How much amplifier power do you realy need?

[Ralf11]

yup - need to look at the Impudence plot of the speaker

[fas42]

1 hour ago, sandyk said:

 

 Are you talking about sustained current capability here ? That is mainly a function of the power supply area.

 

Yes. Most people don't appreciate that the majority of power supplies used in power amplifiers are quite sub-standard; which is why these components all sound so different when asked to deliver higher volumes to 'difficult' speakers.

 

The fantasy that the voltage rails merrily sit at the designed levels, irrespective of what the amplfier is asked to do, hangs on grimly ... simulating real world amplifiers, with real world power supplies, and real world mains power shows what a mess it actually is - the voltage rails go in a  merry dance, and form a quite remarkable audio signal in their own right. Many years ago I did this as an actual experiment - use an ordinary power amplifier to drive a load, and tap its power supply rails to drive the line inputs of another setup, and listen via speakers. As the first amplifier was asked to deliver more and more power, the monitoring rig would go from almost dead silence to a tremendous, awful cacophony of jarring noise - the power supply of the first system was in a chaotic state.

[audiobomber]

3 hours ago, sandyk said:

 

 Are you talking about sustained current capability here ? That is mainly a function of the power supply area.

Both; sustained current to be able to supply the load, but also the amplifier's power supply needs to provide instantaneous current for transients, with no sagging. 

[audiobomber]

2 hours ago, mansr said:

There is a simple relationship between current, resistance, and power. With reactive loads, it gets a little more complex, but not terribly so.

It is not nearly so simple to achieve with a reactive load, that's why a cheap receiver rated for 200W sounds anemic compared to a 25W Pass amp.

[GregWormald]

The above ½ dozen or so comments give some credence to the utility and sound qualities of current drive amplifiers when matched to appropriate speakers.

 

When my active speakers were upgraded to current drive the improvement was worth the cost.

[Confused]

14 hours ago, audiobomber said:

IMO this is a simplistic and deceptive video. Watts per channel is next to useless as a measure of an amp's ability to drive a loudspeaker. Current is what really matters, not voltage. I will forever recall hearing a 40W amp blow away a 200W amp in a store demo decades ago. A real eye-opener.

 

In the video they strapped the amp channels. Monoblocking is for sound reinforcement, not for home stereo; lots of voltage, but compromised current capability, thus inferior SQ. If they had vertically biamped instead, the sound would have improved by leaps and bounds, and the amps would still not have run out of steam.  

 

How about the infamous Apogee Scintilla, which only a handful of amps could handle? One of the most famous pairings was the 25wpc Classe. Most high-powered amps totally crapped out with this 1-ohm load.

 

I have two stereo amps from the same manufacturer rated at 50Wpc into 8 ohms. My speakers are 89dB/w/m, and 4 ohms. Driving the speakers with one stereo amp sounds fine, even at the highest levels I care to listen. Adding the second amp in vertical biamp mode increases sound quality markedly, with improved bass control, soundstage, instrument separation and detail. Watts per channel does not explain the difference in these two setups.

So how were the amps delivering 750 watts if they were not capable of producing current?  For a given voltage watts and amps (current) are directly proportional to each other.  Don't get me wrong, I fully agree that current matters, and that "wpc" ratings are often highly misleading, but @mansr is right, there are simple relationships between current, impedance and power. (and volts for that matter)  If you have lots of current, you will by definition have lots of watts, assuming you have some volts too of course.

 

The point being, I agree with you re current, and bi-amping can be a better option than bridging, depending on the speakers impedance characteristics, but the video still has some interest I think, in that the amps are showing some surprisingly high power figures, in what seamed a fairly normal "real world" situation.

[Confused]

As an open question, how does amplifier power, current demand and so on relate to the dB "A" weighted scale?

 

The Laptev Sea track used in the video is has a lot of low bass content, right down to frequencies more felt that heard, so not just below 100Hz, but dipping below 20Hz.  Looking at the a weighted scale (see graph below), these low frequencies "sound" -30dB down to human hearing at say 40Hz, even more as you drop to 30 or 20Hz.  -30dB is a lot!  So if you take the power needed to reproduce 1000Hz at 90dB(a), then you would need fully 1000 times the amplifier power for an equally loud sounding sound at 40Hz.  I am thinking aloud a little bit here, but could this be a factor in what we are seeing in the video?

