8/4/2 Ohm power doubling at low volume?

[TankTop]

Do well designed amplifiers (if rated for low impedance) double their power at lower output? For instance an amplifier rated at (completely making up numbers) 70/130/150, would it double at 30/60/120? Just curious if this is a relevant spec if you have a relatively low impedance speaker? For instance I have Contour 20’s which dip to around 2.9 ohms, if an amplifier can’t maintain consistent power even at moderate volumes should it be upgraded?

 

[RayDunzl]

Do well designed amplifiers (if rated for low impedance) double their power at lower output?

I'd vote "yes".

Almost.

Whatever resistive losses there are would double and double again with the increase in current.

 

[MZKM]

I’ve wondered the same. My gut response would be yes, and that it downtown double down at max wattage as it simply can’t produce that much.

 

[MakeMineVinyl]

In a perfect world, yes. But real world power amplifiers face restraints such as price, and thermal dissipation. Building an amplifier which can honestly mimic a perfect voltage source (i.e. double power at 1/2 impedance) costs a LOT of money and very few amplifiers realistically can do this for any reasonable price a typical consumer can tolerate. The problem is worse in multi-channel amplifiers. Class D amplifiers have an easier time with this requirement than other types.

So in the end, is this aspect important enough for you to spend the extra money?

 

[NTK]

Doubling power when impedance drops by half means that the amplifier behaves like an "ideal" voltage source -- meaning it can maintain the same output voltage regardless of load impedance (and therefore current draw). Properly designed amplifiers will behave like an ideal voltage source until its current output capacity is reached.

 

[ahofer]

There was a whole thread on this started by a guy with B&W speakers*. It's not a great criterion, and many of the expensive AB amps sandbag their specs to make it work (for instance, my Bryston is advertised as a 100W@8Ohm amp yet tests at 135W, allowing them to honestly say 200 into 4 ohms and show the magic "doubling"). The important thing is that the amp have the power/current you need at the low impedance point in your speakers' measurements, not the ratios between the claims.

* it appears he got banned or something, as the thread only has responses now. It got kind of annoyingly repetitive after a while. I think the answer he really wanted was "yes, that is the holy grail". and he was going to keep asking until he got it ...."what about the doubling?,,,yeah, but the doubling... I see, that means that the doubling is really important, then..."

 

[DVDdoug]

For instance I have Contour 20’s which dip to around 2.9 ohms,

Speakers (nd headphones) are tested/spec'd with a "constant voltage" source (the voltage is independent of the load). It's assumed amplifiers work that way, and it's generally true.

The speaker can have flat frequency response (based on voltage) even though it's consuming more power/current where the impedance is low, and less where the impedance is high.

 

[TankTop]

I read through all the answers and may have missed it but specifically would a properly designed amplifier rated for 2ohm double all the way down at roughly 50% rated output?

 

[MakeMineVinyl]

I read through all the answers and may have missed it but specifically would a properly designed amplifier rated for 2ohm double all the way down at roughly 50% rated output?

An amplifier which was designed to do that would.

Really, your category 'properly designed' is so broad and open to interpretation as to be pretty much useless. Its like saying a 'properly designed' car should do such-and such.

 

[dshreter]

I read through all the answers and may have missed it but specifically would a properly designed amplifier rated for 2ohm double all the way down at roughly 50% rated output?

It's not clear what this question means.

• What do you mean double all the way down at roughly 50% rated output?
• A "properly designed" amplifier will perform to its rated specifications or better. That doesn't necessarily mean power doubling, though it could be designed and specified to achieve that.
• "I have Contour 20’s which dip to around 2.9 ohms, if an amplifier can’t maintain consistent power even at moderate volumes should it be upgraded?" If an amplifier is clipping at moderate volumes in your system, then I would say yes it should be upgraded.
• I have a feeling this is a hypothetical issue. Is your amplifier actually presenting issues with driving your speakers?

 

[TankTop]

It's not clear what this question means.

