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Tube Watts vs. Solid State Watts - Any credence?

I've heard numerous times that Tube watts are not the same as Solid State watts when it comes to amps running speakers. For example, a 70 watt tube amp provides more power than a 140 watt solid state amp. Is there any credence to this or just sales talk and misguided listeners? If so, how could this be? One reason I ask is a lot of speakers recommend 50 - 300 watts of amplification but many stores have 35 watt tube amps or 50 watts tube amps running them. More power is usually better to run speakers, so why am I always hearing this stuff about a tube watt is greater than a solid state watt?
djfst
bifwynne
2,206 posts
 
Al and Ralph ... help me with the terminology. When I think of headroom, I think of an amp's ability to handle short term dynamic transients, which is part of real music ... especially classical music.

To be more specific, my tube amp has a 1040 joule power supply. In my "un-technical" way, I interpret that as "head room" because my amp presumably can handle short term power demands that exceed its rated power output of 150 wpc, subject to FR, speaker impedance and tap output impedance. Am I mixing and matching terms and concepts here??
melbguy1
1,438 posts
 
ss watts can be misunderstood as well. "Qaulity" watts matter more than quantity. The more important consideration if the amp's ability to drive a low impedance load and remain dynamically stable. Thus the power supplies are critically important. You want a large, highly efficient multi-regulated power supply with a high current and a good damping factor. For example, my Vitus SIA-025 has 'only' 25 watts kl.A power & 100 watts kl.A/B (switchable), but has a very efficient 1.4kVa UI-core transformer which is capable of controlling low impedance loads running in kl.A mode. Big transformers also tend to be heavy, and the SIA-025 weighs 42kg. Compare that to the Accuphase E-600 which weighs 24.7kg. I can't think of any other class A ss which integrated amp which can do that. Similarly tube amps can have some pretty hefty transformers. I know the Absolare 845 mono blocks were driving the S5's comfortably at CES last year.
almarg's avatar
almarg
9,670 posts
 
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
bifwynne
2,206 posts
 
Thanks Al ... the problem is my misuse of terminology.

Perhaps a better way for me to think about my amp's performance window is that it does not appear to choke when asked to deliver power. Perhaps that is because I am not really tasking the amp all that much during "normal" operations.

In my layman's way of thinking, what I think of as "headroom" is my perception that if the amp is making say 25 to 50 watts of power during "normal" operations, a transient peak that pushes the amp out to 100+ watts is well within its rated power capability. Not headroom in the technical sense, but headroom insofar as the amp can produce a lot of power at reasonably low distortion numbers and remain stable.

Another fine point from the Atkinson report is that my amp will produce rated power in those cases where the load impedance matches the nominal tap value, i.e., an 8 ohm load plugged into the 8 ohm tap; or a 4 ohm load plugged into the 4 ohm tap. However, the amp will not produce rated power where there is an impedance mismatch between load and nominal tap value.

That said, in my case, my speakers have a 4 to 6 ohm saddle in a good part of the low frequency range, say 70 to 500 Hz. Impedance goes vertical past 700 Hz.

I surmise that most of the power demands placed on my amp fall within those goal posts. Given the foregoing, the 4 ohm taps make the most sense from a impedance and power matching perspective. And as an aside ... my rig sounds the best to my ears off the 4 ohm taps too.
almarg's avatar
almarg
9,670 posts
 
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
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