Tech: 40W Speaker on a 50W channel?

Subject: RE: Tech: 40W Speaker on a 50W channel?
From: Bernard
Date: 18 Aug 04 - 04:00 PM

Unfortunately, most modern domestic HiFi amps don't seem to have VU meters/LED ladders on them, so you can't get an idea of the actual output drive (okay, let's not have a VU versus PPM debate!). In pro-audio we can judge how hard we are driving a system very accurately, and usually use a compressor-limiter as an extra safeguard.

Dave summed it up - no distortion or overheating = probably okay. Brown smelly smoke from the speakers is not a good sign, though!! That can happen even when the system sounds sweet...

Common sense is the order of the day!

Subject: RE: Tech: 40W Speaker on a 50W channel?
From: Dave Bryant
Date: 18 Aug 04 - 05:08 AM

If it's running OK and sounds fine - stop worrying. Unless you're running the amp on full power you shouldn't be anywhere near damaging either amp or speakers.

There are several ways of measuring amplifier power, both using watts. Peak Power is the power of the maximum transient it can put out (based on the voltage at the top of a sine wave), RMS (Root Mean Sqare) power is based on the average power delivered - 50w Peak is equal to about 25w RMS.

Most amplifier manufacturers tend to be rather over-optimistic about the power output of their product and you might find that your amp doesn't actually produce anyway near it's claimed output.

The other thing is that for most music the maximum power is only produced very ocasionally and even running at full 50w power, the average power is unlikely to exceed 20w - unless you're inputting something like a constant sine-wave test signal.

Running 4-6 ohm speakers on a 8 ohm otput shouldn't cause any problems unless you're using high power levels - if the amp appears to be getting hot, there could be a problem, but usually there's plenty of spare capacity in the power supply and output circuitry to handle this at medium power levels. Also using lower impedance speakers would probably produce a higher output volume, which you would probably compensate for, by turning down the volume control.

IF IT SOUNDS FINE WITH NO DISTORTION AND THE AMP ISN'T OVERHEATING, THEN YOU SHOULDN'T HAVE ANYTHING TO WORRY ABOUT.

Subject: RE: Tech: 40W Speaker on a 50W channel?
From: Richard Bridge
Date: 18 Aug 04 - 03:55 AM

Amps and speakers are rarely as fussy as theorists would like you to believe. If it sounds OK it probably is OK. Try it quietly. If it starts to distort, and if turning down does not immediately cure it, end the experiment.

Alternatively, on the figures you give, put a 2 ohm 10 watt resistor in series with the speaker. It won't go as loud, but that is not your concern.

Bear in mind, if the experiment is a disaster, what do you lose? You have to buy new speakers, which you were going to do anyway.

Subject: RE: Tech: 40W Speaker on a 50W channel?
From: s&r
Date: 18 Aug 04 - 03:02 AM

PDQ - an overdriven amp is more likely to damage speakers I would agree with. An underpowered amp won't do any harm until it is overdriven. Otherwise I agree with you

Stu

Subject: RE: Tech: 40W Speaker on a 50W channel?
From: The Fooles Troupe
Date: 18 Aug 04 - 01:30 AM

But ya Gotta Burn da Gunk OUT occassionally - put it in first and do 100 MPH!

Where can I do that?

Any side street!

:-)
(apologies to Bill Cosby!)

Part of the problem with the original question is that the world of technology keeps changing rapidly. I do find that this place seems to have very knowledgeable people who can explain advanced concepts in fairly simple terms - would that have anything to do with the fact that they are musicians as well? :-)

Robin
"Musos are better at anything!"

Subject: RE: Tech: 40W Speaker on a 50W channel?
From: treewind
Date: 17 Aug 04 - 07:16 PM

Of course you can run a 50W amp into a 4-6 ohm 40W speaker.

"I've got a 160mph Ferrari. Does that mean I can't drive in in a 30 mph zone?"

Anahata

Subject: RE: Tech: 40W Speaker on a 50W channel?
From: pdq
Date: 17 Aug 04 - 07:03 PM

s&r - no, I am correct (and not "partially"). An underpowered amp is more likely to damage speakers than is an overpowered one. Both require the same caveat: keep the bleepin' volume down.

Repeat, a 40W speaker is properly matched to an 80W "valve" amp or a 160W solid state one. This is a rule set by stereo users and is not found in any book I know.

Not all solid state amps want to see a lower load than the "nominal" load at which the amp is rated. A 50W amp is rather small, by modern standards, and will have only one pair of output devices. Higher powered amps, say 150-400W, will have 5 to 10 pair of output devices with a concominant lowering of the output impedance and a more efficient energy transfer into lower impedance loads. Huge amps often deliver the most power (and obviously the most amperes) into loads of less than 1 ohm.

