Question about impedance, 2 cabs plugged in the head amp.

Reynald

Reynald

Dec 31, 2021
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#1
Let's say my amp is 1200 Watts at 4 ohms and 2 ohms, you noticed it can handle 4, 2.666 and 2 ohms of load.

#1
800 Watts cab @8 ohms and 400 Watts cab @8 ohms.

#2
800 Watts cab @4 ohms and 400 Watts cab @8 ohms.
#1 and #2 are grand total 1200 Watts combined cabs

What is the right impedance to handle 800 Watts in the bigger cab and 400 Watts in smaller cab, and no damage the smaller cab or amp head?

#1 or #2?
 
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#3
#1 will split the power evenly, leaving the door open to overpowering.

#2 will distribute the power 1/3 to the 8 ohm cab and 2/3 to the four ohm. Seems a better choice, depending on the driver configuration.
 
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#5
The power will split itself between the two cabinets, you don’t have to do anything but plug them both in. The power split was explained succinctly by basscooker.

basscooker said:
#1 will split the power evenly, leaving the door open to overpowering.

#2 will distribute the power 1/3 to the 8 ohm cab and 2/3 to the four ohm. Seems a better choice, depending on the driver configuration.
Click to expand...

Which exact amp you have is a very relevant question as several of the 1200 watt class D amps have 2 power amps and do indeed handle things like this differently. We would hate to give you incorrect advice.
 
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#6
The reason I asked the question is because the 8 ohm cabinet combined with the 4 ohm cabinet results in 2.67 ohms and the output power could be different depending on HOW the amp handles the different impedances. There are several ways, including a pair of 4 ohm capable power amps or a single 2 ohm capable amp or it may involve an impedance selector switch.

It could be about 1000 watts at 2.67 ohms, meaning that your 4 ohm cabinet would receive ~660 watts and your 8 ohm cabinet would receive ~330 watts.

It might also be 1200 watts at 2.76 ohms and the result would be different.
 
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#7
Sid the Kid said:
The power will split itself between the two cabinets, you don’t have to do anything but plug them both in. The power split was explained succinctly by basscooker.



Which exact amp you have is a very relevant question as several of the 1200 watt class D amps have 2 power amps and do indeed handle things like this differently. We would hate to give you incorrect advice.
Click to expand...
Not a biamp, jus a single amp. Not a bridge mode, just a simple single bass head amp.

Just looking to avoid damage at the 400 Watts cab if I would like to plug a 800 Watts cab and pump up to max volume.

Looks like 800 Watts cab should be a 4 ohms cab.

Total load will be 2.66 ohms and both cabs and amp will work well.
 
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#8
agedhorse said:
The reason I asked the question is because the 8 ohm cabinet combined with the 4 ohm cabinet results in 2.67 ohms and the output power could be different depending on HOW the amp handles the different impedances. There are several ways, including a pair of 4 ohm capable power amps or a single 2 ohm capable amp or it may involve an impedance selector switch.

It could be about 1000 watts at 2.67 ohms, meaning that your 4 ohm cabinet would receive ~660 watts and your 8 ohm cabinet would receive ~330 watts.

It might also be 1200 watts at 2.76 ohms and the result would be different.
Click to expand...
Yes, I just meaning about power distribution on cabs depending of cab loads. My example could be an amp head with no load switch selector, just a single output, 500 Watts at 8 ohms load, 1000 Watts at 4 ohms and 1200 Watts at 2 ohms, just a single amp, not biamp.

Thank you for your your answer, it helps me.

I was looking at GC the Mark bass MBR58R 104 bass cab (800 Watts), that looks lightweighted and it looks like the 4 ohms version is better than 8 ohms version. But could be combined with my 8 ohms 2x10 400 Watts bass cab, but I wouldn't like to damage the smaller cab.

For rehearsals I could transport the 2x10 cab, for medium venues or monitor 4x10 could be the option, but for bigger venues both cabs could achieve a max of 1200 Watts with no damages.
 
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#9
Reynald said:
Yes, I just meaning about power distribution on cabs depending of cab loads. My example could be an amp head with no load switch selector, just a single output, 500 Watts at 8 ohms load, 1000 Watts at 4 ohms and 1200 Watts at 2 ohms, just a single amp, not biamp.

Thank you for your your answer, it helps me.

I was looking at GC the Mark bass MBR58R 104 bass cab (800 Watts), that looks lightweighted and it looks like the 4 ohms version is better than 8 ohms version. But could be combined with my 8 ohms 2x10 400 Watts bass cab, but I wouldn't like to damage the smaller cab.

For rehearsals I could transport the 2x10 cab, for medium venues or monitor 4x10 could be the option, but for bigger venues both cabs could achieve a max of 1200 Watts with no damages.
Click to expand...
But you said in your first post 1200 watts at 4 or 2 ohms which is why the details are so important.

