Ohms question.

[sloop_john_b]

In shopping for a new bass amp, having massive amount of headroom is key. Not that I will ever actually need 500 watts of power, but it's nice to know it's there if I want it.

Anyway, I noticed that some bass amps will say, for instance, "500 watts at 4ohms, 300 watts at 8 ohms". I have an 8ohm cabinet. Is this deceiving? Would having a 4 ohm cabinet really give me 200 extra watts of volume or am I not understanding this correctly?

[cjj]

Yes, you are understanding this correctly, it's very possible to put out more power into a lower ohm rated cabinet. It has to do with how the output circuits are designed, but the basic way to think about this is using the following calculation straight from any electronics textbook:

P = V^2/R

In words, that's power equals voltage squared divided by the resistance. So, for a given output voltage, cutting the resistance in half will double the watts consumed. The actual drive circuitry is rarely able to completely accomplish this, so you generally don't get quite double the power for half the speaker impedance, but you can easily design the output stages to work this way...

[sloop_john_b]

Thanks CJ! I can't say that I've seen many 4ohm cabinets. Any reason for that?

[jps]

There are many 4Ω cabinets out there, most of them are SVT 810 cabs!

I have two 4Ω cabs, a Dr. Bass DRB212 and a custom LDS 286.

[beatlefreak]

Thanks CJ! I can't say that I've seen many 4ohm cabinets. Any reason for that?

Many people use two 8Ω cabinets - As they're connected in parallel, you get 4Ω.

[johnallg]

There are a lot of 4 ohm cabinet makers. My Ampeg 410HLF cabs are 4 ohms. I'm waiting for winter to end to take the other cab over to the practice house to try them together with the SVT-2 head.

[BAD RONBO, KiLLeR DWaRfS]

just like the old svt ...you could run it with one 4 ohm 8x10 cab but when you added the second cab the jack would automatically switch the output transformer to deliver the full output at 2 ohms ! getting very close to a dead short, boyz !

[jps]

getting very close to a dead short, boyz !

I think it is different with tube amps as opposed to solidstate amps, where it is getting close to magic smoke time. A lot of SS amps are made these days to go down to 2Ω, but I think that is taxing the amp a bit much. Tube amps are designed to work just fine with 2Ω loads so long as the output transformer is built with this in mind.

[cjj]

Actually, if designed right, SS amps will have no problem with 2, 1, or even lower impedance cabs. The big difference is that tubes basically are voltage switching devices, so you have to have a transformer to match the high voltage/low current (high impedance) to the low impedance high current speakers. This generally limits you to just a few load combinations and requires some sort of switching to get the right transformer tap.

Transistors are, by nature, current switching devices, so they can directly drive the currents required by the speakers. The "automatic" switching for different loads is nothing more than designing a circuit that can handle the lowest impedance load desired. Putting higher impedance speakers will automatically consume fewer amps, and thus, fewer watts too.

Of course, economics dictate that the design not have too much "over kill", so yeah, when running 2 ohm loads at maximum power, you're generally pushing the design closer to the limits...

[johnallg]

SS amps ability to deliver to a load are limited by the current capability of the output devices (before destroying themselves) and the current output capability of the power supply. The "liveliness" or "punch" of the sound is determined by how fast all that current can be delivered.

[BAD RONBO, KiLLeR DWaRfS]

would that analogy apply to a street fight or boxing or and argument with the wife ? (on her end, i mean !)

[jps]

would that analogy apply to an argument with the wife ? (on her end, i mean !)

Sounds like you have some experience in this!

[FretlessOnly]

All the previous replies have good info; I've been researching this for a while, for what it's worth:

If all you ever want or need is a single cabinet, try to find a 4Ω; that way you'll max your amp's output. But, as Kris pointed out, two 8Ω's in parallel = 4 Ω, so you still get the higher wattage from the head, albeit split in half to each of the cabinets. One of the key questions is: which is the better set-up? I'll use my GK 700 RB-II as an example. 320W into 8Ω, 480W into 4Ω. So, what this fundametally breaks down to is:

320W into one 8Ω cabinet; or
240W into each of two 4Ω cabinets

I have two 8Ω cabinets, and the larger setup certainly is much bigger sounding, but since the smaller setup is all I really need (it's quite loud with a GK Neo 212-II), I found that either one is fine and not lugging around that second cabinet is nice.

Can someone define headroom definitively? Is it the difference between amp output and speaker watt rating? What is the "proper" differential?

[johnallg]

All the previous replies have good info; I've been researching this for a while, for what it's worth:

If all you ever want or need is a single cabinet, try to find a 4Ω; that way you'll max your amp's output. But, as Kris pointed out, two 8Ω's in parallel = 4 Ω, so you still get the higher wattage from the head, albeit split in half to each of the cabinets. One of the key questions is: which is the better set-up? I'll use my GK 700 RB-II as an example. 320W into 8Ω, 480W into 4Ω. So, what this fundametally breaks down to is:

320W into one 8Ω cabinet; or
240W into each of two 4Ω cabinets

I have two 8Ω cabinets, and the larger setup certainly is much bigger sounding, but since the smaller setup is all I really need (it's quite loud with a GK Neo 212-II), I found that either one is fine and not lugging around that second cabinet is nice.

Can someone define headroom definitively? Is it the difference between amp output and speaker watt rating? What is the "proper" differential?

John, do you have the speaker ohms/watts listed backwards?? I would think it would be 320W into two 4Ω cabinets and 240W into one 8Ω cabinet. Also, even though the power is divided between the two 8Ω cabinets, the power is still delivered to the load and also the two 8Ω cabinets would have more radiating cone area and thus more acoustic power out. In general. And why you say it is bigger sounding - more amp wattage and more acoustic output from the larger cone area.

Headroom is the short term (much less than a second) ability of an amp to output more power than it is rated for. For example, an amp with a power supply that has a lot of current delivering capability and is rated 100W but with a 3dB headroom rating means over a very short time period it can deliver 200W. That could happen on the initial attack of a note when the bass string is plucked.

[beatlefreak]

What he meant to say is

320W into one 8Ω cabinet; or
240W into each of two 8Ω cabinets (4Ω total)