything and your speakers don't move, then the cable is bad.
You can test for an intermittent cable by keeping the battery on the phone plug while swinging the wire like a jump rope. If the cable is good, the speaker will remain in its battery activated position and not make any noise. This test can be especially handy after making new cables or repairing old ones, and it can also be used to check speaker phasing - more on that soon. (A couple of related notes: Holding a battery on a phone plug continuously will drain the battery quickly, so don't overdo it. Conversely, this test will tell you if you have a dead 9-volt; if you know the cable is good but the speakers don't move - toss the battery.)
Before you turn your amp on, make one last check to be sure all of your cables are connected properly—especially your speaker cables. If you have a loose speaker cable that gets plugged in while you're playing, it could cause your AC or speaker fuse to blow. For reasons like this, it's always wise to carry several spare fuses.
Now you're plugged in and switched on, but during soundcheck you find out you're not quite loud enough to carry the room. A member of another band generously offers the use of his speaker cabinet. That's great, but there are three questions you should ask before you hook it up: (1) What's the impedance of your cabinet, and what's the impedance of his? (2) What will the total combined impedance be? (3) Is that a safe load for your amplifier?
When an additional speaker is hooked up to a system, it's generally connected in parallel to the other speaker(s);see Fig. 2. This is the case when you use the extension speaker jack(s) on your amp, or the in/out jacks (if provided) on the input panel of your cabinet. As you add speakers in parallel, the total impedance the amplifier "sees" becomes less. Also, other than plugging in an additional speaker in an existing jack, no other wiring is necessary. In a series arrangement, on the other hand, the existing wiring must be broken and the new speaker inserted; in this case, the total impedance becomes greater. Let's look at parallel impedance, since it's more common.
To figure out the total impedance of two or more cabinets of equal value hooked up in parallel, divide the impedance of one cabinet by the number of cabinets:
impedance of one cabinet / number of cabinets = total impedance
Let's say the enclosure you own is 8 Ohms and the cabinet you borrowed is 8 Ohms. The formula is 8 / 2 = 4, so the total impedance will be 4 Ohms. Likewise, if you had four 8 Ohm enclosures, the total impedance would be 2 Ohms ( 8 / 4 = 2). Now, it may be just your luck that the second cabinet is 4 Ohms and yours is 8 Ohms. No problem - just think of the 4 Ohm cabinet as two 8 Ohm cabinets (we know this is true from the first example), so you now have, in effect, three 8 Ohm enclosures. The formula is 8 / 3 = 2.67.
The owner's manual that came with your amplifier should state the lowest (or minimum) impedance your amp is designed to drive. This may also be indicated next to the speaker output jacks. Since the total impedance of the cabinets you want to use is 4 Ohms, your amp must have a minimum load rating of 4 Ohms or less. This being the case, you turn off your amplifier, hook up the second cabinet, and then turn your amp back on. To your disbelief, the sound is not louder—in fact, it's tinny with no body, and turning up the gain makes it sound even worse.
Most likely, the two cabinets are out of phase. This means that while the speaker cones of one cabinet are moving out, the cones of the second cabinet are moving in. The net result is little or no sound at all. To check for this situation, get out that 9-volt battery again. Turn off the amp and unplug the speaker cable from the amp, leaving the other end still connected to the cabinet. As shown in Fig. 1, touch the plus (+) side of the battery to the tip of the phone plug and the minus (-) side of the battery to the sleeve of the phone plug. When you do this, the cone(s) in the cabinet should move out. When the battery is disconnected, the cone(s) will go back to their original position. Now repeat the procedure with the second cabinet; chances are, the cone(s) will move in the opposite direction—in. If so, the speakers are wired out of phase.
The guy who loaned you the cabinet is really upset, because it's brand new. He even made a new speaker cable for it himself. Hmmm. Take your battery and recheck the phasing of both speakers, using your speaker cable. If they check okay, then the homemade speaker cable is miswired—that is, plus and minus have been reversed. Give the cable back to its owner and suggest that he rewire the connectors (or buy a properly wired commercial cord). Whenever you replace a speaker or have one replaced, use this test to make sure it has been properly installed in the cabinet. You should also check all new or repaired cables the same way.
Thanks to all your trouble-shooting, the night turns out to be a complete success. And, as an added bonus, you get an offer from the club to be their house stage technician!
The next day, you set out to find a second speaker cabinet. Before going, you should make a list of all the items pertinent to your additional enclosure, including: impedance, power-handling capacity, function, and price range. In the owner's manual of your amplifier, it says the minimum load is 4 Ohms. Since your existing cabinet is 8 Ohms, you know you can add one more 8 Ohm speaker safely ( 8 / 2 = 4). You could also add one, or even two, 16 Ohm cabinets: two 16 Ohm cabinets in parallel have the same total impedance as one 8 Ohm speaker.
To get the most efficiency out of your system with the fewest cabinets, your best choice is to add one 8 Ohm enclosure. Since the two cabinets are of equal impedance, each will receive half the power your amp can deliver. If your amp puts out 200 watts RMS at 4 Ohms, then each cabinet will receive 100 watts RMS maximum under clipping. (Clipping is the point where the power amplifier runs out of headroom and begins to distort.) If you had four 16 Ohm enclosures, each one would receive a maximum of 50 watts RMS under clipping. It should be noted that continuous clipping is very harmful to speakers, especially in a bass system: the lower the note, the longer the duration of DC content in the clipped signal. To understand what happens under this condition, remember what your speakers did when you applied the 9-volt battery to them. Now imagine what 20 or even 50 volts would do at the rate of 40 times a second! The results can be overheating, disfiguring of the voice coil, overall fatigue, and—eventually—complete failure.
After figuring out the power rating you need, look at the power specifications of the speaker cabinets carefully—they can be deceiving. Some companies rate their speakers in "music power" or "continuous program" or just plain "watts." The only specification you should be concerned with is "watts RMS." The term RMS is short for "root mean square," and I won't bore you with the complete mathematical explanation; suffice it to say that a rating in watts RMS indicates true power. In musical terms, this means that with a cabinet rated at 100 watts RMS, you can play 16th-notes on your 100 watt amp at full volume (under clipping) constantly for extended periods. If the literature on any cabinet you're considering does not have this specification, call the manufacturer and find out before you purchase it.
Last, but not least, ask yourself why you're buying this cabinet in the first place. What do you need: More volume to cut through in live situations? Lower lows for better articulation on your 5-string? Crisper highs for slapping? If the most critical need is for more apparent volume in live situations, a smart choice would be a cabinet known for good punch and midrange response, such as a 2x10 or 4x10. (The ear "hears" midrange and higher frequencies up to about 3KHz better than low notes. That's why a solo on a 50-watt guitar amp blows away your 100-watt or even 200-watt bass amp.) For this reason, a 1x15 or 1x18 may not be a good choice, especially if the cabinet you currently own fits this description.
Now that most of the parameters have been defined, go to your local music store and ask what they offer in an enclosure that has good punch and midrange response, 8 Ohm impedance, can handle at least 100 watts RMS, and is in your price range. Try out everything that fits this description, preferably with your own amp and instrument. And while you're there, don't forget to pick up a good speaker cable along with a couple of spare 9-volt batteries. You're going to need them.
After playing guitar in surf and psychedelic bands during the 60's, Steve Rabe turned his talents to musical electronics, working for Cerwin-Vega and then Acoustic Control Corp. After joining Acoustic in 1973, he helped to design more than 20 new products and personally directed the maintenance of Jaco Pastorious' Acoustic 360 amplifiers.
In 1984, Rabe started his own company, SWR, which has become the world's leading manufacturer of bass amplification systems.