The idea is SS amps are more powerful and better able to drive most speakers. So by adding tube warmth with a tubed pre you're still able to use the speaker of your choice with those SS watts. As always ymmv and all cases are not the same. Most of all use whatever floats your boat, and don't get weighed down by dogma.
Another factor in that paradigm is that tubes in a pre generally last 1000s of hours, while in many power amps, their lifetime is more limited. For that reason, cost of NOS power tubes is much higher. So total operating cost can be higher for a tube power amp than a tube pre.
For me it was experimenting w/gear back in the 80's when I came across the big SS amp/tube pre combo & it always was a good matchup. Over the yrs. I've discovered that a well implemented design, be it SS or tube, will give you the sound you desire.
I agree w/Jond "don't get weighed down by dogma" and will also add my mantra of "trust your ears".
If you want to know the technical rationale for choosing a tubed preamp and SS power amp, tubes operate at hundreds of volts and are much more efficient at voltage gain. Solid-state devices like transistors operate at lower voltages, like tens of volts, but due to their lower output impedances, are much better at driving current, which is what your loudspeakers need for bass control and high volumes. Since most voltage gain is found in preamp and power amp input and driver gain stages, tubes are a better bet for these applications, whereas for power amp output stages, SS devices are superior. This doesn't always translate into sonics that we like, note SET amps that have very high output impedances but many like the sonics of these amps. Like you noted, in most applications it is better to use a tube preamp and SS power amp, especially with loudspeakers that present difficult loads to the power amp.
I agree with Tvad. Much depends on the specific character of the components you're considering. Of all the combos I've tried, I'm happiest with my SS preamp driving a PX-25 tube amp. While the tube/SS pairing is touted as being more likely to be successful, nothing beats figuring out what your sonic preferences are and evaluating specific combinations from that point of view. Following a "generally applicable" approach isn't helpful if the particular combination you're evaluating doesn't float your boat.
The primary reason for going with tube pre into solid state amp is that it is much easier to get a lot of power from solid state than tube circuits. While I am not a fan of solid state amps of any power rating, I am also not a fan of most high powered tube amps as well. A lot of high powered tube amps sound hard and brittle and less natural than comparably powered solid state amps.
But, when feeding low-powered tube amps, the linestage issues become more mixed. I've heard a number of tube linestages that sound good in certain setups and quite bad in others; and the differences are often unpredictable. One really must try the specific pieces together. The solid state linestages I own and have tried in several systems (Placette Active and Levinson No. 32) seem more consistent and predictable than tube linestages I have heard.
But, when you find a good tube-into-tube arrangement, there really can be magic.
I think the original rational, at least sometimes, was $'s. Now with very good high power tube amps available for less than insane cost (Rogue, used Bat, etc.) I'm not sure that the original rational is valid. Imho.
I'm a purist. Tubes (SET) all the way. Pre to amp. It works for my ears. SET, particulary, very magical, holographic, you've heard the whole nine of adjectives: and they are true. I wouldn't let an SS pre amp touch my SET amp. Kind of like putting slip covers over a beautiful new sofa. Of course, there are probably SS pre amps that can do the job that my pre does, but I gots to draw the line somewhere in my sonic journey.
I do agree with you that all tube, is the way to go. I currently use a SET amp as well.
I have, however, heard a number of non-SET amps that are really good alternatives. One is a near one-of-a-kind OTL that is about 20 years old that the owner was will to part with for something like $70k. Another astonishing amp I heard was a Western Electric pushpull amp in a massive vertical rack arrangement. The owner had only one amp (i.e., mono) and cannot find another to make a stereo pair. I heard it in a side-by-side comparison with an Audionote (uk) Kageki (the SET I own). It was a better amp, but, then again, it costs almost four times what the Kageki costs.
I have heard a number of fine systems with solid state linestages in them, but, these systems are so different from mine and I cannot say what contribution to the overall sound was made by the solid state linestage. Still, some of these linestages, like the Connoisseur and the Naim 552 I heard, must be pretty good if the systems sounded pretty good.
Tz7, I am an engineer but not an electrical engineer, so maybe an EE type can weigh in here. My understanding of MOSFET's is that their transfer function is more like that of tubes than that of bipolar, HEXFRED, or other transistor types. They probably are also used in circuits with a lot less global negative feedback than those of bipolars, whose distortion tends to be odd- and higher-order, thereby requiring more feedback to minimize distortion.
>> 12-27-06: Rlawry >> Tz7, I am an engineer but not an electrical engineer, >> so maybe an EE type can weigh in here. My understanding >> of MOSFET's is that their transfer function is more >> like that of tubes than that of bipolar, HEXFRED, or >> other transistor types.
the reason that MOSFETs can be made to sound more like tubes (compared to BJTs) is that the MOSFET current-voltage transfer function follows a square law. Thus, MOSFETs end up creating more even-order distortion (just like many tubes) than odd-order distortion (just like many tubes). The BJT's current-voltage xfer function follows an exponential curve. Thus, BJTs end up creating more odd-order harmonic distortion that can sound strident.
>> They probably are also used in circuits with a lot less >> global negative feedback than those of bipolars....
this is not a true statement at all! The type of device (MOS or BJT) has nothing to do w/ the amount of negative feedback used in a circuit. It has more to do with the amount of gain in the circuit & what parameter the designer is trying to control (output impedance, distortion, noise, etc). Just FYI: vacuum tube circuits also use (global)negative feedback - some use a lot & some use very little & others something in between. There are several tube amps that use zero global NFB & guess what........................ there are several s.s. amps that use zero global NFB. So, it is all design-based & not device-based.
The preamp signal is smaller,more delicate and fragile,so it benefits from the qualities of tubes.The amp signal is less fragile and so can more readily tolerate the disadvantages of transistors while benefitting from the better bass and oomph of transistors.
dc generated by some preamps can damage some ss amps, so ss pre and tube amps may be safer.
sonically, i prefer tubed pre and tubed amp.
however, if my back were to the wall and i could find a lush tube pre and felt confident that a ss amp would not be damaged, i would select that combination.
i would hope i could find a ss amp that didn't have the obvious signature of transistors.
The preamp signal is smaller,more delicate and fragile
No it isn't. Although yr thinking *should* be logical, it so happens that the line signal is usually higher than what goes into the amp. There's attenuation involved in the preamp, remember? And most amps are overdriven with the standard 2V output from a cdp, for example:). Cheers
Alright- I'll bite- The reason tube preamps are often preferred over transistors is that they loose less of the signal. Transistors are pretty good at loosing low level detail so if you have transistor preamp driving a tube amp you will be missing a lot more of the signal: once the detail is lost you can't make it up downstream.
A better illustration of this principle is using a cheap microphone with an excellent recorder or a really fabulous mic with a cheap machine. The latter will turn out a better recording every time: you can't make up for lost detail downstream.
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