# Trying to Understand Amp Power

Discussion in 'Amps and Cabs [BG]' started by mikarre, Nov 19, 2003.

1. ### mikarreGuest

Okay, if you have an amp that's 250 watts per side, I understand that's 500 "total" watts, but it is really the same as having a 500 watt amp? I'm thinking in terms of real world power and volume, like if you're standing in a room with the thing. Would a 250 watt per side amp be as loud as a 500 watt amp, or only as loud as a 250 watt amp?

I understand that cabinets, acoustics, the room, stages of the moon, etc all play a role here, but I'm saying all things being equal...say a stack with one 250w side going into a 4x10 cab, and the other 250w side going into a 1x15 cab versus the same stack with all 500 watts going into both cabs.

2. ### notanaggieGuest

Sep 30, 2003
Well, assuming two of the same cabinets, theoretically the two 250 would be the same as 500.

In reality, to my ears and gut, one 500W amp into a speaker sounds louder and ballsier than two 250s. Probably an engineer could "prove" that it isn't but I still wouldn't believe it.

The 500 works the speaker that much harder, and that seems to be magic to me. I don't know how to explain it, but I feel it.

If you have an amp with 2 x 250, can yours be bridged? That would let you compare ....if you have suitable speakers.

If you use two different speakers, comparisons sort of go out the window, I think.

3. ### nicoli

Apr 4, 2002
This depends... I would think that amps rated at 250 watts/side are rated for that power at a 2 ohm load (4 ohm cab each side), which would mean the full 250 watts/side wouldn't be going out at only a 4 ohm load (8 ohm cab each side).

However, if the 250 watts/side is with a 4 ohm total load, in theory, they would be the same.
Assuming the cabs are both 8 ohms the 500 watt head is going to have half the power going to each cabinet as well; not "the full 500 watts going into both cabs".

4. ### xyllionCommercial User

Jan 14, 2003
San Jose, CA, USA
Owner, Looperlative Audio Products
Well, I'm an engineer and I won't argue with you on this point because there is an awful lot of variables that determine how loud you sound. The amp power rating is only one number in a large equation.

What is even worse is the amp power rating often is a worthless number. There is no standard for determining an amps power rating. Some companies are better than others when it comes to giving you a useable number. Also, often times the amp power rating is based on a frequency that is higher than what comes out of a bass.

Although it pains me to say it, the only way you can judge the volume capability of a given rig is to try it. This is one of the reasons why bass players often keep changing their rigs. Also, it is one of the reasons why we tend to by amps with such large ratings. If you buy more than you need, then you will be happy, but if you buy less then you will be shopping again soon.

With this all in mind, it is actually possible to put together a rig rated at 150W that kicks ass. It also possible to put together a 500W system that doesn't cut through.

My recommendation is to buy used. Then if you choose to change your rig, you can do so without losing money.

5. ### Bob Lee (QSC)In case you missed it, I work for QSC Audio!Gold Supporting MemberCommercial User

Jul 3, 2001
Costa Mesa, Calif.
Technical Communications Developer, QSC Audio
I'm not clear on what you're trying to say, but let me point out that an 8-ohm speaker is an 8-ohm load, not 4 ohms, and a 4-ohm speaker is a 4-ohm load.

If you have a two-channel amp and you put an 8-ohm speaker on each channel, you will have an 8-ohm load on each channel. If you put a 4-ohm speaker on each channel, each channel will have a 4-ohm load. The idea is that you have two separate channels and two separate speaker loads.

If you're talking about a one-channel amp, then parallel-connecting two 8-ohm cabinets to its output will give it a 4-ohm load, while two parallel 4-ohm cabs will constitute a 2-ohm load. With a mono amp, though, you wouldn't say it's rated for X watts per side, because it doesn't have sides.

6. ### Bob Lee (QSC)In case you missed it, I work for QSC Audio!Gold Supporting MemberCommercial User

Jul 3, 2001
Costa Mesa, Calif.
Technical Communications Developer, QSC Audio
Okay, if you've got a two-channel amp rated at 250 watts/ch into, say, 8 ohms, that's equal to having two separate 250-watt mono amps. Each channel or each mono amp can drive a cabinet, and you have independent control over the loudness of each cabinet so you can match them, make one louder than the other, etc.