Although the convention is to rate amps into 8 ohms for home stereos, many (if not most) design them for a compromise value of 6 ohns. Many speaker makers also fudge a bit here and make a speakers whose crossover frequencies are correct for 6 ohms also. So in reality you probably have a perfect match at 6 ohms each, but there is no way to prove it!

Answer? What you have is perfectly fine, just do not turn the volume up past the point at which the music sounds clear and clean. End of problem.

Subject: RE: Tech: 40W Speaker on a 50W channel?
From: s&r
Date: 17 Aug 04 - 06:16 PM

Matching suggests that the output impedance of the amp is = to that of the speaker. That ain't so, except in ancient transformer coupled low power record decks and such. The amp output impedance to drive 8 ohm speakers is substantially lower than 8 ohms - more like 0.1 or so. This is a nice article if you are interested

The 4/8 ohm mismatch is largely irrelevant unless the amp is going to be driven near its maximum.

Do it Clinton

Stu

Subject: RE: Tech: 40W Speaker on a 50W channel?
From: Pete_Standing
Date: 17 Aug 04 - 05:48 PM

Of course, even if the impedence of the amp and speakers are matched, the efficiency of the speakers (and the sensitivity of your ears) will determine how the volume is adjusted and hence how much power is flowing through the speakers. It is possible to determine the power flowing through the speakers by using an oscilloscope or a multimeter, but that is overkill, just be sensible!

The premise that if you halve the impedence you will double the power output of the amplifier is not stricly true, otherwise logically the amp could deliver 200 watts into 2 ohms. Amplifiers will have a maximum power output into a specified impedence. With a valve amp, the output transformer may be tapped for different impedences and the power at each impedence is the same. It is imperative that the correct total impedence of the speakers (and dummy loads) matches this. For all but the very oldest of transistor amps, an output transformer is not usually part of the design and therefore the impedence of the speakers (which should be matched to maintain a balanced stereo image) can very within a tolerance which is typically 4 to 16 ohms.

Subject: RE: Tech: 40W Speaker on a 50W channel?
From: JohnInKansas
Date: 17 Aug 04 - 05:33 PM

Actually, Clinton, if you can find the little pads, the answer is YES. On the other hand, if you're going to radio shack they have some pretty good - and pretty cheap 60W 8 ohm "bookshelf" speakers. Just like the ones I got when the kid blew my old ones away.

John

Subject: RE: Tech: 40W Speaker on a 50W channel?
From: JohnInKansas
Date: 17 Aug 04 - 05:31 PM

I thought I changed it, but of course in the vacuum tube amp, fixed voltage into half the resistance is twice the current and TWO times the power - in the ideal case, not four times.

John

Subject: RE: Tech: 40W Speaker on a 50W channel?
From: Clinton Hammond
Date: 17 Aug 04 - 05:29 PM

Wow...

so... ya... the short answer I guess, in this situation is... no....

Thanks John!

LOL

Subject: RE: Tech: 40W Speaker on a 50W channel?
From: JohnInKansas
Date: 17 Aug 04 - 05:24 PM

Well, Clinton sort of answered himself while I was compos(t)ing; but my babblings may be useful anyhow. The 4 ohm speakers can be used if you can find the right "cheap accessory" (see following).

For a normal "home system," an actual "acoustic power" of 7 to 10 watts will be as much as you want. Anything more than that in small enclosed spaces risks damage to your hearing. Really cheap speakers can be quite efficient, so quite a bit of cheap commercial/consumer stuff actually has 7.5 or 10 watt speakers built in. As you progress into "better" quality speakers, the speakers become less efficient, so you need more "electrical watts" to get that "10 acoustic watts."

Running an amp that has significantly higher power rating than the speakers is quite common, works very well in normal circumstances, and shouldn't cause you any problems. I ran a 40 W/channel amp into a pair of 7.5 W speakers salvaged from an old phonograph for several years - until my son, having bought a new bass guitar and NOT buying an amp, snuck into my place to try it out. Full gain - one WHANG - cones shattered, wires melted. (No damage to the amp.) "But dad, I never turned it up above 3!" ... "%$@#!!"

The 40 W speakers should work fine with a 50 W amp, but the 4 ohm vs 8 ohm mismatch is a little more troublesome.

