Any amps that "double-down" into 4 ohms in a magazine/lab tests? (1 Viewer)

[Ricky T]

First, what does "double-down" really mean? To me, it means if an amp clips at say 100 watts into 8 ohms with 1.0%, then it would test at 200 watts into 4 ohms, with the same distortion. I was under the impression that only amps like big boulder monoblocks can really do this.

Even some amps that "spec" doubling into 4 ohms, don't show it a lab test...ie, Classe monoblocks down to their 40lb cav-75 75x6 amp.

Can someone point to/show me links to lab tests with amp doubling into 4 ohms?

 

[Mifr44]

Ricky,

I don't have that link, but I would like to add that it has been discussed that many manufacturers state a very, very conservative output at 8 ohms to make it seem like it almost doubles at 4 ohms.

For instance, you might see an amp that is rated at 200 W @ 8 ohms and 375 W @ 4 ohms. Looks good, wouldn't you say? Upon testing the amp, it is determined that it was actually 225 W @ 8 ohms and 360 W @ 4 ohms for the stated THD values.

I will try to find the post where this was discussed.

Michael

 

[Peter Johnson]

It's not electrically possible that the amp can double down.

When you half nominal impedance, current increases. Internal losses, as well as cables losses etc increase as according to P=i^2*R, as i increases, power losses increase.

The rails inside the amp also sag a little when driving a harder load. The better the PS, the less it will sag. This will also prevent it "doubling down".

As Michael said, any amp whose specs imply the amp can double down, are either understated for the 8R load, or just plain false.

 

[Chris PC]

As Michael said, any amp whose specs imply the amp can double are either understated for the 8R load, or just plain false.

That makes sense. If you have 6 or 8 ohm speakers that are not terribly in-efficient, I would be happy if the amplifier put out higher wattage under a lower current load, but i wouldn't expect it to be "double". My older amp actually only put out the same 50 watts into 8 or 4 ohms, but at least it could do it and not actually put out LESS power into 4 ohms than 8 ohms. Some lousy amps actually put out less into a lower resistance than the higher 8 ohm that they were rated for. They can't deliver current. This you want to avoid

 

[chung]

It's not electrically possible that the amp can double down.

When you half nominal impedance, current increases. Internal losses, as well as cables losses etc increase as according to P=i^2*R, as i increases, power losses increase.

The rails inside the amp also sag a little when driving a harder load. The better the PS, the less it will sag. This will also prevent it "doubling down".

Maybe not exactly doubling, but close enough. The whole idea is, of course, that the power supply has to be stiff enough, so that it does not sag when it is required to supply the higher output current. If the amp is designed to handle the current requirements for a 4 ohm load (some amps are designed to even handle 2 ohm loads without dropping output voltage), I don't see any reason why the power into 4 ohm loads would not be very close to twice that into 8 ohms.

Example: Output resistance of power amp = .05 ohm (damping factor of 160, not unreasonable). Let's say the amp delivers 100 W into 8 ohm. That means the output voltage is 28.46V rms. The current required from the amp is 3.557A rms. When used with a 4 ohm load, that amp will deliver 197.5 W, assuming the amp can deliver 7.027A rms. That is a factor of 1.975 times; not exactly 2, but very close (only 1.23% off). And if the output resistance is less, than the number gets even closer to 2.

The speaker cable loss is immaterial, because that is part of the external load. The internal cable loss can be made arbitrarily small; just need good heavy ga. cables and well designed supplies.

The key is that the amp has to be designed for driving low impedances, i.e., plenty of current capability.

 

[Norm Strong]

As I may have already mentioned here, or on another forum, it's better to think of it as "halving up" rather than doubling down.

If an amplifier will put out 200W into a 4 ohm load, it's a 200W amplifier--and you're paying for a 200W amplifier. Better you should ask why it only puts out 100W into an 8 ohm load!

 

[Peter Johnson]

No doubt it can get close..the best ones can *almost* double. But they can never double it.

I think its best to make the distinction clear as many people are misinformed on the matter and some may even use it as a final decision choice.

 

[Chris PC]

Norm,

I don't follow your thought process. The limiting factor of todays amplifiers is pretty much the current they can deliver. When an amp is rated at a given ohms, say 4 or 8 ohms, the biggest unknown is what it does under less resistive loads, not more resistance. If an amp puts out 200 watts into 4 ohms, it can't put out more into 8 ohms, you expect it to put out less wattage at the speaker because of the higher resistance.

Conversely, if an amp is rated 100 watts into 8 ohms, you have no idea what the amp puts out at 6 or 4 ohms because you have no idea how much more current the amp can supply based on the 8 ohm rating alone. If its a good amp, you should expect it to put out, for example anywhere from 1.0 x the wattage into 8 ohms up to the roughly 1.975x as stated by the above poster.

Perhaps a poor amplifier rated at 100 watts into 8 ohms is putting out its maximum voltage at its maximum current. When you connect a lower resistance load, you end up having to reduce current and that means the "100 watt into 8 ohms" amp now is putting out less than 100 watts.