Compare it to a mono amp rated at 500 watts into 4 ohms. You would connect the two 8-ohm cabs in parallel to make a 4-ohm load. You would not have individual control over the cabinets; if one was inherently louder than the other, you would have that same difference no matter how loud you play.

Which is louder? In either case, it all depends on how much of the total 500 watts you actually use, which depends on where you set your gain and volume controls, how hard you play, and other factors.

7. ### Bob Lee (QSC)In case you missed it, I work for QSC Audio!Gold Supporting MemberCommercial User

Jul 3, 2001
Costa Mesa, Calif.
Technical Communications Developer, QSC Audio
In the world of power amps, there are FTC and EIA power ratings. The FTC spec is far more rigorous than the EIA. The all-but-obsolete IHF power rating is not used except by a scant few manufacturers who want to hide their amps' inability to deliver continuous power. Germany had a standard called DIN, but I don't know how widely used it is any more. The standards allow some leeway; for example, each manufacturer can arbitrarily define the threshold of clipping as a certain percentage of THD of the test sine wave.

Ignore "peak" and "music" power ratings. They are meaningless. If anyone starts selling you on peak or music power, tie him up with his Monster Cable and stick him in a closet.

Here are some Web pages worth reading:

http://sound.westhost.com/power.htm

http://www.qscaudio.com/press/in_news/cssbypq.htm

http://www.1388.com/articles/tech_underAmp/

At one company I used to work for, the owner insisted that we add "heavy metal power" specs to the power amp spec sheets, with numbers he picked out of the air. I told him that was stupid, but he wanted it done anyway. I started sending out résumés.

That's true. Reputable companies tend to understate their power ratings. Seedy brands may tend to round their numbers up a lot when calculating power.

8. ### mikarreGuest

I understand what everyone is saying, but I guess maybe I should ask my question a different way:

If I am playing in a band and one 250 watt amp isn't cutting it, would adding another 250 watt amp be as significant of an ungrade as switching to a 500 watt amp, or would I still have the same problems I had with a 250 watt amp?

9. ### Bob Lee (QSC)In case you missed it, I work for QSC Audio!Gold Supporting MemberCommercial User

Jul 3, 2001
Costa Mesa, Calif.
Technical Communications Developer, QSC Audio
It depends on how you use the amp(s) with your cabinet(s). Without those in the equation, it's impossible to tell. What do you have in mind?

10. ### JMXVorsprung durch Technik

Sep 4, 2000
Cologne, Germany
AFAIK DIN or sinus (sine) rating is still widely used (maybe except by the PMPO rating faction ) and is roughly comparable to the RMS rating.

11. ### Bob Lee (QSC)In case you missed it, I work for QSC Audio!Gold Supporting MemberCommercial User

Jul 3, 2001
Costa Mesa, Calif.
Technical Communications Developer, QSC Audio
Yes, the DIN power spec is similar in many ways to the EIA spec, enough to be practically interchangeable. Do you know if there is a CE or IEC amplifier power rating standard, JMX?

12. ### uglybassplayer

Aug 24, 2001
New Jersey
I think I understand what mikarre is attempting to find out. How about if we use this as an example...

Let's say mikarre has a "B250-15" combo amp with a single 15" speaker from XYZ Amp Company. It's rated at 250watts RMS.

In terms of "power and volume", does it make a difference if he were to:

A) Get a 2nd "B250-15" and feeds his bass signal to both amps.

vs.

B) Upgrade to the "B500-15", which is XYZ Amp Company's 500watts RMS version of the same Combo.

13. ### JMXVorsprung durch Technik

Sep 4, 2000
Cologne, Germany
Do you mean this?

http://www.1388.com/articles/tech_underAmp/

14. ### Bob Lee (QSC)In case you missed it, I work for QSC Audio!Gold Supporting MemberCommercial User

Jul 3, 2001
Costa Mesa, Calif.
Technical Communications Developer, QSC Audio
Thanks, JMX, but it's my understanding that those are for measuring loudspeaker power capabilities, but not power amps. I don't know why those were mentioned in an article on amplifier power.

