how much current is enough?

I know that to gain 3db of additional headroom from your speakers you need to double the power from your amp...given that, its easy enough to calculate how many watts will be suficient to reach your speakers rated max volume level before compression...or to see if your amp will play loud enough for your particular listening habits...
and i know know that watt/sq.rt of resistance = current ...so i can calculate how many amps my amplifier will put out into various loads, but i dont know how many amps are enough for a given speaker...or if the difference between say 10 amps and 12 amps is a lot?

with regards to a specific difficult to drive speaker the Totem Mani 2 how much current is enough??

its specs are
4 ohm nominal...it drops down to 2 ohms, all resistive
81 db sensitivity at 1w/1m

and it uses an isobaric configuration which i gather makes it require more current than normal..but again i dont know why or how to calculate it...

so from a mathematical point of view how do i figure this out...

and in terms of the ability of am amp to put out lots of current what specs are the most important to look at?

thanks
mbacinello
12 responses
 10-06-2004 12:49amIMO you don't need to know that detailed current output.Amp selection is controlled most importantly by the sensitivity of your speakers and your highest normal listening levels in YOUR room. Because your speakers are VERY insensitive AND low-impedance, you should look for a very-high-power amp that doubles its output power into 4 ohms. I'd say 250WPC into 8 ohms and 500WPC into 4 would be a good starting point.If you love tubed amps, I recommend 2 stereo amps of at least around 75 + 75 watts. An amp like the (discontinued and hence 'affordable') conrad-johnson Premier Eleven is 70WPC into 8 or 4 ohms; with the 2 channels paralleled that turns into 140 watts into 4 or 2 ohms. In this case, using a stereo amp with channels paralleled is superior to using a mono amp of the same (combined) power rating, because the stereo amp will deliver that same combined power into half the impedance. If 140 'tubed' watts doesn't sound like enough to you, start with a higher-power stereo amp. For instance, an older c-j Premier Four will deliver 200 watts into 2 or 4 ohms from its combined 100-watt channels. Of course for stereo, you'll need 2. I'm sure there are many more-current tubed amps that will do similar things and that you'll soon read about them from fellow 'Goners.. 10-06-2004 3:27amHi I always thought that running a stereo power amp in mono configuration(even if it was designed with that in mind)degrades the sound to a degree.My Levinson 27.5 is capable of turning into a monoblock but reading it's manual clearly indicates that is not advisable.That,s why there are separate monoblocks to begin with.My two and a half cents. 10-06-2004 4:39amWell, your Levinson is solidstate and those I mentioned are vacuumtubed. GENERALLY, when SS amps are 'monod', the channels are bridged so that the maximum power quadruples, not doubles. However, each channel drives only half the single impedance, so bridged SS amps are NOT high-current amps. The channels of tubed amps are paralled so they're generally able to increase current output enough to combine the output of the 2 channels. BTW, I understand that SS amp channels can be combined this way, but the only manufacturer I know of to recommend it is B&K.For Mbacinello's VERY insensitive and low-impedance speakers, I am NOT recommending a bridged SS amp.. 10-06-2004 11:10amNumbers dont really tell the whole story about any given speaker/amp combination. That said, your speakers are notoriously power(current)hungry, and most amps dont produce a lot more current when bridged. You really have to try an amp with your speakers to see how they play together, sometimes the amp with more modest specs outshines the one with numbers that suggest it would be the better amp for the load. I can think of worse problems than having to find the right amp for the fine speakers you own. Good luck. 10-06-2004 1:14pmanother simmilar question...so if the mani-2's (which i dont as yet own...but am considering) are 81 db sensitive...that means thatthey will play 81 db with 1 watt, 84 with 2, 87 with 4 etc... that means that to get to the 110db...which there spec sheet says is there maximum volume level they would need 1024 watts of power...two things...is the sensitivity measured at 8ohms? such that a four ohm nominal speaker draws twice as much power from the amp?, in which case those numbers should all be double and it would draw 2048 watts at 4 ohms to play 110 db?? and secondly....thats way more watts than the speakers can handle, does that mean that they could never actually play as loud as there specs suggest??thanks for indulging this speculation...not that i'd ever listen at these levels id just like to get a handle on the math governing the speaker amp connection... 10-06-2004 4:06pmi would not assume that the SPL output level is log linear with input power. the mechanical dampening in speaker systems probably isn't graphed in a straight line.note from your exercise that if SPL is important, more efficient speakers are far cheaper than kilowatt amps. large speakers with many drivers tend to be the most efficent. 