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Amp/cab Ohm difference

rcthorp

rcthorp

Dec 3, 2013
10
0
#1
Hello everyone, first time poster here. I'm shopping for a new amp, my first real amp. Currently I'm looking at:

Head: Eden WT550, Hartke 5500C, or GK Fusion 550

Cab: Probably or 4-10 cab matching brands with whatever head I buy (a little neurosis involved with the match there I'll admit), unless I come across some more information or get to play through something that's amazing.

My question is this: I do not understand the ohm difference that is stated in the specs of heads vs cabs. Most of those heads will say something like 500 watts at 4 ohms, while the cabinets will say some wattage number and then list then impedance at 8 ohm. For instance the Hartke 4.5 XL 4x10" 400-Watt Bass Cabinet w/5" Driver notes it's impedance at 8 ohm. Can I plug any of the heads I mentioned into a cab like that without worry of blowing anything up or do I need to by a 4 ohm cab (I don't know if those exist, I could just be misunderstanding the concept here)? Am I correct in assuming if the cab is wired for 8 ohm then pumping 4 ohm to it could be detrimental? I get that if I used two 8 ohm cabs then the amp would "see" 4 ohms, thus giving me close to the stated wattage on their marketing. I just don't understand how it works for just a single cabinet.

Realistically, what I'm proposing of buying up there is overkill for what I could see myself doing in the near future. I currently don't have a playing out option but I'd like too and I'm trying to setup my gear so that I'm ready for that occurrence.


My budget is around 1000.00 new or used and I want a head with a single cabinet, probably a 4-10.
 
#2
The two 8 ohm cabs will get you the full 500 watts. If you only run one 8 ohm cab, the power output will typically be about half. So around 250 watts delivered to a single 8 ohm cab so you should be fine running a single 400 watt, 8 ohm cab. If you want to spend less money this would be a good option. Then you could add another 8 ohm cab down the road and take advantage of all the power the amp is capable of delivering.
 
#5
The amp only "sees" the load, it doesn't 'know' how it's connected. If you connect two 8 ohm loads to the amp, it will see 4 ohms. If you hook a 4 ohm cab to the amp, it will see 4 ohms. On a multi-speaker cab, the speakers individually can be a variety of 8 and 16 ohm loads connected in some form of series/parallel. Whatever wiring magic the manufacturer has done to generate the net cab load is invisible to the amp.

Basically, you need to watch both load and power - ohms and watts respectively. Don't go below the load specified by the amp manufacturer but use the lowest allowed load to get the most power out of the amp. Avoid using an amp that can overpower the speaker's rating - at some point, you will try to push more power through the speaker than it can handle and you will fry it. Rule of thumb here is the amp should be twice the power of the speaker. Note that there are two types of power ratings - Peak power rating or program power rating is the maximum instantaneous rating. RMS (root mean squared) is a form of average power. You should only compare like measures of power.

There's lot more engineering subtleties to this but if you stick to the above rules, you should be OK.
 
#7
"Rule of thumb here is the amp should be twice the power of the speaker."

Really? I'm not trackin' here. Are you saying if you get a 800 watt rms amp at 4 ohm one should buy a 400 watt cab at 4 ohm? I get that the cab in an 8 ohm would work, at least would match watts for watts (that's assuming the amps wattage cuts in half at 8 ohm).
 
#8
rcthorp said:
"Rule of thumb here is the amp should be twice the power of the speaker."

Really? I'm not trackin' here. Are you saying if you get a 800 watt rms amp at 4 ohm one should buy a 400 watt cab at 4 ohm? I get that the cab in an 8 ohm would work, at least would match watts for watts (that's assuming the amps wattage cuts in half at 8 ohm).
Click to expand...

No it is absolutely incorrect. That little piece of tripe was a misquote from a JBL white paper that was written for HF horns in PA use. NOT bass or even midbass cone speakers and is passed around like poison candy.
 
#9
rcthorp said:
"Rule of thumb here is the amp should be twice the power of the speaker."

Really? I'm not trackin' here. Are you saying if you get a 800 watt rms amp at 4 ohm one should buy a 400 watt cab at 4 ohm? I get that the cab in an 8 ohm would work, at least would match watts for watts (that's assuming the amps wattage cuts in half at 8 ohm).
Click to expand...

No an amps rated capacity isn't necessariy 1/2 because the load is 1/2. The Peavey amp I just sold was 500 watts at 4 ohms and 350 watts at 8 ohms. What's 'tossed around' here on the forum is that the speaker should be rated at twice the wattage as the amp. The problem with having an amp rated higher than the speaker is that the temptation for more volume will eventually lead to blowing out the speaker. There's lots of anecdotal stories about folks blowing out speakers on combos at 1/2 volume or less but doesn't mean the amp was working at 1/2 it's rated power.
 
#10
Watts / power doesn't always indicate volume. The cab's sensitivity at lower power (db /w /m) is a good and general indicator of the loudness a speaker will sound. I have a 15" cab with 103 db /w/m and it's seriously loud at very low power levels. It's 8 Ohm and at only a few watts it can keep up with lower sensitivity cabs which have to be driven at higher powers to match. Typically a 3db increase in output requires a double of wattage power. So a 97db/w/m cab will need 4 times the power to create the same air movement as the more sensitive cab.
It is wise to have a cab which has a higher power rating than you amp but not essential. But putting a lot of power though a cab in the hope to make up for a low sensitivity rating is not a good idea, which is how guys blow speaker cones at quite low volume levels.
If you can find a loud cab, which can handle your amp power rating (4 or 8 Ohm) then you can dial in the decibels you need without the worry of blowing your amp or cones.
 
