Need help with an ebay purchase!! and a AMP - AVS Forum
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post #1 of 10 Old 09-11-2012, 09:46 AM - Thread Starter
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Hi everyone i'm new to this so please help

As part of my collage course work one of the tasks is to create and buld your own speaker system...

I wanted to go my own way as everyone else is making cheap, tacky speakers supplied by the collage its self. Listens to them and they are sh*t.mad.gif Obviously i'm on budget and these are my choices:

http://www.ebay.co.uk/itm/230818597727?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649#ht_3599wt_1041

Willy, the man who sells them, said he has had excellent feedback form these but i need a AMP to work with these under 100W. Can anyone help me out with a AMP????

Hope anyone can help me as it would be really appreciated.!! smile.gif

Tom

if you want you can always email me if you want a little more info? tdheywood@yahoo.co.uk







http://www.ebay.co.uk/itm/230818597727?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649#ht_3599wt_1041
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post #2 of 10 Old 09-11-2012, 11:13 AM
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Because speakers are rated at 100W, does not mean you need to use an amplifier <100W. Speakers do not behave like a fuse whereby when you exceed rating *poof* they are destroyed.

Local ebay will turn up hundreds of integrated amplifiers or older AVRs at a suitable price.
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post #3 of 10 Old 09-11-2012, 11:17 AM - Thread Starter
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Oh so i can use a AVR to power the speakers becuase after all they will next to my TV's AVR?

Thanks for your help.. Im not clued up on this
Tom
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post #4 of 10 Old 09-11-2012, 12:18 PM
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I didn't know collages offered courses? wink.gif work on those phonetics playa.

Could you maybe swing something similar to this and start over?

http://www.diysoundgroup.com/speaker-kits/small-kits/os-speaker-kit.html

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post #5 of 10 Old 09-11-2012, 03:21 PM - Thread Starter
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Haha :-) that's hat I'm looking for but not the housing as it needs to be build myself from scratch :-( can you give me an idea of what type of AMP I need for the speakers I was going to buy?

Hope you can help me as I'm a little confused...

T
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post #6 of 10 Old 09-11-2012, 07:15 PM
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So I'm grading your course work and I see you bought a speaker kit, with a pre-assembled crossover. Right away even if it looks and sounds good the best grade you get from me is a C+, If you want an A you need to do some reading on crossovers and build your own from resisters, inductors (coils) and capacitors.
Show a worksheet where you worked out the formulas for the values of the resistors. Impress me and show the resource you used to calculate the size of wire and the number of windings that went into the coil.
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post #7 of 10 Old 09-11-2012, 10:38 PM - Thread Starter
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The speaker quality doesn't matter.... It's the casing that does as it needs to be build with strong joints such as dove tail... I'm not botherd for any A* sounding speakers providing it works.

(my marker is old)
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post #8 of 10 Old 09-16-2012, 03:54 PM
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Quote:
Originally Posted by A9X-308 View Post

Because speakers are rated at 100W, does not mean you need to use an amplifier <100W. Speakers do not behave like a fuse whereby when you exceed rating *poof* they are destroyed.
Local ebay will turn up hundreds of integrated amplifiers or older AVRs at a suitable price.

I NEVER buy under powered amps to drive speakers with as they often run out of headroom(especially the sub bass and bass frequencies eats up amp headroom FAST) and CLIPPING fries the speakers.

An amp with slightly more wattage the the rms of the speaker is what to go for. That way the amp has extra headroom before clipping. And as music is NOT a continuous sine wave or continuous tone, it's dips and peaks in wattage, it won't blow the speaker.
Clipping however can put more than twice the power or more into the speakers than they can handle(which overheats/burns out the voice coils of the speakers), and is also a very distorted signal, not a clean signal.

For pa speakers and the higher quality expensive car subwoofers(not the budget subs) I use an amp 1.2-1.5x the rms of the sub or speaker.

For hifi speakers,studio or home cinema speakers I use an amp 1.2x the rms of the speaker or sub, no more. As pa speakers and high quality expensive car subs have much higher temperature bigger voice coils and vents around the magnet able to handle more accidental misuse/abuse than normal hifi speakers.
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post #9 of 10 Old 09-16-2012, 05:33 PM
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Quote:
Originally Posted by BIGmouthinDC View Post

So I'm grading your course work and I see you bought a speaker kit, with a pre-assembled crossover. Right away even if it looks and sounds good the best grade you get from me is a C+, If you want an A you need to do some reading on crossovers and build your own from resisters, inductors (coils) and capacitors.
Show a worksheet where you worked out the formulas for the values of the resistors. Impress me and show the resource you used to calculate the size of wire and the number of windings that went into the coil.

