Under powered cabs...problem?

[Jaruquin]

I have a GS212 that's 8 ohm cabinet ..... and I use it on occasion with the Tone Hammer 500 or my ThunderFunk 550-B .... it's plenty loud for a bar gig ..... if you get the 8 ohm cabinet, you'll have the ability to get another 8 ohm cabinet, so you'll load most recent manufactured amps correctly at the 4 ohm rating

With the Tone Hammer at 8ohms its only pushing 250w? You find you have enuff volume like that?

 

[jastacey]

Yes ... it works for the band I play in ...... we don't play at a ear ringing sound level ... the ToneHammer 500 is rated pretty conservatively ..... if you playing with a loud drummer who has no dynamics and a guitar player who like to play at the 12 volume setting, then no ....you'll need a Fender Bassman 1200 running at 2 ohms ...to blow then into submission

 

[Mark Reccord]

Correction: RMS power peaks at 1.41 times its rating. However, modern amps are rated at peak power, so a 500W amp can only do ~355W RMS without clipping/limiting. If it can handle full saturation it can do 500W square wave no problem, but 500W square wave will fry a 700W RMS speaker if you either try to do it or let some bonehead crank your gear.

The difference with square wave vs RMS is that the voice coil has to endure 41% more current without any increase in cone excursion, so there's more heat without more air moving around to cool the coil. Using an amp's limiter to fool a speaker into cheating its excursion limit just lowers the thermal tolerance of the voice coil.

I think you are misunderstanding things a bit here. That's not meant to be a slam at all as the concepts involved are pretty advanced, but I do feel I should clarify.

Like Bob Lee mentioned, there isn't any such thing as RMS power, the term has been co-opted from its application for voltage and current. The correct term is continuous (or continuous average) power. No biggie as that's just a bit of terminology, however you're off on the mathematical concepts a little.

With a sinusoidal waveform, the peak voltage and peak current are ~1.41x the RMS values. With a square waveform the RMS values of current and voltage are the peak values. Power is a function of the square of voltage (or current) so the power of a square wave is 1.41^2 = 2 times that of a sine wave of the same amplitude. This is hard fact and not the least bit debatable (consult any circuits textbook for confirmation). So yes, assuming the power supply can keep up (not usually the case) an amp driven 100% into clipping can approach 2x its rated power. That being said, this doesn't happen in the real world because an amp even driven into the hardest clip doesn't put out true square waves and because most power supplies will sag under that kind of condition which limits the total power output.

I think you'll find that any reputable manufacturer of amplifiers will state the continuous (RMS) power for their amplifiers, not the peak. Especially the pro manufacturers (ie QSC, Crown, et al).

As for your assertion that a 500W square wave will blow up a 700W speaker, well that depends on a few things. Assuming the speaker is honestly rated for thermal power and that it can mechanically handle the power (neither is usually the case ), then no a 500W square wave won't blow a 700W speaker. When it comes to thermal characteristics, speakers don't care about waveform. At all. Any 500W alternating signal would have 200W of thermal power handling margin in this instance.

Now if you said a 500W rated amplifier (honest rating) can blow up a 700W speaker, that would be true as the amp could well produce in excess of 700W if it were driven hard enough. In the real world, a speaker's power handling is limited by excursion long before its thermal limits are reached (at low frequencies particularly) and 90%+ of cone speaker failures are excursion related in the real world. Even a burnt VC is often due to overexcursion as that condition allows the coil to spend time outside the gap which derates its thermal power handling ability. Cooling in speakers is primarily due to convection to the magnet assembly. The motion of the driver is a minor factor in most cases.

 

[Bob Lee (QSC)]

Yes, granted, I screwed up the terminology. I know how to use gear within an inch of it's life without damage, explaining it isn't my forte. Similarly your explanation of square wave harmonic content adds to 104%. We all **** up explaining what we do day in day out without having to explain.

Please note that I clearly indicated using approximate figures; using more than one significant digit would be hard to justify in describing audio power in a waveform.

Correction: RMS power peaks at 1.41 times its rating.

Still incorrect.

However, modern amps are rated at peak power, so a 500W amp can only do ~355W RMS without clipping/limiting.

That also is incorrect.

The difference with square wave vs RMS is that the voice coil has to endure 41% more current without any increase in cone excursion, so there's more heat without more air moving around to cool the coil.

You have an incorrect notion of cone excursion.

Using an amp's limiter to fool a speaker into cheating its excursion limit just lowers the thermal tolerance of the voice coil.

A limiter cannot possibly "fool" a loudspeaker into anything, including "cheating" its excursion limit.

