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Your gonna kill me for raising this again... Spk-watts.


Josh 2

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Hi all,

 

I'm absolutely certain that this has been covered many times, in fact I've been searching through to forum and I have found answers, but I am either being really stupid or the info just doesn't collate. Could you folks kindly help.

 

I do realise that the output of a speaker is not effectively measured by watts and that SPL is the best efficiency guide. I also understand that PA systems should have the speakers rated at approx half of the amps watt output, so I think I am going someway down the right track. However, something somewhere just isn't falling into place with me.

 

I want buy some speakers to match a Samson amp. The amps manual says 2x500W at 4Ohm @ 0.1% THD at 1KHz and 1x1000W at 8Ohm Bridged. Okay, I understand the bridged concept of a joined/single channel output so that isn't the problem.

 

Where I am getting stumped is the speaker adverts/specs 'sometimes' quote just watts, sometimes RMS, or sometimes RMS and Peak watts. Again, so far so good, but I go off and read about RMS and PMPO, come across many calculations that I don't understand and then discover that RMS and PMPO are both arbitrary and fairly useless measurements. However, seeing as my amp manual isn't specifying whether it's output is being quoted as RMS or Peak… what do I look for/consider in speakers?. Or are amps always quoted by convention as RMS or Peak and I just haven't realised!

 

Sorry to go on, but could someone please tell me, in plain terms, what spec speakers I should be considering for 2 channel (not bridged) operation with this amp, both in a single speaker per channel and a two (full & sub) speaker per channel configuration.

 

I'd really appreciate any/all advice.

 

Thanks and seasons greetings,

Josh.

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Match the amp to the speakers you decide to buy, not the other way round. You'll end up going round in circles otherwise.

 

Amp specs vary massively from manufacturer to manufacturer. You will also find many that quote the output power, as the power at 1kHz. This isn't a true test of the amp as bass frequencies require significantly more power from the PSU to attain the same output as at 1kHz.

 

With amps, it really is a case that you get what you pay for. Two amps that both claim 1250W per channel at 4R will probably have very different performance depending on the manufacturer. I'm sure others will have their own opinions but MC2, Lab Gruppen, Crown, Crest, QSC generally have some of the best on the market, but again, within their own ranges their is variation. MC2 MC series are some of the best sounding amps around for mid-high, but the E series will blow them away when being used to drive bass. Likewise with crown - the XTi Series are a very nice budget amplifier, but the bottom end performance is woeful when compared to the iTech

 

Since you already have the amp in this case, going by its rating alone you'll want some speakers that are rated at around 250W-300W at 4R (that's 125W-150W at 8R).

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The amps manual says 2x500W at 4Ohm @ 0.1% THD at 1KHz and 1x1000W at 8Ohm Bridged.

 

You're safe to assume that it will output roughly 125W per channel in to eight ohms. In practice, it'll probably be a little less than this.

You'd normally, as you've stated, allow an amp to be able to output 150 - 200% of the speaker's rated RMS power handling, so you'd be looking at a speaker power handling of 50 - 75W (8 ohms) or 100 - 150W (4 ohms). As Steve said though, it's much better to pick the speakers that will do the job for you and then select an appropriate amp.

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Thanks Steve,

 

I have to admit, I hadn't considered and didn't realise what it meant when the spec said @ 0.1% THD at 1KHz (I'm off to read up on what 'THD' means!), but what you say makes sense in that lower frequencies take more power to drive. Gee, why does everything with amp/speaker ratings sound more and more like manufacturers snow jobs to me!!!

 

However, this does fit in with my original questions. Seeing as the amp specs do not state RMS or Peak for it's output rating (should it, shouldn't it… am I missing something here!!??) , should I be looking at speakers rated 250-300 watt 'RMS' or 250-300 watt 'Peak'?.

 

Also if I used two speakers (say full and sub) per 500 watt channel, should I split the available power equally, 125-150 x 2 or should the subs take a higher proportion, say 200 for the sub, 100 for full range… or even the other way around with less for the subs?

 

Thanks,

Josh.

 

EDIT after seeing Shez's reply.

 

Hi Shez,

 

Thanks to you too,

 

I gotta admit, I'm now more confused (my fault) Steve is talking 250-300 watt at 4 Ohm, have I got you correct at saying 100-150 watt at 4 ohm?

 

My sincere apologies for being so thick with this.

 

Josh.

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I gotta admit, I'm now more confused (my fault) Steve is talking 250-300 watt at 4 Ohm, have I got you correct at saying 100-150 watt at 4 ohm?

 

Steve has suggested 250 - 300W speakers to be driven by a 250W / channel amp. Accepted wisdom is that that amp should be rated at between one & a half and twice the RMS power handling of the speaker. So I'm disagreeing with Steve on this one I'm afraid.

 

Also if I used two speakers (say full and sub) per 500 watt channel, should I split the available power equally, 125-150 x 2 or should the subs take a higher proportion, say 200 for the sub, 100 for full range… or even the other way around with less for the subs?

 

Not quite that simple I'm afraid... Efficiency comes in to it again - if you used a very efficient sub & a not very efficient top, you'd end up bass heavy. Vice versa (but same power handling) and you'd be bass light. I've only ever used cabs from the same manufacturer designed to work together like this before; I suspect finding cabs from differing manufacturers with the right power handling and sensitivity(efficiency) might be tricky.

