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Passive Speaker question

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Passive Speaker question

Postby Soculese » Sat Jul 20, 2019 2:17 am

This is not my thing so I am posting for advice. I was just given an amp to use to build a small PA. It has an output of:

Stereo:
225 Watts at 8 Ohms
350 Watts at 4 Ohms

Bridged
800 Watts at 8 Ohms.


What wattage speakers can I hook up to this and not damage any of the equipment?
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Re: Passive Speaker question

Postby Martin Walker » Sat Jul 20, 2019 8:55 am

Hi Soculese, and welcome to the SOS Forums! 8-)

I've just approved your first post, so hopefully you'll now get lots of helpful replies :thumbup:


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Re: Passive Speaker question

Postby Sam Spoons » Sat Jul 20, 2019 9:11 am

Soculese wrote:This is not my thing so I am posting for advice. I was just given an amp to use to build a small PA. It has an output of:

Stereo:
225 Watts at 8 Ohms
350 Watts at 4 Ohms

Bridged
800 Watts at 8 Ohms.


What wattage speakers can I hook up to this and not damage any of the equipment?

Standard advice is to rate the amp 1.5-2.0 x the rating of the speakers to allow peaks to be reproduced without distortion so 4 ohm speakers around 200-250 watts continuous or 8 ohm ones of 125 or so watts would be about right. Avoiding damage to the HF drivers is, though, a matter of using your ears and a little common sense (LF drivers are usually more robust). Avoid audible distortion and you should be ok, distortion saturates the HF and kills tweeters.

By modern standards that's quite low powered (my pub band FOH rig is almost 3000 watts all told) so let us know what your planned use is and we can probably give some guidance WRT speaker sizes (15" + Horn for DJ, 10" + Horn for vocals for example).
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Re: Passive Speaker question

Postby ef37a » Sat Jul 20, 2019 9:22 am

Please give the make and model of the amplifier. Many "budget" amps have "optimistic" power specifications but if we take your figures as true....

The speaker need s a power rating of 300W minimum if of 8 Ohms impedance and around 500W if 4 Ohms but even here there are traps!

Many speakers are given a "programme" rating and a "peak" rating and that is fair to some degree but to be dead safe go by the lower programme rating. The actual power handling capacity of a speaker is very hard to define with any precision and depends greatly on the programme content. The better quality manufacturers tend to be conservative.

You might be told that the speaker can have a LOWER power rating than the amplifier's capacity becuase you want to avoid PA clipping. I have never agreed with this position. VERY hard to hear speaker distress (till it is too late!)

Sound reproduction starts with a consideration of the Sound Pressure Level you want/need to produce at the ears of the audience. The speakers are therefore selected to be capable of producing that level, with a few dB of headroom. Now you can select an amplifier based on the sensitivity of the speakers.

That ^ of course is an ideal scenario. You have an amp so find some speakers that will not burn out at max welly. With that sort of power in hand a speaker of 95dB/W/mtr sensitivity (about average?) will produce around 120dB SPL at one metre for a 200W input. That is very loud of course but 20mtrs away will have fallen off quite a bit.

I am sure the top sound guys here will tell you that such a system is really little more than a "starter kit"?

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Re: Passive Speaker question

Postby Wonks » Sat Jul 20, 2019 9:38 am

ef37a wrote:You might be told that the speaker can have a LOWER power rating than the amplifier's capacity becuase you want to avoid PA clipping. I have never agreed with this position. VERY hard to hear speaker distress (till it is too late!)

That's not the reason for that selection at all and never has been. It's to maximise the output from the speaker given the typical live sound sources played back through the speaker, so if you've got a 600W rated speaker, then you use an amplifier powerful enough to play back a signal with an average of up to 600W, so normally 900W-1200W. It's amplifier clipping you primarily want to avoid because of the extra HF energy created, which the tweeter will struggle to cope with.

But you do have to know what you are doing as heavily compressed sounds with low dynamic range, or loud synth square waves, will take you above the acceptable average power output limit at lower mixing desk gains than standard vocals. guitar bass, drums will. Peak meters won't tell you this info.
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Re: Passive Speaker question

Postby Wonks » Sat Jul 20, 2019 9:48 am

As Sam said, that's a relatively low powered amp by today's standards, and I'd be tempted to use it for powering some passive monitors, and getting some more powerful active speakers for the main FOH speakers.

