A Bass Players Guide to Matching Bass Amp Heads and Cabinets - johnnycoxmusic.com/
I should start by saying that I'm not an electrician or audio engineer. I'm just a bass player, and this is the first time I've done a Bass Practice Diary video that doesn't feature any bass playing. It's also the first time that I've ever had all of my bass amp heads and speaker cabinets together in the same room.
Combo Amplifier or Amp Head and Cabinet?
I believe that having a separate head and cab has numerous benefits over using a combo amplifier. Combo amps are only practical when they're small. I own a couple of bass combo amplifiers, but I didn't feature them in this video. I use them for rehearsals and small gigs because they're small and they save space. However their use is limited. Once you get up to a medium sized bass rig, combo amplifiers become heavy and impractical.
Having a separate head and cab (or cabs) gives you many more options. Lightweight solid state bass amp heads like the Markbass heads and the Warwick LWA 1000 are small and very light, and easy to transport. And you can match your head up with different sized cabinets or combinations of two cabinets to match the requirements of your gig. Bass amp heads also give you the potential to access much more power (Wattage) than combos.
However, "with great power comes great responsibility", and there are a couple of things that you need to know if you're going to safely unlock the potential of using a bass amp head with speaker cabinets.
Ohms and Watts
Those things are Ohms (Ω) and Watts (W). It's simple stuff, but back when I was trying to work all this stuff out, it always amazed me how hard it was to find this information put together for bass players in a simple to follow guide. I'm not an electrician and I have no qualifications in audio engineering, but this is my simple guide to Ohms and Watts for bass players.
All you need to know about Ohms, is that bass speaker cabinets are rated at either 4 Ohms or 8 Ohms. You need to know whether your cabinet is rated at 4 or 8 Ohms before you use it. If you run two speaker cabinets together (in parallel), then the Ohm rating halves, not doubles. So, two 8 Ohm speakers running in parallel makes a total of 4 Ohms. Virtually all bass amp heads are designed to run at a minimum of 4 Ohms. So you shouldn't run two 4 Ohm speakers together. That would be 2 Ohms, which would be below the minimum level that the amp is designed to run at. You should also never switch your amp head on without a cabinet connected, because that would be zero Ohms.
RMS stands for Root Mean Squared. The RMS power rating (Watts) is two things. It's the amount of continuous power that your amplifier is capable of putting out. And it's also the amount of continuous power that a speaker cabinet is capable of handling. Before you use a speaker, you need to know how many Ohms it's rated at and the RMS power rating. I've heard that some amp companies exaggerate the power handling capabilities of their speakers. They give the maximum Wattage that a speaker can handle, rather than the Root Mean Squared. RMS is the amount of continuous power it can handle. You need to make sure you know the RMS value for the Wattage.
How are Ohms and Watts Connected?
The number of Ohms your speaker is rated for will effect the RMS power that your amp head puts out. When amp manufacturers give the power ratings of their amps, they always give the RMS rating at 4 Ohms. However, if you read the technical specs for any amp head, it should give two RMS ratings. One for 4 Ohms and one for 8 Ohms. For example, the Warwick LWA 1000 is so called because it has the potential to run at 1000W RMS at 4 Ohms. At 8 Ohms it's only 500W RMS.
So, in order to unlock the full power of your amp head, you need to run it either into a 4 Ohm speaker or into two 8 Ohm speakers. However, it's not necessarily a bad thing to connect your amp head to just one 8 Ohm speaker. In fact, that's what I do most often when I take my amps out for rehearsals or gigs. The problem with running very high powered amps at 4 Ohms is finding speaker cabinets that have high enough RMS ratings. To safely match a 1000W bass head at 4 Ohms you'll need a really big rig. Either two 8 Ohm cabinets rated at at least 500W RMS each, or a massive 4 Ohm cabinet, like an 8x10 with a rating of at least 1000W RMS.
Can You Match a Higher Powered Amp with a Lower Powered Speaker?
Surprisingly, yes you can. Some people will tell you, that if you have a bass amp head rated at 800W RMS for 4 Ohms and a 4 Ohm speaker rated at 500W, then the head is too powerful for the cab. It seems logical that an 800W amp is too powerful for a 500W cab. But it's actually fine, as long as you don't push the speaker really hard. If you play through the amp well below full volume, it should be fine. The speaker will let you know if you're pushing it too hard, because it will start to sound bad.