Diablo Posted March 24, 2014 Share Posted March 24, 2014 Looking at different heads and amps it all comes down to how the impedances stack up to give the total power out from the amp. Cabs in series you add the ohms of each, cabs in series you use the 1/ formula http://www.bbc.co.uk/bitesize/higher/physics/elect/resistors/revision/1/ Now looking at the spec sheets from some head manufacturers they say it is fine to mix, for instance a 12 + 12 + 8 + 8 = 2.5 ohms when all in parallel. Or a 12 + 6 = 4ohms, so if people have mixed cabs on their spec sheets, what is the reason it is somewhat frowned upon to mix impedances? I realise you will end up with different voltages, but is it not the power that matters given a bit if V=IR and P=I squared R? Is there a guide where one should keep impendances within a few ohms (ie 12 and 8, or 8 and 4, or 4 and 2) rather than stick a 12 in parallel with a 4 for instance giving 3ohms total? Cheers, Rich Quote Link to comment Share on other sites More sharing options...
paul_5 Posted March 24, 2014 Share Posted March 24, 2014 In theory you're fine to mix impedances provided that the end result is greater or equal to the minimum required impedance of the amp. In practice bass cabs tend to either be 8Ω or 4Ω cabs, so it limits the scope for mixing somewhat. Guitar cabs can sometimes be as high as 16Ω, but I don't think I've ever seen a bass cab with that much impedance. Quote Link to comment Share on other sites More sharing options...
Bill Fitzmaurice Posted March 25, 2014 Share Posted March 25, 2014 [quote name='Diablo' timestamp='1395702213' post='2405404'] I realise you will end up with different voltages, but is it not the power that matters given a bit if V=IR and P=I squared R? [/quote]What matters is that all the drivers receive the same power, and that's assuming they're identical. If they're not identical the possible permutations are considerable, and most of them aren't good. A favorite combination is a 1x15/4x10, but if each ten is receiving the same power as that one fifteen it's a bad news situation. Bottom line, the best option is to use identical speakers, that way you won't get into trouble. Quote Link to comment Share on other sites More sharing options...
Phil Starr Posted March 26, 2014 Share Posted March 26, 2014 Hi Rich, Bill is of course quite right. In many ways though this is something you could do in the privacy of your own home but not something to do in public If each of the oddments of speakers you are adding can handle the voltage swing and hence the power of the amp then the speakers should come to no harm. If the overall impedance is above the minimum allowed for your amp then the amp shouldn't be damaged. The question is however, why would you do this? Other than curiosity of course. Amps may power into very low impedances without immediately blowing the amp but they usually run hot and the power available reduces as they run hotter, many designs will be less reliable if you are running them at full power into low impedance and the internal resistance of the amp and more significantly the leads will reduce damping and may introduce audible distortion. You can engineer around all these problems as an amp or system designer but it's an expensive option for a bass stack. There's a reason most systems settle on 4ohms as a good compromise. Mixing speakers is more complex than just impedance and power handling. If one speaker is significantly less loud than another then you won't hear very much of it and there is little point it being there so adding a 16ohm speaker to a 4ohm one of similar sensitivity (per watt) won't add very much to the sound. It would be similarly pointless to add a 92dB/w speaker to a 102dB/W speaker for the same reason. Adding any extra speakers will have a negative effect on the off axis response and mixing speakers of different types will have slightly unpredictable effects on frequency response, often losing the character of both speakers. If you were engineering your perfect bass speaker then you wouldn't go about it by mixing impedances, you'd start with a driver that sounded the way you wanted, if you need more sound then you'd try to make it louder by using a bigger motor and better suspension or by adding more, identical speakers. If I can use an analogy mixing speakers like this is a bit like a formula one team deciding the way to win a race is by bolting a sports car onto their race car for a bit more power. Having said that if you have a room full of speakers trying them in every combination possible sounds like a good evening's entertainment to a nerd like me Quote Link to comment Share on other sites More sharing options...
