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For tube sound, which is more important: preamp or power amp?

I have always loved the “tube sound” - warmth, midrange, soundstage. Through the years (since about 1975), I have owned many tube and solid state amps and preamps, in various combinations. Presently, I have a tube amp and a solid state preamp. But like most of you, I am thinking of making changes, again.  Not to cloud the discussion, the specific brands are not important.  I also listen to acoustic music, females vocals, love mini monitors, EL34s, NOS tubes, and don’t care that much about bass.  So you can see that my taste fits the tube sound very well. But I have had systems that are too warm, not enough dynamics or details, and fat in the low end, too.

okay, now to the discussion.  To produce the tube sound, which is more important: the preamp or the power amp?  Let’s talk in general, and (if possible? May not be) not tied to one specific piece/brand/model of equipment.  I know there are exceptions to any general rule.  Not sure if it makes a difference to your comments, but I have no phono and am running line stage only.

As an attempt to prevent the conversation as going in a big tangent, let’s assume equality of price/quality. i.e. not comparing a $10k power amps contribution to a system to that of a $1k preamp.  Let’s also assume that the amp (tube or solid state) can drive the speakers just fine, such that compatibility does not limit the decision. And ignore mono blocks versus stereo amp differences.  

two follow ons: I have  the perception that preamps give you more bang for the buck - meaning that it takes less money to get a great tube preamp compared to a great tube amp.  Agree/disagree? And second, I have never owned a tube dac or CD player, and will assume that tubes in either of these is less critical than in a preamp or power amp. Agree/disagree?

i am interested in your thoughts.

Bill
meiatflask

Showing 11 responses by atmasphere

atmasphere's avatar
atmasphere
12,099 posts
 
BTW I like all tube for classical and jazz, but for rock you gotta have that SS pre-amp in there to drive that bass.
That's pretty subjective- I find that my tube amps can play bass better than any solid state amp on my system. I play a lot of rock too. My speakers go to 20Hz and are 16 ohms so tubes play them easily. On top of that, my amps are full power to 2Hz, so no hint of a rolloff at any frequency the speakers can play.
atmasphere's avatar
atmasphere
12,099 posts
 
My understanding is that one of the reasons that low feedback may be beneficial, is that the feedback is very slightly out of time with and that the human ear is extremely sensitive to this.  
Like all things, I am sure that the selection of feedback is a compromise, as is everything concerning the design of an amplifier right down to the quality of components and build quality.
Its not a timing issue so much as a matter of distortion generation (while suppressing feedback, it adds some of its own). But the ear is indeed quite sensitive to this, because the distortion generated are higher ordered harmonics and the ear uses those to sense sound pressure. Combine that with how music energy is distributed over frequencies and that's about as sensitive as it gets due to the Fletcher-Munson curve!

I prefer zero feedback, but 20 db of feedback is easier to listen to than 10 db or so. There's a sort of 'in-between' hump where a moderate amount of feedback results in audible problems that lower or larger amounts don't have.

You are correct that there is a bit of compromise with the use of feedback, and frankly most designers fail to get it right as there are more variables than the old school formulas suggest!


atmasphere's avatar
atmasphere
12,099 posts
 
Is it possible that one of the reasons that Valve amps sound so good is that they tend to use much lower amounts of feedback that most transistor amps?
I have noticed that the sound seems to open out as I select lower amounts of feedback.
You can either run high amounts of feedback or very low amounts; in between (about 6db up to about 15db) its application can be really detrimental.

Feedback, while suppressing distortion, also adds some of its own, and the type it adds (harmonics 5th and up) is pretty audible. Even though its a very small amount, its audible because the ear uses those harmonics to sense sound pressure, so it has to have a 130db range!

Tubes (triodes in particular) are very linear so it is possible to build zero feedback tube amps much easier than it is to do so with solid state.

So, quite often, yes, this is one reason tubes can sound better than transistors.
atmasphere's avatar
atmasphere
12,099 posts
 
^^ When it comes to that, my preference is tubes all the way...
atmasphere's avatar
atmasphere
12,099 posts
 
Getting the two amps to work together properly will be a bit of a challenge. Unless you use an active crossover, I think you will find that to a very difficult task. The problem with using an active crossover of course it that its not actually tailored to the speaker, and every one that I've heard has imposed its own signature, which is why I try to avoid them unless they are used in a subwoofer.
atmasphere's avatar
atmasphere
12,099 posts
 
Also, looking the specs of most tube power amps it seems the S/N ratio isn't nearly as good as SS amps. The THD numbers aren't as good either with 1% at full power not uncommon. SS amps' specs look considerably better. Is this typical?
The problem here is that the ear puts a different weighting on what harmonic distortion is present. IOW, it almost does not care about the 2nd harmonic, but cares quite a lot about the 7th and other higher ordered harmonics. So a tube amp tends to have lower orders to which the ear is less sensitive; solid state amps have more of the higher ordered harmonics than tube amps.

The ear translates all distortions into tonality. So the 2nd contributes to 'warmth'; the higher orders contribute to brightness and hardness. Both are colorations!

The bottom line here is that although solid state amps appear to have less distortion, the distortion that they do have is far more audible and objectionable to the human ear. This is why tubes are still around decades on after they were supposedly 'obsolete'.
atmasphere's avatar
atmasphere
12,099 posts
 
can you guys recommend some good 100W+ stereo tube amps to pair with an Emotiva XSP-1 preamp?
Yes, but a budget is the first place to start.
atmasphere's avatar
atmasphere
12,099 posts
 
@schubert you might try a set of JJ blue glass KT88s.


atmasphere's avatar
atmasphere
12,099 posts
 
I think if a tube is on, it's wearing out…I'd like to see what at atmasphere says about this.
If it has B+ on it, then that is the case, slower if there is no B+ (filaments degrade over time as well).

Al's comments reflect the actual reliability, but in this case a KT120 is in use and in practice they don't seem to hold up as well as the KT88. I suspect that the tubes in this amp will hold up only slightly longer than in an amplifier where the tubes are biased a bit hotter, assuming the tubes don't have a defect which can shorten their life, which many modern tubes do.
atmasphere's avatar
atmasphere
12,099 posts
 
+1
atmasphere's avatar
atmasphere
12,099 posts
 
Hmm. What is "tube" sound?
Smoother and more detailed than transistor, more lower ordered harmonics which are harder for the ear to detect. Less higher orders that the ear easily detects as harshness.

************************

If you really really have to make a choice of which way to go, my money is on the tube preamp.

The reason is simple: I don't care how good your amp or speakers are, if the preamp looses definition, there is nothing you can do to replace it downstream. Tubes simply make more detail than transistors (and also without brightness); they are easier to listen to for hours too, so send that to your amplifier.

The problem here of course is if the amp isn't up to the task, you won't hear what the front end of the system is doing. I do regard the use of a tube preamp with solid state as a compromise.

If you really want to hear all that is on that LP or CD, get a tube amp to go with the tube preamp.

Some people think that they don't want the tube amp because of reliability or heat concerns. Tubes are easy to deal with (they are in sockets after all) and heat is a function more of what class of operation the amp employs rather than whether or not it has filaments. A solid state amp biased to the same level of class A operation will make 90% of the heat that a tube amp does. IOW, many tube amps run hotter because they are biased harder.

Transistor amps tend to be biased at very low currents; that's the main reason they make less heat. 
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