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Old 9th April 2015, 11:11 AM   #1
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Default ok, blew-up my speakers :(

sad story, blew up pair of speakers. my mistake, ignored some things. this is a noname sound system with speakers (brand labels reads as A♥WA)

i have been using this amp for years for pc music/movies. i tested the amps speaker out and dc reading is 0mV for left 10mV, which i believe is acceptable, right? so i tested with disco remixes and after like 15minutes or so, i heard harsh sounds from speaker and amps speaker protection keeps tripping off. but its too late, the left speaker is blown while the other one is shorted.

i retested amps dc and its still 0mV and 10mV on each channel. is it possible to know how much wattage the power amp is giving off? since i believe the speakers blew because its overrated(60watts, 4ohms 4" diameter cone). i have another speaker form sony SS-L90VH which is 8 ohms, but i can only search from net for its rated output at 120 RMS and i dont want these speakers to suddenly blow also. i dont have schematic for the amp but it is based on C4468/A1695 pair with 30-0-30v 150VA toroid. will my speakers possibly be safe?

thanks.
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Old 9th April 2015, 11:29 AM   #2
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sounds like you played too loud for too long with a pair of speakers not designed for that duty cycle. Ratings for cheap gear are usually just numbers and do not link to the ability to handle the power all the time.
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Old 9th April 2015, 11:41 AM   #3
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Make and model of amplifier would help.
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Old 9th April 2015, 11:47 AM   #4
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Quote:
Originally Posted by wertz View Post
sad story, blew up pair of speakers. my mistake, ignored some things. this is a noname sound system with speakers (brand labels reads as A♥WA)

i have been using this amp for years for pc music/movies. i tested the amps speaker out and dc reading is 0mV for left 10mV, which i believe is acceptable, right? so i tested with disco remixes and after like 15minutes or so, i heard harsh sounds from speaker and amps speaker protection keeps tripping off. but its too late, the left speaker is blown while the other one is shorted.

i retested amps dc and its still 0mV and 10mV on each channel. is it possible to know how much wattage the power amp is giving off? since i believe the speakers blew because its overrated(60watts, 4ohms 4" diameter cone). i have another speaker form sony SS-L90VH which is 8 ohms, but i can only search from net for its rated output at 120 RMS and i dont want these speakers to suddenly blow also. i dont have schematic for the amp but it is based on C4468/A1695 pair with 30-0-30v 150VA toroid. will my speakers possibly be safe?

thanks.
Sorry about your problem, but it sounds like you had a good time!

That amount of DC offset is OK for speakers. The cap to the tweeter will stop it from hitting tweeter anyway. The amp is probably fine, it is behaving as it should by protecting itself when connected to a short circuit.

Measuring amp's power is usually done with an oscilloscope, you connect a known resistor load, then increase the voltage rails with a variac until the signal starts to clip on the oscilloscope, then compute the power based on the output voltage into the load.

If amp protection is tripping, then there is a short in the speaker. Probably a shorted voice coil in one of the drivers, probably the woofer. This is the mode by which most speakers fail. The voice coil overheats when power rating is exceeded, melts the nylon insulation on the voice coil conductor, then the coils short together increasing the load until the amp protects.

You can't rely on power ratings to prevent speaker or amp damage. The only way to avoid blowing up speakers is to listen to them and don't push them. If they sound bad then they are overdriven and heading toward failure. The speakers are probably rated for 25W. Look on the individual drivers for power rating and nominal impedance. Now you know these speakers aren't loud enough for your needs. But it can work the other way too, a speaker with too low impedance played too loud can overheat a transistor. A small tube amp can distort so badly when overdriven that the HF signal can melt and damage planar ribbon tweeter, that's a case of amp too small breaks the speaker! It happened to me.

You'll need to replace the broken woofer, but if you can't identify the exact part then we can help you choose a suitable replacement part.

Open up the speakers and check everything out to see where you really are. Inspect the crossover and wires for signs of burning or melting and check that all wire connections are tight, like at speaker posts and tight connectors on the drivers. Check any electrolytic capacitors (metal cans) for bulging or leaking. Disconnect one wire from each driver, one at a time, and check their resistance with ohmmeter across the two driver tabs. A shorted voice coil will show <2ohms DC resistance. An open voice coil will show 0 Ohms. You have to disconnect one lead of the driver in order to measure the voice coil resistance.