• What do you mean double all the way down at roughly 50% rated output?
• A "properly designed" amplifier will perform to its rated specifications or better. That doesn't necessarily mean power doubling, though it could be designed and specified to achieve that.
• "I have Contour 20’s which dip to around 2.9 ohms, if an amplifier can’t maintain consistent power even at moderate volumes should it be upgraded?" If an amplifier is clipping at moderate volumes in your system, then I would say yes it should be upgraded.
• I have a feeling this is a hypothetical issue. Is your amplifier actually presenting issues with driving your speakers?

I’m running a Marantz 5014, for TV I run it crossed over at 80hz to a subwoofer but for music I run it pure direct. I’ve compared with my HD6XX’s and even at moderate volumes the Contour 20’s give up a little low end. I know it’s not a fair comparison but A/B test with music like Krudder or Primus with good stand up bass or bass guitar it seems like I loose a little detail and output. This is even at moderate volumes.

 

[restorer-john]

An amplifier really cannot double its actual output power as the impedance halves.

As impedance halves, the current doubles and the voltage drops across the output transistors and emitter resistors increase. That means your overall voltage swing becomes less. As power is a function of voltage and current, doubling is impossible.

Getting close to theoretical doubling requires absolute minimum voltage dropping devices (parallel up a heap of them) and a very tightly regulated supply. But you will never achieve perfect doubling of power when impedance halves.

The way manufacturers have been claiming it is by under rating the 8R and 4R figures to give the illusion audiophiles want to buy into.

 

[Doodski]

The way manufacturers have been claiming it is by under rating the 8R and 4R figures to give the illusion audiophiles want to buy into

Yes, all true. Even my beloved Kinergetics Research KBA-75 fudges the numbers to get a good linearity specification of, "Doubling down."

 

[MakeMineVinyl]

Getting close to theoretical doubling requires absolute minimum voltage dropping devices (parallel up a heap of them) and a very tightly regulated supply. But you will never achieve perfect doubling of power when impednace halves.

On our highest end amplifier, we use separate power supply rails in the driver stages and gazillions of output transistors to mitigate this problem.

 

[TankTop]

On our highest end amplifier, we use separate power supply rails in the driver stages and gazillions of output transistors to mitigate this problem.

What amps do you build?

Also someone else mentioned class D amps handle higher current better, this confuses me as they’re missing all the massive power supplies of a high end A/B amp.

 

[pjug]

At low volume I would think it should be able to put out constant voltage. You can test it fairly easily (at low volume).

Record something, then put a resistor in parallel with your speaker (e.g. 8 ohm resistor in parallel with 8 ohm nominal speaker) and record again. See if there is any drop. At low volume with real music you shouldn't need a very high power rating for the resistor.

 

[Doodski]

What amps do you build?

Also someone else mentioned class D amps handle higher current better, this confuses me as they’re missing all the massive power supplies of a high end A/B amp.

The power supplies appear smaller because as the frequency of the power supply operation increases the inductors and capacitors get smaller for the similar task. Lower frequency of say 60 Hz and that requires a large transformer for power transfer. Then in a class D with a switching power supply they can operate at 50KHz or 200 KHz. Just examples of frequency they vary...

 

[dshreter]

I’m running a Marantz 5014, for TV I run it crossed over at 80hz to a subwoofer but for music I run it pure direct. I’ve compared with my HD6XX’s and even at moderate volumes the Contour 20’s give up a little low end. I know it’s not a fair comparison but A/B test with music like Krudder or Primus with good stand up bass or bass guitar it seems like I loose a little detail and output. This is even at moderate volumes.

Amplifiers distort when they run out of power. The low end doesn’t just get wimpy. Power might be your problem, but I’m skeptical.

 

[TankTop]

Amplifiers distort when they run out of power. The low end doesn’t just get wimpy. Power might be your problem, but I’m skeptical.

I agree, I was thinking if I’m only at 25-30 max at 8 ohms then it shouldn’t have an issue at 90-100 watts at 2 ohms even if it isn’t rated for it.

 

[Doodski]

I agree, I was thinking if I’m only at 25-30 max at 8 ohms then it shouldn’t have an issue at 90-100 watts at 2 ohms even if it isn’t rated for it.

For the type and price of these speakers perhaps you might like a better power amp instead of a all in one AV unit. The impedance and phase is challenging.