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How are speakers matched with audio amplifiers? (avoiding overloading either)


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#1 vivitern

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Posted 19 October 2017 - 11:14 PM

I know a similar question title has been asked, but I believe that does not answer my question (and I couldn't think of a better way to phrase the question).

I'm a bit confused by how exactly an amp can overload a speaker, and vice versa.

My guitar amplifier speaker is  CDM-10008 (See http://www.kynix.com/Parts/4380/CDM-10008.html.)

If I understand correctly, the output amplifier (should) outputs a fixed voltage signal output regardless of what load is placed on it. If this step is wrong, please correct me.

So if there is a fixed voltage signal, (say, +-15V, i.e. 30V of swing) and if the speakers' impedance is ~8? (I understand it will vary with frequency but say it is around this figure), then how is the wattage varying with different amp combos even though the impedance is roughly the same? Is it that the voltage increases with higher wattage amplifier/speaker combinations.

For example a 10W combo with an 8? speaker vs a 100W amplifier connected to a 4 speaker cabinet wired for 8? impedance (parallel connect 2 series pairs of 8? speakers), the 100W is obviously louder. Is it that the output voltages of the 100W amplifier is more? Else how can you get the wattage to increase if you are keeping voltage and impedance constant?

What would happen if you connected the 10W amplifier directly to a 4 speaker cabinet? Would it overload the amplifier? Or just play it quieter? Theoretically, if the voltage is the same and the impedance is still 8ohms, the wattage should be the same, i.e. 10W through the 100W rated speakers.

If so, is this true then: when we say 10W 8ohm speaker, we mean its able to handle maximum peak voltages of (P=V^2/R, V=sqrt(PR)) ~9V. Whereas for a 100W 8ohm speaker, it is able to handle peak voltages of ~28V?

In what situation can you harm a speaker? By connecting a too powerful an amplifier to it? But then isn't that something that is recommended by many people? (Amplifier output of atleast 2x the speaker rating). If so then the voltage output of an amplifier is not fixed? It varies according to which speaker is connected to it? (even though the impedance is the same?)

In what situation can you harm an amplifier? By connecting too high a wattage speaker to it? Then why do I see so many people posting videos on youtube of 1/2Wguitar amplifier builds connected to big wattage rating 4 speaker stack speakers or atleast even 2 speaker combinations?

First of all, it's pretty rare for a speaker's impedance to be anywhere close to flat. The impedance curve normally looks vaguely like this:

ZY550.png

The peak is fs, the speaker's free-air resonance. The rated impedance is the first minimum in the impedance curve above resonance. The DC resistance will usually be a little lower than that, but not typically a lot lower (e.g., might be around 6 ohms for a speaker rated at 8 ohms impedance). The DC resistance is also affected by other factors though--for example, a speaker intended to handle more power will typically have thicker-gauge wire in the voice coil which will reduce the DC resistance, but have almost no effect on impedance at higher frequencies.

When you mount that driver in a box, you typically add at least one (and often a couple) more, smaller peaks at lower frequencies that reflect the resonant frequency of the cabinet and any ports it may have.

I'm not sure where you got the idea that voltage is constant (or even close to it). Like any other circuit, P = I * E. So, for example, one watt through an 8-ohm speaker is 2.83 volts (square root of 8, since P = E2/R). Perhaps you're thinking of the fact that most amps will be rated for a maximum voltage swing (but it'll usually be higher than 16 volts).

As to what happens if you connect a 10 Watt amp to 4 speakers (presumably in series-parallel to maintain the same impedance), you'll typically gain at least a little efficiency, because most speakers are at least somewhat non-linear. For example, a speaker might be rated at 92 dB SPL at one watt (under some standard testing conditions). In theory, that means it should produce 95 dB SPL with 2 watts of input, or 102 dB SPL with 10 watts of input. In reality, three or ten more dB of input won't usually produce (quite) three or ten more dB of output though. By separating the power from the amplifier into four separate speakers instead of one, you'll minimize this effect, so you'll get (slightly) more acoustic output for a given amount of electrical output from the amp.

As far as too powerful an amp damaging a speaker: it depends. If you completely overpower a speaker, yes, that can happen. For example, if you connected a 500 watt amplifier to a little 3 inch speaker and just cranked it up to anywhere close to maximum power, the speaker would almost inevitably fail fairly quickly. Depending on the design, it's a little hard to be certain what would fail first -- you might overheat the voice coil, and a wire would simply vaporize, or you might generate a stronger magnetic field than it's designed for, and push/pull the speaker cone further than intended and destroy the surround (in my experience, voice coil failure is a lot more common though).

Much more common is destroying a speaker by driving an amp past its rated power. This is particularly problematic with bipolar amplifiers, since they tend to have quite harsh clipping characteristics. Here, however, you're saved by the fact that intentionally producing various forms of distortion is pretty common, so when you're dealing specifically with a guitar amp and speaker, you're not quite so likely to destroy things (very quickly anyway). With something like a normal stereo, clipping will typically increase high-frequencies in the signal a lot in a hurry -- that, in turn will result in a lot more of the power going to the tweeter than intended, which may destroy it very quickly.

Hurting the amplifier depends. The short summary is that failure in a solid state amp will typically happen if you connect too low impedance of a speaker. That will try to draw more current than the amp can deliver, leading to overheating and (if you go too far) melting down the output transistors.

Conversely, tubed amps tend to be damaged more often by connecting too high impedance of speakers. The amp is designed for the speaker to load the output. Without enough loading from the speaker, the amp will produce higher voltage than intended. When/if a speaker wire comes loose, you effectively get infinite impedance almost instantly. Depending on the design, either your protection circuit kicks in and shuts down the amp, or else the last sound you hear before repairing the amp is a loud pop as the output tubes burn out.

Maybe  I can try to hook 250 ohm headphones directly into tube Amp's speaker output and see what will happen.



#2 AkakmanH

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Posted 20 October 2017 - 05:32 AM

The color of the print on your post is so dim it is impossible to read.



#3 Braegnok

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Posted 20 October 2017 - 06:58 AM

+1 cant-read-dont-care.jpg


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#4 phil69

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Posted 20 October 2017 - 08:56 AM

The color of the print on your post is so dim it is impossible to read.

I couldn't read it either until I clicked and dragged my mouse to the bottom as if to copy. But he should still change his print color.


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#5 wildman2

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Posted 20 October 2017 - 04:31 PM

May get more replies if ppl can read the text. here is a link that may answer some of your questions...http://www.regiscoyn...spk_cab_wiring/


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#6 Fight Game

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Posted 23 October 2017 - 08:55 AM

I'm guessing it's a fake account and the person will never be back.  It's that color, probably cause it was copied and pasted.  It was their first post, talking about speakers at an overclcokers site.  Not that we don't talk about that here, but it's very odd to create an account to talk about that.


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#7 Fight Game

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Posted 23 October 2017 - 08:57 AM

And I was right. I searched the text, and found it at another site, almost 4 years ago. Word for word.

https://electronics....ng-either/90213

Edited by Fight Game, 23 October 2017 - 08:59 AM.

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#8 wevsspot

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Posted 26 October 2017 - 12:07 PM

Well.....  I changed the font color on the OP's post and now it's readable.  However, since the post was questionable to begin with, and he/she included a link to an external site, I just gave swift kick to the OP.  


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