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Amp output and cab power, dumb question

Discussion in 'Amps and Cabs [BG]' started by julo, Jun 23, 2006.

  1. julo


    Jan 18, 2005
    Boulder, CO

    I'va done a search but too many threads with "amp" "output" power" "cab"...

    I have a MAG 300 and the MAG 4*10.
    The head is rated 300W @ 4 ohm (from what I read here) and the cab is rated 450W and is 8 ohm. I'd like to add a 15" cab, probably the MAG 115 which is rated 250W and is 8 ohm.

    my Qs are:
    1) Are the 250W enough for the amp?
    2) The Mag head has 2 jack outputs for cabs, should i plug each cab in 1 output?

    This has must have been discussed before, but could not find anything.

    thanks for the help.

  2. Figjam


    Aug 5, 2003
    Boston, MA
    1) yes, i believe each cab will get half of 300.
    2) That is one way to do it, yes. The other way would be to run the head into one cab, then from that cab, go to the other cab.
  3. julo


    Jan 18, 2005
    Boulder, CO

    That's what I was thinking too.

  4. brandonsbass


    Jun 25, 2006
    Here's how it works my man,
    You have 300 watts at 4 ohms. If you run the amp into an 8 ohm cab you won't get the full 300. So, in order to power to cabinets it will be best to run the speakers in paralell. This will reduce the ohms to 4. That means you need to run a cable directly from your amp to each of the cabinets. The amplifiers power will be split into two 150 watt "streams" for each cabinet which your cabinets should have no problem handling.

    There is also a way to run cabs called series. This is done by running from one output on the amplifier then connecting to the first cab, and then running a cable from the first cab output into the second cab. Whne you run in series you add the ohms and it would put you at 16 ohms with your current cab setup. And I would say that much resistance would be far too much for you to get any useful performance sound out of your amp.

    The formula for calculating each of these numbers is
    Z= (Zn*Zy)/(Zn+Zy) ex. your cab set up (8*8)/(8+8)=
    64/16=4 ohms

    series 8+8 16 ohms. These formulas will come in handy in the future if you need to connect a lot of cabs to a more powerful amp.

    Bass it up man,
  5. Actually, as Figjam said, that would be running the amp in parallel as well. Running an amp in series involves some different or modified cables i believe. Someone want to drop some more knowlege on us?
  6. funkinbottom

    funkinbottom Supporting Member

    Apr 23, 2006
    Northern CA.
    Most amps (if not all) output (speaker jacks) are wired parallel as are the jacks on most cabs so your amp will see two 8 ohm cabs as a 4 ohm load whether you run out of both outputs into both cabs, or one output ibto the cab,out of the cab into the second cab.
  7. The short answer is no. you'r asking a car engine to drive a truck/lorry, 300 watts is not really enough to drive a 700 watt speaker set-up at stage performance levels, and may well start to break up when driven hard causing possible damage to speakers or early shutdown of your amp.
    For around the same price that you will pay for a named cab you can build one and put a power amp module in it, this is what I do.
  8. Lowtonejoe

    Lowtonejoe Supporting Member

    Jul 3, 2004
    Pasco, WA
    No, that's not a very good analogy.

    It takes no more 'effort' on the amps part to power a 50w cab or a 5000w cab.

    His amp will drive the cabs just fine.

    His only 'trouble' will be clipping if and only if he turns up that loud. That is the exact same problem that all of us have no matter what gear we are using.

    If the problem is clipping, then there is a need for a bigger amp. If the speaker is 'farting' then a higher powered cab is probably needed.

    Adding speakers will make the rig a little louder anyway and big rigs always look cool.


  9. It is quite incorrect to say that a 5k handling voice coil takes no more power to drive it than a 50 watt voice coil, think of the difference between the power required to drive a 40 watt lite bulb and 150 watt, to say it is the same is grossly incorrect!
    A 500 watt voice coil requires far more power than a 50 watt voice coil to work to it's optimum.
    Most speakers today only fart when they are receiving a clipped signal.
  10. Speakers "fart" when they are driven too hard.
    A 700 watt cab will sound fine with 300 watts, as Lowtonejoe says, the only problem in this situation is when the amp is turned up to clipping level in an attempt to get maximum volume. You cannot "underpower" a cab, but you can cause problems by supplying it with a clipped signal.
    That's not to say that a particular cab might not sound as good as it can with a low input, but it won't be damaged, and it will still sound fine. 300 watts will drive a 700 watt cab set-up and sound good (although it might sound even better with a lot more power)
  11. Lowtonejoe

    Lowtonejoe Supporting Member

    Jul 3, 2004
    Pasco, WA
    Another terrible analogy.

    Not to mention bad info.


  12. Light bulbs are rated by the power they actually consume when operated at a specified voltage, while speaker voice coils are rated by the amount of power they can handle. Two completely different ways of rating power.

    The most pertinent metric for how much power a driver requires is its sensitivity. A 5kW (not that there are many of those) speaker rated at 100dB 1W/1M sensitivity will be exactly as loud as a 50W speaker rated at 100dB 1W/1M given the same input power. So how does it require more power?