10-06-2004 6:44pm"Another simmilar question...so if the mani-2's (which I dont as yet own...but am considering) are 81 db sensitive...that means that they will play 81 db with 1 watt, 84 with 2, 87 with 4 etc. That means that to get to the 110db, which their spec sheet says is their maximum volume level they would need 1024 watts of power."Yes, your process is correct but I didn't check your arithmetic."..two things...is the sensitivity measured at 8ohms?"No. 'Sensitivity', a term that became popular with solidstate amps, is the acoustic output measured 1 meter away from the speaker when driven with a signal that produces 2.83 volts into an 8-ohm test load (which happens to be 1 watt) and driven by a constant-current amp, which means solidstate, not tubed. 'Efficiency' is measured with 1 watt of input INTO THE APPROPRIATE LOAD, and that term is NOT used much anymore. So...sensitivity is measured with 2.83 volts into 8 ohms while efficiency is measured with 1 watt (into the load equal to the speaker's rated impedance). See below about Totem's rating."...such that a four ohm nominal speaker draws twice as much power from the amp?"Yes, generally."...in which case those numbers should all be double and it would draw 2048 watts at 4 ohms to play 110 db?? and secondly, that's way more watts than the speakers can handle. Does that mean that they could never actually play as loud as there specs suggest??"Well, sort of. As merge03 indicated, these things never work out exactly because, at least partially, they're based on subjective measuring techniques.Merge03 also is absolutely correct in his statement that high listening levels are FAR easier and less expensive to achieve with more-sensitive speakers than with very-high-power amps. Also, it seems that achieving a sense or feeling of dynamic 'ease' is much-more difficult with low-sensitivity speakers and high-power amps than with higher-sensitivity speakers and less amplifier power.BTW, the Totem Mani-2's sensitivity is eighty-FIVE, not 81dB, at least according to their website. That will reduce your power requirement to 300 - 400 watts for 110dB. And it appears to me they specify it inconsistently--"Sensitivity 85 dB/W/m.". If they're claiming to indicate efficiency, that indeed is stated in output at 1 meter with 1 watt of input. If they're claiming to indicate sensitivity, that's measured with an input of 2.83 volts into 8 ohms. There's a real difference.Also, and this is STRICTLY personal, and I know NOTHING about Totem speakers, so PLEASE don't anyone overreact to my comment here: any audio-equipment manufacturer that says "Regardless of specifications, Mani-2 channelizes primal energy and force into being" goes WAY down on my shopping list. That had to have been written by some marketing idiot who specializes in obfuscation, not elucidation.And Blkadr is correct in writing that you need to try different amps with the speakers you love; I too should have emphasized that.Personally, if high-quality sound at high listening levels were important to me, I'd start with a more-sensitive speaker. 10-07-2004 8:10pmThere website does state they are 85 db /w/m, however when stereophile reviewed them they found them to be 80.7 db/w/m... 10-07-2004 9:26pmFinding those reviews is the proverbial search for the needle in the haystack.What's the exact term 'Sterophile' used for its sensitivity/efficiency rating? 10-08-2004 12:52pmthey use the term sensitivity...the mani-2 original..reviewed in 1995 measured 80.7db(b)/2.83v/m the mani-2 signature is 84.5db(b)/2.83v/m 10-08-2004 4:13pmStereophile measured SPL at one metre. Probably at 1kHz, BTW which is an indicative but hardly final measurement. It is however useful./ Since CURRENT was mentioned, and adding onto Jeffreybehr informative post, the 4ohm nominal indicates the need for twice as much current as in the standard 1W/8ohm/(1kHz) ratings... for 2ohm, four times the current all else being equal. As such, it seems difficult to achieve 110db (spl) with those speakers UNLESS the drivers are extremely resistent both mechanically and thermally. Or maybe, the 110 db is an indication of in room spl levels, both speakers driven (even then, each speaker would have to stomach nearly 1/2kW rms). Maybe they mean "peak" (1-3ms)? Or, the 80db spl is wrong??? 10-09-2004 4:39amWith 82 dB speakers, and these same speakers are of relatively low impedance, you better buy a "Mack Truck" of an amplifier. That is, IF you want to play loudly with good dynamics while maintaining proper control over the drivers. If you want to know how to REALLY rate an SS amplifier in terms of REAL output power and how well it can control a load, take a look at this post i made over at AA about a week ago. Amplifier power ratings from most manufacturers are typically pretty useless as they can print whatever they want. Seeing actual third party test results can be VERY informative, IF the right tests are performed AND one knows how to read those results. As a side note, try reading this post about power output and loudspeaker control that fellow Agoner' Abe posted over at AA. Many of his comments along with those of a few others may help you to better understand what it is you're looking for in an amp. Sean>