spliting a 2x12 into a 2(1x12)

Col_Decker

Can it be done? Have you done it? Did you die, did we all die?

 

Say fer example that I was to get a 2nd amp head, could I run both heads into one 2x12 cab, but with the drivers each dedicated to an amp? The drivers in the cab are 32ohm each, the amp loads needs to be 16ohm, so a 16ohm power resitor in parallel across each driver would needed I guess.

Then I could run a A/B/A+B switcher from my pedal board to switch between the two heads

thoughts??

not_the_dj

Yes you will die. That might not be related to your amp GAS of course.

not_the_dj

...similar thread also going on today: http://www.thefretboard.co.uk/discussion/13416/2-heads-1-cab#latest

Col_Decker

nah thats slightly different form what I want to do, he wants to switch amps between a 2x12. I want to run both amps into a 2x12 as a pair of 1x12s.

 

in it

ICBM

Col_Decker said:

Say fer example that I was to get a 2nd amp head, could I run both heads into one 2x12 cab, but with the drivers each dedicated to an amp? 

Yes.

Col_Decker said:

The drivers in the cab are 32ohm each

Are you sure? Although 32-ohm drivers do exist, it's much more likely that it's two 8s in series.

Col_Decker said:

Then I could run a A/B switcher from my pedal board to switch between the two heads

If you want to switch rather than run both at the same time, really you need the cabinet internally divided so both speakers are not in the same chamber. If they are, in a closed cab, the un-driven speaker will try to act as a 'passive radiator', which will usually be out of phase with the driven speaker and will sound odd.

Col_Decker

32ohm not sure actually, if they are 8ohm I guess I'll need an 8ohm resitor in series.

 

Spliting that cab ... didn't think of that. Good point, though it may have sunk my plans

ecc83

Those 32 Ohm drivers must I think have come from a bass rig? I know of a 4x12 that was sold some years ago that was 32R and thus gave a resultant 8R when paralleled (SERIES pllel is NEVER used for "hi-fi" and is a bad idea for bass). 

Thus, they might be quite valuable?

But yes, you can load them with an R of equal value and approximately the same power rating.

Dave. 

ICBM

Col_Decker said:

32ohm not sure actually, if they are 8ohm I guess I'll need an 8ohm resitor in series.

Spliting that cab ... didn't think of that. Good point, though it may have sunk my plans

The problem with using resistors to change the effective impedance of a speaker cab is that it changes the frequency response, because speakers don't have a fixed resistance, they have an impedance which varies (quite drastically) with frequency. An "8 ohm" speaker can easily vary from about 6 ohms to about 40 ohms over the audio range.

Do your amps not have 8-ohm outputs?

ecc83 said:

Those 32 Ohm drivers must I think have come from a bass rig? I know of a 4x12 that was sold some years ago that was 32R and thus gave a resultant 8R when paralleled (SERIES pllel is NEVER used for "hi-fi" and is a bad idea for bass). 

You should tell that to all the companies which make bass 4x10"s with series-parallel wiring .

Ampeg definitely use 32-ohm drivers (10s) in the SVT cabs though.

Col_Decker

Do your amps not have 8-ohm outputs?

aye it does but it needs to 'see' 16ohms iirc (Orange Rocker 30), so I can't just use one of the 8ohm sockets on its own.

I could be wrong of course, I'm justing thinking out loud atm

ICBM

Col_Decker said:

Do your amps not have 8-ohm outputs?

aye it does but it needs to 'see' 16ohms iirc (Orange Rocker 30), so I can't just use one of the 8ohm sockets on its own.

Yes you can. The 8-ohm jacks are just in parallel.

Danny1969

I use a 4 x 12" split all the time with 2 amps, no probs with close mic'ing. 

If you use a resistor to match a needed lower impedence then the resistor will turn your watts into heat rather than volume ... which might be useful to you or not

A wirewound resistor has some reactance not as much as a speaker though

57Deluxe

I asked this a couple months back and got these answers...

http://www.thefretboard.co.uk/discussion/10217/pondering-moment-cabs#latest

ecc83

"You should tell that to all the companies which make bass 4x10"s with series-parallel wiring"

Yeah, said it was a "bad idea" ! Lots of companies have lots of them! 

Yes, IC, I certainly agree that SERIES resistors to build out a speaker Z are very bad form. Not overhappy with parallel but, when the Devil drives...? 

Dave.

ICBM

ecc83 said:

Yes, IC, I certainly agree that SERIES resistors to build out a speaker Z are very bad form. Not overhappy with parallel but, when the Devil drives...?

Very rare that you'd need to. Higher than the right impedance is not an issue for a solid-state amp, and the loss of power with a valve amp going into double the right impedance is less than throwing away half of it into a resistor at the right impedance…

Given that the tone will suffer too - you'll lose much more top-end than bottom, because at high frequencies the resistor is much lower impedance than the speaker - I don't really see the point!