Diablo Posted March 26, 2014 Author Share Posted March 26, 2014 [quote]In many ways though this is something you could do in the privacy of your own home but not something to do in public[/quote] I'm not sure if that implies I risk blowing stuff up, letting magic smoke out, or just looking rather foolish by investing my time into the worlds worst bass rig! I'm just waiting for some speakon sockets to turn up. Just to clarify that I'm not completely mad, I've got a 700W 2ohm amp which is plenty. Plan is to use a 12 ohm 200W cab (6x5), and an 8 ohm 250W cab (2x10) as a start point giving 4.8 ohms total at around 400W. That should leave plenty of headroom on the speakers and the amp. If that does not do the trick I will add another 2x10 at 8 ohms to give 3ohms total and just under 600W, which again is keeping the amp and cabs all within their limits. Why am I doing this? Decibels. Me vs two guitarists and 1 drummer has not been a fair competition so far, so for the gig this Friday I'm going armed Quote Link to comment Share on other sites More sharing options...
badboy1984 Posted March 26, 2014 Share Posted March 26, 2014 In theory you are fine but will it sound good thats another question. Mixing speakers might not sound good. Quote Link to comment Share on other sites More sharing options...
Bill Fitzmaurice Posted March 26, 2014 Share Posted March 26, 2014 [quote name='Diablo' timestamp='1395822943' post='2406619'] Me vs two guitarists and 1 drummer has not been a fair competition so far, so for the gig this Friday I'm going armed [/quote]You're probably still under-gunned. Look at the diameter of your low E string. Compare it to that of a guitar low E string. For the very same reason that your low E string is fully twice the diameter, and therefore four times the cross sectional area, of the guitar string your speakers need to have that same size relationship compared to what the guitar player has in order to play as loud. Quote Link to comment Share on other sites More sharing options...
Diablo Posted March 26, 2014 Author Share Posted March 26, 2014 Frankencab is ALIVE bwhahahaha My ears are ringing from my home testing, 500w LED just flicks on occasionally, we are good to go. Will report back with pics of mods when I get back from rehearsal tonight. Cheers for all advice and info, all good stuff and I appreciate it. I also know somewhat understand why one of the guitarists with an 80w amp can drown out my 300w effort. Tonight, however, he's gonna get his guts thumped by me. Cheers, Rich Quote Link to comment Share on other sites More sharing options...
Phil Starr Posted March 26, 2014 Share Posted March 26, 2014 Who is it that can't hear the bass? I gig regularly with a 2x10, nothing particularly special either in terms of efficiency. I can be louder than the drummer and I don't need any more than that unless I'm playing outdoors. Your guitarists should probably try and find their volume controls, they don't have to play on 11. It's possible that everyone else can hear the bass perfectly and at the right volume, You may not because a 2x10 isn't very high and is probably pointing at your knees or thereabouts. you hear better with your ears By all means take the extra cab, you'll get the benefit of raising one of your cabs to ear level so you will hear yourself much more clearly than currently. Or you could tilt your cab. I'd probably go for the extra 2x10 with all the speakers in a vertical line but if you have all three cabs you could try both combinations. Quote Link to comment Share on other sites More sharing options...
Phil Starr Posted March 26, 2014 Share Posted March 26, 2014 Enjoy Quote Link to comment Share on other sites More sharing options...
Diablo Posted March 26, 2014 Author Share Posted March 26, 2014 (edited) I'm just going to start a new thread with pics to show what I got up to. It worked great. As you will all probably know it is not a case of turning the other guitarists down, the overall sound volume is dictated by the drummer who only seems to have a switch that is either "off" or "loud". So the guitars turn up to be heard, and even with a kickback cab pointing at my head my old Laney amp was not up to being heard. I could feel it though the floor, but that is a very rough sound that does not tell you much about your sound. More in the other thread coming in a few mins.... Cheers, Rich P.S. - guitarists both said "WTF", then with amp vol on 6 from 10 I could hear every note I played clear as you like. Job done. Edited March 26, 2014 by Diablo Quote Link to comment Share on other sites More sharing options...
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