EBS_freak Posted January 31, 2009 Share Posted January 31, 2009 [quote name='Bill Fitzmaurice' post='396115' date='Jan 31 2009, 01:41 PM']In that case the difference at small signal will be precisely 3dB, and the difference at full output, assuming the amp is able to drive both to full output, will be precisely 0dB. Again, the only advantage to the 4 ohm is if the amp has the ability to drive the 4 ohm cab to full output and not the 8 ohm, and if your amp can't driver an 8 ohm 2x10 to full output you really need a larger amp, because you've got no dynamic headroom.[/quote] It can drive a 8 ohm or 4 ohm 2x10 to the limit of each cab but requires more output from the amp to make the 8 ohm get there. The TD650 has more than enough guts to drive 2 4ohm cabs to their limit and 4 8ohm cabs to their limit with some left over. I think there was a bit of confusion - if I have an 8 ohm cab, play, then switch over the cab to 4 ohm without touching any of the controls, the 4 ohm is louder... but thats presumably because the amp is putting more out at those same settings...? Quote Link to comment Share on other sites More sharing options...
Bill Fitzmaurice Posted January 31, 2009 Share Posted January 31, 2009 [quote name='EBS_freak' post='396474' date='Jan 31 2009, 04:49 PM']if I have an 8 ohm cab, play, then switch over the cab to 4 ohm without touching any of the controls, the 4 ohm is louder... but thats presumably because the amp is putting more out at those same settings...?[/quote] At average settings, twice the power. Which requires drawing twice the current from the amp, which contributes to additional heating of the amp components. A free lunch there is no such thing as. Quote Link to comment Share on other sites More sharing options...
alexclaber Posted February 1, 2009 Share Posted February 1, 2009 [quote name='Mr. Foxen' post='396034' date='Jan 31 2009, 11:24 AM']Could it be the sweep of the volume pot changes at different loads? I don't really know how they work. Would its be % of total power as it goes arounf (although 100% might not be at the end) or would the greater max output be further round the dial?[/quote] The sweep of the volume pot doesn't change, you still get the same voltage at the speaker outputs. However by halving the impedance load you double the amount of power coming out of the amp at the knob setting because twice the current is flowing. But you won't be able to turn the volume knob as high without the amp running out of current delivery. [quote name='EBS_freak' post='396474' date='Jan 31 2009, 09:49 PM']I think there was a bit of confusion - if I have an 8 ohm cab, play, then switch over the cab to 4 ohm without touching any of the controls, the 4 ohm is louder... but thats presumably because the amp is putting more out at those same settings...?[/quote] Exactly! Like comparing two amps within the volume knob in the same place and claiming that whichever is louder is more powerful, so comparing different impedance cabs without seeing how loud the amp can push each without break-up is not so much inaccurate as pointless! [quote name='monquixote' post='396022' date='Jan 31 2009, 11:06 AM']I don't know if it is something that is more apparent with class D amps like the Mark Bass. A 3 dB lift should only be perceived as 25% louder (I think) and I would anecdotally suggest it seemed to make more of a difference than that.[/quote] The LMII isn't Class D. [quote name='monquixote' post='396022' date='Jan 31 2009, 11:06 AM']using a cab with decent sensitivity speakers like an EBS makes as much or more difference as dropping the impedance.[/quote] It makes more difference. As does using a cab with speakers that can handle truly handle the power by having enough excursion ability. [quote name='monquixote' post='396022' date='Jan 31 2009, 11:06 AM']I've still got stacks of power on hand with the LMKII. I usually have it set so the gain is well back from the clip level with the master on about 40% to play with a loud drummer and I've had it up to 3/4 out of interest which still sounded undistorted, but had everyone running for the exit with bleeding ears![/quote] If you still have stacks of power with a 4 ohm cab then an 8 ohm cab would be just as loud. You can't claim that a 4 ohm cab is louder than a matching 8 ohm cab unless one of these cabs is taking the amp to its limits! Alex Quote Link to comment Share on other sites More sharing options...
