Amp Heads and Cabs: How?

Discussion in 'Amps and Cabs [BG]' started by Vince S., Jul 18, 2003.

  1. Vince S.

    Vince S. Resident Former Bassist

    Jan 24, 2003
    Ive been looking at combo amps for a while, and I realized that I needed a rig, because a combo just wouldnt cut it for volume and sound. So Im very new to the whole rig deal.

    How are heads connected to the cabs? What is impedance?

    Lets say I have a amp head at 300W 4ohm, and a cab at 400W 8ohm..would that work?
  2. miccheck1516

    miccheck1516 Guest

    Feb 15, 2003
    alright some guy will answer this better than me but here goes.

    Heads, are what powers the cab. Moost of them are stable at 8 ohms or 4 ohms.

    Most heads are more powerfull at 4 ohms.

    If you have a 1000watt head and a 200w cab, you could blow your cab, make sure that your cab has more power than your head, unless you know not to push the cab too far.....

    With your example, you would be ok but you wouldnt get 300w out of the cab/head combination.
    If you got a 4ohm cab rated at 400w, you would get 3oo wats out of it because there is less resistance and so more power (or wattage) can get to the cabnet.

    For most heads, if you want to use the full power output you will need one 4ohm cab or two 8 ohms cabs

    2 8 ohms cabs connected to the same amp result in an overall impendence (or ohmage as some people call it) of 4 ohms.

    Some power amps are stable with a 2 ohm load (ie two 4 ohm cabnets) but i doubt youll be looking into that right now.

    Generally it is safer to have a cabnet which is rated at more power than the head which you are usinh (in my opinion) This way you wont blow the cabnet, or get unwanted distortion.

    Like i said above, your head can be more powerfull than what is recommended for the cab, but you need to be careful.

    Now lets see how many mistakes i made when the big guys get here
  3. If your amp is a 4 ohm head, that most likely is the minimum impedance you can use... Your cab has the capability of handling an amp that can throw 'up to' 400 watts, but you will not pull 400 watts from it. You won't even pull 300 watts from the amp as you're not running at 8 ohms... But the good part is, you'll be pulling out about 150-240 watts from the head with that single 8 ohm cab. YOU CAN ADD ANOTHER 8 ohm cab and use them simultaneously. Doing so will give you a 4 ohm load. Some amps can handle 2 ohms loads, but many can't. Impedance is the resistance, otherwise notated as ohms...

    Say your amp has a minimum impedence of 4 ohms. you can run ONE four ohm rated cabinet OR two 8 ohm cabinets. A single 4 ohm cabinet will pull more watts out of your cab being louder than a single 8 ohm cabinet (in general)... BUT running two 8 ohm cabs will still bring you to a 4 ohm load, and you wont be pulling anymore power out of the amp anyways, but you will have a louder output from your speakers because you will have more of them...

    I'm sure there's a FAQ's section here somewhere...

    but in general:

    1 4 ohm cabinet = 4 ohm load
    2 4 ohm cabinets = 2 ohm loads (never do this unless you're amp is capable, read your user's manual... just because your amp is a good brand and expensive doesnt mean it can do 2 ohm loads...)
    1 8 ohm cab = 8 ohm load
    2 8 ohm cabs = 4 ohm load

    you get the picture. If you want to pull the most power out of your amp in a single cabinet, it will need to match the minum impedance of your amp to pull out the most power. But if you want it to be louder, getting 2 8 ohm cabinets will bring it to a 4 ohm load being louder with the same power. But you can use a single 8 ohm cabinet with your 4 ohm head... you wont be pulling the most power your amp can give though, but it's perfectly safe!
  4. By the way, Solidus, what town are you from? I see you're from Connecticut:)... I'm in cheshire.