Your example may not be asking the question that you think you are asking. For example, if the amp does 500 watts at 8 ohms and 1000 watts at 4 ohms, the 2 ohm rated power does not make sense UNLESS there is a 2 ohm impedance switch of some sort. There are technical reasons for this, but you seem to be avoiding the question for some reason.
 
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#10
agedhorse said:
But you said in your first post 1200 watts at 4 or 2 ohms which is why the details are so important.

Your example may not be asking the question that you think you are asking. For example, if the amp does 500 watts at 8 ohms and 1000 watts at 4 ohms, the 2 ohm rated power does not make sense UNLESS there is a 2 ohm impedance switch of some sort. There are technical reasons for this, but you seem to be avoiding the question for some reason.
Click to expand...
Ok. It is the Epifani UL901 1st gen that does not have a 2 ohms switch. And spec says minimum load 2 ohms. Maybe not achieve 1200 Watts at 2 ohms but still the point is to not damage overloading the smaller cab.


Specs

Configuration
Single Channel Preamp
Class-D Power Amp
Switch-Mode Power Supply

Power Rating
1000 Watts RMS
Minimum Load: 2 Ohms

Connectors
Input: 1/4-inch TS Phone Jack Passive: 470KΩ
- Active: 200KΩ
Output: Combination SpeakonTM and 1/4-inch
Send: 1/4-inch TS Phone Jack 200KΩ
Return: 1/4-inch TS Phone Jack 50KΩ
Tuner Out: 1/4-inch TS Phone Jack
Foot Switch: 1/4-inch TRS Phone Jack
Direct Out: Low Impedance XLR w/ Level Control
Pre/Post EQ: Selectable Switch sets DI
Output to Pre/Post EQ
 
#11
Reynald said:
Ok. It is the Epifani UL901 1st gen that does not have a 2 ohms switch. And spec says minimum load 2 ohms. Maybe not achieve 1200 Watts at 2 ohms but still the point is to not damage overloading the smaller cab.


Specs

Configuration
Single Channel Preamp
Class-D Power Amp
Switch-Mode Power Supply

Power Rating
1000 Watts RMS
Minimum Load: 2 Ohms

Connectors
Input: 1/4-inch TS Phone Jack Passive: 470KΩ
- Active: 200KΩ
Output: Combination SpeakonTM and 1/4-inch
Send: 1/4-inch TS Phone Jack 200KΩ
Return: 1/4-inch TS Phone Jack 50KΩ
Tuner Out: 1/4-inch TS Phone Jack
Foot Switch: 1/4-inch TRS Phone Jack
Direct Out: Low Impedance XLR w/ Level Control
Pre/Post EQ: Selectable Switch sets DI
Output to Pre/Post EQ
Click to expand...
Ok, your original post was a non-sensical fabrication. That’s why it didn’t make any sense.

The original UL901 is 750 watts at 4 ohms and 1000 watts at 2 ohms. Based on experience, it will be about 500 watts into 8 ohms.

At 2.7 ohms it will deliver about 875 watts, split 1/3 to the 8 ohm cabinet and 2/3 to the 4 ohm cabinet.

Details are important.
 
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#13
pgpete said:
According to this JBL technical note - it is better to have a higher power amplifier that is not driven into clipping, than "dimeing" it and hoping the cabs will cope with it.

Loading…

adn.harmanpro.com
Click to expand...
No sir, you are perpetuating bad advice.

This tech note only applies to pro audio applications, where with proper protective limiting, is fine. It does NOT apply to MI applications like bass guitar.

If you go back to the original tech notes that this paper was updated from, for MI applications it’s recommended to derate the power handling capacity by 50%. This is due to the lower crest factor of MI signals like guitar and bass. Since JBL stopped making speakers for MI application several decades ago, they removed the references to MI use.
 
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#16
pgpete said:
Im running an ampeg SVT7pro (1000WRMS @ 4 ohm into a Ampeg SVT HLF410 (500WRMS @ 4 ohm) and havent blown it up yet - Im not diming it either.
Click to expand...
Consider yourself lucky and/or fortunate.

That’s not the original tech notes, it was also printed after JBL exited the MI industry.

I have posted the original JBL white paper here in the past. I also worked in the industry back then, all OEM users of the JBL MI products had to agree with the derating in order to have warranty protection against claimed defective drivers. Since they were used by the pallet loads, manufacturers couldn’t afford to suffer excessive uncovered failure claims.

This is a clip for MI applications from that original JBL paper that I posted years back. The original paper is no longer available on line that I could find but I have a copy in my shop.

