> From: Pamela Leming <[log in to unmask]>
<SNIP>
> I have a Pioneer stereo receiver that I use to connect stereo speakers
> to my PC. Everything seemed to work fine for the first couple of
> days. Then, I was playing back some sound (wav) files and all of a
> sudden, no more sound. Windows Media player would still play the sound
> file but nothing coming out of my speakers. I fiddled with the
> speakers for awhile, but nothing. Thinking my sound card was somehow
> locked up again, I rebooted. I opened the computer, thinking maybe the
> PCI slot was bad or something. I swapped my video card and my sound
> card. I did notice that both of them were a little (actually more than
> a little) warm to the touch. Plugged the speakers back in, still nothing.
> Then I got out an old cheap pair of speakers I had and plugged them in.
> They worked so I figured it wasn't my sound card that was the problem but the
> stereo speakers. I went out and bougth some new PC speakers for about $60. I
> plugged them in and everything worked fine...for about 30 minutes. Then the
> light on the speakers went out and I couldn't play any sounds. At the time I
> wasn't playing any sounds, but the speakers were on and connected to the sound
> card. I did play a couple of system sounds to make sure they were working ok
> and went on a website where I know the site comes up with music. Thats when I
> noticed that the light on the speaker was NOT on and no sound.
> Rebooted, still nothing. Got out the old pair of cheap speakers again,
> plugged them in, and they work! I've had them on for about 20 minutes
> now and still working and I'm at a website that plays continuous
> music. So, just as a test, I plug my NEW speakers back in...no sound,
> no nothing. Plug the old ones back in, it works. I am at my wit's
> end on this one. I know the brand new speakers work cause I
> used them for a while tonight then they just conked out on me.
I have a nasty suspicion that the speakers that failed were amplified
speakers (with electronics and a power supply that plugs into the wall).
The "old cheap" speakers that still work are plain, unamplified, "just
speakers".
If I am correct, the amplified speakers were designed to plug directly
into the sound card. By using the stereo amplifier, you got more volume
out of the speakers. Unfortunately, if you turn up the stereo volume too
high, you can exceed the maximum input limits of the amplified speakers,
blowing chips (that's why the LED went out and both speakers failed at the
same time). The amplified speakers can probably handle something like
12 VAC input (guessing, using the + and - 12 VDC available from the PC
to run the sound card).
Picking numbers "out of the air" for example, a typical impedance
(complex "resistance" for AC) for a speaker is 4 Ohms.
for: P=Power in Watts P=ExI E=IxR
E=Voltage in Volts P=E**2/R
I=Current in Amperes P=I**2xR
R=Resistance in Ohms
for each of a pair of 25 W speakers, P=25=E**2/4
E**2=100 E=10
for the output of a 400 W stereo amplifier P=400=E**2/4
and a 4 Ohm speaker E**2=1600 E=40
and a 100 Ohm amplifier input P=400=E**2/100
E**2=40000 E=200
Some ICs have input impedances over 20 MegOhms. The stereo amplifier
could not put out millions of volts (fortunately!), but it could still
blow a chip at a fraction of what it can put out.
Seeing as you already have a (presumably reasonably high quality) stereo
amplifier, your cheapest solution would be to go with the "old cheap"
speakers you already have. What are their ratings? If they are rated
at 25 W each, and the stereo is rated at 200 W/channel, you have some
choices: 1. Never use them at more than 25 W output (max. 10 VAC for
a 4 Ohm speaker).
2. Pump up the volume 'til they blow.
3. Calculate the appropriate resistance such that the rated output
gives the rated power to the speaker, and install it in series
with each speaker. (This will protect the speakers, but waste
LOTS of power heating some big resistors - that could be 175 W!)
P=I**2xR 200=I**2x(R+4) (R in series with 4 Ohm resistor)
subtract 25=I**2x4 (same I through 4 Ohm resistor)
--------------
175=I**2xR I**2=25/4
175=25/4xR R=28
4. Buy new, high-quality "just plain" speakers rated for the same
(or maybe slightly higher power than the stereo amplifier.
5. Buy new amplified speakers (or repair the old ones), and use
them directly from the sound card.
One way to confirm that the amplified speakers are dead is to plug them into
another system with a working sound card and speakers.
Hope this helps.
Boyd Ramsay
[log in to unmask]
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