15. ### Bob Lee (QSC)In case you missed it, I work for QSC Audio!Gold Supporting MemberCommercial User

Jul 3, 2001
Costa Mesa, Calif.
Technical Communications Developer, QSC Audio
Okay, that's a good clarification. The biggest unknown differences between the two situations are the speakers. Presumably it would be a choice between two lower-power speaker systems and one much higher-power system. The sensitivity, tone, polar response, etc., of the higher-power one might differ greatly from the lower-power ones. In addition, having two side-by-side speaker sources will alter the horizontal dispersion of sound; stacking them will tend to make the horizontal dispersion more like that of a single cab, but will alter the vertical dispersion (but you, your band, and your audience are in a mostly horizontal plane about the speaker).

16. ### j_sun23

Feb 24, 2003
Baton Rouge LA
No, no, no. I think what Mikarre is asking is whether there is any difference between running two cabs with 250 watts each from two amps (or amp channels) or having one amp with 500 watts running both cabs, daisy chained.

I'm no expert, but here's my attempt at a simple explanation. Key word, simple. i.e. not scientifically correct, simplified for explanation. Oh, and pay attention to ohm loads!

If both cabs are 8ohm cabs then an amp with a rating of 500w @4ohms (be it a mono amp or a bridged amp) will deliver half its power to one cab and half to the other (250w per cab).

If you have a 2 ch. amp making 250w/ch @8ohms, with one cab on each channel, then once again, you've got 250w per cab.

When you hook up two 8ohm cabs in series, daisy chained, whatever you want to call it, it effectively becomes a 4ohm load, which is why you'd need an amp with 250w @8ohms per ch. to deliver the same amount of power to each cab as a 500w @4ohm amp would with the cabs daisy chained.

Which is louder? well, ignoring the fact that it depends on other variables, for the sake of an answer, I'll say neither if what you're talking about is like the example I gave.

Hope this helps and I'm not too far off base.

17. ### Bob Lee (QSC)In case you missed it, I work for QSC Audio!Gold Supporting MemberCommercial User

Jul 3, 2001
Costa Mesa, Calif.
Technical Communications Developer, QSC Audio
In that case, my second post in this thread addresses the question.

18. ### xyllionCommercial User

Jan 14, 2003
San Jose, CA, USA
Owner, Looperlative Audio Products
At the risk of being a suck up,
I would like to say that I appreciate that QSC takes pride in their work and rates their amps with care.

I'd rather not risk slander and name the makers that I feel take liberties with the numbers. If you search around TB, I'm sure you can get a feel for which amps deliver the power that they claim and which do not.

And as Bob Lee said, your speaker cabinets have a huge impact on the volume that you get from a given amount of power.

19. ### nicoli

Apr 4, 2002
Sorry, lack of sleep. The point of my post was essentially what J-Sun just posted.

But, I think the other part if worded better, is that many amps that one sees advertising their peak power rating are different from the typical head rating.

That is to say, a 2 channel power amp rated at 500 watts will likely be putting out that much power on a 4 ohm bridged mono load, one cabinet. If he's driving two 8 ohm cabs and using each channel independantly, the total output is likely to be something like, say, 120 watts per side, or 240 total.

If he was using a head rated for 500 watts, it most likely was putting out 500 watts total into a 4 ohm load from the two cabinets.

This could be what he's talking about as far as seeming like more power is coming out of a head than a 2 channel power amp. Hope that cleared things up a bit.

20. ### j_sun23

Feb 24, 2003
Baton Rouge LA
Actually, I think if you re-read your post

You'll see that it wasn't very clear as far as answering mikarre's question.

That's not to say it was wrong or wasn't useful and informative.

I just thought that mikarre was asking if there's a difference, powerwise and volumewise, between powering 2 cabs with 250watts a piece from two separate amps/channels, or chaining the cabs and feeding them 250 watts a piece from a single mono 500 watt amp.

All the confusion seems to be from mikarre's not being, or being able to be, more specific about the ohm ratings and configurations of the cabs and the amp/s.

My answer to what I thought was his question was an unsure NO, assuming the example I gave.