 

[Hank Frankenberg]

Doubling down is a good indication of a beefy power supply section as well as a robust output section that can handle the increased current. For instance, Aragon doubles down and is a favorite among the power amp fanatics

 

[Dennis B]

Chung,

Please help me out here, when re-doing your calculations, for the 100W @ 8 ohms case, 28.46V means the total impedance the load sees is 8.10 ohms, which is neither 8 + 0.05 in series (=8.05) nor in parallel (=0.0497), if I remember correctly (?). How is it that you're combining the output and load impedances?

Thanks.

 

[chung]

Dennis:

The 28.46V rms is the equivalent open-circuit voltage of the power amp. In other words, I am modelling the power amp as an ideal voltage source, whose magnitude is 28.46Vrms, in series with a 0.05 ohm resistor, which is the internal output resistance of the amp. Total load seen by that ideal voltage source is 8.05 ohm. Voltage across the 8 ohm load is 28.46*8/8.05 Vrms. Power into the 8 ohm load is (28.46*8/8.05)^2/8=99.99W. Obviously some round-off there.

 

[Arthur S]

Ricky

I think I have a review of an Aragon amp. I'll look for it and let you know.

Artie

 

[GordonL]

I agree it's all marketing BS. Any amp can be presented as "doubling-down" if its 8 ohm output is understated. Take, for example, the Parasound 2205AT. Their literature puts it at 220wpc @ 8 ohms and 300wpc @ 4 ohms. What if they had specified it as 150 wpc @ 8 ohms? Ricky, forget about what you've read about an amp doubling-down being better - it doesn't mean squat. BTW, this is not a putdown on Parasound, substitute any other name of your choice.
FYI, Boulder takes a non-standard approach to their specs. They show the same power output at 8/4/2 ohms. Its all marketing.

 

[Ricky T]

Guys,

I agree, true doubling down is very rare indeed. I had an argument with one of my friends, who INSISTS that all classe amps doubling down into 4 ohms. He said the baby 40 lb Classe CAV-75 75x6 is better than ANY receiver or midfi amp because it doubles into 4 AND again into 2 ohms. Huh?! I called classe and their people said the same thing. Whatever Classe...here are some lab tests:

Stereo Review, July 1998. Four amps, each 3 channels driven at 1 khz for 0.5%THD plus noise.

Adcom 7500 = 182 into 8 and 278 into 4, ratio ~ 1.5

Harman Kardon Signature 2.1 = 118 into 8, 187 into 4, ratio ~ 1.6

Classe CAV-75 = 81 into 8, 132 into 4, ratio ~ 1.6

Sunfire cinema grand = 244 into 8, 400 into 4, ratio ~ 1.7

Stereophile Jan 2001. the Classe CAM 350 did 450w into 8, and 780w into 4, a ratio of 1.73. Not even this monster monoblock can do 2.00.

Take four cars, in three of them spec zero to 60 in 6.0 sec, and the fourth in 5.0 sec. A race car driver drives all four on the same track on the same day, his best times are 6.0 for all four cars. Who cares if the fourth mfg said his car can do 5.0?

 

[chung]

Stereophile Jan 2001. the Classe CAM 350 did 450w into 8, and 780w into 4, a ratio of 1.73. Not even this monster monoblock can do 2.00.

That Classe amp is rated at 350W@8 ohms. So it clearly can do 700W @4 ohms at the rated OUTPUT VOLTAGE.

It just means that the output voltage reserve (or margin) is more at 8 ohms than at 4 ohms. So the Classe guys are correct in stating that it delivers twice the rated 8 ohm power into 4 ohms. I don't think this is marketing BS at all. Another manufacturer would probably spec. that amp as a 450W@8 ohm amp instead of 350W. Classe is being conservative, which is good.

 

[Ricky T]

Chung,

How are the classe amps any different from other amps in terms of power into 8 and 4 ohms? Don't you think other amps also have more reserves into 8 ohms?

 

[chung]

How are the classe amps any different from other amps in terms of power into 8 and 4 ohms? Don't you think other amps also have more reserves into 8 ohms?

No, if you look at amps in a receiver, for instance, very rarely will you see much reserve. Sometimes those amps don't even meet specs. Those amps may not be designed with driving 4 ohms loads as one of the requirements. They may only provide the voltage rails to get the proper output swings, but lack the current to support lower impedance loads.

On the other hand, the Classe amp, like other top solid-state amps, is clearly designed to drive lower impedances, down to 2 ohms. These mono-blocks and stereo amps are much more capable than multi-channel HT power amps, IMO.

 

[Saurav]

It still doesn't matter, because no one can hear a difference in a level matched DBT, right?

 

[Ricky T]

Chung,

Let me phrase it this way.

AmpA specs at 100 and 170.

AmpB specs at 85 and 170; AmpB literature and marketing/sales people use "our amps are superior because they double-down into 4 ohms" as a major selling point.

BOTH amps test at 100 and 170. Tell me again, what is so "special" about AmpB?

 

[chung]

Let me phrase it this way.

AmpA specs at 100 and 170.

AmpB specs at 85 and 170; AmpB literature and marketing/sales people use "our amps are superior because they double-down into 4 ohms" as a major selling point.

BOTH amps test at 100 and 170. Tell me again, what is so "special" about AmpB?

No difference, except Amp B has a more conservative 8 ohm rating.