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Recommendations for powering Klipsch KM2 speakers
random_asdf
post Dec 25 2011, 09:04
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Hi all,

I'm looking to move my speakers to my desktop, but my amplifier is current a rather large receiver/amp.

Does anyone have any advice on something smaller? I've been looking at t-amps like the sonic impact and some chinese brands but I'm somewhat skeptical of the build quality of the latter.

Not looking for anything expensive, just something to power my speakers that's in a small package to fit on my desk. Below are the specs of the speakers:


SPECIFICATIONS
BUILT FROM: 1994
BUILT UNTIL: 1997
DIMENSIONS: 13" (33cm) x 9.25" (23.5cm) x 9.25" (23.5cm)
ENCLOSURE MATERIAL: Medium density fiberboard construction (MDF)
ENCLOSURE TYPE: Bass reflex via rear-mounted port
FINISHES: Black vinyl
FREQUENCY RESPONSE: 50Hz-20kHz(+-)3dB
HIGH FREQUENCY HORN: Exponential Cone
NOMINAL IMPEDANCE: 8 ohms
POWER HANDLING: 50 watts maximum continuous (200 watts peak)
SENSITIVITY: 90dB @ 1watt/1meter
TWEETER: K-89-K 1" (2.54cm) Polymer dome compression driver
WEIGHT: 15 lbs. (6.8kg)
WOOFER: K-1007-K 6.5" (16.51cm) ICG cone
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kraut
post Dec 26 2011, 18:33
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For the price - just try it. A cheap amp for 40 - 120$, and you might use the parts to upgrade your toaster if it goes wrong.
http://www.ebay.ca/itm/New-SMSL-SA-S1-TA20...=item1c222327b7
http://www.ebay.ca/itm/TOPPING-TP-32-TA202...=item19c9486e11
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MichaelW
post Dec 26 2011, 22:45
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I did just that--replaced an AV receiver with one of those little amps (mine is labelled S.M.S.I.) that I bought off a local auction site. Mine works fine with a pair of KEF bookshelf speakers at low volumes. I don't play them loud, so I don't know where it would run out of steam. As kraut says, these devices are very cheap, and if it's not satisfactory, you could probably find a use for it (small sound system in the loo?)

This post has been edited by MichaelW: Dec 26 2011, 22:45
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random_asdf
post Dec 27 2011, 01:34
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sounds good to me. I don't plan on playing them too loud so I think i'll be okay.

thanks for the help.

-edit-

The two amps you linked seem to be rated at 15-20W per channel @ 8ohms. Shouldn't I be looking for an amp that can provide 50-100W per channel since my speakers are 50 W continuous?

This post has been edited by random_asdf: Dec 27 2011, 01:51
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pdq
post Dec 27 2011, 02:29
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Running speakers at less than their maximum rated wattage is not a problem. What you really need to look at is their efficiency. With such low wattage amplifiers you need fairly efficient speakers.
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random_asdf
post Dec 27 2011, 02:45
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that would be the sensitivity rating, correct?

What would I want to power these speakers then since they're rated at 90db/watt/meter?
-edit-
if I use this table http://www.decoaudio.com/deco_audio_dbw.html and http://en.wikipedia.org/wiki/Sound_pressur...pressure_levels

then would it be correct to say i'd regularly only really need the amplifier to provide 0-2 watts for most regular use?

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kraut
post Dec 27 2011, 02:50
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QUOTE
Running speakers at less than their maximum rated wattage is not a problem. What you really need to look at is their efficiency. With such low wattage amplifiers you need fairly efficient speakers.

Of course it is not a problem to run speakers below their max. Unless you run them too low to hear SOUND...
He sets up at the desktop, so 20W should be enough to at least blow your ears off partially.
90dB per meter, at 20 watts the speakers are capable of 102db if I got my dBs right (each doubling of power adds 3dB spl)

This post has been edited by kraut: Dec 27 2011, 03:03
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random_asdf
post Dec 27 2011, 20:19
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That makes sense.

