How important is doubling down power to 2 ohm or 1ohm on Hi Fi Speakers on high dynamic range music?

[dlaloum]

The hand-waving explanation is that phase determines how voltage and current "peaks" are related. If the phase angle is 0 degrees, voltage and current are exactly in phase, and the power factor is 1. Think of a sine wave with both voltage and current in phase so the curves overlie. Power peaks where the sine waves peak, voltage and current, then falls off since they both "fall" together. The amplifier's power demand is also 1 (relatively).

As the phase angle increases, voltage and current peak at different times, sort of "spreading" the power (the product of voltage and current) over more of the cycle, and apparent power increases. At 45 degrees, current and voltage peaks are a quarter-cycle apart, and you get a series of peaks one after another (alternating current and voltage peaks). The power factor drops to sqrt(2)/2 = 0.7071, meaning real and reactive power are different due to the phase difference, but the amplifier actually delivers twice the average power (power demanded is 2). Instead of two peaks over each cycle (one positive, one negative) now you get four. As the angle goes above 45 degrees the difference in current and voltage peaks lessens again and the amplifier's power drops back towards 1.

HTH - Don

Edit: This is not technically rigorous, take "hand-waving" literally, but helps provide a picture that might help understand why phase matters.

Thank you!

Yes I understand the phase/frequency/level relationship, what I was missing was how this then interacts with V and W... (A) - and how the demands are then spread over time, and not limited to the specific momentary peak.

 

[dlaloum]

Understanding Ohm's Law, Impedance And Electrical Phase 101

Do you ever wonder what makes a loudspeaker "difficult to drive"? Do you wonder what's so special about an amplifier that is stable into a 4-ohm load? Audioholics looks to answer these questions here.

www.audioholics.com

Thank you!

 

[EdW]

@EdW very interesting. Thank you.

Watt being what is expensive, I'm wondering can I go weaker with my new purchase, but it would seem I shouldn't. I should stick in the 90-110w range. Mr. Oclee said it's not even the lowest impedance dip that is most power hungry.

Later edit: here it is "the most difficult frequencies are either side of the impedance dip where the magnitude is still fairly low and the phase angle deviates from zero. Such impedances make the amplifier output stage work much harder and can cause protection systems in the amp to trigger"

So there's more to worry than the dip.

Jack Oclee-Brown is always worth listening to and his employer, KEF, arguably make some of the best loudspeakers out there at most price points.
I still maintain that what we need to understand is the conditions which cause the amplifier to deliver most current. This is most likely to occur at low/mid frequencies at points where the impedance of the speaker goes through various minima. At high frequencies we know that we won’t be delivering 100’s of watts, music isn’t like this and besides which the tweeter will burn out if subjected to more than a few watts. So impedance minima here rarely stress the amplifier.

With non-switching amplifiers it is known that a reactive load causes an increase in dissipation in the amplifier relative to a purely resistive load and this is covered in Keith Howard’s article which I referenced earlier in this thread:

Heavy Load: How Loudspeakers Torture Amplifiers

Why, in loudspeaker reviews, is impedance measured (assuming that the magazine in question bothers to measure anything)? Generally, for one principal reason only: to establish whether the speaker presents an "easy" or a "difficult" load to its partnering amplifier. In the design context, much...

www.stereophile.com

I also mentioned earlier Douglas Self’s book “Audio Power Amplifier Design”. This book and Bob Cordell’s book “Designing Audio Power Amplifiers” describe the issue in detail and put it in context of the overall design considerations for an amplifier.
A class D amplifier will not be so adversely affected by reactive loads.

 

[peng]

Thank you!

Yes I understand the phase/frequency/level relationship, what I was missing was how this then interacts with V and W... (A) - and how the demands are then spread over time, and not limited to the specific momentary peak.

Since you understand the phase/frequency/level relationship already, just take a look of the power formula and Ohms law:

Power (Watt) = (Vrms) X Current (Irms) X Cosine of the Phase angle between Voltage and Current (that is: Cos ø, ø is the angle in degrees) so yes there is a clear relationship. Vrms is in volts, Irms in A (amperes) obviously.

V = IR for a resistor load, and V = I X |Z|, for a reactive load, where |Z| is the magnitude part of the impedance in ohms, so the phase angle does not change the relationship between Voltage, Current, and Impedance in ohms. That is, if V = 16 V, |Z| = 8 ohms, then I = V/|Z| = 16/8 = 2 A whether the load if R, for a resistor, or Z, for a combination of resistor, inductor or capacitive reactance it doesn't matter, Ohm's law still applies the same way in terms of the relationship between ohms, applied voltage and current flow.

Just a note: Stereophile sometimes would mention a sort of equivalency (based on the increased heat dissipation in the amp's output stage) when considering the loading on an amplifier, by sort of inflating the calculated current flow into a highly reactive load such as speakers with large phase angles, in order to show the much more severe load to a power amplifier for the same current.

 

[Head_Unit]

Stereophile sometimes would mention a sort of equivalency

You mean the EPDR? Like in this link https://www.stereophile.com/content/heavy-load-how-loudspeakers-torture-amplifiers-page-2
that @EdW posted above? Which I always incorrectly thought of as the resistance that would draw the same amount of current, not power dissipation. I guess that would be Equivalent Current Draw Resistance or ECDR to coin a new phrase ha ha.

Then on top of that is the consideration of how much transient current can be drawn. Ottala's paper referenced in the Stereophile link said an 8 ohm speaker could draw transient current equivalent to a 1 ohm speaker. A later paper I recall found no current draw greater than the DC resistance would indicate; I seem to recall a later paper did find some high transient currents. This area would be ripe for some new investigation.

 

[Doodski]

First the obvious admission - I am not an Electrical Engineering Geek.....(yes I did do some in Uni days but that was a long time ago)

Typical sensitivity measurements are done at 1W or 2.83Vrms... the former is dependent on impedance, which varies with frequency... the latter is not... but I doubt that most speaker sensitivity measurements are done wideband (using spots, sweeps, or pink noise)

I haven't yet been down the rabbit hole of working out what SPL I actually need at each part of the frequency spectrum... and I have a feeling that knowing this will potentially change my perspective of what amp power/impedance handling my system/speakers require.

With regards to "doubling down" - I have noted that several well regarded high current designs, (which don't double down) have the following factors:

Power @ 8ohm
Power @ 4 ohm = (Power @ 8 ohm) x 1.7
Power @ 2 ohm = (Power @ 4 ohm) x 1.5

I wonder whether in real life, the above is what most "double down amps" actually deliver, and they simply quote the 8ohm figure more conservatively to boost the 4 ohm and 2 ohm figures?

Crown XLS2500/2502 have 440W / 775W / 1200W - if it were to be quoted in "double down" marketing mode that would be 300W / 600W / 1200W

The class D amps and PWM power supplies don't do doubling down very well at the lower impedance. A unregulated linear power supply can be stiff enough to not droop much when under load and in the case of one linear output power amp that I owned it was class A from zero to 75W/ch. It doubles down all the way to 300w/ch @2 ohms. Just because it doubles down to 300w/ch does not mean they manipulated the specs. In this case it really does double as per ohm's law and it really puts out 75w/ch @ 8R.