Mythbusters #1 - amplifiers

There are almost as many myths about amps as there are about loudspeakers, so we are now going to attempt to debunk them!

Myth #1 - A 1000W amp will be so ridiculously loud that it's stupid.

Truth #1 - When driving a perfect loudspeaker a 1000W amp is only twice as loud as a 100W amp. When driving a real loudspeaker that suffers from thermal and mechanical power compression a 1000W amp will be less than twice as loud as a 100W amp, probably only about twice as loud as a 50W amp at best.

Myth #2 - The higher the cab power rating, the more powerful an amp you need to prevent 'underpowering'.

Truth #2 - Generally the larger the cab, the higher the power rating, because it's determined by the power handling of each voice coil added together, the bigger the voice coil the greater the power handling (bigger or more expensive speaker tends to equal bigger voice coil), and the greater the number of voice coils the greater the total power handling. The larger the cab and/or the greater the total cone area, the higher the sensitivity and thus the LESS the amount of power you need to reach gig SPL.

Myth #3 - If your amp isn't powerful enough you risk blowing your speakers through underpowering.

Truth #3 - You will never blow woofers or mid-bass drivers, i.e. the 8"/10"/12"/15"/18" speakers found in a bass cab, because your amp isn't powerful enough.

Myth #4.1 - Clipping blows speakers because it creates DC or square waves.

Truth #4.1 - Clipping does not cause DC. Clipping does not cause square waves.

Myth #4.2 - The square waves caused by clipping mean that the woofer has to stand still all the way in, then instanteneously leap all the way out, then stand still, then leap all the way back in again and so on, which causes the woofer to break due to the harsh movements and the lack of cooling when the woofer is stationary.

Truth #4.2 - If you were to apply a square wave to a perfect loudspeaker it would move steadily out and then steadily back in, because the cone position is the integral of power over time, which gives you a symmetrical triangular wave. However all woofers and mid-bass drivers have sufficient inductance that the square voltage wave from the amp does not result in a square current (and thus power) wave due to the lowpass nature of a series inductance.

Myth #4.3 - Clipping hugely increases the power output from the amp due to the square waves

Truth #4.3 - Clipping only increases the power output at higher frequencies. Due to the lowpass nature of the voice coil inductance very little of this extra power is delivered to woofers. However this extra high frequency energy can blow tweeters or midrange drivers if the clipping is heavy and prolonged. Most rigs will clip at some point during a gig, some on the majority of louder notes - however this gives times in between those moments of extra HF energy for tweeters to cool down.

Myth #4.4 - Clipping is really dangerous to speakers because of the extra harmonics and thus HF power it generates.

Truth #4.4 - The extra harmonics and HF power generated by power amp clipping are no different to those generated by preamp clipping or distortion or fuzz pedals. This kind of distortion rarely sounds good through tweeters, and thus the unpleasant sound tends to protect them from being blown as most sensible people turn the tweeter down before damage occurs.

Myth #4.5 - Clipping is always bad so you should turn down if clip lights flash.

Truth #4.5 - Every bass player will cause their amp to clip at some point on the gig, the dynamic range of bass guitar is too large for this to be avoided. Frequent clipping is not a problem. Only near-constant clipping is an issue due to the lack of cooling time in between 'clips' for the tweeter/midrange driver. If the clip light is almost permanently on, or you can hear the clipping almost constantly, then turn down or reduce your lows.

Myth #5 - A 600W amp will be louder than a 400W amp.

Truth #5 - Not necessarily.

Myth #6 - An amp that is louder than another amp is thus more powerful.

Truth #6 - Not true at all! Due to the deficiencies of the human ear, plus the complexities of psycho-acoustics, an amp that has more distortion and/or more midrange in its sound will sound louder than an amp which is cleaner and has more even response through the mids, even if the latter is somewhat more powerful.

Myth #7 - A 600W amp is more powerful than a 400W amp.

Truth #7 - It depends on how the specs are determined. An amp which is rated at 600W RMS with 5% THD at 1kHz will be less powerful than an amp which is rated at 400W RMS with 0.05%THD from 20Hz-20kHz. Test both amps the same way and apply the same qualifiers and the 400W amp becomes a ~650W amp or the 600W amp becomes a ~350W amp. Watts are watts - but unfortunately humans measure them and then marketing departments 'translate' the measurements ...

Myth #8 - A heavier amp will have more 'slam' in the lows than a truly equal-power lightweight amp.