Last question then, I think. How can these amplifiers work with speakers of different impedances? I.e. 4-16 ohms as opposed to just 8 ohms. Does this just mean it's designed to deliver 20W but in reality this is only for 4 ohms. This would then mean at 8ohms it only delivers 10W since P=V^2/R. Or is this more of transmission line impedance matching between the speaker and the amplifier?

ex:
http://www.ebay.com/itm/HiFi-Audio-Stereo-...#ht_2102wt_1135

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pdq
post Dec 27 2011, 22:47
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The maximum power delivered does vary with load impedance, but it's not that simple. There is an optimum load impedance at which the amplifier delivers the maximum wattage, usually somewhere between 4 and 8 ohms. Above and below that the maximum power will be less.
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random_asdf
post Dec 27 2011, 23:16
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QUOTE (pdq @ Dec 27 2011, 16:47) *
The maximum power delivered does vary with load impedance, but it's not that simple. There is an optimum load impedance at which the amplifier delivers the maximum wattage, usually somewhere between 4 and 8 ohms. Above and below that the maximum power will be less.


My main concern is that if it's an issue of impedance matching there will be a reflection of the input signal which can distort the transmitted signal to the speaker. I guess it would be negligible since the signal probably propagates at around 2/3 the speed of light, so its just an issue of power delivered.

That said, does anyone know anything about this? http://hyperphysics.phy-astr.gsu.edu/hbase...o/imped.html#c2
"But modern audio amplifiers are active control devices, and the impedance matching of the amplifier to the loudspeaker is no longer considered best practice."

i.e. how reliable/established are these "active control devices." Can I expect the chinese brands to be of passable quality?

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kraut
post Dec 27 2011, 23:29
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QUOTE
My main concern is that if it's an issue of impedance matching there will be a reflection of the input signal which can distort the transmitted signal to the speaker. I guess it would be negligible since the signal probably propagates at around 2/3 the speed of light, so its just an issue of power delivered.


I did the calcs for fun at one time, and you don't have to believe me, do them for yourself: at the frequencies involved impedance matching is no big issue. Reflections have importance at the MHz range, i.e. in digital and radio transmission.

QUOTE
Can I expect the chinese brands to be of passable quality


Most of if not all consumer electronics you buy nowadays are made in China, albeit (hopefully) under QC from the Brand owner.
I had a lot of Behringer stuff among other consumer material, which all worked splendidly - just some with some issues as to longevity.
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random_asdf
post Dec 28 2011, 02:56
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Ah. That's good to know. On both parts. Thanks!

And do amplifiers generally operate linearly up to the maximum wattage? i.e. would I want to buy an amplifier that can supply more power so that I'm guaranteed a more linear amplifier output at lower wattages?

i.e. this http://www.dealextreme.com/p/60w-stereo-au...-red-91350#open full view

it's 30 watts/channel instead of 10-20
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MichaelW
post Dec 28 2011, 03:46
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I've been searching for stuff I vaguely remembered. It seems that it is possible to damage speakers if you try to get more volume than your amp is really up to: you can get clipping, which will damage the drivers. I don't really know about this, but you could try http://www.jblpro.com/pub/technote/lowpower.pdf

OTOH, you are unlikely to need much power for your intended use. I found a calculator at http://www.crownaudio.com/apps_htm/designt...ect-pwr-req.htm As this is a Harman company, the calculator is likely to be reliable. Making some assumptions: the speakers will be on the desktop, so say 1 metre from your ears; you are probably going to be playing music while you're working, so for long periods of time, so to avoid hearing damage you will want to limit volume to about 85 dB SPL (and if you're working, you're probably not going to want to rock out, anyway); your speakers, IIRC, have an efficiency figure of 90 dB per watt. According to the calculator, that means you need about 2 watts (per channel, I assume).

This is just as well, because the wattage figures quoted for those devices will be peak music power (as oppose to RMS, quoted by the solid companies), at an unspecified level of distortion, subject to rounding, with a following wind, and after clearance by the sales department.

I've got an S.M.S.I. branded amp. As far as physical construction goes, it seems to be solid--I can't judge electrical construction, and that's all hidden anyway, but it's driven my speakers satisfactorily at moderate volumes in desktop usage without trouble and without signs of strain. But if you want anything louder, you might be well advised to hang onto the AV receiver for party duty.
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pdq
post Dec 28 2011, 04:47
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Generally distortion will increase gradually as you approach the maximum wattage, as opposed to remaining low almost until the point at which the output clips.

