Stereo amp question

[BopEuph]

I just decided to get a bigger pair of speakers for my laptop, since my old 5.1 speakers died on me a few months back.

I just got a pair of bookshelf speakers; they are 50 watts at 8 ohms. Even as an electric bassist, I was never very good about figuring out all of those numbers. What I'm looking for is a stereo amplifier that could power these speakers. The only power amp I own is a QSC 1500, which is two channels at 750 watts. Yeah...overkill.

I'm just wondering what combinations of numbers I could get for a power amp that will let these speakers sound decent? If I want more headroom (like a two channel 200 watt amp at 100 watts each channel), what would the resistance need to be?

Nick

[BopEuph]

I should add...I see the 100 at 8 ohms products, but I was just wondering what other combinations could be used. I just want to try to get the most bang for my buck.

Nick

[iiipopes]

First of all, to review, the speaker rating is the wattage or power it is designed to handle at the specified impedance. Therefore, any clean amplifier rated to deliver the same power at the same impedance should work.

First caveat: it is alright to go above the speaker's rated power on an amplifier, so long as you don't increase the gain to the point you are pushing the speaker with more power it is designed to handle. A misconception is that all that power is being used all the time. In fact, for normal living room listening, most amplifiers, no matter what the rating, with reasonably efficient speakers, are only pushing about one to ten watts until you turn it up loud enough to the point it gets difficult to carry on conversation.

Second caveat: DO NOT power the speakers with an amplifier that has a power rating significantly below the rated power of the speaker. If you do, and turn it up, before you run out of speaker, you will run out of amplifier. This causes clipping, which is like sending direct current straight through the speaker coil, which will cause it to heat up and eventually burn out.

So, just about any amplifier rated at 50 to 100 watts per channel RMS without clipping at the designated impedance will drive the speakers just fine.

[BopEuph]

Thanks.

Now what happens if the amp has 4 ohms? Does that mean a 25 watt per channel at 4 ohms is right, or do I go the other way?

Just wondering what happens when the impedance changes.

Nick

[iiipopes]

Thanks.

Now what happens if the amp has 4 ohms? Does that mean a 25 watt per channel at 4 ohms is right, or do I go the other way?

Just wondering what happens when the impedance changes.

Nick

All things being equal, using ohm's law, if impedance goes down, power goes up because current demand goes up as voltage usually remains constant in the output stage of stereo amplifiers. So, theoretically, an amplifier that delivers 50 watts at 8 ohms delivers 100 watts at 4 ohms. However, this is rarely the case, due to limitations in the output transformers for those amps which have them, or from other loading from other types of amplifiers and the related circuitry, including any feedback or resonance circuitry to keep the amplifier behaving over a really wide frequency response and the appropriate damping of the output so it doesn't "overshoot," causing spikiness in the output our ears interpret as an upper mid/lower treble boost (oversimplified, but this is the essence).

If you run an amplifier rated at 4 ohms and you connect an 8 ohm load, you will lose efficiency, power, and generate heat. You won't immediately hurt the amplifier, but over time, if pushed, the heat generated will kill something in the amplifier. Conversely, if you run an amplifier rated at an 8 ohm load and connect a 4 ohm load, you will be asking the amplifier to put out more current than it is designed to do, and it will heat up quickly and can kill especially an output transformer or fry an output MOS-FET, which is a standard power output transistor for solid state amps.

So matching the impedance of the speaker to the rated output impedance of the amplifier is pretty much on top of the priorities when sorting this stuff out, right in front of matching the power of the amplifier with the power a speaker can handle. Now, the good news: most American stereo amplifiers and some hobby amplifiers are designed to push 8 ohm loads, and are designed with some leeway so that they handle slightly lower loads.

For example, some time ago I purchased a solid state small Vox guitar amplifier for home recording for my Rickenbacker 12-string. It drives an 8-ohm speaker with about 10 to 15 watts cleanly, which is just right. The amplifier has a jack for a second speaker cabinet, but when you plug in, it has the defeat finger in the jack that automatically disconnects the main speaker. I didn't like that. So I researched the technical "white paper" on the particular output transistor it used. It turns out the particular output transistor is designed to handle both a 4-ohm load and an 8-ohm load, and is marginally more efficient at 4-ohms, but due to its internal design really doesn't pass any more current because it's designed with a high-end rolloff sooner than a stereo amplifier since there is not much usable frequency content above @8kHz on an electric guitar. So, I bridged the cut-out finger, plugged in another 8-ohm speaker, and it is the loudest, cleanest little amplifier for its class of any you've ever heard. But it's obvious why the factory put in the cutout finger in the external speaker jack: they didn't want some yahoo mistakenly plugging in a 4-ohm load, thinking he was adding in series instead of parallel, and have to replace the amplifier under warranty after it fried. So the amp, stock from the factory, chugs along happily with the 8-ohm load and its internal speaker designed with a voice coil to dissipate any heat from driving the amp into clipping, and the output transistor can happily digest and spit out the current needed for a 4-ohm load if the player is not being careful about matching speaker impedance to rated amplifier output impedance, or for guys like me who want to "hot-rod" the amp to its fullest after studying the technical literature to see what is feasible.