 

Or to put this another way, it is easy to say I have speakers with an efficiency of 90dB 1 w / 1 m, so I do not need more than say 10 w for my normal listening level of 90dB at 3m, but if the bass frequencies need an order of magnitude more power to reach 90dB(a), how exactly does this work?

 

 

[mansr]

8 hours ago, audiobomber said:

It is not nearly so simple to achieve with a reactive load, that's why a cheap receiver rated for 200W sounds anemic compared to a 25W Pass amp.

Ohm's law generalises nicely to AC and reactive loads.

[PeterSt]

As far as I know, the power required for playing "music" is unknown hence it is undoable to calculate it. Keep in mind this is not about test signals. The reactive load is more or less important, but still not helpful to calculate it for music.

 

1 hour ago, Confused said:

but if the bass frequencies need an order of magnitude more power to reach 90dB(a), how exactly does this work?

 

You assume that the bass frequencies need more power. Maybe this is so, maybe not (my experience tells it is the high frequencies - but apply some math and you will know it for sure).

Still the plot is in that quote because it is about an unknown number of frequencies at the same time. No math exists for that.

 

Maybe someone can point to a nice article about how it's done - I am willing to learn.

[PeterSt]

Something I miss in the discussion:

 

It is about the PSU all right and how it is capable to deliver a constant high current (FWIW). But what I do not see passing by is that the capacity for current is depicted by heat dissipation;

If a PSU can deliver for 0,5 seconds, it can deliver infinitely when the heat dissipates as fast as heat is added.

 

Add to the above that the PSU (and more) can slowly collapse at the same power requirement because of it getting less efficient the more hot all becomes. So the current requirement gets higher because of that, and this in itself implies more heat again.

You could say that this is the reason a PSU must be fairly over sized. But in the end it is about sufficient dissipation.

 

Things get a little bit more complicated when other elements than the PSU (like a transistor etc.) heat the space in the cabinet and with that heat up the PSU again. Same thing as above, but now implied by other parts getting hot. Or, the PSU heats up those parts and amplification collapses because getting less efficient.

 

Heat is never your friend ...

[sandyk]

17 minutes ago, PeterSt said:

Heat is never your friend ... 

 

Which is why with my DIY 15W/Channel Class A amplifier, whose 2 large heat sinks dissipate a total of  more than 80 Watts of heat, 2 separate dual polarity + and - 20V regulated supplies are mounted in an external 2U rack case, along with 2 separate 18-0-18V toroidal transformers for the Class A Preamp which is powered via the  low voltage AC supplies.

[jabbr]

48 minutes ago, PeterSt said:

Still the plot is in that quote because it is about an unknown number of frequencies at the same time. No math exists for that.

 

Maybe someone can point to a nice article about how it's done - I am willing to learn.

 

The math exists of course. The complex form of Ohm’s Law is your friend. 

 

There are two different approaches

 

1) measure the output impedance of both your amp & speaker at each frequency and power level from say 20-20 kHz (or wider if you want) and then use the Fourier transform of you input signal — you have the digital file. 

 

2) SPICE

 

You can measure or compute power dissipated by power supply, amplifier and speakers each using complex Ohm’s Law and  the total power is the sum of power from each frequency.

 

 

[R1200CL]

18 hours ago, Ralf11 said:

Current is what really matters, for the example you chose and some other panel design types (my Maggies for example).

I had Some fun using a clamp on meter on my speaker cables. I think I was close to 15A. 
Martin Logan Odyssey. 

[Summit]

2 hours ago, PeterSt said:

As far as I know, the power required for playing "music" is unknown hence it is undoable to calculate it. Keep in mind this is not about test signals. The reactive load is more or less important, but still not helpful to calculate it for music.

 

 

You assume that the bass frequencies need more power. Maybe this is so, maybe not (my experience tells it is the high frequencies - but apply some math and you will know it for sure).

Still the plot is in that quote because it is about an unknown number of frequencies at the same time. No math exists for that.

 

Maybe someone can point to a nice article about how it's done - I am willing to learn.

 

Yes the bass need more power than the mids and treble to get the same SPL. The lower the frequency the bigger and slower is the audio weaves. Low-frequency (bass) sound has long wavelength (many meters long) while the high-end waves (treble) has much shorter wavelength of just a few millimetre.

 

It is easy to comprehend that to move a lot of air, to produce a 20 Hz bass tone, you need much much more power than to move a 10 KHz treble tone. Low bass has so much energy that it can be heard on long distance and thru concrete walls.  

