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Newfoundfreedom

What's the worst that could happen? (Getting ohms wrong)

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I know ohms ratings have been discussed at great length in other threads, but what I'm wondering is, what would likely happen if you did get it wrong? 

Specifically, I've just bought 2 PA subs, and I was informed by the seller that they're 8ohm, but there's no actual labelling on the subs to confirm this. I know I could (and will) open up the cabs and look at the actual speakers to make sure, but it kind of got me thinking, what actually happens if you get it wrong. 

My PA (solid state) has 2 channels at 4 ohms minimum rating per channel. I'm currently running 2x8 ohm speakers, which can either be run in separate channels, or linked together to give 4 ohms into a single channel. No problems so far. Now if I add the 2 bass bins and let's say for the sake of argument, they're both 4 ohm (giving i believe a 2 ohm resistance into a single 4 ohm channel) what would it actually do to the amp. Are we talking sudden and immediate damage? Would it depend on the volume put through? Would I get any warning first? Are we talking catastrophic failure, or just a blown fuse? Do modern solid state amps have any kind of built in tolerance? I know you shouldn't do it. I just have no idea what would happen in the real world if you did. 

 

 

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A load that's too low in most cases will trigger protection circuitry that shuts the channel down, often resetting itself after the heat that caused it to shut down dissipates. If you don't have protection circuitry, rare these days, the amp could blow. You should not be using subs plus mains on the same channel . You should be using an electronic crossover and a separate amp for the subs, or one channel for the mains and one channel for the subs if the loads per channel aren't too low.

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15 minutes ago, Bill Fitzmaurice said:

A load that's too low in most cases will trigger protection circuitry that shuts the channel down, often resetting itself after the heat that caused it to shut down dissipates. If you don't have protection circuitry, rare these days, the amp could blow. You should not be using subs plus mains on the same channel . You should be using an electronic crossover and a separate amp for the subs, or one channel for the mains and one channel for the subs if the loads per channel aren't too low.

Yes. Perhaps I explained badly. I will be running mains through one channel, and subs through another channel. No crossovers required as both will have separate EQ. 

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8 hours ago, Bill Fitzmaurice said:

You still need a crossover. EQ doesn't provide the necessary roll off slope or depth of out of pass band attenuation.

Perhaps I'm miss using the term subs.

They're bottom PA speakers, which are still full range, but EQ"d to favour the bass end of the spectrum. They're not actually subs in the sense that you're talking about. 

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2 hours ago, Newfoundfreedom said:

Perhaps I'm miss using the term subs.

They're bottom PA speakers, which are still full range, but EQ"d to favour the bass end of the spectrum. They're not actually subs in the sense that you're talking about. 

It's a free country and you're welcome to do it any way you like. You may or may not get it to work doing it that way. Bill was explaining the way to get the best result. Ideally, you need to be able to control the frequency ranges and relative amounts of power going to the subs and tops. If they are very different in terms of efficiency/sensitivity, you will struggle to balance them. Used power amps (especially old school - i.e. class AB/heavy) can be picked up very cheaply these days, so it's worth picking one up. An inexpensive basic crossover from someone like Behringer isn't going to break the bank, either.

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1 minute ago, Dan Dare said:

It's a free country and you're welcome to do it any way you like. You may or may not get it to work doing it that way. Bill was explaining the way to get the best result. Ideally, you need to be able to control the frequency ranges and relative amounts of power going to the subs and tops. If they are very different in terms of efficiency/sensitivity, you will struggle to balance them. Used power amps (especially old school - i.e. class AB/heavy) can be picked up very cheaply these days, so it's worth picking one up. An inexpensive basic crossover from someone like Behringer isn't going to break the bank, either.

I must profess my total ignorance here. I've never used subs in my life before so it's all brand new to me. I wasn't disagreeing with Bill. I just don't really understand what you mean by balancing. The subs are on a totally separate channel so don't need to be balanced with the tops. Two subs are on one channel daisy chained with their own EQ, the tops are on a separate channel daisy chained with their own EQ. If you mean balancing the sound then I've done this by adjusting the volume on each channel until the blend of the two sounds right. It all sounds fine to me. 

Is there are chance you could post what you mean by a crossover. Is it something that goes inline, or would it be permanently wired into the actual speakers? 

Also I don't understand why you say I need a separate power amp for the subs, when the PA is a mixer and power amp combined, and the subs have their own dedicated, powered channel. 

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Ok I know I'm only answering myself now, but  I've just been studying up based on the advice here, and I've just found a Behringer super X pro crossover locally (well, in the country at least) for about 55 quid. So I'm going to take a punt on that and see (or rather hear) the difference. 

Thanks for the advice. 👌

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The worst huh? OK The power amp will fail if the load is too low. In that failure the amp outputs one of it's rails which blows all of the drivers in the attached cabinets. 

 

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Thanks all.

I've run the speakers flat out for a couple of hours today without issue, so I'm fairly confident that the seller was telling me the truth and they are indeed rated at 8ohms. 

I did check the drivers first but there was no markings on them, and I don't have an ohms tester, so it was a case of suck it and see. 

 

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On 13/01/2021 at 22:31, Newfoundfreedom said:

I did check the drivers first but there was no markings on them, and I don't have an ohms tester...

...which tells only the DC resistance of the coil, not the impedance.

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This is why amp designers spend so much time and effort designing protection circuitry, but even so, protection circuits are not perfect and failures can occur.

Edited by agedhorse

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23 hours ago, BassmanPaul said:

Digital multimeters are really cheap and any serious electric musician should own one or more.

 

2 hours ago, itu said:

...which tells only the DC resistance of the coil, not the impedance.

Fairly 'reliable' at around about 2/3 of the total nominal impedance rating for a driver in a cab.

If you measure DCR, the driver impedance in cab is going to be the next rounded off common figure on the larger side of your DCR.

Eg, DCR is 5.8ohm, '8 ohm' driver.

I don't know how well that works for OP's PA cabs with crossovers and compression drivers hooked up.

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Just 'cos they're working doesn't mean they're going to keep doing so if you have the connections wrong.

Edited by BassmanPaul
Bad speling.

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I use the same crossover, it's really good. I like having the choice as to where the crossover occurs. I use it in a bi-amp set up for bass. 

 

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See my post above, just because something "works" doesn't mean that it's correct and not causing a problem (or a problem that will rear it's head down the road). 

For those who claim that all failures in an amp under warranty is because the amp is defective, this is a good example of why this statement is not always true.

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Just to go off piste for a moment. I arrived for a gig and the boss was putting out the PA, powered by an eight channel Yamaha as I recall. Something made me notice the speaker leads and they were long coax instrument leads. I told the guy he shouldn’t be using those and he just wouldn’t believe me. He said that’s what he always used. So we started the gig which went well. Until there was a distinct plasticky smell in the air, when the amp went up in flames. 

So, yes, the load on the output stage is critical.  Best not to monkey with the ohms.

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