[BopEuph]

That helps quite a bit. Now I feel better about looking for a good power amp. Can't wait to see how these babies sound with the power behind them.

Nick

[Ricko]

In the pro sound arena the rule of thumb is to power the speakers with an amplifier rated at 2x the rated capacity of the speaker - translated - for a speaker rated at 100 watts, you should be using an amplifier of at least 200 watts. In the pro sound world - under-powering a speaker is more dangerous than overpowering a speaker.

As for the impedance - most consumer speakers are rated at 8 ohms - it's the 'standard' of the industry. Most consumer amps are designed to operate with an 8 ohm load, most are fairly safe with a 4 ohm load. Because the consumer grade stuff isn't designed as 'heavily' as the pro equipment - you should not run a consumer amp with a 4 ohm load wide open. Heat will be a problem first, then the equipment will begin to fail.

Good luck.

Ricko

[Rick Denney]

The power rating of speakers is mostly smoke and mirrors. At different frequencies and with different transient characteristics, a given speaker may or may not run out of excursion.

Okay, let me try that in English. The limit on power for most loudspeakers is the amount of available travel (excursion) on the drivers. If you thump too loudly, the speaker cone will move too far, and it will bottom out on the case of the speaker voice coil. This will cause a loud mechanical rapping sound. If you hear that, turn it down! The next thing you hear might be the compliance of the speaker cone disintegrating.

At lower frequencies, more excursion is required to achieve a given sound output depending on the equalization of the system, and so those percussive thumps are most likely to find the limits of the speaker's excursion. If you are a person who wants the system to thump, get both bigger speakers and the amp to drive them. If you keep the bass amplification down to realistic levels (as when playing recorded acoustic music), you can often drive speakers to higher listening levels than you can stand even with modest power. 20 clean watts into reasonably efficient speakers can be ear-splitting.

As has been mentioned, the real danger is overdriving the amplifier. The amp runs out of the power needed to complete the waveform, and chops off the peaks (aka "clipping"). Those flat peaks are indeed direct current, but even worse are the sharp corners going into them. Those sharp corners represent extremely high spurious frequencies that resonate in the circuitry of the tweeters and can cause them to overheat. An overdriven amp will kill a speaker in a hurry, and far more easily than too large an amp.

You could even drive your speakers with your 750-watt PA amp, if you keep the volume low enough to avoid bottoming out the speaker. But high-power amps are designed to different standards than home stereo amps, which usually means more distortion and noise, but also much greater durability and power. You'll find that distortion and noise will be worse with a very large amp that is underdriven.

Power ratings for amplifiers is also smoke and mirrors. They rate the power based on amplifying one single frequency tone to the point where total harmonic distortion exceeds their specified value, and then round up. They then design the circuit to survive that power level (if the amp is any good). The result is that an amp usually cannot match its specs when playing real music. Even at the volume setting that produces 100 watts on their test bench, the thump of thumpy music might overdrive the amp at that same setting. That's why it's better to get an amp rated for higher power than the speakers.

Rick "who runs a 250-watt/channel amp into speakers nominally rated for 100 watts with no hint of issue" Denney

[iiipopes]

Rick "who runs a 250-watt/channel amp into speakers nominally rated for 100 watts with no hint of issue" Denney

And I have done the same thing, gotten overenthusiastic, and literally ripped the surrounds off the drivers. $65 later, I keep the volume moderated.

[Rick Denney]

Rick "who runs a 250-watt/channel amp into speakers nominally rated for 100 watts with no hint of issue" Denney

And I have done the same thing, gotten overenthusiastic, and literally ripped the surrounds off the drivers. $65 later, I keep the volume moderated.

Acoustic suspension speaker designs help there, using the pressure inside the cabinet to limit excursion. But I have replaced the bass drivers in these speakers, too. I think it was because they got too old, though.

Rick "that's my story and I'm sticking to it" Denney

[iiipopes]

Yes, age may have been a factor. These were in a ported sub, so no sealed cabinet to limit excursion, with paper cones, and the surrounds were foam. The replacements were the upgrade that came out in this model a couple of years later, with impregnated cones and butyl surrounds. The slightly stiffer compliance has raised the resonance of the sub from @ 125 Hz to @ 160 Hz, so I'm going to have to re-port that part of the room (the space behind a diagonal shelving in a corner) to bring the resonance back down and lower its peak, but that's part of the fun of tailoring a system to a room.

It's the same thing that happens when the uninformed, wanting a "vintage" bass guitar tone, but also wanting road worthiness, take old Ampeg B15 Portaflex bass amps, and instead of having them reconed properly, shove a modern stiff-coned driver into the small cabinet, and wonder why they can't get rid of the resulting mid-bass peak instead of the relatively smooth response, albeit with some high-end rolloff, of the more compliant cone.