Truth #8 - Whichever amp has the stronger power supply (more reserve power, more resistant to rail sag, better current delivery) will have the more slam. With traditional class AB amps with mains frequency transformers then there tends to be a correlation between weight and effortless power delivery. Once you get into class G, class H, class D and/or switch mode power supplies (SMPS) then all bets are off. Don't make simplistic assumptions, use anecdotal evidence and preferably real world testing. Read between the lines of all anecdotal (forum!) evidence as little of this is collected with a scientific mindset.

Myth #9 - Having an amp rated at 1000W and a cab rated at 200W means you have lots of headroom.

Truth #9 - Headroom is not the difference between amp power rating and cab power rating, but the difference between the maximum clean SPL your rig can produce and the normal max SPL required for your gig. This should be measured in dB. The best way to have lots of headroom is to have many large high-quality loudspeakers. Throwing high power at smaller cabs usually results in overdriving the cab due to a lack of excursion limited power handling.

Myth #10.1 - Your amp should be twice as powerful as your cab.

Truth #10.1 - This is normal procedure for quality PA systems but you're not putting a full band through a PA system, you're putting a bass guitar through a bass rig. So no.

Myth #10.2 - Your cab should be at least twice as powerful as your amp.

Truth #10.2 - This is normal procedure for guitar amps, to prevent overdriven sounds with low crest factor overheating voice coils and killing speakers. But you are not playing guitar through a guitar amp, you are playing bass through a bass amp. So this is wrong too.

Truth #10.3 - For cleaner sounds you will need more headroom in the system (real dB headroom, not amp:cab power-rating fake headroom). For dirtier sounds headroom is irrelevant because part of the dirt is the compression/distortion from running out of headroom. If running dirtier sounds then it's sensible to err towards having no more power available from the amp than the cab is thermally rated to handle. If running cleaner sounds you can safely use a higher power amp because the crest factor and thus peak:average power ratio will be larger. It is continuous excessive power that kills speakers, loud bursts do not matter because it takes time for the speaker to heat up and then it usually has time to cool down before the next peak hits it.

Myth #11 - There is no point using a very high-power amp unless your band is incredibly loud or you have a huge stack of speakers.

Truth #11.1 - If your band is incredibly loud then the only time to use an incredibly powerful (1000W+ amp) is if you are using a speaker with extremely high excursion-limited power handling, which counts out almost all cabs on the market except those made by Barefaced. If you are using a huge stack of speakers then they will be able to handle the power but you won't need the power because of the high sensitivity resulting from the size of the stack.
Truth #11.2 - If you like to have true extended lows and/or downtune your bass or play low B or lower, then a very high power amp plus a Big Series cab will result in huge max SPL, tone and bottom. Due to the extended lows from a reasonably compact loudspeaker cabinet the old (G1 & G2) Big Series cabs required about twice as much power to hit equal SPL as similarly sized conventional cabs. So as many bassists would agree that 300W into a 2x12" is a sensible amount to handle any gig that isn't all out rock, you would thus require 600W into the G2 Big Baby to hit similar SPL with much deeper lows, whilst if you play LOUD rock gigs and use 500W into a 4x10", then 1000W into the G2 Big Twin will give you similar SPL again with much deeper lows.

Myth #12 - If you have a limiter on your amp you should always use it to protect your speakers.

Truth #12 - If your amp truly has greater power output than your speakers can handle thermally and you are pushing it frequently to limiting, it is safer to leave the limiter off so you get an audible sign that you are demanding high power from your amp. A good limiter circuit can allow you to get another 3dB out of your amp (i.e. twice as much power) which can cause woofer meltdown (this has happened to two Big One woofers).

Myth #13 - If your amp and cab are both rated at 500W then you can push them as hard as you like without risk of failure.

Truth #13 - Very few loudspeakers can handle as much power without excursion-related problems as they can without thermal issues. Typically the ferrite magnet drivers used in bass cabs over the last decade or two exhibit excursion-limited power handling less than 20% of their thermal power handling, whilst more expensive neo magnet drivers in more recent bass cabs are closer to 30-40% excursion vs thermally limited power handling. The Barefaced cabs have unusually high excursion-limited power handling, which is detailed on the spec sheets. Consequently if you push a 500W cab really hard with a 500W amp and prefer big bottomed sounds, you can cause mechanical failure.

 

Truth #14 - You can safely use any amp with any cab, regardless of power ratings, but if you hear distortion or farting, turn down the volume or the bass knob.

This distortion/farting is a warning sign that the cab is reaching its excursion-limited power handling. Clearly if your band is so loud that you can't hear your cab at all then you won't be able to hear the audible warning signs!

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