Titan 48

[gdougherty]

Mark, most larger amps are typically rated to 2 ohms stereo, 4 ohms bridged. By this point, most of us are aware of "nominal" impedence, minimum impedence, and the constant varying impedence of a system under operation. Practically speaking, combined speakers calculated to be 5ohms, nominal, are just a hair away from 4 ohms. Their actual difference in operation in comparison to speakers combined to a nominal 4ohms will be miniscule. So, yes, the amp would be pushed "right to the edge of it's bridged performance." In light of recent technical information presented on this forum regarding the ill effects of heat on amplifiers, it seems to me that it would be irresposnsible at this point to continue to condone running amps at or near their minimum impedence load.

That being said, I ran my Peavey CS800s at 2ohms per channel for over 2 decades, and they reliably kicked ass. I guess I was ignorant and lucky.

What accelerates failure of circuit components is heat.
The old axiom is for every increase of 10° C over ambient temp, the life of a part is shortened by 1/2.
Decrease the impedance, and you increase the current flow, more current greater heat. Resistance increases with heat - it's a vicious cycle and can lead to thermal runaway.
Keep amps as cool as possible and they last much longer.
I ( and friends ) have amps that are still working fine after 25+ years

It seems to me that the 2ohm load isn't really the problem. The problem is the voltage and current run through at 2ohms and the heat it generates. If the parts are of similar quality, class H and Class D amps will also help since if you're pushing 1000W out of an amp but it's only 30-40% power efficient, you're generating a lot of heat from wasted energy like with a class A/B. If your amp is 70-90% efficient, you'd be generating a lot less waste heat. If you regularly did gigs outdoors somewhere like Phoenix then I'd definitely consider staying away from 2 ohm loads. If your amp is constantly on the verge of clipping at 2 ohm loads then that doesn't sound good either. As with Bill's recommendations on limiting, you have to take into account what you actually need and protect your gear accordingly. Contrary to the popular mantra around here that you don't need 1.5-2X RMS power in your amps, that overhead and gentler use may be why some people have very old amps that continue to run reliably.

[Mark Coward]

overhead and gentler use

Exactly, not just with amps but with the entire system. Just because you could squeeze out a few more db, doesn't mean that you should do so. If you're using a system that's "over-engineered", then in normal use it's just coasting along.

[Mark Coward]

Practically speaking, combined speakers calculated to be 5ohms, nominal, are just a hair away from 4 ohms. Their actual difference in operation in comparison to speakers combined to a nominal 4ohms will be miniscule. So, yes, the amp would be pushed "right to the edge of it's bridged performance." .

The rest of your previous statement was about the driver's minimum impedance:

given the 3015LF's Re of 5.31ohms, you have impedence dips below the amp's rated minumum (4ohms, bridged). So, you're pushing your amp right to the edge of it's bridged performance.

I have not seen any proof that a speaker's minimum impedance can cause damage or adverse affects to an amp. The vast majority of 8 ohm speakers will have Re around 5 ohm - 6 ohm. Regardless of whether you want to run 2 ohm loads or not, there is no reason to calculate amp loads based on Re IMHO. Where is the proof of these "impedance dips" affecting an amp in actual use?

Ron Elliot's articles can seem incomprehensible if you don't have a background in electronics ( especially AC and amplifiers ), but the bottom line is Heat is the enemy.
It doesn't matter if an amp gets hot because of inadequate heat dissipation, from a blocked or inadequate heat sink, a failed fan, poor air circulation, summer sun or excessive current from low impedance: Amp life is shortened.

Thanks Sydney, this makes perfect sense.

[Mikey]

I have not seen any proof that a speaker's minimum impedance can cause damage or adverse affects to an amp. The vast majority of 8 ohm speakers will have Re around 5 ohm - 6 ohm. Regardless of whether you want to run 2 ohm loads or not, there is no reason to calculate amp loads based on Re IMHO. Where is the proof of these "impedance dips" affecting an amp in actual use?

Impedence varies with frequency, so when your program material contains frequencies which coincide with the drivers' Re frequencies, the drivers are operating below nominal impedence. So, if your program material happens to contain a lot of material at those frequencies, your amp will be seeing a lot of impedence below "nominal", causing it to operate at yet higher temps.

[Mark Coward]

Impedence varies with frequency,

This is true, and can be tested.

so when your program material contains frequencies which coincide with the drivers' Re frequencies, the drivers are operating below nominal impedence. So, if your program material happens to contain a lot of material at those frequencies, your amp will be seeing a lot of impedence below "nominal", causing it to operate at yet higher temps.

Where do you get this information, can you prove that this occurs? I don't believe it can happen in typical use as there will always be a range of frequencies reproduced therefore an average of impedance - unless you're running a pure sine wave in a test environment.

[Mikey]

Mark, the whole issue becomes moot if you take the thermal issues into consideration and plan your system accordingly. If you want to run 2ohm loads, or very close to it, that's up to you. As I said, I did it for years without knowing any better, and it was never an issue. But I don't do it now, and "probably" won't again. "Your mileage may vary"

[Bill Fitzmaurice]

Mark, the whole issue becomes moot if you take the thermal issues into consideration and plan your system accordingly.

I agree, you have to leave some headroom somewhere. If you go with minimal power don't do so at the minimum load as well. Either overpower or underload so you aren't hitting the amps with a demand for both maximum voltage and current capability at the same time. But as to the minimum impedance issue, Mark's right, complex musical waveforms don't behave in a manner where you have to be particularly concerned about it. If it was a problem amps would be blowing left and right with commercial subs.

[James R]

Well tonight I used the 2 of the 4 T48's I have done,I was very cautious used my older QSC USA series amps with my Dbx DRPA.
Ran a sine wave to double check the voltage at the amps, set my limiter a little more than needed. And boy I was loving the
clean bass and kick ,I'm still using my SP4G tops ( for now ) All I gotta say is ............. Thank You Bill! can't wait until I have all 4 up and running
Then some new tops probally the otops 12's

Thanks Again,
Jim