Does amplifier wpc @ 4 or 2 ohm really matter at "low" volumes?

[wyn palmer]

All other things equal - the answer is no.

Well, I don't agree in principle.
At low power levels where current limiting does not occur then what matters (ignoring distortion in its various forms) is the output impedance.
The max power versus load tells you nothing about the "small signal" output impedance except in the case B where the power is explicitly doubled when Rload drops from 8 ohms to 4 ohms (the output impedance is exactly zero which is unlikely to say the least, but it really means that Zout<< 4 ohms).
To a first order the gain vs frequency of the amp/speaker combo is set by the output impedance of the amp and the input impedance of the speaker. Tube amp transformer taps are selected to reduce the output impedance (and as a consequence, reduce the output voltage) in order to drive lower impedance speakers and are generally sort-of constant output power versus 4 ohms or 8 ohms.
What however, is lost here is that both the speakers and amplifiers have impedances that can be highly dependent on frequency, so the rated power can be very misleading. It is possible for amps to have a lower/more constant over frequency small signal output impedance and yet not have anything approaching the doubling of max power into half the load impedance.
Remember also, if I limits then the power, I^2*R is halved when R is halved.

 

[mhardy6647]

Remember also, if I limits then the power, I^R is halved when R is halved.

I^2R

 

[wyn palmer]

I^2R

Whoops. Typo. Thanks.

 

[zaibatsu]

Usually Class AB amplifiers has Class A operation only for few milliamps, which I find very weird...

Not weird at all, the purpose of AB amplification isn't to give you any amount of actual Class A listening - as you hint, a few mA of bias isn't enough to provide any useful listening margin in pure Class A. The need for the small bias current in Class AB is because transistors are a) nonlinear in general, b) incredibly nonlinear around the cut-off point. If you had zero bias the transistor would constantly be dipping into a very poor part of its operating range and distortion would be high. So the purpose of that AB idle current is to keep the output transistors turned on - in fact this will be applied to any small signal transistors (particularly BJTs) in the signal path - with many of the smaller signal parts usually being biased much higher.

Lets make it more simple: Lets say listener in previous example listens music at ~1W (very common situation)

I think in theory you actually have the right idea when it comes to your original question. You've understood the scenario correctly, in theory with all else equal, your scenario *should* result in the same listening experience. In reality, in my experience (as slimecity mentioned, and as Nelson Pass hints to the diy community) some lower powered amps particularly with lower output device counts can even sound superior - particularly if they are treated to the same high-headroom power supplies as higher powered amps (which isn't very common, unfortunately).

However as everyone else said, in many cases the higher powered amps are simply built better in many other areas (esp. PSU) and most good speakers do have deep impedance dips which place very peaky demands on the amplifier - just as real music isn't a constant peak-limited waveform either. So in short, while I think you have understood the theory correctly, in real life it's not applicable to much unless you are designing and building your own amplifiers. The reason is because even in two amps of the same "family" from a commercial brand (eg. a 75W and 100W with "all else equal", even on the spec sheet), there are almost always vast differences in areas that are not represented on the specsheet.

So in short - trust your basic knowledge and understanding - but realise the limitations of that, and let your ears decide.

 

[EngineerNate]

I think zaibatsu hits on a key point. Unless you're buying boutique gear, it's highly likely that a 15W amp from manufacturer A is going to have less engineering put into it than 150W amp from manufacturer A. That starts at the PSU and works through the amp to the output.

Maybe that's why some here really prefer low power stuff from the early days. A 30W amp from 1970 was likely much nearer the TOTL models than a 30W amp from the late 70s onward.

 

[Gibsonian]

Well I've ran some very low impedance loads into some not so stable amps and for the original question, the answer is no, you don't an amp rated for low impedance loads to do this if you run at low volumes all the time. As was stated, if you don't exceed the volt/amp (and obviously the voltage is safe here) capabilities of said amp it will work just fine.

Quoted for truth and emphasis.

Doubling down of amplifier output only occurs on paper and in the minds of the un-informed.

