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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
atmasphere's avatar
atmasphere
11,753 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.
iscm
40 posts
 
atmasphere
I am curious that you suggest that over 15db of feedback need not be detrimental. Perhaps I miss-understood. 

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.

Some things, I am sure, are just down to plain old fashion. 
atmasphere's avatar
atmasphere
11,753 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!


dhite71
312 posts
 
I have not auditioned any tube preamps, but purchased a pair of ARC Classic 120s from here last year.  Prior to that I had a single Parasound HCA 2200ii and then a 2nd running them in mono.  This took me to 750wpc SS with a very well respected pair of amps with all the specs for great bass and dynamics.  I have 86db floor standers.  The ARCs at 110 wpc outshined them in every aspect....including bass!  The output level was higher, more dynamic and more detailed.  The smoothness of the sound is where I think the lack of ear fatigue comes in.  I'm only listening to vinyl.  Fast-forward 9 months and I'm wondering about SS again so I just picked up a B&K 200.2 and it sounds fantastic.  It reminds me a lot of the Parasound.  I listened for a day and then reconnected the CL120s and it was the same transition as before.  I think I got very lucky in my choice of tube amp for my first experience since they each have 8 6550s as output tubes.  Newer amps have fewer with a higher output/tube and maybe thats why they are so strong....not real sure.  I would love to compare my ARCs to another tube amp, but that's not in the plan for now.  I encourage anyone who hasn't had a tube amp in their system to figure out a way to make it happen.  
johnjohn
42 posts
 
Avanti1960 mirrors my own findings. Tube power amp definitely makes a bigger imprint on the overall magic than a tube pre-amp. Not only that, it is much easier and less fussy for a solid state pre-amp to drive a tube power amp. Not so the other way around. Tube pre-amps, even good one with strong output, will choke if asked to drive SS amps with input impedances much lower than, say 50 kohm or so. That basically rules out half of the SS power amps out there. 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.
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