Question about Wattage/Speaker rating

gordonf238

Quick question. I have a pair of RT25i bookshelf polks. In their specs, it's recommended that I use an amp rated at 20-100 w/channel. Their nominal impedance is 8ohms.

As of now, I have a crappy Optimus (Radio Shack brand) pre-amp/amp powering them. It's a mere 25w/channel unit.

Now here's my question. I run a stereo signal from my computer's sound card LINE-OUT, into this Optimus pre-amp/amp, then into my Polks. Most of my music seems to play fine. I get occasional clipping in the tweeter with classical/vocal/piano music however (but not other types of music). Even when I lower the volume, I still hear the clipping (mostly trebles). Clipping which I don't hear in headphones, so I assume it's the **** Optimus unit being driven into clipping, not the speakers, correct?

Would this clipping be eliminated if I got a more powerful, 100 w/channel 8ohm amplifier like this?

http://www.crutchfield.com/S-Q6mTGMfLbK0/cgi-bin/ProdView.asp?g=10450&I=580M282

Could I run a line out from my computer's sound card, directly into this amplifier, then out to my speakers? Or do I really need a pre-amp? (that would cost more $$ which I unfortunately do not have at the moment).

I'm ready to buy that Onkyo unit, but I thought I'd ask you whether it's my underpowered Optimus unit which is causing the occasional trebles to clip.

My second question. How is it that the 25w/channel Optimus unit can drive my Polks with great bass given that it's such a weak pre-amp/amp? Is this due to the fact that the line-out from my soundcard is already amplified?

I guess I'm just somewhat puzzled by the logic. I would think that if speakers are 100 watts each, then you need a 100 watt/channel amplifier, yet a 25 watt/channel amp, which I have now, powers them just fine.

In any case, thanks for any advice you may have.

wallstreet

Originally posted by gordonf238
I get occasional clipping in the tweeter with classical/vocal/piano music however (but not other types of music). Even when I lower the volume, I still hear the clipping (mostly trebles).
.
.
.
I guess I'm just somewhat puzzled by the logic. I would think that if speakers are 100 watts each, then you need a 100 watt/channel amplifier, yet a 25 watt/channel amp, which I have now, powers them just fine.

In any case, thanks for any advice you may have.

These 2 statements are contradictory. Clipping indicates your amp does not power your speakers "just fine". In fact, you're probably damaging your speakers. Buy a bigger amp and be done with it.

unc2701

Actually, it sounds like a input voltage problem... the optimus is expecting a voltage of 200mv on the inputs; the computer sound card can put out more than that voltage (so it can drive headphones)... when the voltage is higher that 200mv, it clips. This clipping isn't as bad as an over-driven amp clipping, but it still sounds bad.

So, yeah, you do need a better amp, but for a quick fix, crank back the volume on the computer and use the volume knob on the optimus to get it louder.

gordonf238

I was under the impression that "clipping" could be classified within two categories.

Clipping of the speaker, i.e. running more power through a driver than it can handle.

and clipping of the amplifier. i.e. when the amplifier tried to aplify music to levels which are past its ability.

Am I wrong about these?

steveinaz

Only the amplifier clips, causing the speaker to distort.

gordonf238

Originally posted by steveinaz
Only the amplifier clips, causing the speaker to distort.

ah i see. then what is the term used for when you run more power through a speaker than it can handle? or am i missing something here?

SuperDave

I think he was trying to say your computer pre out level is too high for the input of your amplifier. Like it was mentioned previously, (using your computer volume control) turn it down to half way and let you amplifier do the rest.

If the input level to the amp is too high, the amp will output a clipped signal.

Just my 2 cents,
SD

unc2701

Yep- superdave nailed it... typically only an amp clips, but when you have a rigged setup (like computer-receiver or walkman-receiver) you can get clipping on the input side- this is not as likely to hurt your speakers since it's sorta like playing a recording of, say, a distorted guitar... but it's still not the best thing for your system.

As for speaker clipping- it's not exactly clipping, but you can "bottom out" your speakers, which is when you put enough voltage through them that they hit their maximum excursion and make a sound like "thwock!" (that's the scientific term, anyway...) But this is distinct from amplifier clipping which is when the amplified sound waves get squared off.

aaharvel

the 'optimus' amp is underpowering your speakers- that's why they're distorting and clipping. Tweeters are always the first to go.

You'd be better off buying an amp that exceeds you're speakers wattage specs than buying an amp that underpowers them.

best thing to do is buy an amp where the rms lies somewhere in the middle to 75% and also has high-quality current and power supplies stapled to it's **** as well.

good luck!



rti-4 fronts
csi-3 center
f/x300i sides
f/x300i rears
velodyne cht-10

h/k 235, adcom gfa-7300, adcom gfa 5300:D

gordonf238

Just to reiterate on the topic. My Denon DRA-395 (80w/channel @ 8ohms) arrived today, along with monster speaker cables.

I've discarded my Optimus and my old speaker cables. I've hooked it all up and WOW! I never thought these $400 polks could play so loud, so clearly, with such amazing bass (amazing bass for bookshelf speakers anyway). I mean who would have thought these small speakers could be such power hogs! Hell the amplifier itself weighs more than both speakers combined.

Lesson learned here (which should go without saying). Don't pair up good speakers with a **** Radio Shack-brand underpowered amp.

nadams

Another satisfied customer of the Club Polk forums

gordonf238

Yes indeed

My only issue, now that the sound is so crisp and clear, is that I hear artifacts and distortions everytime the hard drive reads/writes. It's not loud, but when listening to music with quieter sections, you can hear this repetitive electronic noise that shouldn't be there.

I know this is somewhat inevitable since the sound card is connected to the motherboard, whose current varies depending on CPU/RAM/HDD load, which inherently produces eletromagnetic variations that are audible.

The only way I can think of getting around this is by using the SPDIF digital output from my sound card instead of the analog one. The only problem is that I can't seem to find a simple, compact unit which decodes SPDIF digital audio into analog.

cfrizz

The best way to get around it is to set up a complete audio system that doesn't include a computer.

I don't care what they say, a pc is not designed to be a home entertainment system.

ndrwcn

i use my laptop to play music, and use an external soundcard, a soundblaster mp3+ (or something like that, picked it up for 40 bucks) which connects to laptop via USB and has an optical output. i have not noticed any interference/feedback/buzz, and am very happy with the setup.

F1nut

Originally posted by cfrizz
The best way to get around it is to set up a complete audio system that doesn't include a computer.

I don't care what they say, a pc is not designed to be a home entertainment system.

AMEN!!!

There is no subsitute for a real hi-fi rig.

TroyD

As a wise man once said:

There is no substitute for radiating surface area and a real man's amp.

BDT

jsdeprey

If you like using a PC, try buying a nice sound card that has digital outputs on it, I have a SB Live card that has a add on that has coax and optical digital outs on it.
You can them run that digital output to your new receiver and you should not hear the problem you are describing.
Most the time that is just because inside a PC is a bad place to go from analog to digital and also most sound cards have pretty crappy analog to digital conversion.