Discussion in 'Amps and Cabs [BG]' started by Preston Smith, Nov 18, 2018.

1. ### Preston Smith

Aug 1, 2016
Hey everyone.

Blew my amp a while back, 225w into 800w cab. Uh come to find out, I should've been bridging the bi-amps for 700w. Duh but I'm studying audio and kinda learning this impedance stuff.

I think it's cause I was pushing it too hard.

150 W head into 800 W cabinet
350w head into 800w cab, is this ok?

In these threads everyone assures that there will be no problem using a small head into a huge cab in terms of watts, to the extent of mocking the OP, but I think they neglect the real issue. Yeah the cabs gonna be fine but what about the amp? What about the amp??

Fight.

2. ### Wasnex

Dec 25, 2011
When bridging you need to be mindful of the amp's minimum impedance rating. Typically the minimum impedance rating doubles when the amp is put into bridge mode. For example, the minimum load might be 4 ohms in dual mono or stereo modes, but 8 ohms in bridge mode.

Here's whats going on. The output section of an amp can only supply so much current before it overheats and incurs damage. The minimum impedance rating is related to this. One side of the amp develops a certain voltage, and when the voltage is applied to the impedance of a speaker a certain amount of current flows that is within the limits of the output section. Assume the amp develops close to its maximum current when driven hard into a 4 ohm load.

When you bridge an amp, you stack two output sections on top of each other so the voltage doubles. This means if the impedance stays the same, the current will also double. But remember the amp was already operating near its maximum current capabilities before you put it in bridge mode, so if the current doubles,the magic smoke will escape. The way to keep the magic smoke where it belongs is to double to impedance from 4 ohms to 8 ohms so the currents stays within limits.

The resultant power, voltage, and current relationships are based on ohms law. Impedance is a special form of "reactive" resistance so you can play around with the following formulas, by subbing (Z) Impedance for (R) Resistance

Last edited: Nov 18, 2018
3. ### Old Garage-Bander

An amp as you describe cannot be damaged by using a cab as you describe, provided the cab and interconnecting cables are in good working order and is the proper impedance match for the amp. Always use the same type of power units when comparing “Watts.” If the amp power is rated in RMS, use the RMS power rating for the cab. You can convert between units, but it’s better if you’re doing an apples to apples comparison.

If you are concerned that under powering a cab will harm either the cab or the amp, then you need not worry. Most of the time, an amp is delivering way below its rated power to the cab and way below the cab’s rating. You might be plugged in, hooked up and playing and set you bass down and walk away. There is very little power being applied at that point. And amps don’t fail under those conditions.

That being said, there are instances where a smaller amp can damage a cabinet rated at much more power.
This would be if you try to use a lot of bass and crank up the amp. Speakers generally need to be derated for their power handling capacity when using lower frequencies such as found in bass cabs. Most manufacturers don’t account for this when they rate cabs for power handling. This is where listening to your speakers for signs of distress is important. If you hear things that don’t sound right, you need to back off.

If biamping gives you an amp with a rated power output of 700 watts vs 225 Watts, the larger rated power amp is more likely to damage the speakers. Then depending on the failure mode of the speakers it could damage the amp. But the speakers need to go first. At that point you are operating into a cab that is no longer operating properly.

I don’t know the history of the under powering myth, but would not be surprised to learn if it came from a slick amp salesman who tried to convince someone with big cab that they need a bigger amp. There is a lot of snake oil being peddled to unwary musicians. There is even some of this that goes on with the power ratings of amps and cabs. You are doing the right thing in asking questions and expanding your knowledge. Hopefully you will get the answers you need here without having to put up with a lot of hype. There are a lot of people here who are much smarter than me. Hopefully they will chime in and hopefully, if I’ve misstated anything they will point that out.

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4. ### agedhorseSupporting MemberCommercial User

Feb 12, 2006
Davis, CA (USA)
Development Engineer-Mesa, Product Support-Genz Benz
There is no problem or additional risk to the amp if it's rated at less output power than the cabinet, but if you are driving the amp heavily into clipping, that may put additional stress on the amp (no matter how powerful it is).

Generally, the cause of failure of an amp is more related to its quality level.

What amp and what cabinet?

Sep 14, 2010

6. ### Preston Smith

Aug 1, 2016
Oh cool interesting info Wasnex and everyone. Bridged is rated 700W into 8ohms and 1000 into 4. Minimum 4ohms.

I never ran it bridged though, always 225W. I worked the amp pretty hard, and the cab is ok AFAIK, I've ran it with other amps since then.

I was unaware there was such an under powering myth, I've heard though that at least in a live sound setting, you can really mess up your amps if you're driving them past their capabilities, i.e. into speakers way bigger than them. Makes sense to me, but I haven't thought into it a ton.

Its a cuh..czzhs..Ccarvin R1000 into a.. shoot I don't want to say, lest I get more star wars pictures posted at my expense.

