Seriously considering tube preamp…opinions?

@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

@atmasphere Thank you for your comment about speakers. It definitely helps.

@almarg You are one tireless explainer! I keep learning.

@russellrcncom I’ll be curious what path forward you take. I began with the idea of a tube/ss hybrid -- where some manufacturer works that out for me. I’ve moved toward the idea of separates, ideally matched by the same mfr or at least of the same type (tube or ss). I have moved away from mixing and matching myself because of (a) the experiences presented here that it could be somewhat tricky to do in a way that has a pleasant outcome and (b) because I won’t have many opportunities to try out combinations, first. The last thing that I want is to just grope in the dark for synergy -- to mix and match and discover, soon after purchase, that I have to try to sell one of the pieces I bought because it doesn’t work. And remember, there is also (c) the speakers to match up with all of this, too. 

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)?

Al's comment is correct- look at the 20Hz output impedance if you can find that spec. **That** impedance is the one that you want to be 1/10th that of the input impedance of the amp. In the case of our MP-1 and MP-3, the output impedance is the same as the frequency response curve (which is to say that their output impedance are the same at 20Hz as 1000Hz) but this is as Al pointed out, owing to a balanced direct coupled output (which to my knowledge is the only other way to support the balanced line standard outside of using a line transformer at the output of the preamp).