Seriously considering tube preamp…opinions?

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).

@russellrcncom 228 posts 02-26-2020 10:27
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.

You can also future-proof yourself by finding a multi-purpose tube preamp that not only mates up well with your solid state amp, and a future tube amp down the road later on.

It happens :)