Amp Speaker Confusion

[dravenzouk]

Ok, I’m confusing myself and need you guys to bap me in the head and get me straight. The situation is about matching power amps to speaker cabinets.

 

The components are as such:

2 – Main cabinets, 8ohm each, rated at “handling” 250W, 1000W peak

manufacturer recommends power amp of 250-500W @ 8ohms (which means a recommendation of 500-1000W @4ohms, yes?)

 

2 – Sub cabinets, 4ohm each, rated at “handling" 350W, 1400 peak

manufacturer recommends power amp of 350-700W @ 4ohms (which means a recommendation of 700-1400W @2ohms, yes?)

 

1 – Amp (call it AmpA) rated at 500W 8ohm, 750W 4ohm, 1200W 2ohm (each side)

 

1 – Amp (call it AmpB) rated at 600W 4ohm, 900W 2ohm (each side)

 

Please tell me if I understand this right.

SCENARIO #1: I run everything from one amp (AmpA). The two mains go into one side, which will be a 4ohm load, meaning this amp channel will deliver 750 watts. The two subs go into the other side, which will be a 2ohm load, meaning this amp channel will deliver 1200 watts.

 

SCENARIO #2: I run two amps. AmpA gets the mains, one into each side, which will be an 8ohm load, and the amp will deliver 500watts. AmpB gets the subs, one into each side, which will be a 4ohm load, meaning the amp will deliver 600 watts.

 

My gut feeling is that it would be better to run 2 dedicated amps (Scenario #2) but when I look at these numbers, it seems that there is either no real difference or that it’s better to run as in Scenario #1. That doesn’t seem right. I’m missing something obvious, right? This is one of those situations where the more I think about (alone) the more I confuse things, and most likely miss the simple answer. So, what’s your take? Which scenario is better? Can I really run everything off one amp? If not, why not? Thanks, d.

[jayselway]

Hi there,

 

If you run with Scenario 1, then you have yourself a mono rig. Unless you have a crossover built into your mixer, or amp, then you're still going to need one in the rack somewhere.

 

If you run with Scenario 2, you get a stereo rig, with one amp channel running one speaker. You still need the crossover in the rack.

 

If I was in the situation with 2 amps available, I'd run them as per Scenario 2. ;)

[dravenzouk]

I guess I'll add a little more info. First, I will be running this as "mono" either way - meaning the same signal will go to both sides, I have no need for any stereo imaging or such. Second, I will be running through a crossover before the amp(s) either way, if for nothing else than to isolate the feed going into the subs.

 

The main thing is that I currently HAVE the one amp (AmpA) and mains, and I'm soon stepping up to adding the subs. I'm wondering whether I need to obtain a new amp (dedicated for subs) as well. It seems "safe" to run Scenario #1, but would it be better to have two dedicated amps? By better, I'm thinking of things like distortion, possible amp or speaker damage, heat build up, amperage draw, etc. Realizing that the one amp is in actuality two mono amps, is it acceptable here to just think of side 1 as my mains amp and side 2 as my subs amp? Of course, money is an issue, as always; if there is no appreciable benefit to buying another amp, why spend the cash?

 

Again, it seems as if having two dedicated amps should be better, I just need to know if that's really true, and more importantly - Why?

 

thanks, d.

[rob.d]

I guess I'll add a little more info. First, I will be running this as "mono" either way - meaning the same signal will go to both sides, I have no need for any stereo imaging or such. Second, I will be running through a crossover before the amp(s) either way, if for nothing else than to isolate the feed going into the subs.

 

The main thing is that I currently HAVE the one amp (AmpA) and mains, and I'm soon stepping up to adding the subs. I'm wondering whether I need to obtain a new amp (dedicated for subs) as well. It seems "safe" to run Scenario #1, but would it be better to have two dedicated amps? By better, I'm thinking of things like distortion, possible amp or speaker damage, heat build up, amperage draw, etc. Realizing that the one amp is in actuality two mono amps, is it acceptable here to just think of side 1 as my mains amp and side 2 as my subs amp? Of course, money is an issue, as always; if there is no appreciable benefit to buying another amp, why spend the cash?

 

Again, it seems as if having two dedicated amps should be better, I just need to know if that's really true, and more importantly - Why?

 

thanks, d.