Krell FPB 300 for example is rated at 300, 600 and 1200 wpc respectively into 8, 4, and 2 ohm loads. Thorough testing on a competent tech's bench will show that the amp actually puts out just under 600 wpc into 8 ohms, a bit over 900 wpc into 4 ohms, and just shy of 1300wpc at 2 ohms.

There are other amplifers out there that come closer to the doubling down trick, but there are none to my knowledge that actually double their output with halving the load impedance.

Thanks for pointing this out. I've seen many manufacturer specs showing the double down, but in actual testing it's pure baloney, i.e. a marketing tool. And it works! I've seen many individuals state within audio forums (won't name any audio site names here of course ) that that is what they look for in an amp, only thing they will buy, etc.

 

[wyn palmer]

Well I've ran some very low impedance loads into some not so stable amps and for the original question, the answer is no, you don't an amp rated for low impedance loads to do this if you run at low volumes all the time. As was stated, if you don't exceed the volt/amp (and obviously the voltage is safe here) capabilities of said amp it will work just fine.



Thanks for pointing this out. I've seen many manufacturer specs showing the double down, but in actual testing it's pure baloney, i.e. a marketing tool. And it works! I've seen many individuals state within audio forums (won't name any audio site names here of course ) that that is what they look for in an amp, only thing they will buy, etc.

They probably are viewing this as a proxy for having a very low output impedance, which as a metric for quality sound is a fallacy.
I drive a pair of hybrid electrostatics plus sub woofers with a 160W/ch tube amp. I use the "4 ohm" output of the amp which actually has an output impedance that increases from a few milliohms at 10Hz to about an ohm at 20kHz- so it looks like an inductor not a resistor. I use a sub 40mohm low inductance interconnect wire and the lowest impedance of the speaker is about 0.9 ohms real at about 20kHz, so the frequency response slopes downwards at high audio frequencies.
It sounds perfectly fine- in fact the gentle roll off at the high end is quite musical. At frequencies below about 300Hz the input impedance of the speaker is a few kohms so the amp output impedance really hardly matters and the distortion/power spec of the amp is largely irrelevant.

 

[Racingh11]

There are several factors that effect wattage vs speaker impedance. The biggest is the current capability of the transformer. Transformer power is primarily determined by core size, then awg of windings. Next is the ESR of the power capacitors, resistance of wiring, connections, trans-impedance of the transistors. Also emitter/source resistors. Some MOSFET amps do not use any source output resistors. All of this directly effects the damping factor. It goes proportionally lower as speaker impedance goes down. An amp that has a high static power supply voltage that drops considerably during load will have good dynamic range, but poor RMS and poor low impedance capability. I believe there is a difference if it is a common-emitter or common-collector design as well. From my perspective, common-collector is superior in actual use compared to common-emitter. The reason being is that common-collector has a "built in" feedback that responds instantly to impedance changes and drives accordingly, where common-emitter relies completely on the negative feedback loop to control the response to change, power supply voltage has some effect as well. Essentially one is a voltage controlled amp with voltage feedback and the other is current controlled with voltage feedback. If you think about the impedance spike at the speaker resonant frequency, the common-collector naturally keeps the voltage amplification where it should be and the speaker will be controlled better, where a common-emitter would raise the voltage when impedance goes up and would become much louder at the resonant frequency if the negative feedback loop wasn't there. Speakers also respond and resonate to sound and reverberations in the listening environment that need to be canceled out. Common-collectors also are better at increasing power supply rejection ratio. Common-emitter may look better as far as slew-rate and distortion specs when tested on a dummy-load, but in actual use it is not.

 

[OMGCat!]

Thanks for pointing this out. I've seen many manufacturer specs showing the double down, but in actual testing it's pure baloney, i.e. a marketing tool. And it works! I've seen many individuals state within audio forums (won't name any audio site names here of course ) that that is what they look for in an amp, only thing they will buy, etc.

The "doubling down" parroting you read on forums has to be one of the most annoying things I run into on forums. I'd also add the whole "negative feedback is the devil" as a close second.

