It's generally assumed by speaker designers that the amp will be a voltage source as opposed to a current source. Consequently, speakers are designed to behave flatly according to voltage.
All that said, I'm driving a pair of Focal 936's with a muscled up First Watt F5. The nice thing about a push-pull amp is you can get a bit more head room without hard clipping. As it sits it has a potential class A envelope of about 80 watts. But, at 8 ohm that envelope drops to 52. My Focals have a low point of 2.8 ohm. That means the class A envelope collapses to a whopping 18 watts. Sure, it will drive more power up into class AB, especially since I deleted the current limiting transistors. It's pretty easy to tell if you're pushing a class A amp out of it's envelope because they get quite a bit hotter. I've been listening to Tool's 10,000 Days here loud enough to feel it in my chair for a cool half an hour at least. The amps is still at idle temperature. It's probably not belting out 18 watts much, if at all. And if it does at that impedance low point it just transitions to class AB.
This thing is about all the amp I need and these aren't the easiest speakers in the world to drive. I have no idea why people need 500 watt amps in a normal sized room with decently sensitive speakers. I'm not sure why people get so worked up about an amp's ability to double it's current into half the load. 6moons reviewed the F5 and noted that if your speakers need more that the F5's 25 watts they're probably poorly designed and I tend to agree with that based on what I'm hearing right now. I seriously think people should listen to a simple, well designed, low power amp before they form opinions about the importance of current delivery ability and kilowatts of power potential. It sounds like @ghosthouse discovered that. I think Fist Watt amps represent exceptional, perhaps unmatched, value in terms of quality for the dollar, and I doubt much of anybody really needs more power. |
Hey... Can I point out that what a designer decides to call his "rated output" is little more than the number he feels like advertising? So the amp will drive 100 watts into 2 ohm. It'll probably have no trouble driving 50 into 4 ohm and 25 into 8. And it will do 4 and 8 quite well because it can probably do better than that 50 and 25 watts. I know for a fact Nelson Pass does stuff like this. It's not like there's some law that stipulates an amp can ONLY do what it's rated for. It makes sense to understate the ratings and show excellent measurements. |
@blindjim
One could just as rationally say amp designers should just build amps that have much more authoritative grip on the speakers. I don't see the reactive nature of many speakers as a problem. A well designed speaker uses those characteristics as a means to an end and it's really the fault of the user who doesn't select the appropriate amp. The goal of a speaker designer is to design a speaker that sounds good, not treats the amp kindly. A competent amp deals with it.
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@georgehifi
Clearthink is just looking for a fight as usual. And you're right. I'm not sure I would trust the manufacturer's numbers for speakers anyways. I don't have much of a problem with an amp maker understating the power capabilities of an amp. That's not nearly as problematic as a speaker company calling thier speaker an 8 ohm device when the bottom third of it's range is clearly 4 ohm and less. |
@miketuason
@almarg
It's called impedance matching. The closer a load matches the impedance of the source, the more efficient the energy transfer. Generally you want the load to be at least twice the impedance of the source to avoid odd coupling effects. The ratio of impedance difference also defines the damping factor which can significantly effect speaker behavior.
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@blindjim
I wish it were as simple as you would like it explained to be. Voltage gain is just a fixed value of the circuit design. How that plays out in dB actually heard depends on how well the amplifier drives the speakers. dB can be used a lot of ways to describe many things. Atmasphere can explain that jazz better than I can. Suffice it to say, an amp with 20 dB of gain can produce vastly different results bases on the speakers it's driving. That might be just barely adequate with some 85 dB efficient speakers and wild overkill for some 103 dB efficient horns.
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@mattmiller
The Grateful Dead did NOT use Mc amps for ALL their live shows. They used them for about 2 years to power the Wall of Sound PA until they retired it. Beyond that they used mostly a variety of Meyer Sound PA rigs all the way to their retirement.
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@almarg
I understand why they use transformers. However, transformers do have drawbacks. Many speakers don't expect the low damping factor transformers provide and they drive a lot of speaker hot in the highs. There's a reason DC coupling is the standard in solid state amps. It gives the output stage maximum control over the load. |
@georgehifi is right. That's what you get when you drive a significantly reactive load with autoformer. Designing a speaker with drivers and crossovers that work synergistically to draw power to fill in holes of inefficiency in the spectrum is an effective approach to solving the problem. Denying the speaker the power it demands to perform that function in the interest of being kind to the amp's output transistors completely negates the benefits gained from allowing the outputs to remain more linear.
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Wouldn't it be nice if speakers makers provided electrical characteristics so you could make a better choice about amps? The way a speaker needs to be driven is why I don't understand why some start with the amp or source when building a system. |
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I've NEVER seen a speaker company provide charts of impedance magnitude or phase angle. I've never even heard of one providing that information upon request. I'm not exactly sure how you would listen for those specifications. I'm not sure buying thousands of dollars worth of test gear and constructing an anechoic chamber so that I can measure a few speakers once or twice a decade makes a lot of sense. It's pretty widely known that basic specs like impedance and sensitivity aren't terribly honest or realistic. You need a lot more information than nominal impedance, impedance low point, cross-over frequencies, and power rating to develop an idea of how much power you need, the appropriate damping factor, and what architecture might be optimum. |
The more I've dug into amp designs and uses the less important watts seem to be. A watt is a composite value derived from voltage and current that really means very little and only vaguely describes an amp.
I've come to a point of not thinking of speakers as "hard" or "easy" to drive. Some should be driven by voltage sources and some should be driven by current sources. I think arguments about current delivery ability and such neglect consideration of the more nuanced nature of a circuit's topology and how it's delivering voltage and current. I think everybody should build an amp and genuinely understand what every transistor, resistor, and cap is doing. Just building something simple like the 5 watt ACA amp is cheap and educational. $300 for a pair of Pass monoblocks! How do you go wrong?
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I'm not sure I've ever seen any transformer coupled amp show damping over 100 into 8 ohm. I'm also not convinced that the drawbacks of feedback are so bad it should be completely avoided. I feel like thats a knee jerk reaction like many people have about guns. It's just a tool to be used. |
@atmasphere
You really have no argument from me on any of those points.
A while back Nelson Pass wrote an article stating something to the effect that amplifiers largely a solved problem and are as much exercises in art as they are science. There are just better and worse amps for some speakers. I don't so much advocate for any feature, spec, or topology as I do some simple honesty on those matters. I'd like the process of component pairing to be less guess work and more analysis. |
Very few amps billed as a class AB amp exhibits a bias of 5-7% of it's total power unless it's specifically touted as being a high bias variation. A typical class AB bias is more like .005 to .01 watts; just barely enough to push the devices into conduction. There is NO significant class A envelope.
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The short answer is no amp is built to have 7 times the headroom of it’s rating.
First of all, 100 watts of average power playing actual music would leave you with severe hearing damage in minutes given your speakers are decently sensitive and you’re in an average sized room with them.
The ability of an amp to provide headroom depends on it’s topology and class as well. A single ended class A amp is pretty limited by its bias. A push-pull class A amp can operate decently outside it’s class A envelope in class AB until the power supply begins sagging voltage and/or current or the outputs release the magic smoke. Class A amps are rated at what their bias is. Class AB amps are rated mostly by the voltage the power supply will reliably deliver. A well built class A 25 watt push-pull amp will usually deliver 100 watt peaks reliably into 8 ohm. A 100 watt class AB will reliably deliver 100 watt peaks into 8 ohm. |