 

[Tuned]

RMS power, co-opted term or not, refers to the average continuous power output of an AC signal. The voltage and amperage values are a separate entity. Yes volts x amps = watts but nobody is saying that RMS volts x RMS amps = RMS watts. RMS is a mathematical term, not exclusive to any specific parameters. If it is the convention of electrical engineers to avoid using the term with power ratings, so be it. I promise not to use it on electrical engineering forums. In the meantime, it's all over the backs of our bass cabinets, so us bassists are gonna use it, m'kay?

Same applies to amplifier RMS power. Regardless of the conventions of engineers, it is a safe bet that a bass amp's rating is the highest number they can possibly justify. To hold them to engineering conventions and expect twice the peak power of the given rating is foolish. By your own admission the people deciding what ratings to put on the equipment are ignoring the engineers, why shouldn't we?

My methods are proven in practice. If my mathematical justification is off, it doesn't mean my advice is wrong. Sorry I tried to make sense of it, meant no disrespect.

BTW "cheating" a speaker's excursion limit referred to using the amp's limiter to clip the waveform just before the driver's excursion limit, which yields more perceived volume, higher voice coil heat, but without increased air volume through the magnet assembly. I found this out the hard way, had a 15" driver jumping plenty, but fried the VC without mechanical damage.

 

[ThisBass]

RMS power, co-opted term or not, refers to the average continuous power output of an AC signal. The voltage and amperage values are a separate entity. Yes volts x amps = watts but nobody is saying that RMS volts x RMS amps = RMS watts. RMS is a mathematical term, not exclusive to any specific parameters. If it is the convention of electrical engineers to avoid using the term with power ratings, so be it. I promise not to use it on electrical engineering forums. In the meantime, it's all over the backs of our bass cabinets, so us bassists are gonna use it, m'kay?

My suggestion:
take a light bulb and different AC signals (sine or pulse or triangle) with same Peak Voltage but different RMS Voltages.

Different RMS Voltages will give you different lightness but Peak Voltage remains all the time the same.


So what would you suppose by analogy to a loudspeaker?

 

[fkh006]

I'm sensing Dunning–Kruger effect in play...

 

[Bob Lee (QSC)]

You are correct that RMS is a mathematical term, but it applies to AC voltage and current only because they have both positive and negative values (power does not). Its use as a description of power is a misnomer, albeit a common one in non-technical or marketing discussions. If you try to apply RMS math to non-RMS parameters, it either falls apart or you develop incorrect conceptions of how and why things happen as they do.

Your idea of what a limiter does is also flawed. Limiters are not voltage clamps, and they are not used to clip a waveform at a voltage threshold. Instead, they use a gain cell of some sort to reduce gain when the signal reaches or exceeds a certain threshold. The idea is to limit peaks with minimal distortion of the waveform shape.

A limiter can be used to keep the signal just below the driver's excursion limits, but its threshold and compression ratio would have to be closely mapped to the driver's behavior with regard to frequency, voice coil temperature, etc., within a given enclosure. That would be best done through DSP designed for a particular powered loudspeaker system.

 

[Bob Lee (QSC)]

a guy that designs amps for qsc

I don't design the amps; we have smarter people here to do that.

 

[ThisBass]

Your idea of what a limiter does is also flawed. Limiters are not voltage clamps, and they are not used to clip a waveform at a voltage threshold.

Probably limiters are the most mistaken features of amps.
IMO limiters are very often mismatched with "soft" peak stop levels.

Instead, they use a gain cell of some sort to reduce gain when the signal reaches or exceeds a certain threshold. The idea is to limit peaks with minimal distortion of the waveform shape.

That's not as easy as it looks like to most of folks.
Limiters should be very fast and just the same time the "sound" should remain harmonious.

A limiter can be used to keep the signal just below the driver's excursion limits, but its threshold and compression ratio would have to be closely mapped to the driver's behavior with regard to frequency, voice coil temperature, etc., within a given enclosure. That would be best done through DSP designed for a particular powered loudspeaker system.

IMO "standard" limiters (and compressors) need some little time to "react". So some signal content remains unlimited/uncompressed.

As an example: it is impossible even with digital working limiters/compressors to prevent/correct clipping in a digital mixing console if random signal peaks are slightly beyond headroom.

BTW of course there are digital working limiters out there with a high performance quality.
The signal is "delayed" a bit to read the signal information first and then the "delayed" signal is "calculated" and reduced.

 

[CL400Peavey]

I don't design the amps; we have smarter people here to do that.

So you cant design me an amp?

 

[Steveaux]

I'm sensing Dunning–Kruger effect in play...