 

[Summit]

It’s not possible to generally say how much power that “you need”. It all relative to which speakers used, the listening level and the size of the room. It is also about which type of amp design you have (SS/tube, Class A/AB/D) and in combination with which transducer. What you really want is a powerful amplifier, and a powerful amplifier isn’t only about the amount of power/watts it can deliver, like others have already explained.

 

For high sensitive horn speakers 3-8 watt is probably all it needs. For moderate sensitive flor standing speakers in a normal sized living room, around 100-200 A/B watts. The only way to really know for sure is to test the amp at home.

 

And then we have the quality of the amp and the first watt principle .

[matthias]

I have passive speakers with a sensitivity of 99dB/W/m and a power amp which delivers around 100W/channel.

Enough headroom to get the illusion of a live concert.

 

Matt

[audiobomber]

6 hours ago, Confused said:

So how were the amps delivering 750 watts if they were not capable of producing current?  For a given voltage watts and amps (current) are directly proportional to each other.  Don't get me wrong, I fully agree that current matters, and that "wpc" ratings are often highly misleading, but @mansr is right, there are simple relationships between current, impedance and power. (and volts for that matter)  If you have lots of current, you will by definition have lots of watts, assuming you have some volts too of course.

 

The point being, I agree with you re current, and bi-amping can be a better option than bridging, depending on the speakers impedance characteristics, but the video still has some interest I think, in that the amps are showing some surprisingly high power figures, in what seamed a fairly normal "real world" situation.

Double the output voltage is available in monoblock mode, current capability is halved because the amp sees half the load impedance. An 8 ohm speaker load becomes 4 ohms, and if it dips to say 3 ohms in the bass like the Nautilus 802, the amps see 1.5 ohms at those frequencies. This is a punishing load, and most amps will not be able to supply sufficient current to deliver full power.

 

Audio Engineer's Reference Book 2.8.3 (2)

"The amplifier may run out of ability to provide the highest peak instantaneous output currents required, even though it may have something in hand for peak voltages. This is called current clipping."

 https://books.google.ca/books?id=XOvf30iChsYC&pg=SA2-PA118&lpg=SA2-PA118&dq=amplifier+"current+clipping"&source=bl&ots=DdxLBLbZWb&sig=ACfU3U3y7TE1ekpKonvy99i6-bsCr7EbBg&hl=en&sa=X&ved=2ahUKEwiJ993K3LTjAhVGOs0KHZCDCRcQ6AEwB3oECAkQAQ#v=onepage&q=amplifier "current clipping"&f=false

 

I would have liked to see an SPL reading in the video. At one point one of the men is shouting very loudly to the other, from inches away, and has to repeat himself to be heard. Clearly these were ear-damaging exposures that I would avoid. Lowering the level by a few dB would radically decrease the power requirements and make better music. Of course that was not the agenda.

 

[Confused]

3 hours ago, audiobomber said:

Double the output voltage is available in monoblock mode, current capability is halved because the amp sees half the load impedance. An 8 ohm speaker load becomes 4 ohms, and if it dips to say 3 ohms in the bass like the Nautilus 802, the amps see 1.5 ohms at those frequencies. This is a punishing load, and most amps will not be able to supply sufficient current to deliver full power.

 

Audio Engineer's Reference Book 2.8.3 (2)

"The amplifier may run out of ability to provide the highest peak instantaneous output currents required, even though it may have something in hand for peak voltages. This is called current clipping."

 https://books.google.ca/books?id=XOvf30iChsYC&pg=SA2-PA118&lpg=SA2-PA118&dq=amplifier+"current+clipping"&source=bl&ots=DdxLBLbZWb&sig=ACfU3U3y7TE1ekpKonvy99i6-bsCr7EbBg&hl=en&sa=X&ved=2ahUKEwiJ993K3LTjAhVGOs0KHZCDCRcQ6AEwB3oECAkQAQ#v=onepage&q=amplifier "current clipping"&f=false

 

I would have liked to see an SPL reading in the video. At one point one of the men is shouting very loudly to the other, from inches away, and has to repeat himself to be heard. Clearly these were ear-damaging exposures that I would avoid. Lowering the level by a few dB would radically decrease the power requirements and make better music. Of course that was not the agenda.