Loading…

www.talkbass.com
 
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#18
As always, @agedhorse, I admire and appreciate your tact, restraint, and patience in these discussions. Many of us, myself included, read an article, or worse, an internet post, that we file away as solid fact, and base our subsequent "educated opinions" on these half-baked/misinterpreted ideas.

We are truly fortunate at TB to have an actual expert and professional in the field of ampification to provide the straight dope to those willing to listen and learn.

Thank you sir.

Jon Brint
2cooltoolz
 
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#19
OK

What people want : - is to buy a (say) a 1000 watt amp and (say) two 500 Watt speakers and no matter how much they turn the volume up and no matter how hard they play they wont blow up their cab and/or amp.
Sadly, as the JBL techical note explains, as the amp starts to clip all the high order harmonics generated by the clipped signal gets sent to the horn(s) in your cab ( via crossover circuit ) and blows them.

I experienced this in the 80s - my garage-band had a 150W vocal PA amp and two 100W cabs - we managed to blow the horns and peizo-tweeters in the cabs. The JBL techncal note was right.

My personal preference in bass tone is a clean sound - with a clean sound I can hear clipping as a form of distortion. and know that I have to reduce volume. I'm not sure I could do the same with the drive and dirty tones a lot of bass players are using these days.

I suspect using drive and fuzz etc reduces the Crest factor that Agedhorse mentioned.

The take away from this is : - dimeing an amp and hoping you wont blow the horns in your cab is where you need luck and fortune.
no amount of impedance matching etc will allow you do drive the cabs with a clipping amplifier without risk.

Having an amp that exceeds the power rating of the speaker is less of a problem as many might think - if you manage the volume levels.

Speaker overexcursion could be a factor but only if you over do it!

And there are exceptions to the rules - Ampeg 810E fridge cabs dont have horns, you can use valve amps with overdriven input stages to to create even (?) order harmonics fuzz
etc etc -
The more you look into it, the more complicated it gets.


-----------------------------------------------------------------------------
ex electronics tech - yes , I had to fix the stuff the engineers designed.
 
#20
pgpete said:
OK

What people want : - is to buy a (say) a 1000 watt amp and (say) two 500 Watt speakers and no matter how much they turn the volume up and no matter how hard they play they wont blow up their cab and/or amp.
Sadly, as the JBL techical note explains, as the amp starts to clip all the high order harmonics generated by the clipped signal gets sent to the horn(s) in your cab ( via crossover circuit ) and blows them.

I experienced this in the 80s - my garage-band had a 150W vocal PA amp and two 100W cabs - we managed to blow the horns and peizo-tweeters in the cabs. The JBL techncal note was right.

My personal preference in bass tone is a clean sound - with a clean sound I can hear clipping as a form of distortion. and know that I have to reduce volume. I'm not sure I could do the same with the drive and dirty tones a lot of bass players are using these days.

I suspect using drive and fuzz etc reduces the Crest factor that Agedhorse mentioned.

The take away from this is : - dimeing an amp and hoping you wont blow the horns in your cab is where you need luck and fortune.
no amount of impedance matching etc will allow you do drive the cabs with a clipping amplifier without risk.

Having an amp that exceeds the power rating of the speaker is less of a problem as many might think - if you manage the volume levels.

Speaker overexcursion could be a factor but only if you over do it!

And there are exceptions to the rules - Ampeg 810E fridge cabs dont have horns, you can use valve amps with overdriven input stages to to create even (?) order harmonics fuzz
etc etc -
The more you look into it, the more complicated it gets.


-----------------------------------------------------------------------------
ex electronics tech - yes , I had to fix the stuff the engineers designed.
Click to expand...
No sir, again you are stating as fact incorrect information.

For MI (musical instrument) speakers, at the time JBL was supplying them to the MI industry, tweeters weren’t commonly used.

The 50% derating of power handling was to account for compressed, distorted (overdriven or clipped) signal that is common in these applications.

When a signal is clipped heavily, the (thermal) RMS power can increase to double the value of a unclipped signal, which causes OVER-powering of the speaker. This was the most common failure mode back in the day when JBL was in that market. In the pro audio and hi-fi markets, distorted and heavily compressed signals were not common.

As amp power increased over the years, a different failure mode became common, mechanical failure where the high power caused damage due to excess movement of the speaker’s soft parts. This is why most bass speakers fail, and it’s made much worse when the amp is driven into clipping in MI applications because when an amp is clipped, the resulting loss of feedback causes the amp to lose control of the speakers movement at each positive and negative peak. The momentum of the moving mass causes even greater movement and faster damage.

Additionally, when the amp is clipped at high power, the thermal power also can also increase by 50% resulting in thermal damage.

It’s called “double whammy” damage in the recone business.
 
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