 

Put simply, imho, Scenario one will work, but scenario 2 is going to save the amp from sweating its proverbials off. I was disappointed to learn recently that even something so reputable as a Crown XLS, with published and rated output to 2 ohm, is actually, in fact, *not recommended* for running a 2 ohm load! (source: negative experience with a bridged XLS802 and a pair of 8 ohm paralleled subs; a colleague's ensuing conversation with a chap at HP UK. Yamaha P series also have a published output at 2 ohm loads- but I remember reading a caveat somewhere that the measurement was based on having that loading on it for 2 seconds (!) only). The moral of this story, is dont always put too much faith in manufacturer's specifications. The more you spend, the more accurate they are, but the visa versa is also generally true. Consider Amp A- how the hell does the power supply manage to pass an extra 450 watts between 4 and 2 ohms, but the drop from 8 to 4 only yields 250? as a very general rule, I'd say the law of diminishing returns will normally apply for that specification. I'm not suggesting its a falsehood, not least because it could be an amp thats particurlarly fond of 8 ohm loads, and what with these days of esoteric high technology/ topology, but hmm, sounds maybe a little fishy to me as its kind of counter-intuitive.

 

A 2 ohm load is putting serious pressure on the components in the amp, and if your running loud and long into the darkness of the night, your probably better off sticking with 4 ohm loading, as this generates less heat and works the amplifier less hard- both of these things are good karma for amplifiers, and by rights they should live longer, healthier, happier lives and also save you the stress of hobbling to the end of a gig because your losing amps to thermal cut out. Also, as determined by the mathematical model of sods law, your bass channel will over heat your (scenario 1) amp, and the whole flippin thing will cut out until its cooled down. With specific reference to the sentiment we know as "utter dismay", no music at all for 5 minutes is much, much worse than full ranging the tops for a couple of tunes.

 

On the flip side, if your ragging two amps at 4 ohms to get the sound pressure, this wont do any part of your rig any good either. Exercise caution, and with a spot of maths you can guesstimate what kind of power you need to achieve the rated max spl of the cabs- this is a good thing to know, as its probably wise (if expensive) to get a system which will produce 3db more than you reckon you'll ever need.

 

Hope this answers the question ;)

 

rob

 

<edit 1- corrected comment about the crown amp. Original post said it was two 4 ohm subs in parallel. Not so.>

<edit 2- typos, grammar, added words and a couple of extra notes>

[Ekij]

If you want to avoid a 2 Ohm load on your amp (which I would), you could wire the two sub cabinets 4 Ohm loads in series rather than in parallel to get back up to 8 Ohms.

 

The only drawback being that fault (cable or cone) would take out both speakers wheras in parallel it would only take out one.

[jayselway]

Ah! More info is always useful!

 

hmmm - what the other guys said! ;)

[Rob_Beech]

Another vote for scenario 2, yes its more money, but yes it will be better on the amps, and if you ever get an amp failure you can always go to scenario one whilst your other amp is being repaired, even if you are left with the smaller amp, you just have to be careful. I'm not fond of 2ohms, most amps feel the same way.

 

I'm not sure if anyone else has noticed but alot of these high powered amps (mainly switch mode) are giving out figures where the rated 2ohm output is LESS than the rated 4ohm output.

 

Take the Crown I-tech 8000, 4000wpc at 4ohms, but only 3500wpc at 2ohms. Clearly this is some way to getting around the 2ohm problem, it is either the manufacturer trying o put people off using them at 2ohms by showing that you dont gain anything, or it some circuit which limits the power so the amps are not driven as hard at such a low impedance, I would guess the second but both may play a part.

 

crown specs

 

http://www.crownaudio.com/amp_htm/itechspecs.htm

 

there are more a few more following similar patterns.

 

theoretically with 100% efficiency the amplifier should output 8000 watts per channel at 2ohms. Clearly this is not so.

 

A note on the amplifiers I use, the Powersoft Digam 7000 particularly has a rating of 1800wpc at 4 and 3500wpc at 2 9very nearly 2x the 4 ohm rating). This is what it Claims anyway, whether that is true or not remains to be seen.

 

Rob

[Simon Lewis]

Furthermore, there is a convention that most of the major amp manufacturers seem to adopt:

 

Power ratings at 8 or 4 Ohm are quoted at (say) 0.01% THD, whereas the 2 Ohm figure is at 1%.

 

For clean power that produces less stress on the amp, stick with a 4 Ohm minimum load where possible.

[Rob_Beech]

And another little addition to what Simon Says,

 

Often the 8ohm and 4ohm ratings are measures 20Hz to 20kHz whereas the 2ohm rating is only rated power at 1kHz (with 1% THD)

 

Rob