Most equipment capable of 50W per channel output would normally allow you to select 4 or 8 ohm speaker outputs. There's no guarantee that yours does, but you need to look and be sure. Sometimes this feature appears as a different set of screw terminals for the alternate speaker impedance, and sometimes as a switch that lets you make the appropriate connection to a single set of jacks. (Clinton answered this?))

If your amp does not allow you to select for 4 ohm speakers, the simple expedient is to put two of the 4 ohm speakers in series on each channel. You MUST make sure that the speakers are "in-phase" if you do this, but that's a trivial thing.

Assuming that you don't want 4 speakers instead of 2, you really should check the local Radio Shack and see if you can find an "L-Pad" to match the speaker impedance to the amp output impedance. Essentially, this is just a pair of (sometimes 3) "resistors" that looks like 8 ohms on the amp side and 4 ohms on the speaker side, all nicely packaged for you. The pad should have a power rating at least as high as the speakers, and you may want to match the amp power rating here if it's easy. The last ones I bought were when vacuum tubes were the rage, but they weren't more than a couple of bucks then. They may be a "catalogue" or special order item now, as they're not as much in demand.

Note that the "couple of resistors" does oversimplify things. The amp anticipates an "inductive load" and just using resistors from your junk box seldome works very well. The commercial pads will give much better results.

If your amp happens to have a vacuum tube output stage, the vacuum tube amplifier is a "voltage gain" device, and produces the voltage needed to produce the desired power in the intended (8 ohm) load device. If you put a 4 ohm load on it, you'll get up to twice the current - or 4 times the power into the speaker. At low power, relative to component ratings, this shouldn't cause real damage, but is "stressful" on components both in the amp and on the speaker. It can also make volume control "touchy."

With the more common transistor output, the transistor amplifier stage is a "current gain" device, and will try to deliver the current needed to produce the desired power in the expected (8 ohm) load. If your load is only 4 ohms, that current will only produce half the expected voltage, and you'll get half the intended power into the speaker. This will force you to set your volume control up substantially to get desired audio volumes - which again stresses components in ways not intended in the design.

Neither of the two kinds of amplifiers is a "pure" current/voltage gain device, so how the actual mismatch effects come out is just a "general tendency" description here. Any vacuum tube output will have a transformer that acts as a significant "ballast" and can change things significantly. A transistor output stage must have a few resistive output stage components to protect the circuit in the event you turn the amp on without a speaker connected, and these may significantly affect the actual "deviant behaviour" you'll see. Either of these design "features" should tend to compensate for minor mismatch between amp and speaker resistances, so you might get by with 4 ohm speakers on 8 ohm outputs at low power levels - but a real fix is strongly recommended. You're not likely to be very satisfied with the sound you get, even if it doesn't damage anything.

John

Subject: RE: Tech: 40W Speaker on a 50W channel?
From: Clinton Hammond
Date: 17 Aug 04 - 04:51 PM

Well, seeing as how (Now that I've looked) the book for the stereo says not to (DO NOT!) use speakers under 8 ohms, and these are 4, I guess it's moot!

But thanks fer the assistance getting there folks!

Subject: RE: Tech: 40W Speaker on a 50W channel?
From: s&r
Date: 17 Aug 04 - 04:45 PM

The impedance of the speakers varies with frequency. Usually with reasonable quality speakers/amps etc there is some tolerance. The connection you describe is unlikely to cause problems.

These are the possibilities:
Excess power into the speakers long term could cause overheating and damage to the voice coil - like heavy rock on max for a good long time.
Excess power into the speakers short term is likely to make them rattle a bit
PDQ is partially right - if a low powered amp is turned up too high in an attempt to get enough volume, clipping will occur - this generates harmonics which increase the high frequency power through the speakers, often destroying tweeters (paradoxically bass overload causes most high frequency harmonics in a typical music programme). This is the opposite situation to yours however; I would agree with the practical solution of using amplifiers of a high power rating.
Doubling or halving the speaker impedance doesn't double or halve the power - the amplifier has current and voltage limitations (internal impedance). Sometimes a 'perfect amp is postulated whose voltage/current is not affected by load.
Often the term matching is used to describe the 'correct' load for aa amplifier - optimum is better.
It all depends as well on what ratings are used - if amp and speakers use RMS wattage ratings, you're OK - if the amp is RMS and the speaker is PMPO or someting similar, forget it

Stu

Subject: RE: Tech: 40W Speaker on a 50W channel?
From: Clinton Hammond
Date: 17 Aug 04 - 04:10 PM

"It would need someone who understands electornics"

Zactly why I asked here!

