Rickresource Rickenbacker Forum

Another newbie question. I am shopping for an amp head and speaker cabinet. I've done gobs of research, played through a number of set ups, and identified the finalists. This is not the issue.

The issue is that I'm not sure I fully understand all I know about power output from the amplifier vs. the power rating of the speakers. Without getting into specific pairings, I've been looking at amps in the 350 to 800W range. I don't really need more than a 2x10 or 12 cab at this point, so most of the cabinets I've looked at and/or played through are rated at 400W RMS.

I've always gone by the rule of thumb that your speakers should be rated at or near twice the power output of the amp. As applied to bass amplification, is this being too conservative? I'll appreciate any guidance I can get from my more knowledgeable peers (that would be, oh I guess every other member here). Thanks.

I used to think along the same lines, coming from a hi-fi/pro audio point of view, but this has been seriously challenged of late from some excellent threads on TalkBass. The basic issue is one of speaker driver (SD) specs. The power handling rating of SDs is the thermal limit of the voicecoil, however, the real world limits are the mechanical limits of a SD, known as Xmax and Xlim, the maximum excursion and the point of damage, respectively, which is far lower than the thermal limits of the voicecoil. In hi-fi/pro audio this is generally not an issue as the speakers are not driven hard, normally, but in musical instrument applications this can become a real problem. Now, I prefer that the speaker system be rated the same or higher than the amplifier's power rating to provide a safety margin and to avoid speaker "farting".

Thanks Jeffrey. Yes................I've combed through quite a bit of the amp/cab power discussion over on TBass. I see a lot of debate on the over/under powering questions. From what I can see, it appears that as long as the amp power rating is not grossly above that of the speaker cabinets, over-power speaker damage is unlikely. For example: an amp generating 300W @ 8 ohms and 500W @ 4 ohms driving a single 2x10 400W/8 ohm cabinet is well within the safe range. Same amp driving a 2x10/400W coupled with a 1x15/400W (both 8 ohms) is also within reasonably safe operating parameters.

I think what has kind of tripped me up is that I see a lot 300/500/800 watt amplifiers that are in my budget range, yet speaker cabs in the same budgetary range either fall into the 400 or 800 watt class. My guess is that, as it applies to speaker cabs, this is simply a function of product standardization at the price point I am working to.

prestoj wrote: For example: an amp generating 300W @ 8 ohms and 500W @ 4 ohms driving a single 2x10 400W/8 ohm cabinet is well within the safe range. Same amp driving a 2x10/400W coupled with a 1x15/400W (both 8 ohms) is also within reasonably safe operating parameters.

It took me a while to wrap my brain around this whole concept, but what you stated here is correct, as far as I have come to understand. You likely already know, but for those who don't, when you use two 8 ohm cabinets in series, you are running at 4 ohms, so you up your amp power, but it is split between the two cabinets.

Formula for parallel cabinet impedance: 1/(1/imp #1 + 1/imp #2). For two 8 ohm cabinets, we get:

1/(1/8 + 1/8) or 1/(1/4) = 4 ohms.

In the scenario above, you are pushing 250 watts into each of the two cabinets. Basically my set-up/wattage (mine is 320W or 480W), but I have a 2x12 and a 1x12. So, it is a trade-off; with two 8 ohm cabinets, you've reduced your overall power to ~250W per cabinet, but you are moving more air with the two cabinets. You also have more headroom (the degree to which you can turn up until you get distortion (not the good kind)) or clipping. Think distortion (bad) vs. overdrive (good at times).

At the end of the day, there isn't a huge difference between running my 2x12 at 320W and the 2x12 and 1x12 at 480W (240W per cabinet), but the sound is certainly bigger w/ the two cabinets.

FretlessOnly wrote:.....when you use two 8 ohm cabinets in series, you are running at 4 ohms.....

That would be parallel, series connection would net you 16Ω.

Thanks for the help Gents............the mystery continues to unravel.

There is quite a lot of misinformation here.
Just use Ohm's law as a base line.
You take a solid state amp that pushes Xwatts @ 8ohm and you connect 2 8ohm cabs in parallel - you take down the resistance to 4ohm, once you cut the resistance by half you double power, so now you have 2Xwatts (sometimes it's a bit less due to design).
It's that simple (but does not apply to tube amps - in tube amps the resistance goes down but they will supply the same amount of power that will be shared by the new speakers).

Simple mess up on my part. Parallel, not series, as I indicated in Ohm's law. No misinformation, just a simple faux pas, as it were.

As Gil said, ideally, when the speaker imedance load is cut in half the power should double, but for economic reasons most amps will not provide that doubling, you will usually see something like 300 watts @ 8Ω/500 watts @ 4Ω, I believe a lot of this has to do with power supply design and implementation. Some hi fi amps will double their power, for example, my Aragon 2004 power amp in my stereo system will do 100 watts @ 8 Ω/200 watts @ 4 Ω/300 watts @ 2Ω (you need a real hefty power supply to keep doubling the power).

FretlessOnly wrote:Simple mess up on my part. Parallel, not series, as I indicated in Ohm's law. No misinformation, just a simple faux pas, as it were.

Sorry, didn't mean to sound nasty.
In any case - getting your amp to push more watts = more power, in Jack's case: 2 cabs @ 8ohm each will draw 500 watt from the amp compared to a single cab that will draw only 300 watt, I wouldn't look at it as "getting only 250watt per cab instead of 300 watt" but rather as "getting 500 watt from my rig instead of 300watt". Your raise your SPL by around 2dB which is quite a lot at these power levels.

Isn't it true that too LITTLE power is often the culprit when a speaker fries? Something to do with square waves coming out of an over-driven solid state amp. In the pro audio world, I've seen recommendations for amps to have double the power of whatever the speakers are rated to handle. Headroom....

That's only if you grossly overdrive the speakers, with an amp that is way underpowered. Under most normal conditions the speaker will reach it's mechanical limits before the thermal limit.

jps wrote:That's only if you grossly overdrive the speakers, with an amp that is way underpowered. Under most normal conditions the speaker will reach it's mechanical limits before the thermal limit.

Yup. Underpowered amp pushed hard into distortion means for most of the time you are putting a LOT of DC current through the voice coils of the drivers and they overheat and melt, or possibly catch fire. Pushing large amounts of power into an equal or lesser power-handling cab will cause the "farting" we all have heard as the speaker reaches it's excursion limit. Continue and it will either tear the cone out or melt the coil.

Like I initially stated, I'm new at this, so thanks to all for weighing in. Having gone through so much info that contains way too much technical minutae, I really appreciate everyone sticking to the general concept. Seems that I get the clearest information on this forum. To paraphrase: get enough amp power to be heard clearly and articulately within your particular playing environment; don't under or over power the speakers. Find the zone.

OK, so can someone help with this then? I missed the lesson on Ohm's Law at school, too busy behind the bike sheds. So, I've got an 8 ohm Ashdown 4x10, and a 1970s standard Marshall 4x12 rated at 16 Ohms which I would dearly love to run together, using my Ashdown ABM 500. Can it be done or will it all end in tears and a hefty repair bill?