Last edited: Nov 18, 2018
7. ### Old Garage-Bander

Think into it... but first how do you define “way bigger?”
Then think a bit more... how does an amp know it’s in a live situation?

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8. ### Preston Smith

Aug 1, 2016
Huh? Clever. A live sound reinforcement situation is what I meant, driving PA speakers that are rated at more watts than your amps can deliver. That would be my definition of way bigger. But sorry I should have been more clear, I'm multitasking.

But to indulge you, an amp might know it's in a live sound setting, as opposed to a practice setting, for the strain you're (or I definitely was) putting on it.

9. ### agedhorseSupporting MemberCommercial User

Feb 12, 2006
Davis, CA (USA)
Development Engineer-Mesa, Product Support-Genz Benz
I spent 40 years professionally operating pro audio amps int speakers rated at higher power. There is nothing in the slightest bit wron with it as long as you have enough rig for the gig.

What speaker, I suspect that your lower power might be better matched than you assume.

10. ### Preston Smith

Aug 1, 2016
Sure, if you had enough rig for the gig, you wouldn't be driving the heck out of the amps right? Agedhorse, what is the rule of thumb matching amps to speakers? Or should I pull out my Yamaha handbook lol.

Huh well if that won't do it I wonder what did. One thing I was thinking is perhaps if I had it in full range mode, and accidentally had the power amp I wasn't using turned up (often the case) and not seeing any load might do it?

11. ### beans-on-toastSupporting Member

Aug 7, 2008
I don’t know what’s going on. There seems to be a rash of blown heads these days on talkbass.

Even with an amp rated at half the power handling capacity of the speaker cab, the tone settings are important. With the EQ set flat under those conditions, you should be fine. If the bass is cranked up, it pushes the speaker cab harder. Some pedals can do the same. Always be careful. Avoid a mechanical flubbing sound.

12. ### Preston Smith

Aug 1, 2016
I usually kept her pretty flat, and my power capacity was well under half. When I cranked it it was always to compete with irresponsibly loud guitar players who then wanted me to be louder than them *winces and turns up a hair* So its harder to tell what exactly was going on.

Anyway, this issue kind of developed and exacerbated over years of abuse and amid other problems with the amp until it just totally crapped out so my speculation here could be way off track.

Last edited: Nov 18, 2018
13. ### Preston Smith

Aug 1, 2016
Acoustic BN410.

What about that rule of thumb regarding matching power amps to speakers in your experience? Yeah you'd want them rated higher, but not wayyy higher right?

Say you don't have enough rig for the gig and push the amp a lot? Say you run an amp without seeing a load like I did with my Carvin?

14. ### agedhorseSupporting MemberCommercial User

Feb 12, 2006
Davis, CA (USA)
Development Engineer-Mesa, Product Support-Genz Benz
Rule of thumb is to power at no more than the RMS rating of the speaker. It's important that you compare both amp and speaker in "RMS" units.

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15. ### Killed_by_DeathSnaggletooth

That meme is from Star TREK, not Star Wars.

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16. ### Preston Smith

Aug 1, 2016
Yeah, agedhorse that's fairly consistent with the handbook in fewer words! Seems the danger in pushing the amp out of limits is sending fugly signals to your drivers and damaging them, not the amp itself. I'll be darned if I can't figure out how to smoke an amp haha.

Anyway, ef need to study this more, sorry for wearing you guys out!

Oh that was just my dry attempt at a wise crack.

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17. ### sawzalotSupporting Member

Oct 18, 2007
Getting popcorn.

18. ### Preston Smith

Aug 1, 2016
Oh no, discussion closed. I hope. Thanks everyone.

19. ### Omega Monkey

Mar 8, 2015
Yeah, I don't know why you think an amp couldn't push a cab that was "way bigger". Every single cab on the market from a reputable maker is rated at how much volume it will put out at 1w from 1m away, and pretty much all of them will do at least "noisy coffee shop" volume levels just with that single watt. If you put in 225w of 3k for more than a fraction of a second and were anywhere within 20 ft, you'd probably be deaf after that.

Your amp failed because its 20 years old and needed more maintenance than it probably got. The power output of the amp and it's relation to the power handling of the cab had zero to do with it. At all. Like not even a little bit. And bigger speaker cabs with higher power handling usually get louder than smaller cabs with lower power handling, so youre likely to get more from a given amount of amp power that way than from a smaller "matched" cab (ie 225w cab).

Any decently designed amp will have protection circuits of 1 or more types specifically to avoid issues with getting pushed hard anyway. So what do you mean by your amp "blew"? Have you for instance checked the fuse/s to see if you have a blown fuse? Have you tried the head hours or days after whatever incident happened to see if the amp was still the same?

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20. ### ficelles

Feb 28, 2010
Devon, England
That statement doesn't make sense to me.

So:

1. "you can really mess up your amps if you're driving them past their capabilities" - is correct.

2. "i.e. into speakers way bigger than them" - is incorrect. In fact the opposite is true.