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Post subject: connecting combo to cabinet ohm problem
Posted: Mon Aug 06, 2012 2:44 am
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Greetings,

i have a 100Watts combo with 8 omh impedance and trying to connect it to a cabinet which currently i want to buy (still haven't decided).

if possible could anyone explain the ohm thing to me, 'cause i heard that you could damage the speaker for a misconduct.


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Post subject: Re: connecting combo to cabinet ohm problem
Posted: Mon Aug 06, 2012 5:41 am
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16 ohms SPK1 parrallel to 16 ohms SPK2 = 8 ohms
4 ohms SPK1 in series with 4 ohms SPK2 = 8 ohms
8 ohms SPK1 parrallel to 8 ohms SPK2 = 4 ohms
8 ohms SPK1 in series with 8 ohms SPK2 = 16 ohms

Parrallel = speaker1 +ve & speaker2 +ve connected to Output +ve and speaker1 -ve & speaker2 -ve connected to output -ve.

Series = speaker1 +ve connected to Output +ve then Speaker1 -ve connected to Speaker2 +ve then speaker2 -ve connected to output -ve.

What is the Cabinet Configuration? Ideally you would like the total Ohms of the cabinet to be 8ohms once wired up - so that you can match the combo impedence. Are you still going to have the combo amp speaker connected when you connect the Cabinet or will it be only cabinet or only combo speaker. Does your combo amp have an ohms selector switch and does it have parrallel speaker jacks. These are all the sorts of things we would need to know to be able to help you.

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Post subject: Re: connecting combo to cabinet ohm problem
Posted: Tue Aug 07, 2012 7:13 am
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thank you AaronK for your fast reply.

i actually have one 100Watts amp 8 ohm combo that i want to connect to a cabinet so that i could have sounds from both ends (combo and cabinet).

there are many cabinets nowadays with great specification that offer good quality sound. unfortunately i do not have any knowledge about ohm/impedance.

lets say that i connect my 100Watts amp at 8 ohm to a cabinet that also have 8 ohm impedance, how about it? did it safely configured?
and what if the cabinet only have 4 ohm impedance?

i have been a guitar player for about 7 years but currently starting to play a bass guitar.
when i am playing electric guitar i do not need to connect my combo to a cabinet because i assume there is no need for that and the sound is loud enough to be heard in an indoor stage. but bass guitar is different, i think the sound is not kicking enough if played from the combo only.
BTW, i have a different combos specifically for guitar and bass.


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Post subject: Re: connecting combo to cabinet ohm problem
Posted: Tue Aug 07, 2012 11:10 pm
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If you connected in series then you would have a total 16ohm speaker on your combo, and if you connected in parrallel (most likely as it is most common for auxillary cabnets on a combo), then you would have a total 4ohm speaker on your combo (which is suppossed to have only 8ohms).

Now if your combo is a Fender Valve Amp then this should be OK iether way as Fender Valve amps usually can handle a 100% tolerance up and down in ohms. BUT with any other brand I would be very careful as some amps can't handle the Impedence Mismatch and could blow the Output Transformer or something.

If your combo is a Solid state amp then lowering the output impedence increases the current(Amps) which is BAD as you could blow the OUTPUT of the AMP. (This would be if you wired your speakers in parrallel.)
Now if you wired speakers in Series a 8ohm solid state amp would have no problems with the 16ohms speaker as it will only draw half the rated current(Amps), but your output power will be halved.

Now if the cabinet is 4ohms this would be bad for all types of amps if you chose the parrallel configuration (total speaker would be 2,7ohms). But in the series configuration it would be just fine (total speaker would be 12ohms).

Do you understand the Parrallel and Series configurations?

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Post subject: Re: connecting combo to cabinet ohm problem
Posted: Wed Aug 08, 2012 6:38 am
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yes, i think i understand the concept of parallel and series and i might want to configure it with series setup.
bass to input in combo amp to output cable to cabinet, right?
correct me if i'm wrong please.

as you would have said before, yes my combo amp is a solid state in which only have 8 ohm impedance available (no switching).

in conclusion, my output power from the cabinet would be halved of what they should be, right?

thank you again for explaining this whole thing to me.


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Post subject: Re: connecting combo to cabinet ohm problem
Posted: Thu Aug 09, 2012 12:59 am
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You can Google Series and Parallel Speaker Configurations to get a good diagram.

The way I see it you have limited options.