OTOH, that does not guarantee that distortion remains low at low signal levels. A poorly designed amplifier can have what is known as "crossover distortion", so it is possible that the higher-wattage amplifier may have higher distortion at normal listening levels than the lower-wattage one.

The idea behind an amplifier damaging speakers when it clips is theoretically possible. Tweeters are generally rated at much lower power than woofers or mid-range drivers, because there is generally much less energy in high frequencies in music. When the amplifier clips, a lot of the energy in lower frequencies is converted to higher frequencies, damaging the tweeters. I don't know if this has ever been observed.
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Kujibo
post Dec 28 2011, 05:31
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A few weeks ago I realized I had an extra pair of JBL S38's (a fairly solid high power "bookshelf" speaker) that have been sitting around unused for quite some time. I figured I should try to hook them up to my PC to replace the Cambridge Soundworks Microworks 2.1 PC speaker system that isn't horrible for an old PC system considering its puny satellites, but I had no proper amp to drive them. Anyway, after worrying about it and doing some research to ensure it wouldn't immediately blow up I just used the sub/amp from the Cambridge system to drive the S38s. It's a fairly ridiculous match but a big improvement on the former. As much as I've always been told long ago to get the impedance matching right I figured as long as you don't try to drive anything hard you are probably OK matching whatever amp to whatever speakers.
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kraut
post Dec 28 2011, 14:46
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QUOTE
And do amplifiers generally operate linearly up to the maximum wattage?


No, when it comes to distortion the progression is not linear. Towards the max out the distortion increases exponentially.

http://www.stereophile.com/images/archivesart/108Bryfig3.jpg

As to impedance: The lower the input impedance of a Speaker, the more amperes it will draw from the amp. The worst case for a SS amp is a speaker that has lower input impedance than the amp output impedance. This would be almost a "short".
Think of it this way: two wires (hot and neutral) connected to a user (lightbulb, etc.) The resistance of the user causes a voltage differential through which a limited current will flow, producing work. If the resistance is eliminated, i.e. touching the wires, the voltage differential breaks down and maximum current flows without producing work.
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dhromed
post Dec 28 2011, 15:53
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QUOTE (pdq @ Dec 28 2011, 05:47) *
When the amplifier clips, a lot of the energy in lower frequencies is converted to higher frequencies, damaging the tweeters.


What is the difference between the clipping you describe and a generated square wave?

Honest question, not a snarky rhetorical one. smile.gif
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kraut
post Dec 28 2011, 16:39
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QUOTE (dhromed @ Dec 28 2011, 07:53) *
QUOTE (pdq @ Dec 28 2011, 05:47) *
When the amplifier clips, a lot of the energy in lower frequencies is converted to higher frequencies, damaging the tweeters.


What is the difference between the clipping you describe and a generated square wave?

Honest question, not a snarky rhetorical one. smile.gif



Clipping can produce harmonics at higher frequency ranges. Harmonics are even or odd order sinusoidal multiples of the fundamental and usually show up as discrete waveforms and not as summed waveforms as in a triangular or a square wave.

A squarewave is the sum of the fundamental plus all the odd order harmonics, and requires per definition: a "clean transition from one state of level to another".
http://en.wikipedia.org/wiki/Square_wave
http://mathworld.wolfram.com/SquareWave.html

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MichaelW
post Dec 28 2011, 17:01
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QUOTE (pdq @ Dec 28 2011, 16:47) *
When the amplifier clips, a lot of the energy in lower frequencies is converted to higher frequencies, damaging the tweeters. I don't know if this has ever been observed.


It seems that Rod Elliott claims to have observed it: http://sound.westhost.com/tweeters.htm
This thread also seems relevant http://www.hydrogenaudio.org/forums/index....=47036&st=0
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random_asdf
post Dec 31 2011, 01:29
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Wouldn't a square wave damage speakers as well? It's effectively a DC signal whenever it's flat, shorting the speaker since it's essentially just an inductor.

What role does higher harmonics play in this? Is it because speakers also get damaged when they try to operate at higher frequencies?

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pdq
post Dec 31 2011, 02:00
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Yes, if you turn up the volume on a square wave you will also damage your tweeters. How likely is that vs. just turning the volume up too high on music, beyond the maximum output of your amplifier?
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