Shockwave Posted October 12, 2009 Share Posted October 12, 2009 [quote name='alexclaber' post='395996' date='Jan 31 2009, 11:30 AM']But what amp were you using? I can guarantee that an EBS 2x10" 4 ohm would be no louder than an 8 ohm one when powered by one side of my PLX 3002 because of the amp having enough power at 8 ohms to take the cab to its limits. Alex[/quote] Interesting, I put a 8ohm 1x15 Black widow cab up to a 4Ohm 1x15 Black widow and the 4ohm was much louder. The problem is, for some reason some amps just prefer running flat out at 4 ohms. Case in point an Ashdown Little Giant. Really quiet when running at 8 ohms. And only really get any power at all when going between 12 and 5 oclock. But the 4ohm seems to be quite loud from the off and just continues to get louder the more the master it turned up. I know many people who also experienced the same thing with the Little Giant. I guess this is something that happens with Class D amps. Hence why i am always concerned about running 8 ohm cabs on their own and why i usually insist on 4 ohms. Also considering the SVT i have on the way, derives most of its power at 2 ohms stereo. So i would always appreciate 2 ohm or 4 ohm cabs whilst bi-amping. If i were running it mono bridged (About 1000watts.) I may be ok with using an 8 ohm cab. But then again i am usually playing at higher volumes then the average basschatter. Quote Link to comment Share on other sites More sharing options...
alexclaber Posted October 12, 2009 Share Posted October 12, 2009 [quote name='Shockwave' post='623880' date='Oct 12 2009, 12:25 PM']Interesting, I put a 8ohm 1x15 Black widow cab up to a 4Ohm 1x15 Black widow and the 4ohm was much louder. The problem is, for some reason some amps just prefer running flat out at 4 ohms. Case in point an Ashdown Little Giant. Really quiet when running at 8 ohms. And only really get any power at all when going between 12 and 5 oclock. But the 4ohm seems to be quite loud from the off and just continues to get louder the more the master it turned up.[/quote] Personally my suspicion is that there's a lot of super-lightweight amps that simply don't deliver the power they should. Chris B and I spent a bit of time pushing three very different rigs to the limit on Saturday and it was very enlightening. I'd like some more time and a suitable venue to continue this testing (though next time with one preamp and then running in through each FX input so we're truly only testing the power amps - note that you can't do an accurate test like this with most cabs as most speakers crap out before high power amps do). Anecdotally we had a chance to try my PLX 3002 into Jake's Schroeder 1212L and my Barefaced Big One and we were all really surprised that the 4 ohm and famously loud Schroeder didn't appear to be much louder than the 8 ohm (I call it 6 because of some crossover weirdness but it's an 8 ohm woofer) Big One. Anyway, here's how it works. You pluck a string on your bass and it generates maybe 0.5V in the pickup. Your preamp stages take that up to maybe 3V. Then your power amp stage takes that up to maybe 10V, or 20V, or 30V, etc depending on how loud you have the amp turned up. That voltage then causes current to flow through your speaker - and if the speaker is 4 ohms then twice as much current will flow, hence twice as much power. But if you turn your amp up some more then you'll get more current to flow hence as much power from an 8 ohm cab. The amount of current an amp can supply is limited, so with a 4 ohm cab the amp will run out of current capacity at a lower voltage than with an 8 ohm cab. So the only time you should get a 4 ohm cab instead of an 8 ohm cab is if your amp cannot generate enough voltage to get your 8 ohm cab loud enough (and it's definitely the amp running out of power, not the cab running out of power handling) - however a better solution is almost always to get a second 8 ohm cab which increases your sensitivity so fewer watts will get you more loudness, and more current flows from a given voltage because the load is now 4 ohms so it's a win win. Something else to consider - when Chris and I were testing the dual Compact rig we kept swapping who was playing. He has his action fairly medium-ish and doesn't pluck softly (not like Ped and his crazy 'breath to fret' Vigiers) - yet when we swapped to me I was more than twice as loud - we had to turn the amp down a LOT! So when I pluck a mf (i.e. standard loudness) note I'm generating about four times as much voltage in the pickup, etc etc. Bear in mind that now amps are so powerful, the biggest issue is how much power speakers can handle in the real world (not the somewhat irrelevant thermal ratings normally quoted). Alex Quote Link to comment Share on other sites More sharing options...