:-)

Subject: RE: Tech: 40W Speaker on a 50W channel?
From: GUEST,Jon
Date: 17 Aug 04 - 03:47 PM

I wouldn't. It would need someone who understands electornics, but at 4ohms, I think you would be doubling the current flowing through the amp.

Subject: RE: Tech: 40W Speaker on a 50W channel?
From: Clinton Hammond
Date: 17 Aug 04 - 03:37 PM

So, it turns out these are 4-6ohm speakers...   Can I hook 'em up to an 8 ohm channel? Especially if they never get very loud?

Subject: RE: Tech: 40W Speaker on a 50W channel?
From: pdq
Date: 17 Aug 04 - 01:49 PM

Here is a paradox concerning amplifiers. More speakers, both professional sound and home stereo, are damaged by having too little power than are damaged by having too much power! See 11:59 post about clipping. A good rule of thumb is that tube power amps should be capable of 2X the speaker's rated power, solid state amp 4X. That means, If I were asked to set up a sysem based around 40W speakers, I would get a power amp of (at least) 160W. You have an amp with too little power, not too much.

Where the volume knob is set is a function of "voltage gain", usually just referred to as "gain", and not a direct indicator of how much power is delivered to the speaker.

Subject: RE: Tech: 40W Speaker on a 50W channel?
From: Clinton Hammond
Date: 17 Aug 04 - 12:44 PM

The amp is 50W at 8 ohms...

I've contcted the store to find out how many ohms the speakers are...

Ta fer the heads up eh folks!

Ya mighta just saved me 50 bucks!

:-)

But lets assume the impedance matches... My original question? The 40Wers will likley be o.k. eh? The volume knob on the stereo rarely if EVER goes past quater-to... Between 20 and 30 outa 100.....

Subject: RE: Tech: 40W Speaker on a 50W channel?
From: Bernard
Date: 17 Aug 04 - 12:30 PM

Don't forget that an amp that delivers 50 watts into 8 ohms will deliver at least 100 watts into 4 ohms... if your speaker is 40 watts at 8 ohms and your amp is 50 watts at 4 ohms, then you some 'headroom' left... the amp will only be delivering around 25 watts.

Ohms law, you see... although it doesn't apply rigidly to amplifier output (there are other considerations such as Total Harmonic Distortion, etc).

Subject: RE: Tech: 40W Speaker on a 50W channel?
From: pdq
Date: 17 Aug 04 - 11:59 AM

If the system sounds "clean" at the volume you like, all is well. If the sound becomes even slighty "harsh" or "dirty", back off on the volume. The objectionable sound is caused by "clipping" and means that the amp has exceeded its maximum output. Clipping causes the production of huge amounts of "harmonics", usually odd-order, which will certainly cause speaker damage if allowed to continue for long period of time.

Subject: RE: Tech: 40W Speaker on a 50W channel?
From: Leadfingers
Date: 17 Aug 04 - 11:44 AM

I have 50 watts out put on my home Hi Fi and run at around 5 or 6 if you think in terms of 10 being max . I can't visualise a situation wher you would WANT a home system to be so loud that a Forty watt speaker would be harmed , unless you were going to have a teenage party and really wanted to annoy the neighbours . The Impedance is more likely to cause a problem with the final stage of your amplifier , but it seems , at least over this side of the pond , that most home system speakers are 8 ohm .

Subject: RE: Tech: 40W Speaker on a 50W channel?
From: Clinton Hammond
Date: 17 Aug 04 - 11:31 AM

Hummm... good point... I'll check that as well...

Subject: RE: Tech: 40W Speaker on a 50W channel?
From: GUEST,Jon
Date: 17 Aug 04 - 11:22 AM

I wouldn't have thought there was a problem as long as you are careful.

One thing to watch though is the impedance of the speakers and amp. If your amp required a min of 8 ohm, it might not take too kindly to being connected to 4 ohm speakers.

Subject: RE: Tech: 40W Speaker on a 50W channel?
From: Clinton Hammond
Date: 17 Aug 04 - 11:10 AM

Sorry... it's for a home stereo that mostly just deals with TV/DVDs these days...   Not a 'gig' sound system...   so pluggin' in isn't really an issue...

I realised as I was reading yer response, I shoulda made that clear!

:-)

Subject: RE: Tech: 40W Speaker on a 50W channel?
From: Pete_Standing
Date: 17 Aug 04 - 11:06 AM

Yes - but a transient like soemthing being plugged in could tip it over the edge.

Subject: Tech: 40W Speaker on a 50W channel?
From: Clinton Hammond
Date: 17 Aug 04 - 10:45 AM

If it's never turned up very loud it'll most likely be fine yes?