Yes, an amp that has a stout power supply is good. Yes, having gobs of negative feedback to correct design short comings or to beef up the THD numbers can be bad. An amp not doubling down (on the spec sheet), or employing local or global feedback does not instantly create a monster that will make your ears bleed no matter how many times you read about it on forums.

 

[62caddy]

Well, I don't agree in principle.
At low power levels where current limiting does not occur then what matters (ignoring distortion in its various forms) is the output impedance.
The max power versus load tells you nothing about the "small signal" output impedance except in the case B where the power is explicitly doubled when Rload drops from 8 ohms to 4 ohms (the output impedance is exactly zero which is unlikely to say the least, but it really means that Zout<< 4 ohms).
To a first order the gain vs frequency of the amp/speaker combo is set by the output impedance of the amp and the input impedance of the speaker. Tube amp transformer taps are selected to reduce the output impedance (and as a consequence, reduce the output voltage) in order to drive lower impedance speakers and are generally sort-of constant output power versus 4 ohms or 8 ohms.
What however, is lost here is that both the speakers and amplifiers have impedances that can be highly dependent on frequency, so the rated power can be very misleading. It is possible for amps to have a lower/more constant over frequency small signal output impedance and yet not have anything approaching the doubling of max power into half the load impedance.
Remember also, if I limits then the power, I^2*R is halved when R is halved.

So many audio fans just cannot resist doing this - redesigning parameters to fit a particular narrative. Somehow, some way they just have to justify - at any cost - of having an oversized megawatt, high-dollar amplifier around and saying how much "clearer", "effortless" it sounds - and all other sorts of meaningless hyperbole.

For sake of argument, let's say amp A really IS more "effortless" sounding (whatever that means) than amp B - why would anyone automatically assume it was on the basis of A being a more powerful than B when it may have simply been a better unit? What if B was out of spec and in need of service/repairs? What if that B was simply a poor performer to begin with?

We all know if the impedance too low for certain models can be problematic. We all know that impedance is dependent on frequency. We all know a bigger amplifier can play louder than a smaller amplifier which will help with dynamic range, etc etc etc. But for the sake of THIS discussion, it is a given that no condition is presenting itself in which either amplifier is being called to operate outside its limits plus another given that all other things are equal!

That said - the OP's question is what sonic benefit(s) is directly attributable to the fact that one amplifier has more output capability that another - and that alone, period.

If there is a scientific basis for example, 1 watt @ 8 ohms being supplied by a 10 watt amplifier and the same supplied from a 1,000 amplifier (identical in all other ways) I would very much like to know what that basis is for one sounding better than the other.

 

[restorer-john]

Let's just say (as an example) the bridge rectifier of the power supply can only handle 4A--and we back-calculate
4A/8 ohm = max output of 128w with a required rail voltage of 32V
4A/4 ohm = max output of 64w with a required rail voltage of 16V

Now let's go the other way--let's just say (as an example) the maximum rail voltage supplied by the power supply is 46V--and we (again) back-calculate
46V/8 ohm = max output of 264.5w and requires 5.75A current
46V/4 ohm = max output of 529w and requires 11.5A current

128W into 8 ohms requires a rail voltage of +/- 45V plus junction losses, not 32V.
64W into 4 ohms requires a rail voltage of +/-22.6V plus junction losses, not 16V.

+/- 46V rail voltage will give 32.5V RMS which (with zero junction drops) will give 132W into 8 ohms.
+/- 46V rail voltage will give 32.5V RMS which (with zero junction drops) will give 264 W into 4 ohms.

You are confusing rail voltages with RMS waveform voltages. The rail voltages need to cover the peak value of the waveform.

 

[wyn palmer]

So many audio fans just cannot resist doing this - redesigning parameters to fit a particular narrative. Somehow, some way they just have to justify - at any cost - of having an oversized megawatt, high-dollar amplifier around and saying how much "clearer", "effortless" it sounds - and all other sorts of meaningless hyperbole.