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I gotta admit, I'm now more confused (my fault) Steve is talking 250-300 watt at 4 Ohm, have I got you correct at saying 100-150 watt at 4 ohm?

 

Steve has suggested 250 - 300W speakers to be driven by a 250W / channel amp. Accepted wisdom is that that amp should be rated at between one & a half and twice the RMS power handling of the speaker. So I'm disagreeing with Steve on this one I'm afraid.

 

I'm really sorry Shez, I'm lost.

 

Steve 'seems' (please correct me if I'm wrong) to be quoting correctly for a 500 watt output and is following the convention at 250-300 watt speakers. I can't quiet see where he's talking 250 watt/channel. What am I missing here!!!

 

Josh.

 

Edit to add:-

 

Just to make my life easy, as I don't think I'm explaining too well... so as to match speakers (watt wise), should I assume the amp manual is quoting RMS or Peak (it doesn't say)?

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I'm really sorry Shez, I'm lost.

 

:blink:

 

Eek - my bad - Sorry for the confusion. For some reason I'd read the amp spec as being 500W total (both channels) not per channel. Sorry Steve - you're right; it's still too early in the morning for me...

 

So. Amp is 500W / channel @ 4 ohms (or a bit less as that rating is at 1kHz only). Suitable speakers are 250 - 300W.

Assume the amp can deliver ~250W in to 8 ohms, so speaker would be 125 - 150W.

 

I am indeed a muppet.

 

Re: amp ratings, I tend to assume RMS. Amps have a definite clip point, whereas speakers can handle short high transients which give the manufacturers more leeway to exaggerate their claims.

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:blink:

 

Eek - my bad - Sorry for the confusion.

 

No problem Shez... why shouldn't something else confuse me (joking) ;)

 

But thanks very much for the clarifcation... I've got it now and am a happy bunny.

 

Best wishes to all.

Josh.

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PMPO tends to be used by PC speaker manufacturers when trying to persuade you that the wall wart can power 500W speakers. ;)

 

:** laughs out loud **:

 

Hmmm yes, a wall wart with a 10" toroidal and regulator inside! :blink:

 

Which as an aside and a bit OT (not important enough for a new topic), raises a question that I have often pondered… In my youth, I used to DJ (old fashioned form) and my kit was a couple of RS valve amps outputting into 100v lines. These amps were only rated at something like 100w, but because of the 100v output I was told, and did, hang as many cabs on it as I wanted, 'seemingly' without noticeable power loss. Is the 100v line system still in use and/or does it have drawbacks so as to make it un-popular… seemed like a good idea to me at the time.

 

I'm also wondering/confused… I've been using of late the above mentioned 2x500w amp and I'll admit it… wired to 2x 600w EV cabinets (I know it's wrong, hence my original questions for new speakers). The thing is the last event our band did was +/-500 people and outdoors, which from reading up, is grossly underpowered amp/spl wise. However, we got away with it, good sound and plenty of spare headroom on the amps output levels… How come???... shouldn't we have been struggling to get a reasonable spl with this gear in this situation!!!

 

Appreciated you guy's are usually talking in mega watts and big gigs and I'm only tinkering at the lowest level, but I am interested.

 

Josh.

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Valve amp was probably closer to 10W ,Root Mean Square , RMS output , regretably the supply of ex cinema valve amps seems to have dried up...

 

100V line has advantage of impedance matching, if you connect 2 8 Ohm speakers in parallel you get a 4 Ohm load another in parallel would give a transitor amplifier too low an impedance to drive. Valve amplifiers have a high impedance output and require impedance matching transformers anyway.

 

With 100V line speakers impedance is in on kilohms,000s, and each speaker has an impedance matching transfomer in it which converts high impedance 100V line to low impedance to feed the speaker.

 

Couple of advantages you can parallel as many speakers as you want and the high impedance load means voltage driven which means long distances are less of a problem, low impedance equals current driven where cable resistance becomes a factor.

So 100V line survives for things like paging and voice evac systems but usually transistor driven.

 

Disadvantage is really fidelity, big chunk of iron driving the speaker which effectively rolls off the bass unless they`re very well made big trafos.

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Hi musht,

 

Thanks for clearing that up... so these were common in cinemas!?

 

I might be wrong, but I think it was 100w, my backup was the same amp in a 70w version. T'was RAF blue in colour, huge knurled rotaries and about the size of a 19" 4U rack unit.

 

I actually made the cabs ¾" chip., for it, 2 x 12" Celestions in each (no porting… I didn't know any better), stuffed full with wadding. I used to travel from the east end of London to Tower Bridge with all this weight in an old mini van, the suspension bottomed out and rock solid… just to do the river boat gigs. Gotta say, to my ears it sounded great and I seemed to have bass enough.

 

Wish I still had that gear, if only for nostalgia.

 

Josh.

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Hi musht,

 

Thanks for clearing that up... so these were common in cinemas!?

 

Josh.

 

Valve amps were common in cinemas and as the buidnigs were getting gutted for bingo halls or supermarkets there used to be quite a few about black age for cinemas, golden age in interesting surplus, then the HI-Fi nuts got wind.....

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