Sometimes some bass-player amp knowledge can pay off in multiple ways. At band rehearsal a few weeks ago, our little PA head started acting up. It worked fine for a few minutes, but then it started to get fuzzy sounding and finally faded to nothing. We swapped out mic cords, but the gremlin kept resurfacing. After a while, I found out that the amp was set in bridge mode, a way of combining its two power amps into a single amp that produces more power than either one alone.
However, that amp section of the PA was rated at a minimum impedance of 8 ohms in bridge mode. Unfortunately, we had two 8-ohm cabs plugged in, which created a 4-ohm load. Aha! There was the cause of our problem. We switched the head out of bridge mode, plugged one PA speaker into each power amp, and the rest of the night was trouble free.
Because most bass amps are solid state rather than filled with lovely glowing tubes, a bass player’s knowledge tends to be different from what a guitarist needs to know—this impedance mismatch problem was just one of the things that gets caught quickly by the alert bass player, but might not dawn on a guitarist. I’ve run across a bunch of these little amp tidbits over the years that have more in common with the bass world, but also have utility for guitar amps and PAs, and that’s the point of these next few columns.
Choose your bass cabs wisely!
In the bass world, there are 8-ohm cabs and 4-ohm cabs, and a bassist is often mixing and matching cabs for a specific gig setting, typically going for combinations of 10" and 15" speakers that can fill the room with the sound you’re after. There are pretty efficient cabs that put out a lot of sound for the power they’re fed, and there are far less efficient cabs that put out a particularly beefy low end. That’s the bassist’s first challenge: getting the right combination of cabs to keep an amp happy and produce the sound you’re after. Combining speakers can result in a different impedance load depending on whether they’re connected in series or parallel. Most amps run down to a 4-ohm load, and a few can handle two ohms. Check your amp and cabs before plugging things together or you might run into a shut-down situation from overheating.
Fortunately, almost all bass amps connect speakers in parallel, and this doesn’t change whether you’re plugging two cabs into an amp or daisy-chaining one cab onto the back of the next. The equation for speakers connected in parallel is a bit of new math, so that 8 + 8 = 4 and 4 + 4 = 2. Just to keep things messy, 8 + 4 = 2.67.
What this means is that to keep your amp happy, you need to keep the speaker load at or above its minimum rating. An amp rated to a minimum 4-ohm load can do fine with a higher total impedance, such as 8 or even 16. At those higher loads, a solid-state amp just puts out less power. For example, an amp rated at 400 watts at 4 ohms might produce only 250 watts at 8 ohms. Conversely, an amp rated at 400 watts at 4 ohms would likely shut down if it’s run at 2 ohms.
Your goal as a bassist is to keep your amp happy by matching your cabs to your amp’s rating. And you want to keep your cabs happy, too. It’s perfectly safe to run a cab rated at 250 watts RMS maximum with an amp that can put out 400 watts—as long as it’s not continuously putting out that much power. In other words, there’s no need to match cab and amp ratings exactly. In fact, using an underpowered amp can cause more problems since it will be straining just to put out distorted sound waves that might be fine for a guitar amp, but spelling doom for a solid-state bass rig.
Stay tuned for more. In the next installment, we’ll talk about cables, gain staging and EQs.
Dan is a professor by day and a bass player when the sun goes down. He plays both electric and upright bass in blues, jazz and pit settings.
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