    That being said, the maximum SPL of the 5kW driver would be 20dB more than that of the 50W one and you would need the 5kW of power to acheive that. This doesn't mean that the 5kW driver requires more power than the 50W one though.
  13. You've answered the point yourself a small amp working beyond its capabilities to produce those DBs will either burn out or damage the speaker and that is why I don't recomend an underpowered amp driving rated cabs well beyond its output, when it is now an industry accepted practice to have upwards of twice the power amplifier power of the speaker ratings, a 500 watt speaker is better off receiving a good quality signal from a 1Kwatt amp than from a 250 watt amp working flat out to produce the same Dbs, I myself used an Eminence 300 watt 15" powered by a bridged 1200 watt power amp for 3 years without any problems.
    By the way the 1k @ 1 watt @ 1 metre went out with the dark ages, that useless way of rating speakers is for inexperienced people to be impressed by and went out when the broadband spectrum became the yardstick.

  14. I notice you didn't address the power rating part of my post.

    I don't disagree that in order to get the most SPL from a speaker that you should use an amp rated for more power than the speaker. However, my point was that the power rating on a speaker is not necessarily an indication of 'required' power. There's nothing inherently wrong with powering a 500W speaker with a 250W amp or a 10W amp for that matter, if that amount of power meets your requirements. People don't always need to wring out every last bit of SPL from their speakers. The point is that a 5kW speaker doesn’t necessarily need more power than a 50W one to reach a certain SPL. How much power a particular speaker needs to reach a certain SPL is not dependent on power handling, it is dependent on sensitivity (assuming that the driver can produce that SPL within its ratings).

    Where did I say anything about 1k? 1W/1M (or 2.83V/1M) is the standard used in broadband sensitivity measurements as well, the broadband curve needs a reference point just as much as one point on it does.
  15. Power handling ratings for speakers is telling you how much power the speaker can safely handle, not how much is needed to drive the speaker. Speakers can be either efficient or inefficient. Speakers with super efficient voice coils can produce good volume at low power levels.
  16. 44me


    Jun 17, 2002
    Bedford, NH USA
    Bassman62 – you are way off base here (no pun intended). The load that the amp sees is the speaker’s impedance. The amp knows nothing about the power capability of the speaker(s). It’s disturbing that you are so adamant with your misinformation. What’s the technical basis for such a strong, yet incorrect position?
  17. Heavier duty voice coils require more power to drive them, a 4" voice coil requires more power to drive it than a 1.5" this is pure logic as a 4" voice coil is much heavier than a 1.5" other wise you've discovered perpetual motion, producing 100 dbs for 1 watt input has no bearing on the output of a 100 watt amp driving a 500 watt cab compared to a 500 watt amp driving the same cab.
    1) 100 watt amp into 500 watt cab, 2 possibile areas of failure.
    2) 500 watt amp into 500 watt cab, 2 possibile areas of failure eliminated.
  18. 44me


    Jun 17, 2002
    Bedford, NH USA
    May be pure logic, but it’s flawed. Let’s get to the fundamentals.

    An amplifier outputs a voltage.
    The voltage is applied across the speaker’s impedance.
    A current develops through the speaker’s voice coil and the amplifier’s output stage defined by:
    I = V/Z
    The portion of Z that is resistive dissipates heat in the voice coil defined by:
    P = I^2 * R

    Where does the mass of the voice coil come in? It effects the efficiency with which the speaker is able to convert electrical energy to mechanical energy. It has absolutely nothing to do with how much the amplifier is loaded.
  19. A 6" voice coil can show the same impedence as a 1" do you seriously think that voice coil size makes no difference.
    I used to field test speakers for Brian McKenzie of Mckenzie Accoustics and later the founder of Precision Devices (Turbosound speakers) when he had his factory in the UK, prior to this Brian worked for Celestion, I met Brian in the mid 70s, when I took my 200 watt 2x15 cab to his factory to ask him why my speakers were flapping/farting when I was only using a 120 watt amp, brian connected my amp to a scope and showed me where it was clipping he then connected a more powerful amp to the scope without any clipping, the more powerful amp played through my 2x15 returned totaly different results than my own amp (no flapping/farting) Brian then went on to explain about speakers not liking clipped signals this often caused the speakers to bottom out damaging the voice coils in the process.
    I field tested many loudspeakers for Brian down the years and was supplied with cabs from Trace Elliot and Turbosound to do so, as well as a Trace Elliot Pre-amp (power amps supplied by Brian) the very first 18"s 15"s 12"s 10"s to come out of Preciosion Devices were field tested by me.
    In the past before his retirement Brian supplied all the leading UK brands including Marshall, a high percentage of his protoypes were field trialed by me
    I used to purchase Brian's protoypes and load them into cabs that I built to sell them off cheap.
    So yes I do think that I've had enough practical and semi technical experience I've built my own gear since 1962 and on a semi commercial bassis since 1975.
  20. Lowtonejoe

    Lowtonejoe Supporting Member

    Jul 3, 2004
    Pasco, WA
    And yet your understanding of what is going on in the real world is so flawed.

    Please stop spreading bad information.

    Not only did you give the OP bad advice, you have also succeeded in hijacking his thread.

    Not good


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