1. is to run a 8ohm CAB by itself (no combo speaker) - cool.
2. is to run 2 16ohm CABS in Parrallel (no combo speaker) - way cool.
3. is to run 1 CAB in series with internal 8ohm Combo Speaker - however this would require you making or buying a connection box that allows you to have your speaker configuration as required without needing to mod your speaker cable or amp. However, because the Speaker Ohms will increase, the full power of your amp will not be reached and the volume split between CAB and COMBO will probably not be identical. Whether or not this difference will actually be audible it cannot say for sure.

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Post subject: Re: connecting combo to cabinet ohm problem
Posted: Thu Aug 09, 2012 3:03 am
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thank you for suggesting Google search. i now know what the diagrams looks like.

it seems that my safe choice is to run on cabinet by itself without the combo.
the cabinet that i would buy only have four speakers and that was the reason i would like to connect my combo to run alongside with it.

say, if each speaker of the cabinet is 4 ohm multiply by 4 speakers equal 16 ohm right?
4+4+4+4=16 ohm
when connected by series configuration,

in parallel setting that would be
(4x4x4x4)/(4+4+4+4)=16 ohm

the result would be the same.
the last thing is to test the setting with an Ohmmeter.


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Post subject: Re: connecting combo to cabinet ohm problem
Posted: Thu Aug 09, 2012 5:07 am
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Your Series calculation is correct, however your parrallel calculation is not correct.

4 x 4ohms speakers in parallel = 1ohm (very bad for your amp)

take first two speakers in parallel 4//4 = 2ohm
take second two speakers in parrallel 4//4 = 2ohm
then both parallel speaker sets in parallel 2//2 = 1ohm !!!!!!!!!!!!

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Post subject: Re: connecting combo to cabinet ohm problem
Posted: Thu Aug 09, 2012 7:05 am
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oh, i understand now.
for each 2 speakers connected with parallel configuration, the impedance would be halved.

thank you for correcting my mistake.
i would check the impedance with ohm meter so that no unwanted result would occurred.

last question,
is it safe to say that the total and final calculation of the impedance tolerance is between 4-16 ohm for both my combo and my future cabinet?

FYI, i would apply the setting on august 11th.
hopefully that nothing bad would happened.

thank you.


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Post subject: Re: connecting combo to cabinet ohm problem
Posted: Fri Aug 24, 2012 1:41 pm
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What amp are you talking about? Most will do 4 ohms.


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Post subject: Re: connecting combo to cabinet ohm problem
Posted: Wed Aug 29, 2012 3:28 pm
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thank you for your reply, brotherdave.

i'm sorry for a late reply 'cause i was on a vacation.

i am an old man right now so i don't have the strength to travel back and forth with a heavy cabinet anymore.

i was a guitarist until recently become a bass guitarist.
a friend of mine told me to grab a roland 120 XL cube bass 12" combo amp 120 Watts and i do so when i look at the price tag while not even have the chance to try it beforehand.
the combo amp has impedance max at 8 ohm and i was buying a fender rumble 12" cabinet 500 Watts which is only about 24 pounds (11 kilograms). the reason i bought this cabinet is because the roland cube does have a good sound but unfortunately not loud enough for my need.

i ended up giging with borrowed larger watts combo yesterdays because of this ohm problem (fear of short circuit).

fender rumble 12" cabinet have two inputs at 8 ohm.
if i connect an external speaker cable from my combo into the cabinet in mono, will it decrease the ohm?


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Post subject: Re: connecting combo to cabinet ohm problem
Posted: Wed Aug 29, 2012 11:12 pm
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Hi Ilava

Just had a look at your Roland 120XL amp specs and it says you can plug in any external speaker into the external speaker jack up to 8ohms. So your cabinet can be 2,4,6 or 8 ohms and it should be fine as long as the cabinet can handle 120 Watts minimum. So the rumble cabinet should be fine as long as you connect those two 8ohm inputs in parralel to give you 4ohms. I was not sure on the Roland wether or not plugging in an external speaker cabnet muted the Cube120XL internal combo speaker. You will have to try it to see.

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Post subject: Re: connecting combo to cabinet ohm problem
Posted: Wed Sep 05, 2012 2:36 am
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Thank you for your respond AaronK.

the Roland manual says only a 'recording out' that will mute the sound coming from the roland combo. There is no detail explanation about the 'external output' on the manual except maximum impedance at 8 ohm and power is 120 Watts or greater. I assume it will not mute the combo speaker though.

the 'ext out' using a speaker cable that seems similar to a guitar cable, only a guitar cable is a shielded cable and not good for high electrical current. There is also American Wire Gauge (AWG) issue regarding a speaker cable.

would you explain to me what this means:
Fender cabinet has this specification:
Inputs: (2) 1/4" parallel wired input jacks
Input Impedance: 8 Ohm

i know that the fender rumble cabinet has 2 inputs at the back of the cabinet and 1/4" means a speaker cable size, but what if i just plug in only one of the input into a fender cabinet from the external output from the roland combo?
is it resulting in an impedance decrease to 4 ohm?


