Mismatched Cab Power Handling Dangerous? (not Ohms)

[fu22ba55]

Luckily, I understand how to manage Ohms properly. An important lesson, learned expensively in my youth.

What I'm NOT certain about is mixing newer / super-high power handling / efficient cabs with older, ceramic EV or Peavey type cabs.

Let's assume I'm using a 300W SVT head (not super-intelligent, but over-engineered for safety).

If I create a 4 Ohm load with the following:

18" or 15", 8 Ohm cab, 200-ish watt power handling
2x10" or 1x12" 8 Ohm cab, say... like 600W power handling

Even though they're both 8-Ohm loads... is there any chance of damaging one or the other cab (or the SVT head) if the two cabs have such grossly mismatched power handling?

I seem to remember doing somethign similar about 15 years ago, and one of the cabs cooked, pretty instantly... and my amp tech said "of course.... this cab can handle much more than the other one..." or something like that, even though the TOTAL power handling of the cab was well above 300 Watts.

Does this have to do with Sensitivity more than power handling?

Or better said... if one cab can handle 600, and one cab can handle 200... will the SVT attempt to deliver MORE watts to the 200 Watt cab, even though both cabs are 8 Ohm cabs?

 

[agedhorse]

The power handling of the two speakers add together, but with one important note... the power handling of the lowest power handling speaker is the limiting factor. Therefore, if you double the power handling of the lowest power speaker (200 watts in this case), the pair can safety handle 400 watts and is safe to use with your amp.

 

[fu22ba55]

The power handling of the two speakers add together, but with one important note... the power handling of the lowest power handling speaker is the limiting factor.

Ah... so another example: Pairing a theoretical 100W cab and 600W cab, the power handing would only be 200 Watts, and an SVT could theorectically cook the 100W cab? Is that math correct?

 

[fu22ba55]

These are the two REAL WORLD cabs available to me that I'd be pushing with an SVT later this week:

EA VL210 (500W, 8 Ohm)
Peavey PV 118 (400W, 8 Ohms)

 

[Wasnex]

These are the two REAL WORLD cabs available to me that I'd be pushing with an SVT later this week:

EA VL210 (500W, 8 Ohm)
Peavey PV 118 (400W, 8 Ohms)

View attachment 7437945

View attachment 7437946

Use either RMS or EIA power ratings

I believe the EA is rated in RMS watts. The Peavey is rated in Program power. The RMS power rating is 1/2 the Program power, so 200W RMS

Two 8 ohm cabs will share usable power approximately equally. Double the lowest power ratings is 2 x 200W = 400W combined usable power.

Another factor is the EA has a 5dB higher sensitivity rating. This basically means that at all power levels the EA will be 5dB louder than the Peavey.

• Calculated SPL of EA at 200W = 123dB
• Calculated SPL of Peavey at 200W = 118dB
  • Combined SPL where these figures are accurate = 126.9dB
• Calculated SPL of EA at 500W = 126.9dB

Specs say the Peavey plays a bit lower.

 

[Wasnex]

Here's the math assuming 300W clean from an SVT

• Calculated SPL of EA at 150W = 121.8dB
• Calculated SPL of Peavey at 150W = 116.8dB
  • Combined SPL where these figures are accurate = 125.7dB

 

[fu22ba55]

Here's the math assuming 300W clean from an SVT

Assuming clean is definitely a miscalculation. There will be fuzz.

But yes, understood, mostly.

Another (ridiculous) option is to run TWO PV 18s and the EA cab. Is it safe (wise) to present a 70's SVT with a 2.67 Ohm load?

 

[Wasnex]

Another (ridiculous) option is to run TWO PV 18s and the EA cab. Is it safe (wise) to present a 70's SVT with a 2.67 Ohm load?

The lead engineer who designed the SVT said it's good with an impedance mismatch, so AFAIK it's most likely fine at 2.67 ohms. Personally I prefer to show a tube amp the expected load.

As far is whether it would be best to set the amp to 2 ohms or 4 ohms I can't say for sure. My understanding is some amps are safer if the impedance is slightly higher than expected and other amps are safer if the load is slightly lower than expected.

There may be some concern about sending heavy fuzz to the EA. That's a pretty HiFi cab, so it may not sound great. Also distortion shifts power higher in the frequency spectrum. I assume the power rating for the mid drivers will be less than 500W...and power rating for the tweeter is probably less than 15W. As you probably know, EA is out of business so if you blow the drivers you can't buy replacements. AFAIK, the mid drivers were pretty much impossible to source even before EA went out of business.

 

[agedhorse]

Ah... so another example: Pairing a theoretical 100W cab and 600W cab, the power handing would only be 200 Watts, and an SVT could theorectically cook the 100W cab? Is that math correct?

