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Mixed Ohm Cab dilemma


D28boy

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Hi Guys wondered if I could tap the collective knowledge of the forum please?

 

I have a Marshall JCM 2000 DSL 100 watt amp together with a 19363V 2 by 12 cab (for home use) and a 1960A 4 by 12 for gigging...

 

Because the 1960A is very directional I thought it might be nice to set the 2 by 12 cab on the other side of the stage in some venues to get better sound dispersion. The Cabs however are different ohms

 

1960 is 16 or 4 mono or 8 ohm left and right stereo

 

1936 is 8 ohms mono.. and 16 ohm left and right when Stereo

 

I reckon I could use them both if I changed the 1960 to stereo (so 8 ohms ) and the 1936 mono (8 ohms) and then ran the amps two 4 ohm outputs. Would that be ok? I expect I'd only be getting two speakers working in the 1960 but I guess that would still be pretty loud.

 

Appreciate any help you guys could give

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Two 8 ohm loads in parallel is 4 ohms.

 

This is from Eminence's website:

 

 

It is important to match the speaker load with your amplifier's output impedance for a couple of reasons. One reason is that you will get maximum transfer of power. The other reason is that you can sometimes approach dangerous conditions for your amp if you have the incorrect load. When in doubt, it is safer to go higher in impedance to help protect the amplifier. The manner in which your speakers (in a multi-speaker system) are wired together determines the overall impedance.

 

Wiring Configuration for 2 Speakers in Parallelwiringparallel2.gif

 

Two 4 ohm speakers = 2 ohm load Two 8 ohm speakers = 4 ohm load Two 16 ohm speakers = 8 ohm load

 

Wiring Configuration for 2 Speakers in Serieswiringseries2.gif

 

Two 2 ohm speakers = 4 ohm load Two 4 ohm speakers = 8 ohm load Two 8 ohm speakers = 16 ohm load

 

Wiring Configuration for 4 Speakers in Parallelwiringparallel4.gif

 

Four 8 ohm speakers = 2 ohm load Four 16 ohm speakers = 4 ohm load Four 32 ohm speakers = 8 ohm load

 

Wiring Configuration for 4 Speakers in Series/Parallelwiringseriesparallel4.gif

 

Four 4 ohm speakers = 4 ohm load Four 8 ohm speakers = 8 ohm load Four 16 ohm speakers = 16

 

 

Can't see how this helps me ?

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Guest Farnsbarns

Can't see how this helps me ?

 

2 cabs in parallel=half the impedance. 2 cabs in series=double the impedence. Same goes with any resistive component.

 

If you set both your cabs up to 8ohms and wire them in parallel you will actually have a 4ohm load. You have a 4 ohm tap on your output transformer so it will be fine if connected to a single 4 ohm tap.

 

Hope that helps.

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Guest Farnsbarns

You need to match your impedance. It is bad to have a 8 ohm cab and a 16 ohm cab being driven by the same amp head. If smoke comes out you did something wrong.

 

Mixing impedance is fine as long as the resulting load is in range. 16 ohms in parallel with 8 ohms gives us 5.33 ohms. Some amps are stable at 4-8 ohms so that would be OK, for example.

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I've mixed impedances my entire life*, completely ignoring the much smarter than me amp techs out there and I've never blown up an amp because of it. Amps blew up, but Marshall and Sound City considered that a "feature" back then, not a result of mixed impedances.

 

rct

 

*I'm mixing them right now!

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Ignorance is bliss. Like Rct for many years I just plugged random extension cabs into the extra speaker jack of my Twin Reverb. Guess I was just lucky I never did any damage. Now that I have learned a little more I am reluctant to try it with my Rivera [crying] On the other hand venues don't really allow much of the volume we used to play at so the amp by itself is usually more than enough.

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Ignorance is bliss. Like Rct for many years I just plugged random extension cabs into the extra speaker jack of my Twin Reverb. Guess I was just lucky I never did any damage. Now that I have learned a little more I am reluctant to try it with my Rivera [crying] On the other hand venues don't really allow much of the volume we used to play at so the amp by itself is usually more than enough.

 

You could plug anything that had some kind of impedance into a Twin, I did it all the time. Todays amps are even better at it, so I wouldn't concern myself with the Rivera either. The whole mismatched impedance thing was because guys didn't have money and didn't know better and would just cobble together cabs made of stereo speakers, hi-fi stuff, am radios, who the hellz knows what they used. Amps would blow up with terarded speaker boxes made up of junk, but not normal speaker boxes. Half the time they weren't put together right, and no speakers actually is death to an amp if it doesn't see some resistance. It became a sort of fact by repetition that you can't mismatch impedances or you will wreck your amp, when really, all along you could plug almost anything legitimate into it, run it for 5 or 6 hours, and be fine.

 

I'm not smarter than amp guys at all, I've just never experienced Sudden Amp Death from mis matched impedance, and if anybody should, I should. Even today.

 

rct

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Thanx for all the input ( pardon the pun ) folks but I do think a lot of people haven't read my post properly...I was confirming that using one of the stereo inputs on the 1960A cab , which is 8 ohms plus the normal mono input on the 1936 cab which is also 8 ohms and using the two 4 ohms taps on the amp would be ok. What I'm doing there IS matching the cab impedance however the 1960A will be running on just two speakers (half of the stereo option). I wasn't sure what effect running the 1960 with two speakers may do to the overall sound of that cab.

 

I could use the cabs as is .....16ohm on the 1960 and 8 ohm on the 1936 and use the same 4 ohm taps. I've read that the overall load on the cabs would be 5.33 which apparently is ok with a 4 ohm tap from the amp. Apparently 2/3rds of the power will go to the 1936 and a 1/3rd will go to the 1960 which luckily will probably about balance the overall volume of the two as the 4x12 would be louder with equal power... Does that sound right ?

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Thanx for all the input ( pardon the pun ) folks but I do think a lot of people haven't read my post properly...I was confirming that using one of the stereo inputs on the 1960A cab , which is 8 ohms plus the normal mono input on the 1936 cab which is also 8 ohms and using the two 4 ohms taps on the amp would be ok. What I'm doing there IS matching the cab impedance however the 1960A will be running on just two speakers (half of the stereo option). I wasn't sure what effect running the 1960 with two speakers may do to the overall sound of that cab.

 

I could use the cabs as is .....16ohm on the 1960 and 8 ohm on the 1936 and use the same 4 ohm taps. I've read that the overall load on the cabs would be 5.33 which apparently is ok with a 4 ohm tap from the amp. Apparently 2/3rds of the power will go to the 1936 and a 1/3rd will go to the 1960 which luckily will probably about balance the overall volume of the two as the 4x12 would be louder with equal power... Does that sound right ?

The numbers look right to me. It might be kind of weird sounding pumping 2/3 of the wattage into the 2x12 and only 1/3 of the wattage into the 4x12. You can try it both ways - running the whole 4x12 and with half of the 4x12 - and see what sounds best.

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