For sake of argument, let's say amp A really IS more "effortless" sounding (whatever that means) than amp B - why would anyone automatically assume it was on the basis of A being a more powerful than B when it may have simply been a better unit? What if B was out of spec and in need of service/repairs? What if that B was simply a poor performer to begin with?

We all know if the impedance too low for certain models can be problematic. We all know that impedance is dependent on frequency. We all know a bigger amplifier can play louder than a smaller amplifier which will help with dynamic range, etc etc etc. But for the sake of THIS discussion, it is a given that no condition is presenting itself in which either amplifier is being called to operate outside its limits plus another given that all other things are equal!

That said - the OP's question is what sonic benefit(s) is directly attributable to the fact that one amplifier has more output capability that another - and that alone, period.

If there is a scientific basis for example, 1 watt @ 8 ohms being supplied by a 10 watt amplifier and the same supplied from a 1,000 amplifier (identical in all other ways) I would very much like to know what that basis is for one sounding better than the other.

Well, in general, because distortion not caused by clipping- that is abrupt hard limiting- increases exponentially with amplitude, two amps with identical max distortion specs but with as you say, 100x difference in power will produce 100x different levels of distortion at 1W, with the high power amp being better.
This may or may not be important. It obviously makes a difference how your system is configured and how loudly you like to listen.
For example, at one point I used a 40W/ch tube amp into a pair of electrostatics and I frequently experienced limiting distortion on musical peaks and a loss of resolution (congestion) at levels approaching full power but not actually clipping. I now have 160W/channel tube amps driving electrostatic panels above 350Hz, a hybrid analog DSP multichannel crossover with two 200W/ch class D amps driving the woofers above about 40Hz and a 400W/ch class D amp driving the 16-40Hz sub woofer, and I never notice any distortion or congestion in the sound- which is, as you say, quite effortless, so nearly 1.2kW of amp power definitely is better than 80W.

 

[bowtie427ss]

Not weird at all, the purpose of AB amplification isn't to give you any amount of actual Class A listening - as you hint, a few mA of bias isn't enough to provide any useful listening margin in pure Class A. The need for the small bias current in Class AB is because transistors are a) nonlinear in general, b) incredibly nonlinear around the cut-off point. If you had zero bias the transistor would constantly be dipping into a very poor part of its operating range and distortion would be high. So the purpose of that AB idle current is to keep the output transistors turned on - in fact this will be applied to any small signal transistors (particularly BJTs) in the signal path - with many of the smaller signal parts usually being biased much higher.


I think in theory you actually have the right idea when it comes to your original question. You've understood the scenario correctly, in theory with all else equal, your scenario *should* result in the same listening experience. In reality, in my experience (as slimecity mentioned, and as Nelson Pass hints to the diy community) some lower powered amps particularly with lower output device counts can even sound superior - particularly if they are treated to the same high-headroom power supplies as higher powered amps (which isn't very common, unfortunately).

However as everyone else said, in many cases the higher powered amps are simply built better in many other areas (esp. PSU) and most good speakers do have deep impedance dips which place very peaky demands on the amplifier - just as real music isn't a constant peak-limited waveform either. So in short, while I think you have understood the theory correctly, in real life it's not applicable to much unless you are designing and building your own amplifiers. The reason is because even in two amps of the same "family" from a commercial brand (eg. a 75W and 100W with "all else equal", even on the spec sheet), there are almost always vast differences in areas that are not represented on the specsheet.

So in short - trust your basic knowledge and understanding - but realise the limitations of that, and let your ears decide.

I seldom use the like button, but i did here.

Very well stated and easily digested post.

Thanks for posting!

