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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
kirkus
502 posts
 
Much as I like prosound woofers, most of them have too much electrical damping (too low Qes) to give good bass extension with a low output impedance (high damping factor) amplifier.
Excellent point - they also tend to have lower mechanical mass for sensitivity, and lower compliance for better power handling, resulting in a higher resonant frequency. 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.

An interesting exception is the JBL 2235/2245, which have mass-rings (weights) attached to the voice-coil former to both increase Q and to lower Fs. But this also makes them much more prone to over-excursion . . . so it's not uncommon for a reconer to remove the weights if there's a problem bottoming-out the voice-coil against the magnet, again raising Fs and lowering Qms in the process.
kirkus
502 posts
 
Pubul57, there's no shortage of material on the web that contains Atmasphere waxing lyrical about his approach to building amplifiers . . . I suggest you look at his website or peruse his posting history on Audiogon if you haven't caught it by now.
shadorne's avatar
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shadorne
7,477 posts
 
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).
kirkus
502 posts
 
You're precisely correct Shadorne . . . and that's why in the pro world, a 12" driver is considered small for a woofer, and usually used for midrange/midbass. Even most pro 18" drivers have a pretty high Fs and low Q compared to a consumer 12" . . . and why most of those pro 18" bass drivers are in W-bins, sugar-scoops, tapped-horns, etc.

But also keep in mind that distortion is very SPL-dependent, and the onset comes on very quickly indeed! So for a smaller cabinet size and better bass extension in the typical domestic environment, having a very clear idea of the maximum intended SPL is crucial for the loudspeaker designer to achieve the requisite performance.
atmasphere's avatar
atmasphere
11,803 posts
 
Pubul57, Kirkus has been right on all his points but one- I've not really talked a lot about the DC coupling issue, even though I am a proponent of OTLs (the two are not always the same thing).

In a nutshell though, I think true DC coupling from input to output to be a bad thing, because every power supply has a low frequency pole, and if you exceed it (which a true DC coupled amplifier can) then you can modulate the power supply with audio from the amp, and that's bad- it gobbles bass impact and adds distortion.

So a frequency pole somewhere in the amp that limits it to a point at least an octave above the low frequency pole of the power supply is a good idea. We take it a step further by having a separate power supply for the driver, which is part of where we get our low IM distortion numbers from.

A few years ago I looked all over the web trying to see if anyone had scanned or posted that GE study, but so far it exists in print form only, no web.
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