    If you want inexpensive cabinets with excellent quality and components comparable to SWR (one of the leading manufacturer's in bass amplification, unfortunately taken over by Fender within the past few month), checkout I bought the Avatar CB115 compact 15" cabinet in an 8 ohm model... my amp is a hartke 3500, and I can deliver 240 watts into that single cabinet, or when I flip the CB115 on its side and put an 8 ohm peavey 410, I can pull a 4 ohm load, 350 watts out of my head...
  5. Scott D

    Scott D

    Apr 21, 2003
    Minneapolis, MN
    Well, you got some of it right, (no harm or anything intended) it is a bad thing to massiviley overpower your cab (like the 1000 watt head and 200 watt cab) but its not a good idea to underpower you cab. There is a general rule- about 2-2.5x more power than the cab is rated is acceptable. so in example, if you had a 200 watt cab, it would be fine to power it with 400-500 watts, but don't go much over there. Also, all heads will be more powerful running at 4 ohms instead of 8 (but thats for solid state power sections only. Tubes are different, they produce the same amount of power at different ohms, but i'm 99.9 % sure that you won't be getting an all tube head.)

    Why its a bad idea to underpower your cabs...

    It can cause distortion. The amp is running too hard and the cab is trying to get all the juice out if it, and this whole process can give distortion.

    Also, if you do have a head that will run 2 ohms, and you have an 8 ohm cab, you can rewire the cab (series/parelell) down to 2 ohms to get the full amount out of your amp. If you will run only 1 cab with your head, it is a good idea to get a 4 ohm cab. But if you want to Upgrade, (like say you want only 1 cab. you get a 4 ohm 4x10, and say you wanted to have a 4x10/1x15 configuration) then you will want to get 2 8 ohm cabs. if you take two 8 ohm cabs, the total impedance comes to 4 ohms. if you have two 4 ohm cabs, your total impedance comes to 2 ohms... etc...

    Good luck man, and if you want a good cheap amp to get you going in the head/cab direction, i have one for sale at about $125 if your interested.

    Good Luck,
  6. miccheck1516

    miccheck1516 Guest

    Feb 15, 2003
    emm 2 8ohm cabs may not be as loud as two 8ohm bas, however you will be using more power.

    Say you are running Two 1x10 speakers both rated at 8 ohms, thats totals a 4 ohm load, however a four ohm 8x10 would probobly be louder as there are more speakers to move the air.
    In the same way 2 1x15 speakers could also be 'louder' that two 1x10 speakers.
    However a 4 0hm 4x10 will be louder than an 8 ohm 4x10 running from the same amp.

    You gotta go and do your physics work on this sorta stuff, and learnthis !!!!(oh god i sound like my old physics teacher.

    The question you asked was VERY broad, read around the amps section of the forum. There is no definitive yes or no answer for all these questions. Id say 99% of the people here know what they know because they have read, and read, and read other peoples posts, commenst and suggestions. Thats how i learned what i know (isnt very much but i get by)
  7. Vince S.

    Vince S. Resident Former Bassist

    Jan 24, 2003
    All: Thanks for helping me with this. I really appreciate it.

    bassist31588: I am in Simsbury, CT. PM or email me if u wanna meet:)

    romeoarmada: I sent you a PM about your amp for sale.
  8. Sorry to revive such an old thread but i too have a question about impedance... Is it possible to mix cabnets with diffrent impedances? for example, a 4ohm and 8ohm cab? what would the combined impedance be? If not, would it be possible to reduce the impedance of the 8 ohm cab to 4 ohms by placing an 8 ohm resistor in the circut of the cabnet?

    Thanks in advance,
    Mollin Man
  9. bigbeefdog

    bigbeefdog Who let the dogs in?

    Jul 7, 2003
    Mandeville, LA

    The equation to use is R1XR2/R1+R2. So, in your example, the combined load is 8X4/8+4 = 32/12 = 2.67 ohms.

    So if your amp can go down to 2 ohms, this is fine. If your amp only goes down to 4 ohms, really bad idea.

    Possible, yes.... but never a good idea. All you'll be doing is burning up (and wasting) some of your power in the form of heat (that better be a big power resistor).

    Better to just run the cab at 8 ohms, than to throw away half your power to a resistor that produces no sound.
  10. thanks for the advice... and yes i'm pretty sure my WT-800 can handle 2 ohm load but i'll have to check it BEFORE i try it...i learn't that the hard way with a combo that i attached my 2x10 too....