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Power output of tube amps compared to solid states

I'm having a hard time trying to figure out how tube amp power output relates to solid state power output. I've been looking at the classifieds for tube amps and I see lots of tube amps with 50w or 60w output, but nothing close to the 250w output typical of solid state amps.

So I have no idea what type of tube amp is required for my set up, right now I'm using totem forests with a required power rating of 150w-200w at 8ohms. The bass is so powerful on these that I have the sub crossover set to 40hz.

My question is, are tube amps so efficient that 50w from a tube sounds like 150w from a solid state? Or will 50w output from a tube severely limit how loud I can play my speakers? If so, are tubes usually meant to be driving super-high efficiency speakers?

I had previously tried a tube pre-amp with a solid state power amp (both musical fidelity) and didn't like the results because the imaging suffered greatly, even though the music sounded nicer from a distance. Now I want to try a solid state pre-amp (bryston) with a tube power amp (no idea which brand to look at), but I don't know how much power output I need or if it will even be possible with my speakers. Does anyone know what I would require?
acrossley

Showing 9 responses by shadorne

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shadorne
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[/quote] In other words, most prosound woofers are overdamped for home audio usage when used with solid state amps. [/quote]

Good point - prosound woofers are often in a vice-like grip of the amplifier and excursions are tightly controlled and the bass is often tight and thin compared to the warmer and more extended sound of consumer designs. Partly this design approach helps protect the woofer from damaging over excursions.
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shadorne
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For instance, my Merlins are said to have critically damped (whatever that means)

Critically damped means a Q of 0.707. It is a function of the driver Qes (electrical Q) and Qms (Mechanical Q) and the acoustic damping from the box (air pressure).

A design that is overall critically damped is the fastest design that returns to the rest or "zero" position without overshooting the rest position when the power is removed.
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04-10-10: Unsound
Shadorne, well done.
The link Atamasphere provided is really worth a look.

Thanks - yes it was very interesting and useful. Please bear in mind that all the experimental results were for "infinite baffles" (no box).

The acoustic suspension is usually the most powerful damping force in a box speaker (until you get below the port resonance in a ported design when it begins to behave like an infinite baffle). This controls whether the speaker will sound "boomy" (underdamped) or "tight" (critically damped) over the designs resonant frequency. In practice it is rare to find a design that is overdamped.

In infinite baffles the amplifier damping along with Qes becomes the dominant factor...and in this case the match with the amplifier is indeed critical as the amp can control the overall system Q to a large degree - to the good or the detriment of the sound.
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shadorne
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THaks Shadorne, that explasins what Bobby was talking about, with which he then says, which is why it works so well with tube amps

Indeed, a critically damped acoustic suspension speaker does not need an amplifier with high damping factor as much as your regular underdamped boomy type designs that will get positively sloppy without amplifier control.
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shadorne
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Another way to think of damping is that a high damping amp "shorts" the speaker woofer voice coil when the signal is "zero" - this means that the voice coil current is maximized (sees least resistance) in response to the movement of the coil through the magnetic field which creates the strongest possible back emf which opposes the coil movement and acts as a "damper" for as long as it takes the coil to come to rest. The more powerful the magnetic field (magnet size/weight) and the bigger the diameter of the voice coil and lighter the cone weight then the better an amplifier is able to tightly control a woofer. (This is why large 4 inch voice coils that are very short (light weight) and sit in a long powerful magnet gap are so desirable. These are extremely expensive as well with a quality 12 inch woofer costing around $1000 and weighing about 25 lbs or more.

Of course at resonance, the impedance of the voice coil rises and electrical damping is not as powerful and that is where the "acoustic suspension" design becomes critical to how the speaker will sound.
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shadorne
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These are extremely expensive as well with a quality 12 inch woofer costing around $1000 and weighing about 25 lbs or more.

I would add that outside of pro applications these kind of woofers are few and far between - buyers will pay for veneer or cabinet work but not for honking great big magnet and voice coil that they cannot even see - of course manufacturers respond accordingly to the customer demand (customer is king).

Without a good expensive woofer the best damping factor in the world is not going to put lipstick on the proverbial pig...
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shadorne
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The problem with increasing the amplifier source impedance for this type of driver (especially in a domestic application) is that while this raises the Q and the bass output, it leaves the resonant frequency unaffected . . . the result is then simply wooly, boomy mid-bass and no increase in bass extension.

I would also add that amplifier tight control is a good thing if one desires overall low distortion (THD). However, as Duke points out, this kind of precision response will be at the expense of bass warmth/extension and coloration. In the end it means that a large 12 inch pro woofer may only get you to 40Hz at 3 db point - whilst some consumer designs can eek out 25Hz to 30 Hz even with a 6 inch ported design (pleasant sounding and good value but with lots of added distortion of course).
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shadorne
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However I find that if I am dealing with an individual who is educated with Voltage rules, that they will generally assume that there are no **other** rules. So I am interested in you thoughts regarding Duke's post.

On the power rules front I think that provided a speaker is well designed smooth load with no large impedance swings and no low impedance dips (i.e. a decent speaker design) and provided it is well damped at resonance (again good practice - although not always followed - even by revered manufacturers such as Wilson)....then the speaker should sound great with a higher impedance amp (i.e. no need for a ridiculous high damping factor if the speaker is well designed). A damping factor of 10 is generally much more than enough and may be far preferable to an amp design with oodles of feedback, a damping factor of 500 and stability problems that result in high amounts of transient or IMD distortion (everyone knows that overly high gain and overly high feedback can bite you when playing real world music to a complex load - no matter what a speck sheet sheet implies).
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shadorne
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A bit off topic here, but, if a speaker manufacturer needs to veer from the criterion you use to describe a "decent speaker design" and needs use something other than a higher impedance amp to achieve better results, what's the harm?

You mean like Infinity or a Totem Mani 2 or a MBL ...well it can be done and with the right amplification great results can still be achieved. However, a "decent speaker design" is NOT one that should present itself as a "stress test" to the amplifier, IMHO.
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