Post a pic of front and back of any blown drivers.
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Old 9th April 2015, 12:00 PM   #5
h_a is offline h_a  Europe
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Sounds like the power rating of your speakers was a bit too optimistic.

Don't rely too much on the ratings anyway, nobody runs 120W_RMS into his/her speakers all the time. Just hang a multimeter or scope on your speaker terminals to see for yourself. Real world power requirements are much lower than that.
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Old 9th April 2015, 12:46 PM   #6
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here are pics of the sound system + speakers...



no other identifying labels can be found like real model numbers, or power rating of amp etc. no labels also on tweeters and woofer, only a sticker at back that says 60watts, 4 ohms. anyways im not gonna use this anyways and instead use the Sony speakers. i found some service manuals and read that these L90VH are paired with system with about 100-120watts. i just want to have a possible idea or estimate on amplifier power so i wont blow these again and since i dont have oscilloscope nor a dummy load resistor.
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Old 9th April 2015, 12:51 PM   #7
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I usually listen to average signals that are 20dB to 30dB below the amplifiers maximum rating.
i.e. a 60W power amplifier is putting out an average of 0.6W to 60mW.
Speakers rated for domestic duty are sometimes rated for domestic duty type of reproduction, i.e. the manufacturer of the speakers receiving only 0.6W on average will work well with a 60W amplifier. The label on the back of the box states 60W. It might even state 150W because the Manufacturer has tested his speakers with a 150W amplifier reproducing non clipped music in a domestic type environment. i.e. average levels 10dB to 20dB below maximum rating.
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Old 9th April 2015, 12:57 PM   #8
AndrewT is offline AndrewT  Scotland
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An amplifier rated for 120W of maximum output power, when fed with nominal mains voltage will produce more output if the test signal is clipped.
A 120W into 8r0 test load is equivalent to 43.8Vpk for an unclipped sinewave.

If the input is increased by 6dB the output is still limited to 43.8Vpk because the supply rails won't allow any more voltage. But the power delivered to the test load will have increased substantially, maybe now measuring 150W.

If the input signal were a square wave, the maximum power output would be 240W
That would do a lot of damage to domestic type speakers rated for 120W using unclipped music.
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Old 9th April 2015, 12:59 PM   #9
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A two channel amplifier powered from an 150w transformer is a small amplifier.
If you push hard this amplifier will clip continuously. This clipping is catastrofic for loudspeakers
keep in mind as general rule,it's easyer to damendge a speaker using small amplifier than using a big one.

Last edited by thimios; 9th April 2015 at 01:04 PM.
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Old 9th April 2015, 01:26 PM   #10
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Many different things are at play here.... First: power handling capacity of drivers. Whatever they say, don't believe the stickers they put on those drivers. There are many different -refined- methods os measuring and specifiyng driver power handling capacity, that can grossly mislead the unsuspecting user. Generally speaking an 8" woofer with 1" dia voice coil is able to handle around 20-30 watts of real power (at the best). If you play soft jazz, or classical music, with occasional high peaks, that's OK, but rock, disco, metal, and similar types of music with high "signal density" can kill a woofer very quickly.
Think about it. The thermal resistance of such a voice coil is (depending on the former's material) somewhere around 4-8 C/W, that means with 30 watts of continous power fed into them they can quickly heat up to 200-240 C, where paper, and the insulation on the V.C. wire starts to burn, and melt.
With black anodized aluminium former the situation is somewhat better, but still, no way they can handle 100 Watts of continous power for too long.
Second, a quite often discussed fact, that a lower power amplifiers can fry a speaker much easier than high power ones. Why? A low power amplifier- when overdriven- quickly runs into clipping mode, that means, the "clipped" portion of the signal shows up on the output as DC, which seriously increases the heat dissipation on the voice coil, burning it sooner or later. An easy way to prevent this, is show some common sense, because clipping means distortion, so when you hear your speakers distorting, just turn the volume down a bit, so you might save your speakers.
Of course this is not easy to excercise in the middle of a heated-up party, but then you must face the consequences.....

I just saw an almost "parallel" comment, where Thimios said the same things....

Last edited by dragonweed; 9th April 2015 at 01:29 PM.
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