Correct

 

[fu22ba55]

EA is out of business so if you blow the drivers you can't buy replacements. AFAIK, the mid drivers were pretty much impossible to source even before EA went out of business.

Noted. Handle with care. I plan to turn the trim pots OFF for the tweeters and not too loud for the mids, fingers crossed.

 

[sunburst79]

I’m curious after reading this and thinking about differing cabs of differing wattage and the same impedance. But also possibly differing frequency response.

What happens to the load the amp sees if one cab gets run below cutoff and the speakers thermal limit. I know the power handling drops and it’s bad for the driver.

Not meant as a thread hijack. But curious as to how frequency response plays in and how or if the presented load changes.

 

[Thumpin6string]

I'm not an EA fan, but I'd take that cab over the Peavey. If you want a 115 or 118 cab, I'd try to find something with closer power handling as the 210. If it was a solid-state head, I think it would be a bit different. I run a 500 watt 210 with a 350 watt 112 with my Eden WT550 and it works great. I think tube amps are a bit more finicky when it comes to speakers.

 

[Wasnex]

What happens to the load the amp sees if one cab gets run below cutoff and the speakers thermal limit. I know the power handling drops and it’s bad for the driver.

FYI Impedance varies by frequency. The way a cab is tuned also has some impact on it's impedance.
Here is the impedance curve of an Eminence 2512 in a large vented design tuned to 44hz.

Ref: https://www.parts-express.com/pedocs/more-info/290-593--eminence-deltalite-2512-cabinet-design.pdf

Not sure if there is a generic answer. Some amps can have problems pushing some speaker combinations.

As a cab is pushed the voice coil heats up and increases the resistance.

A speaker run over it's thermal limit is going to overheat and fail. When the voice opens up, the amp only sees the remaining good speaker.

FYI, The low frequency mechanical limit of many bass cabs is lower than their RMS power rating.

 

[sunburst79]

FYI Impedance varies by frequency. The way a cab is tuned also has some impact on it's impedance.
Here is the impedance curve of an Eminence 2512 in a large vented design tuned to 44hz.
View attachment 7438305
Ref: https://www.parts-express.com/pedocs/more-info/290-593--eminence-deltalite-2512-cabinet-design.pdf

Not sure if there is a generic answer. Some amps can have problems pushing some speaker combinations.

As a cab is pushed the voice coil heats up and increases the resistance.

A speaker run over it's thermal limit is going to overheat and fail. When the voice opens up, the amp only sees the remaining good speaker.

FYI, The low frequency mechanical limit of many bass cabs is lower than their RMS power rating.

I’m aware that impedence changes with frequency. And that speakers can act oddly at their limits. And that overheating can cause issues. I suspected that the resistance goes up. If one driver should blow depending on how they are wired would or might change the impedance. Or just go open.

Speaker damage aside it seems like the presented load would normally go up if one popped.

Just something to consider if perhaps running an amp hard and pushing a lot of low frequency.

 

[JJR58]

As to the OP.
I’m doing something similar.
The 210 is rated at 200, while the 112 is rated at 300.
Both 8ohm.
So the amp is running at 4ohms at 800 watts.
Way overkill, but I’m also very careful.
I never push this rig hard, as there is no need to.
Also have the proper eq to tame what ever needs to be tamed.

 

[agedhorse]

I’m curious after reading this and thinking about differing cabs of differing wattage and the same impedance. But also possibly differing frequency response.

What happens to the load the amp sees if one cab gets run below cutoff and the speakers thermal limit. I know the power handling drops and it’s bad for the driver.

Not meant as a thread hijack. But curious as to how frequency response plays in and how or if the presented load changes.

Generally not an issue, it’s still close enough.

I’m aware that impedence changes with frequency. And that speakers can act oddly at their limits. And that overheating can cause issues. I suspected that the resistance goes up. If one driver should blow depending on how they are wired would or might change the impedance. Or just go open.

Speaker damage aside it seems like the presented load would normally go up if one popped.

Just something to consider if perhaps running an amp hard and pushing a lot of low frequency.

Overthinking, though worthy of discussion.

Below the cutoff (if low enough) the impedance can fall some but as the VC temperature increases the impedance rises so there will be some (or mostly) cancellation between the two mechanisms.

 

[sunburst79]

Generally not an issue, it’s still close enough.


Overthinking, though worthy of discussion.

Below the cutoff (if low enough) the impedance can fall some but as the VC temperature increases the impedance rises so there will be some (or mostly) cancellation between the two mechanisms.

Oh I’m completely overthinking and it would seem like the resistance going up would act as a bit of a failsafe. But worthy of discussion. Which was why I asked.