Thanks Erik. But I'm still not sure I understand and maybe I won't. Are you saying that driving a high efficiency low impedance mid/tweeter section will be no easier for the amplifier than driving a full range lower efficiency speaker that dips to a similarly low impedance in the bass? (assume we're talking about cone/dome speakers, not ESLs/panels etc)
Amplifier stability with very low impedance, high efficiency mid/tweeter section???
I've been looking for this information everywhere but can't seem to find a clear cut answer. I understand that a very low minimum impedance like 2Ohms is a very difficult task for most amplifiers to drive and may even damage it. But it's also my understanding that this is a current, not a voltage problem. In other words, say we had an MTM speaker that was used ONLY as a midrange from 200hz up, so it wouldn't have to play bass where most current is required. And say it also has a very high efficiency of 97db but also a very low impedance of 2 ohms. Would this be a problem for most amps? Could it damage the speakers? I'm thinking no since the amplifier wouldn't have to put out much voltage or current to output sufficient SPL. But what's the actual answer????
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Think of this another way.
You have multiple gates you must pass. You have absolute maximum current and voltage swings which the amplifier can output before failure, or power supply exhaustion. So you solve those issues by making sure the output never exceeds either. OK, but then the issue is, is the output flat? Unless the amplifier has a very low output impedance and the driven impedance is very flat, probably not. |
Here is a typical measurement from a tube amplifier: https://www.stereophile.com/content/doshi-audio-evolution-monoblock-power-amplifier-measurements Take a look at the black line which is the amp output with a simulated speaker load. As you see, the output tracks the speaker impedance. The ratio between the speaker impedance and the amplifier’s output impedance (i.e. damping factor) determines how pronounced that deviation is. |
OK. I understand that speaker impedance fluctuates with frequency. I think what you're pointing out is that a low dip in a speaker's impedance (no matter at which frequency) is going to present a problem for a typical amplifier to track a flat frequency response because most amps cannot double their current when dropping from 4 to a 2ohm load (and tube amps halve their current). And that a low impedance can blow up an amplifier even if it's not playing current demanding material (bass), correct? I also didn't know that an amplifier's output impedance fluctuates with frequency response... |
It's a power and heat sinking problem. The lower the load the amplifier sees the more heat has to be dissipated by the output transistors. Transistors have a maximum operating temperature and they'll fail at that temp so amps are designed with thermal cutoffs well below that limit if the heat sinks get too hot.
But... if you lower the volume then there is less heat dissipated and if that volume level is loud enough (courtesy of the high SPL of the speakers) and the amp is happy and not overheating at that volume level, then it's not a problem. The amplifier does not care what the SPL rating of the speaker is, that is between the speaker and it's crossover and internal volume -- the amplifier sees an inductive load only. To give you an idea of the heat sinking requirements: say I want an amplifier with only one pair of output devices, 50 watts at 8 ohms, biased low A/B, with 36 volt rails. At peak signal input (say 1.2 volts) the power the transistor dissipates at peak is about 45 watts. At 2-ohm loading it's 150 watts. Can't do it, needs more pairs of transistors so the amp is limited in its power output to not lower than 4 ohms.
But if you cut the input voltage with the volume pot down to, say, 200 millivolts -- and that is loud enough for you -- the power dissipated at 2-ohms is 50 watts, which is within the max power spec at 8 ohms. |
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