 

[62caddy]

Well, in general, because distortion not caused by clipping- that is abrupt hard limiting- increases exponentially with amplitude, two amps with identical max distortion specs but with as you say, 100x difference in power will produce 100x different levels of distortion at 1W, with the high power amp being better.
This may or may not be important. It obviously makes a difference how your system is configured and how loudly you like to listen.
For example, at one point I used a 40W/ch tube amp into a pair of electrostatics and I frequently experienced limiting distortion on musical peaks and a loss of resolution (congestion) at levels approaching full power but not actually clipping. I now have 160W/channel tube amps driving electrostatic panels above 350Hz, a hybrid analog DSP multichannel crossover with two 200W/ch class D amps driving the woofers above about 40Hz and a 400W/ch class D amp driving the 16-40Hz sub woofer, and I never notice any distortion or congestion in the sound- which is, as you say, quite effortless, so nearly 1.2kW of amp power definitely is better than 80W.

Here we go again, the goalposts have been moved to fit the narrative - again. What part of "all other things equal" do people not understand.

The fact that one amplifier is distorting more than the other is not the issue! Not all high powered amplifiers necessarily have less distortion than lower powered amplifiers - and certainly it is not due to mere fact of being a higher powered amplifier. This is not a difficult concept to understand.

Go up to McIntosh and speak to the engineers there.

 

[damacman]

Again, science has no place in audio ...

 

[62caddy]

There's a difference between science and religion.

 

[Racingh11]

Well, in general, because distortion not caused by clipping- that is abrupt hard limiting- increases exponentially with amplitude, two amps with identical max distortion specs but with as you say, 100x difference in power will produce 100x different levels of distortion at 1W, with the high power amp being better.
This may or may not be important. It obviously makes a difference how your system is configured and how loudly you like to listen.
For example, at one point I used a 40W/ch tube amp into a pair of electrostatics and I frequently experienced limiting distortion on musical peaks and a loss of resolution (congestion) at levels approaching full power but not actually clipping. I now have 160W/channel tube amps driving electrostatic panels above 350Hz, a hybrid analog DSP multichannel crossover with two 200W/ch class D amps driving the woofers above about 40Hz and a 400W/ch class D amp driving the 16-40Hz sub woofer, and I never notice any distortion or congestion in the sound- which is, as you say, quite effortless, so nearly 1.2kW of amp power definitely is better than 80W.

Seems like there are two questions going here; whether 4 ohm sounds worse at low volumes on a 8 ohm rated amp, and does a high power amp sound better at low volume than a lower power amp. I guess the true test would be; If you took a high power amp with +/-100 volt supply at the rail and did a distortion test and a listening test at low volume with 4 and 8 ohm speakers. Then try the same tests on the same amp, but supply the rails with +/-30 volts and see what differences can be found. I would suspect there may be slight differences between measured distortion, but no perceived difference between 4 and 8 ohm speakers. If the 4 ohm speakers don't demand more current, voltage, and slew-rate to reproduce the input sound than it is capable of, then it will play the same. The only spec that would change significantly would be damping factor, and at low volumes where the speaker cone does not travel as fast or as far for any given frequency, it would not lot be heard. The S/N ratio would also decrease slightly do to less voltage swing over the noise floor because of the lower volt/ higher amp 4 ohm setup. But again, the difference in voltage between the two at low volume is only slight compared to what it would be at higher power. Say 8 ohm is at 2.83v and 4 ohm is at 2 volt so they are both played at 1 watt, the difference is only 0.83v. Compare this to 50watt where the difference would be 5.86 volts. My guess is there is not a person that has hearing good enough to find any difference and most people hear with a placebo-effect to fit what they believe is better. Another version of this test could be to use the same amp, but change only the negative feedback ratio so that the gain of the amp changes the maximum power it is capable of and check distortion and listen to it.

 

[SoCal Sam]

Good 2 and 4 Ohm ratings are indicative of a high speed amp. If you are into objective accuracy, like I am, then this rating is important.

 

[wyn palmer]

Good 2 and 4 Ohm ratings are indicative of a high speed amp. If you are into objective accuracy, like I am, then this rating is important.

I'm sorry. I have no idea what you mean by this comment. Please expand.

 

[Racingh11]

I'm sorry. I have no idea what you mean by this comment. Please expand.

I guess he values top speed over 0-60 times. lol
The only thing I could perceive that comment to mean would be slew-rate and rise times.