Shockwave Posted October 12, 2009 Share Posted October 12, 2009 [quote name='alexclaber' post='623903' date='Oct 12 2009, 12:54 PM']Personally my suspicion is that there's a lot of super-lightweight amps that simply don't deliver the power they should.[/quote] I would have to agree on this. [quote name='alexclaber' post='623903' date='Oct 12 2009, 12:54 PM']however a better solution is almost always to get a second 8 ohm cab which increases your sensitivity so fewer watts will get you more loudness, and more current flows from a given voltage because the load is now 4 ohms so it's a win win.[/quote] Very true, but my problem is exactly that i can only use one cab max, need to learn to drive etc. [quote name='alexclaber' post='623903' date='Oct 12 2009, 12:54 PM'](not like Ped and his crazy 'breath to fret' Vigiers)[/quote] I know! I thought i was the lightest fairy bass player there. But Ped takes it to a new state light! Quote Link to comment Share on other sites More sharing options...
Mr. Foxen Posted October 12, 2009 Share Posted October 12, 2009 [quote name='Shockwave' post='623962' date='Oct 12 2009, 01:49 PM']Very true, but my problem is exactly that i can only use one cab max, need to learn to drive etc.[/quote] In which case, you don't need a 1000w amp. Quote Link to comment Share on other sites More sharing options...
Bottle Posted October 12, 2009 Share Posted October 12, 2009 Hey Alex Good to finally catch up with you on Saturday. Can put a face to the name now! Glad I got the chance to see how my GK head faired into a single Compact (would have been even better to see how it stacked up on [i]two[/i] Compacts, but we'll leave that for another day ) Anyway, my EBS cab is 4Ohms (which is a bugger) - just wondered how that would compare to a pair of neo 2x10 cabs (both 8Ohms). Ta, Ian Quote Link to comment Share on other sites More sharing options...
Shockwave Posted October 12, 2009 Share Posted October 12, 2009 [quote name='Mr. Foxen' post='624013' date='Oct 12 2009, 02:31 PM']In which case, you don't need a 1000w amp.[/quote] Dammit Olli, shut the hell up I am in the process of ordering a Barefaced Vintage So you can get off my back about getting a decent cab! Quote Link to comment Share on other sites More sharing options...
Mr. Foxen Posted October 12, 2009 Share Posted October 12, 2009 [quote name='Shockwave' post='624016' date='Oct 12 2009, 02:34 PM'](With tweeter)[/quote] Just had to go put your neon pointy mark on something otherwise cool and sensible. Quote Link to comment Share on other sites More sharing options...
Shockwave Posted October 12, 2009 Share Posted October 12, 2009 [quote name='Mr. Foxen' post='624019' date='Oct 12 2009, 02:35 PM']Just had to go put your neon pointy mark on something otherwise cool and sensible.[/quote] And i knew you would have had a go at me for putting my neon pointy mark on something otherwise cool and sensible. Quote Link to comment Share on other sites More sharing options...
chris_b Posted October 12, 2009 Share Posted October 12, 2009 Unless I was looking at 6 or 8x10's or 2x15's I would always get an 8 ohm cab. If you play quietly then 4 or 8 doesn't matter, but if you want to play louder you just add another cab and get more speakers shifting air, which seems to me to get a better tone as well as more volume. At the Bash one Compact sounded good and I could use that in many situations but 2 Compacts just sounded "better", even with the volume down. Quote Link to comment Share on other sites More sharing options...
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