Same watts at 8 and 4 ohms?

@unsound +1

This is the false statement, and is the typical statement of amp manufacturers that can’t double down with their amps.

This statement is false.

I recently serviced out a Realistic receiver with 18 watts. Even though it has no chance of doubling its power at full power, at power levels less than full output, it does so with ease, for the simple reason that despite its inadequate supplies and heatsinks, the amp behaves as a voltage source. Its only that last 3dB into half the impedance that is being touted as such a great thing- and really has very little effect if any on how the amp actually sounds.


Any conversation otherwise really points to not understanding how a voltage source works, and why its important. Now its been pointed out that no amp can really double its power at full output when the load is halved. This is true because of a thing called Ohm's Law. It has to do with the output impedance of the amplifier being finitely more than zero. Whatever that value is, it will always prevent the voltage amp from perfectly doubling power as the load impedance is halved ***at full power***.


Since actually doubling power at full power can't be done, how important is it? If sound quality is your goal, its likely that the amplifier topology will play a bigger factor in the sound of the amp,  on account of how it makes distortion, and how audible that distortion is, especially at lower power levels where the amp spends most of its time with real music signals. Its also unimportant because you can't operate the amp constantly at or near full power due to the dynamic nature of music. 

Ideally, a power amp should be able to deliver twice as much power at half the impedance. The way it can achieve this is if:

This statement is false, on account of the fact that list following it is not what is needed. What is important here is that the amp is able to act as a voltage source and to that end, it does not have to double power as the load impedance is halved- at full power. Any amp that has an output impedance low enough will effectively double power as impedance is halved, with the exception of when the amp nears clipping. That is the only time that doubling power might be an issue, and only important if you have a low impedance load and you really for some reason need that last 3dB, which is not a very audible increase to the human ear.

This 'doubling down' thing is mostly marketing and **waaaaay** overplayed.

what I want to know is what one hears with a 17 Hz bass response like the talons? How is that better than a speaker with a bass of 23 Hz? This is coming from the perspective that there is not that much recorded material able to provide 17 Hz.

You're right- but there are some recordings that do have that sort of information. There's a recording of the Saint Saens Organ Symphony on EMI that has 16Hz pedal tones. Its not the only recording with notes that low.

If your amp puts out 150 watts into 8 ohms and 200 into 4 ohms, it will drive your speakers just fine if you only need 100 watts into the 4 ohm frequencies to drive them as loud as you want to listen to them.

Is that right, @atmasphere ?

It is- and more to the point, in this example above, even though the amp doesn't double power **at full power** into 4 ohms, we can easily see that it does increase. If this amp has feedback, its output impedance will be low enough to enable it to act as a voltage source- all the way up to its rated power into 4 ohms. So what if that isn't double the power of the 8 ohm rating?? We are only talking about the amp at full power. At any other power level it will be just fine, meaning that it will indeed double power as the load impedance is halved. This holds true for impedances of less than 4 ohms too- but one must be increasingly aware of the limitations of the amp when coupled to such loads. As long as the load is easy to drive (such as Wilson speakers, which are easy despite their low impedance in the bass) it won't be an issue.


Now someone touched on a different issue- that lower impedance loads cause amps to make more distortion. This is true, and is why you really don't want to make an amplifier (especially a solid state amp) work hard for a living! When it makes more distortion, especially if solid state, most of that will be higher ordered harmonics and IM distortion, both to which the ear has a keen sensitivity! IOW if **high quality audio** is your goal, regardless of your amplifier, a higher impedance load will do better justice to your amplifier investment. OTOH if **sound pressure** is your goal, you have that slight 3dB increase in volume if your speaker is half the impedance. IME the latter isn't worth it, but I like the stereo to sound nice :)

it's doubtful that they double down their power rating. Yes?

Yes, but as I pointed out, that is only a 3dB issue. Another way of looking at this is that if the rated power is enough to do the job, its no worries. It really is that simple.

@n80

Here's the deal:

With most speakers its expected that the amp will 'voltage drive' the speakers (note that I said 'most'- not all speakers behave this way!).


Now if an amplifier 'voltage drives' the speakers, IOW is a 'voltage source', the power it makes into 4 ohms will be double that of 8 ohms, with a marketing exception: it may not do that at full power. So if you are driving the amp alternately into 4 and 8 ohms, right up to nearly its rated power the 4 ohm power levels will be double that of the 8 ohm levels. But right about that point of full power this phenomena stops as the power supplies and/or output devices can't support that power doubling for that last 3dB.

And that last 3dB has little to do with how the amp sounds or for that matter, how it performs on the bench. The amps you mentioned have no worries acting as a voltage source in this conversation BTW.


It does suggest that a corner was cut somewhere, but in the grand scheme of things a relatively small corner, since 3dB simply isn't that audible to the human ear in terms of sound pressure! OTOH, that first watt is far more important as most of the time that will be where the amp is used most and it is entirely unaffected by this sort of thing.

Now if you really want that doubling of power right up to full power into 4 ohms, be my guest but you will be spending a bit more for that last 3dB, and if you happen to own 8 ohm speakers it won't do you a bit of good. My advice is listen to the amp and see if you like it, if so take it home and see if you like it there, if so, fuggettaboudit and enjoy the music :)