Seriously considering tube preamp…opinions?

@tvad 90db. Ok. (I guess 88db isn't better enough.)

Bookshelves: I began with bookshelves because I wanted new desktop monitors. Then, I opened up to the idea of a larger room/space. In the meantime I bought a sub. As I got deeper in, I realized that I really wanted a listening space. So, I guess bookshelves are a remnant of the earlier parameters — and I think they do offer flexibility for future placement. But if I was to be pushed, well, I guess I'm now at the point where I'd entertain floorstanders. Here come the Maggies! (Just kidding. I think.)

@hilde45  The thing you have going for you with the Salk speakers is they have a moderate 8 ohm impedance with no nasty phase angles or dips. This makes the job of the amplifier easier. Jim is pretty conservative about his ratings- I suspect what passes for 83 dB or so for him is 86 or 87dB with some other brands- it depends where on the curve of the speaker you measure its sensitivity!

If I can make a suggestion: When working with **any** amplifier (tube, solid state, class D), its distortion will be lower if it is not asked to work hard! To this end, your amplifier investment dollar will be better served by a loudspeaker that is higher impedance (8 ohms as opposed to 4, and 16 as opposed to 8) because the amplifier will make less distortion. This is both easy to hear (as the distortion is higher ordered harmonics and addition IMD) and easy to see in the specs. When you can hear it and measure it, its real :)  In the case of tube amplifiers with output transformers, the higher impedance will allow the output transformer to operate more efficiently. When the OPT (OutPut Transformer) heats up, its converting amplifier power into heat :(  When driving lower impedances, not only does the OPT operate less efficiently, it can also lose bandwidth. On the bottom end this can be up to an octave of bandwidth loss between 4 ohms and 8!


On top of this loudspeaker efficiency is also important, particularly in the case of tubes, as tube amplifier power is more expensive (the industry went solid state not due to performance but cost reasons decades ago). This also helps the amp to make less distortion since it doesn't have to make as much power. A 3dB change in efficiency doesn't sound like much to the ear, but that is a difference of twice the power in the amp! So when you go from 83dB to 89dB, to make the same sound pressure the amp need only be 1/4 the power. The $$$$ implications should be obvious.


The less distortion, the more the presentation will sound like real music.


One other thing: lower efficiency loudspeakers are inherently less dynamic. This is due to heating of the voice coil- which in turn reduces its sensitivity. The more power you give it, the worse this becomes. There tends to be less thermal compression in higher efficiency loudspeakers.

I'm in a similar situation (looking for a tube pre / SS amp combo.) and I was wondering if there are tube pre's that are more suited to SS amp matching in terms of impedance matching.  Do I just go by the 1/10 rule (output impedance for the pre)?  This would narrow my search down considerably if I could find a tube pre that is SS amp friendly.  

Do I just go by the 1/10 rule (output impedance for the pre)?

No.

To assure impedance compatibility the 10x rule of thumb guideline should be applied at the audible frequency for which the output impedance of the component providing the signal is highest. In most cases impedances are specified at a mid-range frequency such as 1 kHz. It is very common for tube preamps to have output impedances at deep bass frequencies that are much higher than that specified value, often 2K or 3K or even 4K ohms. That rise at low frequencies results from the output coupling capacitor that is used in the majority of tube preamps, and also in a few solid state preamps. The impedance of a capacitor increases as frequency decreases.

So ideally the 10x rule of thumb guideline should usually be applied based on the output impedance at 20 Hz, in the case of a tube-based preamp. If as is often the case the component’s output impedance at 20 Hz is not known, and is not indicated in published measurements (such as those Stereophile often provides), and if the use of an output coupling capacitor in that component cannot be ruled out based on available information, to be safe a considerably higher ratio than 10x should be used, something like 50x or 75x IMO.

This applies, btw, to tube-based source components as well as to tube-based preamps.

Also, to clarify a common misconception I should add that failing to meet that guideline does not necessarily mean that there will be an impedance compatibility problem. It depends on how much **variation** there is in the output impedance over the frequency range. But meeting that guideline (at all audible frequencies) assures that there won’t be an impedance compatibility problem.

I should note also that a significant number of tube-based preamps use coupling transformers at their outputs, and therefore avoid that low frequency impedance rise. Although depending on the specific implementation transformer-coupled outputs might have downsides of their own. Atma-Sphere MP-1 and MP-3 preamps, btw, cleverly avoid the use of both capacitors and transformers at their outputs. (I’m not sure about the UV-1). As far as I am aware they are unique in that respect, among tube-based preamps.

Best regards,
-- Al