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Post subject: Re: connecting combo to cabinet ohm problem
Posted: Wed Sep 05, 2012 4:56 am
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Hi - you only need to use one of the 1/4" jack inputs on your CAB to connect to your combo. According to the specs those 2 1/4" jacks are internally connected to each other in parrallel so that you may daisychain another CAB off it in parrallel if you wanted to. The CAB on its own is 8ohms but if you daisychain another 8ohm CAB using the other 1/4" jack then the total ohms = 4.

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Post subject: Re: connecting combo to cabinet ohm problem
Posted: Fri Sep 07, 2012 6:31 am
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OK I read the manual for the Roland 120XL. The specs are somewhat ambiguous. It says the amp is 120 watts but doesn't state minimum Ohms of the power amp or what the power yield is when only using the built in speaker or what the Ohms of the combo speaker is. The features section does state that the combo speaker does NOT mute when an external speaker is connected and to use an 8 Ohm minimum external cab that is capable of 120 Watts minimum. This leads me to believe that the combo speaker is 8 Ohms also, that the power amp is 120 watts at 4 Ohm, meaning that when an external cab is added it does 120 Watts RMS. I'm just extrapolating this based on what the ambiguous specs and what little the features section does say.

Logically that means when using only the built-in speaker the wattage is roughly halved since the amp is pushing only 8 Ohms, typically this arrangement would yield about 60 watts. A 120 Watt RMS 4 Ohm capable amp pushing an 8 Ohm load yields 60 Watts RMS as a general rule. This really is sort of low power for anything short of home practice or quieter rehearsals or gigs with acoustic guitars and no drums. If you want to use this combo for anything else you are going to have to use an external cab to get more power from it or direct out to a PA or line out to a slave amp.

1. You should ONLY connect an 8 Ohm cab to this combo's external speaker jack. Nothing else.

2. You should only use a SPEAKER cable with a one-quarter inch plug to plug into the amp and whatever plug is needed to plug into the cabinet you use. Typically the cab connector in a low power application like this will be another phone plug but some cabs use Neutrik-Speakon connectors which is fine. Either way. Just make sure it is a SPEAKER cable, not an instrument cable. I like using speakon/Neutrik cables for a couple of reasons, one of which is you can't confuse it for an instrument cable. No more power than we are talking about here you could even buy two phone plugs and use AC zip cord from a hardware store for the cable.

As to what external cab to use, with low power like this the more air you can move the better. So add up the total inches of the speakers involved and the more inches of speakers the more air you move and the louder it will become. For example a single 12 cab added to your combo's 12 equals 24 inches. A single 15 added to your combo's 12 equals 27 inches. A 2x12 cab added to your combo's 12 equals 36 inches. A 2x15 cab added to your combo equals 42 inches. a 4x10 cab added to your combo's 12 equals 52 inches.

With such low power from your power amp, it really doesn't have to be much of a cab power rating wise. In fact if you over push the power amp section it will clip and clipping will damage speakers anyway no matter if they are 1000 watt capable speakers or 120 Watt capable speakers.

I'd either be looking for something more powerful or patching into the PA if the PA is capable of handling the bass and just use the combo as a stage monitor instead of a stage amp. I don't like moving big gear either and a small combo used as a stage monitor patched into a decent PA array works fine for me, but I need more than 60 Watts for that which in this instance means adding an external 8 Ohm cab.

Another option and the one I'd consider if I was using this combo would be to replace the combo's 8 Ohm speaker with a 4 Ohm speaker to max out the power amp for a 120 Watt output in stand alone mode. I wouldn't do that if the amp is under warranty as that would void the warranty. In doing the speaker swap you'd also need to disable the speaker out jack completely, or else replace it with a jack that mutes the built-in combo speaker because the amp would already be maxed out by the internal speaker alone and you couldn't use both an external cab and the built in speaker together. And sure as shooting somewhere down the line someone would plug an 8 Ohm cab in and blow up the power amp. A speaker swap would make the combo louder in stand alone use. Most combo amps these days come with a 4 Ohm speaker built in to get the most power possible out of the combo and they have no external speaker jack. Then someone always complains about not being able to plug in an external cab into them not understanding why you can't! It is a vicious cycle.


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