How do you even know what the objective of this entire thing is? If everyone else is using stuff provided by the school, I would say he is ahead of the group by seeking out something different. I don't think he provided enough information for you to say one way or the other what he "should" be doing to impress you.

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post #10 of 10 Old 09-18-2012, 09:45 PM
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Forgot to post this earlier.

There are two main failure modes with speakers; over excursion and overheating.

Damage by overexcursion need not even need to be caused by a signal that even meets the rated power of the driver or an amp. It can be a short transient so that the speaker exceeds Xmech, or by a large amplitude signal below tuning of a ported enclosure and no filtering - I saw several damaged from poor phono cartridge/arm matching over 30 years ago.

Overheating is caused by the average energy supplied to the coil and how quickly it can dissipate it. The coil does not care what signal inputs the energy, merely that it does. The coil will heat very quickly with applied energy, but will dissipate it more slowly. Most coil formers and/or cones are not metallic so do not aid in heat sinking, so the heat is left to escape by convection across an air gap from the coil to the magnet structure and thereby to the surrounding air, usually inside an enclosure. Failure here takes place often by delamination of the coil as the glue softens, the former failing and either of these causing rubbing on the inside of the magnet gap, causing further damage or jamming the cone. Sometimes the coil will go O/C but I have seen this far more often in coils that don't show signs of major overheating.
Hence my comments that a speaker does not act like a fuse - because it doesn't especially with some mild over powering or clipping.
Quote:
Originally Posted by ItchyTasty42 View Post

I NEVER buy under powered amps to drive speakers with as they often run out of headroom(especially the sub bass and bass frequencies eats up amp headroom FAST)
That statement is only contingent on using amps to drive speakers wideband. It certainly does not apply to complete active systems, like my home of PA. Especially WRT sub bass (as this is an HT discussion), apply as much to systems with actively crossed subs which would be the case for almost any HT.
Quote:
Originally Posted by ItchyTasty42 View Post

and CLIPPING fries the speakers.
To imply this is always or even mainly the case is clearly hyperbolic.
Quote:
Originally Posted by ItchyTasty42 View Post

And as music is NOT a continuous sine wave or continuous tone, it's dips and peaks in wattage, it won't blow the speaker.
And this is precisely the point.

With most HT source material having a peak to average ratio of 20dB or more, if you adjusted your system so that a 100W amplifier was just below clipping, the amount of energy applied to a voice coil would be only a couple of watts at most. If I used a 1kW (and caused no over excursion etc) and adjusted it the same, there would still only be 20-50W of equivalent heating in the coils.

waveform_zpsfbfb8c26.gif

Note, so there is no misunderstanding, that I'm talking about clipping of the amplifier not about clipping/compression of program material which because of the very low peak to average ratio offers such a dense with some combinations of amplifier and speaker, it's conceivable you could damage the speaker whilst never clipping the amplifier.
Quote:
Originally Posted by ItchyTasty42 View Post

Clipping however can put more than twice the power or more into the speakers than they can handle(which overheats/burns out the voice coils of the speakers),
The only way that you can get twice the rated power of an amp into a speaker is by so massively overdriving a sine wave so that it becomes as near as dammit to a square wave. This is hardly a common occurrence in actual use, let alone testing. Unless of course you intend to do it (in which case why not simply use a square wave?) or the tester is an idiot.
Quote:
Originally Posted by ItchyTasty42 View Post

and is also a very distorted signal, not a clean signal.
Well, duh. It's now a(n almost) square wave with a decreasing amplitude (predominantly) odd order harmonics.
Quote:
Originally Posted by ItchyTasty42 View Post

An amp with slightly more wattage the the rms of the speaker is what to go for. That way the amp has extra headroom before clipping.
Quote:
Originally Posted by ItchyTasty42 View Post

For pa speakers and the higher quality expensive car subwoofers(not the budget subs) I use an amp 1.2-1.5x the rms of the sub or speaker.
For hifi speakers,studio or home cinema speakers I use an amp 1.2x the rms of the speaker or sub, no more. As pa speakers and high quality expensive car subs have much higher temperature bigger voice coils and vents around the magnet able to handle more accidental misuse/abuse than normal hifi speakers.
You are free to use whatever rule of thumb you like, but but 1.2-1.5x is pretty irrelevant if the purpose is to stop any clipping. It will realise only 0.8 to 1.8dB of difference, trivial really.
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