If you are just doing vocal+acoustic guitar in coffee shops and don't need much volume, then it will be OK, but for anything more than that, you really need to think about using a more powerful system.

Designing a PA around a piece of kit you've been given isn't really the best way to go about things.

But tell us what you'll expect to be putting through it, the style (s) of music and the size of venues you expect to use it for, and we can give you a guide as to what might be suitable kit.

You can put together a very good PA system for not much money these days. You can't do it for nothing, but certainly for a lot lot less than you would have paid 20 years ago.
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Re: Passive Speaker question

Postby ef37a » Sat Jul 20, 2019 9:57 am

Wonks wrote:
ef37a wrote:You might be told that the speaker can have a LOWER power rating than the amplifier's capacity becuase you want to avoid PA clipping. I have never agreed with this position. VERY hard to hear speaker distress (till it is too late!)

That's not the reason for that selection at all and never has been. It's to maximise the output from the speaker given the typical live sound sources played back through the speaker, so if you've got a 600W rated speaker, then you use an amplifier powerful enough to play back a signal with an average of up to 600W, so normally 900W-1200W. It's amplifier clipping you primarily want to avoid because of the extra HF energy created, which the tweeter will struggle to cope with.

But you do have to know what you are doing as heavily compressed sounds with low dynamic range, or loud synth square waves, will take you above the acceptable average power output limit at lower mixing desk gains than standard vocals. guitar bass, drums will. Peak meters won't tell you this info.

Thank you Wonks (knew I should be in trouble!) I am certainly not going to bandy words with my youngers and betters.

My stance is that "amplifiers will ALWAYS clip," sometime and so the HF unit should be capable of handling that, within reason.

I think the concept of amplifier ratings greater than speakers came from the hi fi era? Sat at home a pair of nominally 50W speakers would be quite safe* on a 100W Quad becuse you are listening critically to the programme and can detect even momentory escursions into clipping. If you are a long way from the speakers or maybe not hearing them at all, clipping will not be noticed.

Certainly you want a guitar amp speaker rated at least 50% over a valve amps output.

*There is however the "day after the day after New Years Day" effect? That is when the punters brought their gear in for repair. All year fine but during the booze fueled revelries, wick was steadily advanced and the distress of amps and sometimes speakers not noticed.

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Re: Passive Speaker question

Postby Sam Spoons » Sat Jul 20, 2019 10:09 am

Of course the best answer for 'non-techie' PA operators is to avoid passive speakers entirely and run powered boxes which are usually capable of looking after themselves ;)
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Re: Passive Speaker question

Postby Wonks » Sat Jul 20, 2019 11:29 am

I certainly wouldn't want to rely on my ears these days to detect HF clipping. Active PA speakers are certainly the way to go for me. Also having a system powerful enough that you never have to turn it all the way up to get the volume you need. Yes, it puts the cost up, but it's well worth it IMO, for ease of mind and the benefits of a system working well within its limits on the sound quality.
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Re: Passive Speaker question

Postby ef37a » Sat Jul 20, 2019 12:02 pm

It is of course the mid frequencies that cause the trouble when they clip the amplifier and generate HF spikes.

This effect was brought home to me very forcibly many, many years ago in a small VO studio.

A pair of Kef Chorals were powered from a 303 (I think) and all was fine, even at quite high levels but if the Ferry S7 was poked into play mode from fast wind, once in a while the screech would pop a tweeter!

Some time was spent gradually increasing fuse ratings until one was found to protect said tweeter. Did not need to be high? T215mA comes to mind.

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Re: Passive Speaker question

Postby Sam Spoons » Sat Jul 20, 2019 2:05 pm

I have Kef 104ABs in the studio now and pop tweeter fuses every so often (usually with a short burst of microphone feedback when setting up for a rehearsal or session). My tweeters are uprated from the old T27s and the fuses are 100 mA. Haven't chanced anything bigger but nor have I blown any tweeters.
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