 

Out of interest, how loud would you estimate the SPL’ s to be?   OK, there is much speculation here, but from about the 6:00 min mark in the video you can just about make out the voices, not what they are saying, when the camera and microphone are about three to four metres away from those talking.  After 7:00 min when the camera closes up on one attendees smart phone to show the track name, you can clearly hear his voice and the camera is maybe half a metre away.  At one point, three of the guys are gathering around very close to the right hand speaker, which to me makes me think that that were running sub 100dB, loud but nothing crazy.

 

From this, I would estimate something a little over 90dB, with peaks under 100dB. This is just a guesstimate though, it is very difficult to judge.  I too would have likes to see an SPL reading, it is easy enough to do with a smart phone app or similar, so it is a very disappointing omission.

[Ralf11]

speaking of heat...

 

amplifiers are of course designed by EE's - and I have noted that EE, EEE majors just hate taking an engineering heat exchange course...

 

I suspect a fair degree of 'low' design could be traced back to the proclivities of undergraduates...

[jabbr]

13 hours ago, Confused said:

So how were the amps delivering 750 watts if they were not capable of producing current?  For a given voltage watts and amps (current) are directly proportional to each other.  Don't get me wrong, I fully agree that current matters, and that "wpc" ratings are often highly misleading, but @mansr is right, there are simple relationships between current, impedance and power. (and volts for that matter)  If you have lots of current, you will by definition have lots of watts, assuming you have some volts too of course.

 

This highlights an important issue, and with other folks saying “current matters” let’s try to pull this all together with Ohm’s Law. 

 

Typically 750 Watts is specified at 8 Ohms. When the typical speaker doesn’t act like a perfect resistor, the actual impedance is not always 8 Ohms. A passage might present a collection of frequencies that presents a 4 Ohm impedance — to maintain 750 Watts, the current needs to double. The amp might not be able to handle that so “750 Watts” doesn’t act like 750 Watts. Conversely a “better” amp might feed 750 Watts into 1 Ohm. That’s 8 times the current!

 

You could look at the power as a function of impedance instead.

[fas42]

What's not being brought into the discussion is energy - your smartphone is a good example of this, with respect to how the battery behaves; hardly use it, and the phone will go for days; but sit down and run apps that work the processor hard - the device will die in a very short order ...

 

Strong bass lines require energy, lots of it; power, which is how fast the energy can be delivered is not so relevant here - and if the power supply can't keep up with the requirement of the energy needed to reproduce the bass frequencies - then the PS if not well designed sags, and audible problems become obvious. Think of your power amplifier's PS as being a battery, with a small charger attached, which keeps trying to top up the battery to the right levels as the demands fluctuate - strong bass will drain the battery fast, like a car's starter motor constantly cranking - will the 'charger' be good enough to keep refilling?

 

I have never found heat to be relevant - correctly designed and implemented heatsinking will always keep this factor under control.

[One and a half]

The measurement for damping factor is completed by resistance only, so those with a multimeter can measure the amplifier end and the speaker and work out the damping factor, to a degree. 

The other issue that affects damping factor is the resistance of the cables, either look up the ohms per km for your cable or a sensitive multimeter using the current\voltage method to measure resistance with a AA battery.

[jabbr]

21 hours ago, One and a half said:

The measurement for damping factor is completed by resistance only, so those with a multimeter can measure the amplifier end and the speaker and work out the damping factor, to a degree. 

 

Some may complete the measurement of damping factor this way but the damping factor is frequency dependent and DF-0Hz is not the same as DF-1kHz ... the damping factor is typically defined as Za/Zs where Za is the actual output impedance of the amplifier and Zs the speaker impedance, so a curve, not a simple value. If the cables matter, either you have a really good amplifier in which case the cables really don't matter or really really bad cables.

[jabbr]

Correction: DF = Zs/Za (Zs - speaker not source), more generally Zload/Zsource

[sandyk]

On ‎7‎/‎15‎/‎2019 at 12:13 AM, Summit said:

Yes the bass need more power than the mids and treble to get the same SPL. The lower the frequency the bigger and slower is the audio weaves. Low-frequency (bass) sound has long wavelength (many meters long) while the high-end waves (treble) has much shorter wavelength of just a few millimetre.

 

 Not necessarily. Note the very fast rise and fall times in the attached from " Yim Hoh_Man-Poem of Chinese Drum."

 

 

Click on the image a few times for a much larger image.