ICEinLAVA's Domain

It always drives me nuts to walk into a club install and see two pairs of JBL SR4719 subwoofers (2x18” each), powered by a single Crown Macro-Tech MA-5002VZ power amplifier. Now I know the amplifier can handle the load, and the speakers can handle the watts; but the match puts both pieces of gear in a non-optimum operating condition. Another club install rig I ran into has well used but well maintained stage monitors of a music-store (MI) category. Unfortunately, the chosen QSC PLX-1202 amplifiers barely get close to the wedges’ RMS ratings.

In this article, I want to get down to basics, because most of us still do not “get it” when matching up sound system speakers and amplifiers. (Also, I humbly apologize to those speaker and amplifier manufacturers that I failed to mention by name. It does not mean that you are lesser in my opinion, but this is a piece about priciple, not product. If they really understand the reasoning behind my examples, the readers can substitute other products by focusing on the power ratings and impedances.)

Through example, I hope to appeal to your common sense via a couple case studies. Let’s go over rule of thumb on matching to answer the SR4719/MA-5002VZ quandary.

Pull Your Thumb Out...

The rule of thumb on speaker driver matching is to size the amplifier maximum power rating in the range from the RMS (continuous) to the program ratings of the driver. Leaning on the RMS side is what I call the “cheapskate” side of the problem, and should be only a temporary solution until a bigger or additional amplifier helps with the load. The program power ratings for most speaker drivers are about twice the RMS ratings, and if everything were perfect, you would choose amplifiers precisely rated at the program power. Remember also that the driver impedance plays a big role, as the nominal rated impedance may better match up separately or together with the power amplifiers ratings, since they do not change in exact proportion to the load impedances.

In the above example, the SR4719 subwoofer has an RMS rating of 1,200 watts and a program rating of 2,400 watts at four ohms. The Macro-Tech amplifier offers 2,000 watts at four ohms, and 2,500 watts at two ohms in the dual channel mode. It seems obvious that one channel of the MA-5002VZ is a great match to a single SR4719, with everything operating near the program ratings. But the install cheapskates decided to avoid buying another MA-5002VZ for the second pair of SR4719s, and loaded the MA-5002VZ at two ohms per channel. So now the amplifier offers 2,500 watts per channel into two SR4719s, or only 1,250 watts each with only 50 watts above the RMS rating. We have an underpowered subwoofer with an amplifier taxed to the limit of its capabilities. Not a good formula for bass-heavy music like techno or hip-hop.

The other “nasty” I saw in this example was the usual single-pair 12-gauge speaker cable running to the first SR4719 then chained to the adjacent second SR4719. Instead, a “two-fer” or splitter Speakon adapter should have been employed at the back of each amplifier’s output connector. Then each sub-woofer presents a 4-ohm load back to the amplifier, and the separate 12-gauge cables have minimal losses and a decent damping factor. Alternatively, 10-gauge speaker wires might also be acceptable.

Having a power amplifier slightly too large for the speakers’ program power rating is not a crime, but it is something to watch out for in operations. In subwoofers, there can be mechanical over-excursion if driven hard. Voice coil damage is possible but less likely than over-excursion. In under-powering speakers, the risk is that overdriving the amplifiers can create clipping or highly compressed (limiting) scenarios in which the amplifiers deliver extra power and overheat the voice coils of the speaker drivers. This comes from the idea that if a speaker cone stays motionless at the ends of excursion too long, the voice coil receives maximum power input (heating), but has little thermo-dynamic means to cool itself because of the lack of end-motion and because it is furthest away from the metals of the magnetic motor assembly. This heat build-up will eventually melt the weakest part voice coil conductor, thus opening the circuit.

Case One

In case study one we depart from subwoofers and go topside. Suppose you have a pair of JBL SR4732 top-box speakers and you want to assess which amplifiers you could chose to power them. If you chose the passive, full-range mode, you have a 1,200-watt RMS power rating. So amplifiers providing 1,200 to 2,400 watts per channel into four ohms will get the job done. If you go for bi-amping, the same applies to the low frequency split on the SR4732s. This is because the pair of JBL 2206H 12-inch LF drivers each handle 600 watts RMS from the recommended 100Hz to 1.2kHz frequency band. On the high frequency split, the answer requires a little intuition and knowledge of the MF and HF drivers. The SR4732 employs the JBL 2447J MF driver with a 16-ohm nominal impedance, and the JBL 2404H driver with an 8-ohm nominal impedance. Because the 2447 does the 1.2kHz to 6kHz band and the 2404 “baby butt” does the above 6kHz band, the 2404’s extra sensitivity is passively matched back to 16 ohms across the whole 1.2kHz to 21kHz split range with a 150-watt RMS rating.

Because power amplifiers normally do not offer a 16-ohm rating, one can conservatively state that one-half the amplifier’s 8-ohm rating applies. So the upper split of the SR4732 requires 150 to 300 watts at 16 ohms, or about 300 to 600 watts at eight ohms. With 16-ohm loads high and 4-ohm loads low, do not be surprised that the same amplifier power ratings can apply to both bi-amp loads.

If we start optimally sizing amps for the SR4732, typical good picks would be like the Crest Pro9200, Crown MA-5002VZ (a touch high) or the QSC PL-6.0II, as these amps hit the optimum spot near the 2,400-watt program power rating at four ohms for the LF split. For the HF split, amps like the Crest Pro7200, Crown MA-2402 or QSC PL230 would be among the candidates. Switching to cheapskate mode, The Crest CA18, Crown CE4000 or QSC RMX-4050HD would get you into adequate operational territory on the LF split. On the HF split, the Crest CA6, Crown XLS-602 or QSC RMX-1850HD would be on the good side of the 300 watts per channel at eight ohms.

Case Two

Now let’s flip things upside down and presume you have one QSC RMX-1450 amplifier to drive stage monitors with. If your wedges are going to be Yamaha Club Series V models, such as the SM12V, then the 250 watts RMS and 500 watts program at eight ohms applies. The question will be how many wedges on each channel “should” you load the RMX-1450 with. Taking the RMX-1450 ratings of 280 watts at eight ohms, 450 watts at four ohms and 700 watts at two ohms yields part of the answer. With a single SM12V per channel, the 280 watts just barely gets you into manageable safety. With two SM12Vs, each receives half of the 450 watts, or 225 watts per wedge. This 25-watt underage is what can be considered playing with fire, or the safety of the voice coils.

With four SM12V wedges, the 700 watts is divided four ways for 175 watts per wedge. This may be passable for political speech rallies, but never close enough for rock ‘n’ roll. This is where the repair shop loves you more than you love them. To resolve this dilemma, consider upgrading to the QSC RMX-2450, which offers 500 watts at eight ohms, 750 watts at four ohms and 1,200 watts at two ohms.

With the RMX-2450, a single SM12V per channel hits the program power mark at 500 watts. With two SM12Vs per channel, the 750 watts divided by two provides a moderate 375 watts per wedge. Even with four wedges, the 1,200 watts by four leaves an acceptable 300 watts per SM12V cabinet. The caveat at two ohms is that the RMX-2450 cannot sustain continuous clipping in that loading, and this also applies to many other brands with similar power ratings.

Now I can hear the obvious rebuttal, “we have run under-powered for years, and haven’t blown up much.” Running on the under-power edge is truly penny wise and pound (and time) foolish. The few extra dollars you have earn, even if you have to wait to choose a larger power amplifier, are well worth it in the long run. As sound system operators, we all fool ourselves with the thinking that the “brand” will protect us from failures. All speakers and amplifiers will fail eventually, but do what you can to make it an age problem, and not an abuse problem.