#11
Rob22315 said:
No an amps rated capacity isn't necessariy 1/2 because the load is 1/2. The Peavey amp I just sold was 500 watts at 4 ohms and 350 watts at 8 ohms. What's 'tossed around' here on the forum is that the speaker should be rated at twice the wattage as the amp. The problem with having an amp rated higher than the speaker is that the temptation for more volume will eventually lead to blowing out the speaker. There's lots of anecdotal stories about folks blowing out speakers on combos at 1/2 volume or less but doesn't mean the amp was working at 1/2 it's rated power.
Click to expand...

I wish there was a "general rule of thumb" to be had. Most cabs are not rated at bass guitar frequencies where their input power limits are most important. "Safest" bet is to have cabs "rated" at twice the amp's power as most cabs will reach their mechanical limits at 1/2 their rated power for bass guitar use.
Here is some reading based on actual facts http://barefacedbass.com/technical-information/mythbusters1.htm. Not all cabs get rated the same way just as not all amps get advertised the same way.
 
#13
Often you would buy amps rated at twice the cab, because older a/b amps were(along with the cabs) grossy overrated. And varied greatly. Now with standards being deployed, along with the advent of digital, I recommend getting an amp 10-20% more powerful than the cab requires. AND CLIPPING IS BAD.
 
#15
Hello:
I'm the Behaviour Police here. So listen up:

the report post button is a triangle with an exclamation point in it, in the upper right of every post here.
hit that to report rude behaviour, rather than engaging in further fisticuffery.

repeat bashy posts in this thread will result in infractions for the lot of you whose mess I just cleaned up. behave better.

not replying, and moving on to another thread, and setting certain users to ignore in your preferences are also working options.
 
#16
B-string said:
I still don't understand the logic of trying to pour 440 to 480 liters (amp power) into a 400 liter container (speaker cab)?
Click to expand...

I do it because amps run most efficiently around 80%, and I refuse to run amps into clipping. Give a speaker more power than it can handle and you'll blow the speaker. I also go off the RMS, being mathmatical and the only comparative you can really use between woofers and amps. Give a speaker to little power and push it into clipping, you'll be buying a new speaker, and amp.

For fun once when I was a kid, I took a 120v 20w bulb and wired it parallel to a woofer and mounted it behind the port. Besides the really cool light show, I got to see how hard the hardest parts were, and how distorted the light became when you ran the amp into clipping. Like the light transients were smooth, and now like your trying to watch while jackhammering.

The lightbulb trick(with a different ligthbulb) is a great way to keep yourself from blowing a speaker. Not only does it give a visible indicator, it power soaks the worst spikes, somewhat like a compressor.
 
#19
Lilduke said:
I do it because amps run most efficiently around 80%, and I refuse to run amps into clipping. Give a speaker more power than it can handle and you'll blow the speaker. I also go off the RMS, being mathmatical and the only comparative you can really use between woofers and amps. Give a speaker to little power and push it into clipping, you'll be buying a new speaker, and amp.

For fun once when I was a kid, I took a 120v 20w bulb and wired it parallel to a woofer and mounted it behind the port. Besides the really cool light show, I got to see how hard the hardest parts were, and how distorted the light became when you ran the amp into clipping. Like the light transients were smooth, and now like your trying to watch while jackhammering.

The lightbulb trick(with a different ligthbulb) is a great way to keep yourself from blowing a speaker. Not only does it give a visible indicator, it power soaks the worst spikes, somewhat like a compressor.
Click to expand...

Interesting thoughts. Speaker rms ratings are at best tested with white noise and usually at 1KHz instead for their maximum safe input power handling. Speaker systems at best are 5% efficient at producing sound from the power input, 95% efficient at producing waste heat. Your light bulb experiment is interesting and could very well demonstrate power compression in the speakers. Power compression is the point where maximum power transfer has been exceeded and excess heat starts being produced. All too often with live bass guitar use, the rated power handling is more like twice what the cone drivers can handle (mechanical limits). Fortunately when not using effects the duty cycle is low and mechanical over excursion can be heard.
 
#20
First, choose the loudspeaker that will give you the sound and volume and coverage that you want.

Second, look at its power rating and impedance. Does it have a "program" power rating? If so, great … that's the loudspeaker manufacturer's roundabout way of suggesting the maximum amp power you should choose, within maybe ±20%. You don't have to abide by it. You can choose an amp with a lower power rating, and as long as you get the sound levels you want without significant clipping, that's fine. You can choose an amp with higher power, but you'll have to be careful because you probably won't be able to use the amp's clip indicator to warn you that you're pushing it too hard.

The "program" power rating is a valid target range for choosing your amp power only as long as you avoid these two things:
  • High average signal power--this could be from heavy clipping or from intense compression or limiting. This tends to cause thermal damage—IOW, burning out or melting the voice coil or other parts of the loudspeaker circuit.
  • Lots of bass boost--The lower the frequency, the larger the speaker cone's excursion. So if you're one who likes a tubby sound with lots of EQ boost in the 30 to 80 Hz range, you're more likely to make your loudspeaker(s) bottom out, which is when the former, the cylindrical piece that the voice coil is wound on, strikes the driver's magnet structure. This results in mechanical damage because it's essentially like hitting the voice coil former with a sledgehammer.
If the loudspeaker doesn't have a "program" power rating, you can usually double its continuous (so-called "RMS") power rating and use that figure instead for selecting your amp power needs.
 

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