Unless you purposely need to protect an under-rated, too-bright speaker?

ChrisMusic

@ecc83 said:  "... bass with series-parallel wiring"  "bad idea" !Not overhappy with parallel but, when the Devil drives...? 

Hey guys, why is parallel a bad thing? is that the same issue in series / parallel (I am assuming so) ?  Thanks

ecc83

ChrisMusic said:

@ecc83 said:  "... bass with series-parallel wiring"  "bad idea" !

Not overhappy with parallel but, when the Devil drives...? 

Hey guys, why is parallel a bad thing? is that the same issue in series / parallel (I am assuming so) ?  Thanks 0k, bit of minefield here! 

As ICBM says, the impedance of a guitar speaker Jodrells about all over the shop. If you therefore put any resistance in SERIES with it (be it a physical component or another speaker!) the series R will attenuate some frequencies more than others, upsetting the response of the speaker*. Putting a resistor in parallel really just keeps the amp happy (and in many cases you need not really bother) it will have very little if any effect on the resultant frequency response. It can also be used to protect a speaker with a lower power rating but if used in such a way make sure the resistor is well over rated and so cannot go O/C.

And yes! All the 4 x12 speaker cabs that have ever been wired series/pllel are technically "wrong" but that is the sound we all know and love! Tis only Rock and Roll!

*This assumes the resistor is a fair proportion of the output Z of the amp. This is not always so and so might make very little difference in some cases.

Just thought. This is probably why certain power soaks suit certain amps since they all must insert some resistance.

Dave.

ChrisMusic

Thanks for your reply @ecc83, still not sure I get the interactive idea of speakers in parallel being bad for bass though, can I prompt you for a little more explanation to kick start the little grey cells (both of them!), Cheers

ICBM

ecc83 said:

Just thought. This is probably why certain power soaks suit certain amps since they all must insert some resistance.

Yes. Actually they often have other components to try to give them a more 'speaker-like' impedance curve as well, or to offset treble loss etc, some of which do that and some actually have the opposite effect at some frequencies! This affects how the amp 'sees' the load, interacts with the damping factor, the negative feedback etc… which all vary from one amp to another - so different attenuators work better or worse with different amps. A bit like speakers do, but maybe even more so. So when someone says they tried an attenuator and it sucked the tone out of their amp, probably they were just using the wrong attenuator - the problem is that without trying various ones (and they're not cheap...) or going by the 'received wisdom', it's hard to know which one is going to work best.

Even the best of them are always going to have an effect at high attenuation (low volume) settings too, because at that point they are also a parallel load on the speaker, which will change its damping and response… there's really no way to avoid all these problems at the same time - ie "you can't get there from here". (Not even counting Fletcher-Munson!) The best you can do is to have it sound different, but good.

benvall

I did it. I went guitar into some drive pedals into nova system. Right out to 18 watt amp, left out to volume pedal into morgan amp. Each amp went into one speaker in a 2x12.

No ground loop issues. Seemed to work well.

ecc83

ChrisMusic said:

Thanks for your reply @ecc83, still not sure I get the interactive idea of speakers in parallel being bad for bass though, can I prompt you for a little more explanation to kick start the little grey cells (both of them!), Cheers

Ah! Missed that din't I! Unlike "lead" guitar, bass rigs should be a bit more "hi-fi" in order to get a smooth, resonance free (ish!) low frequency response. This is done by the amplifier having a very low output impedance, effectively a short circuit across the speaker terminals*. This produces "Electro-Magnetic Damping" and controls the otherwise wild excursions of the cone at resonance. 

For this EMD to work effectively the speaker circuit resistance must be very low, the speaker should "see" less than one Ohm when connected to the amp. Plainly, putting another speaker in the circuit largely destroy this damping. (this property of an  amplifier is called its "Damping Factor" and is the ratio of the output resistance to the load impedance. Much fuss is made of having a high DF but anything passed about 10 ceases to be of much use. Speakers have DC  resistance themselves after all!) 

*If you have access to a "neked" speaker cone try gently tapping the cone with the speaker unconnected to anything. You should get a soft "boom" or even "boing". Now short the speaker terminals and tap it. Not only will you now just get a dull "thud" but I am sure you will feel that cone is far stiffer. 

Not only did/do bass player get fussy about S/Pll speakers but many eschewed ported cabs because these can be very "boomy" if not properly designed. Some peeps even say you CAN'T GET decent bass from a reflex speaker, period! 

Dunno about that but it is a fact that ported cabs offer little protection against high, subsonic signals.

Dave.

ChrisMusic

Thanks again @ecc83, I follow all that OK, but...

Surely two same spec 16Ω drivers in parallel is seen as the same thing as a single 8Ω one? or whatever impedance is still halved at resonance?  Is that not really the whole story?  I think I am missing something here.  help...