Tube Watts vs. Solid State Watts - Any credence?

Large amounts of dynamic headroom can be viewed two ways: The amp can put out a considerably greater amount of power for relatively brief periods of time than it is rated to deliver continuously, or the amp can **not** sustain power outputs that are close to its maximum instantaneous capability for longer than relatively brief periods of time.

I agree with Ralph/Atmasphere that in general it would be wise to consider a particularly "good" dynamic headroom spec to be a caution flag.

It's also worth noting that relatively low powered SET amplifiers are often considered to be especially good when it comes to reproducing musical dynamics (due particularly to how their distortion characteristics vary as a function of signal level, as Ralph has explained in the past), yet as class A amps they have zero dynamic headroom.

Best regards,
-- Al

Mapman, my point is that without further information one doesn't know whether to view a "good" dynamic headroom spec as the glass being half full or being half empty, so to speak.

And my perception over the years and decades is that a "good" dynamic headroom spec often (although not always) correlates with lesser quality (and less expensive) designs.

Is there an amp not capable of putting out more power cleanly for a shorter period of time?

Most well designed class A amps would be good examples, as Ralph indicated. In fact with a class A amp, as I understand it, the more power is continuously delivered to the load, the less power the amp will dissipate internally, and therefore the cooler it will run internally. Which suggests the possibility that in some cases a class A amp might even be able to deliver slightly more power continuously than briefly.

Re your last paragraph, I would certainly concur.

Best regards,
-- Al

Yes, that all sounds right to me, Bruce. It's a very powerful, robust, and undoubtedly dynamic sounding amp. Despite, and in a sense because of, the fact that it has essentially no dynamic headroom in the technical sense.

Best regards,
-- Al

Hi Mapman,

In your last sentence you probably meant to say "drawing" rather than "delivering," the former (referring to AC **input** power) being consistent with class D's very high efficiency. While dynamic headroom, of course, has to do with the difference between short-term and long-term **output** power capability.

I don't have a great deal of familiarity with class D designs, so I'm hesitant to comment. But I seem to recall reading that some class D modules have limitations (measured in minutes) with respect to how long they can sustain their rated maximum output power. And also that amplifiers from different manufacturers using the same class D module will often have considerably different max power specs. So it seems to be a different ballgame when it comes to class D, that may be hard to pin down with respect to dynamic headroom. Not surprisingly, given the radically different technology.

Best regards,
-- Al

Djfst, the differences in power which you cited all amount to less than 1 db, in most cases much less than 1 db, which is negligible. Even a doubling of power would amount to only 3 db, which while easily perceptible is not a huge increase.

In addition to researching which of those tubes others have reported to be preferable in the particular amp from a sonic standpoint, preferably with SF speakers, you might try asking Kevin Deal at Upscale Audio for his recommendation. In addition to being a very knowledgeable seller of tubes, he is the USA importer/distributor for PrimaLuna.

Regards,
-- Al

Bruce (Bifwynne), from John Atkinson's measurements of your ARC Ref-150:

All taps behaved similarly when it came to the maximum output power. Into a load twice the nominal tap value, the Ref150 clipped (defined as 1% THD) at 90W (19.6dBW, fig.4). Into the nominal tap value, it clipped at the specified 150W (21.75dBW, fig.5), but with a higher level of distortion. Into half the tap value, the amplifier clipped at 80W (13dBW, fig.6), but with even higher distortion at lower powers. It is important, therefore, to use the transformer tap that best matches your preferred loudspeaker.

And from ARC's specs for the amplifier:

150 watts per channel continuous from 20Hz to 20kHz. 1kHz total harmonic distortion typically 0.6% at 150 watts, below 0.03% at 1 watt. Approximate actual power available at ‘clipping’ 160 watts (1kHz). (Note that actual power output is dependent upon both line voltage and ‘condition’ i. e.: if power line has high distortion, maximum power will be affected adversely, although from a listening standpoint this is not very critical.)

So since the spec for the clipping point (presumably corresponding to the amp's maximum instantaneous power capability, for some reasonable amount of distortion) is negligibly higher than its maximum continuous power capability, the amp's dynamic headroom is close to zero.

The way to look at it is that the very high energy storage capability of its power supply helps the amp to achieve a continuous power rating that is close to its clipping point, rather than being significantly less than its clipping point (as it would tend to be in the case of an amp having a significantly less robust power supply). As well as perhaps providing other benefits, such as minimizing the extent to which the perceived dynamics of the amp might be compromised by sluggish responsiveness of the AC supply to abrupt increases in demands for current.

Note Ralph's earlier statement that "if class AB and without much power supply, for a brief instant the amp will be able to make more undistorted power than its constant power spec." Or putting it the other way around, if class AB and without much power supply, the constant power spec will be much less than what the amp can supply for a brief instant.

Best regards,
-- Al