Amp and speaker matching question

[mousedadrummer182]

Ok, so I just want to clear up some things, would appreciate some help!

 

I know that its bad to underpower speakers, eg using 350w RMS speakers with a 200w RMS amp. Is this because the speakers would try and draw 350 from the amp thats only supplying 200? Therefore making it distort? Or is there another reason that the speakers distort when underpowered?

 

 

To be safe, Ive read that an amp should be used thats 150-200% the RMS power of the speakers, to give the amp lots of headroom etc. So, if (for example) I was using a 700w RMS amp with 350w RMS speakers, would I then have to only turn the amp volume up to 50%??? Otherwise, the amp will surely be over-powering the speakers. Although Ive also read that the amp will always provide its maximum RMS wattage, and the volume is just regualting the input signal (and that the amps run best at 75-100% of this control)....so Im quite confused!!

 

Somebody please help clear this up for me,

 

Thanks

[Peter Chivers]

Ok, so I just want to clear up some things, would appreciate some help!

(1)

I know that its bad to underpower speakers, eg using 350w RMS speakers with a 200w RMS amp. Is this because the speakers would try and draw 350 from the amp thats only supplying 200? Therefore making it distort? Or is there another reason that the speakers distort when underpowered? To be safe, Ive read that an amp should be used thats 150-200% the RMS power of the speakers, to give the amp lots of headroom etc.

 

(2)

So, if (for example) I was using a 700w RMS amp with 350w RMS speakers, would I then have to only turn the amp volume up to 50%??? Otherwise, the amp will surely be over-powering the speakers. Although Ive also read that the amp will always provide its maximum RMS wattage, and the volume is just regualting the input signal (and that the amps run best at 75-100% of this control)....so Im quite confused!! {snip}

 

(1) The speakers will only draw the power that the amplifier is capable of providing for the impedance load (3) attached to the speaker connection. The distortion generally comes from the amplifier being overdriven to try and get more volume out of the system.

 

(2) You are correct that a volume control simply reduces the input signal that goes to the power amplifier. However the power output by an amplifier is related to the size of that input signal. An amplifier has a figure known as its 'sensitivity' which defines a voltage at the input that with the volume control set to full causes the amplifier to produce its full rated output. When you reduce that input voltage, you will reduce the power produced by the amplifier.

 

Most professional loudspeakers are capable of providing more than their rated power (beware of budget brands! :blink: ) without suffering damage so long as the distortion is minimal. However when the signal becomes distorted there is an increase in the energy being passed to the speaker - energy which becomes heat in the voice coil leading to burnt out voice coils and hence dead drivers.

 

(3) A power amplifier delivers its full rated power into a loudspeaker load which has an impedance value that is the same as specified for the amplifier rating - for many amplifiers this is a 4 ohm load. If the loudspeaker load is higher, for example 8 ohms, the amplifier will produce a lower output power - as a very rough approximation for this example, about half of the rated power output at the correct rated impedance.

 

Take a read through some of the articles in the sound category on the Blue Room WIKI and perhaps a trip to the local reference library for more information and detail.

Hope this helps

Peter

[TimmyP1955]

It's fine to use any amp into any speaker. You just have to avoid clipping the amp (if "underpowering") or overdriving the speaker (if "overpowering"). When the sound quality starts to change, you are doing one or the other.

[DangerousMark]

Erk...

 

Well, I wasn't responsible for the purchasing decision, but we're running 200w speakers (JBL Control-1) with 150w amps (Denon DN-A100) here. Had thought it was more of an issue if you either had

a/ overpowered amp

b/ massively underpowered amp

c/ mismatched impedence

 

AND turned the volume up high. Then chance of (a) blowing speakers or (b) overheating the amp. (c could be either way depending on how its mismatched as it effectively causes one or other effect, to massively simplify it.... hence how tape-destroyingly-hot my old 4x40w Sony headunit got when I, as a naive novice, connected it to some scrap 200w, 8 ohm home stereo drivers PVA'd to the underside of a MDF parcel shelf (yeah, pure class - but when stretching to afford the running costs of an "expensive" £650 car, and knowing bugger-all about how to do it right, you don't buy new purpose-built stuff :)). Not only having it underpowered for the speakers, but it was also expecting 3- or 4-ohm kit. Lots of heat as it tried to make enough power, very little sound coming out, thankfully I replaced the craptacular OEM front speakers with proper items after that escapade. The 4x 25w kit built into the car that followed it was significantly louder at the back...

 

I would have thought slight underpowering wouldn't make much difference so long as you weren't thrashing it (ie maxing the volume - as far as I can tell you're not really meant to do that unless it's a quiet input source, bringing it up to the level that a normal one would be at half-volume or less B-)), as it would basically be much the same as turning the volume control down a bit on a more powerful amplifier. Could be wrong though.

 

EDIT: Pants, that's what I get for posting a reply in a thread I opened in the browser last night without hitting refresh first :** laughs out loud **:

[Chappie]

we're running 200w speakers (JBL Control-1) with 150w amps (Denon DN-A100) here.

From what I can see, the DN-A100 is only rated 70W/ch at 4R, so this is substantially under power for a Control 1. I would be careful running it hard, but at lower levels it will be fine.

 

Had thought it was more of an issue if you either had

c/ mismatched impedence

 

(c could be either way depending on how its mismatched as it effectively causes one or other effect, to massively simplify it.... hence how tape-destroyingly-hot my old 4x40w Sony headunit got when I, as a naive novice, connected it to some scrap 200w, 8 ohm home stereo drivers PVA'd to the underside of a MDF parcel shelf. Not only having it underpowered for the speakers, but it was also expecting 3- or 4-ohm kit. Lots of heat as it tried to make enough power, very little sound coming out, thankfully I replaced the craptacular OEM front speakers with proper items after that escapade. The 4x 25w kit built into the car that followed it was significantly louder at the back...

I don't think you are really understanding the principles of loudspeaker impedance here. An amplifier is not expecting to "see" a certain load and they are not built to drive a certain load. They have power values stated at certain standard impedances and have a minimum impedance which they are capable of driving, which is related to the amps ability to dissipate heat. An amplifier will generate more heat the lower the impedance attached to it. A higher impedance will make it run cooler not hotter.

 

Image pushing a big rock along the ground. The size of the rock relates to impedance and the effort required to move it relates to the amps power. When you have a big, heavy rock (8 ohm for example) there is more resistance to movement and you have to push hard and it moves slowly (low power output). If you halve the size of the rock (4 ohm) you don't have to push so hard and you can push it faster (higher power output). Halve it again (2 ohms) and you can run with it! (max power output). The faster you are moving, the more heat you produce. If you get to the point where you are running so fast you cannot keep cool, you will overheat and die.

 

This isn't supposed to sound patronising, its just an easy way to simplify certain principles.

 

There is no such thing as "mismatched impedance" when it comes to loudspeakers. Any standard amp will drive any impedance as long as it doesn't fall below its minimum threshold and causes it to overheat.