Seriously considering tube preamp…opinions?

+1 Tvad. That has been my experience as well.

Assuming, as you alluded to, that the specific tube amp and the specific speakers are good matches. And in that regard it should be kept in mind that different tube amps differ widely in terms of output impedance, and correspondingly in terms of damping factor, while most solid state amps have near zero output impedances and relatively high damping factors. Thus for a given speaker some tube amps providing a given power capability may be suitable matches, while others providing the same power capability may not be.

In general, speakers having relatively high nominal impedances and/or relatively small variations of impedance as a function of frequency (i.e., relatively flat impedance curves) tend to be less critical in that regard than speakers not meeting those criteria. My impression is that the Salk speakers hilde45 has ordered fall into the less critical part of that spectrum, although I haven’t seen an impedance curve for them.

So choosing the right tube amp for a given speaker involves a bit more complexity than choosing a solid state amp, but if chosen properly the results can be very rewarding.

Best regards,
-- Al

In the case of an amplifier having a triode/ultralinear switch, the ultralinear position will, roughly speaking, usually double maximum power capability compared to the triode position (i.e., a 3 db increase), while probably also affecting sonics in some manner that depends on the design of the specific amp. If an amp you may consider provides adequate power for your purposes but lacks such a switch I wouldn’t consider the lack of the switch to be significant.

Also, FWIW, the only amps I’ve had which provided such a switch were two classic oldies, Marantz models Two monoblocks (late 1950s) and Nine monoblocks (1960s), which I owned during the 1990s. In both cases I preferred the sonics of triode mode, although the 18 or 20 watt capability of the Two in triode mode was not sufficient to handle some classical symphonic recordings having particularly wide dynamic range, with the 90 db speakers I had at the time.

Re fixing locally vs. shipping to the manufacturer, I suppose factors to consider would be the weight of the amp and (if it is bought used) whether or not original packing materials are available, either from the seller or by ordering them from the manufacturer.  But as a general rule of thumb I would say it is preferable to have an amp fixed by the manufacturer.

Best regards,
-- Al

That seems like a good change well worth the increased cost.

+1.

Best regards,
-- Al

I haven't ever listened to Zu speakers, and I couldn't find published impedance curves or other measurements for the Omen models. But based on the specs and on what I've read about them over the years I'd expect them to be very versatile with respect to choice of amplification, while at the same time being revealing of the intrinsic sonic character of the particular amp.

Also, some of the Omen models have 12 ohm nominal impedances, while others are rated at 8 ohms. And while solid state amps will typically have maximum power capabilities that are close to 1/3 less into 12 ohms than into 8 ohms, given the high efficiency of those speakers that won't usually matter. I suppose, also, that from a sonic standpoint there may be some cases where a 12 ohm load might be less than optimal for the 8 ohm tap of some tube amps.

Best regards,
-- Al
 

Danvignau 2-28-2020
Tubes add serious harmonics to terrible recordings to make them sound better. They also add harmonics to good recordings to make them sound worse.

Tvad 2-28-2020
While a bit of an exaggeration, this is true, although tube amplification often sounds better at clipping output level (as @atmasphere has explained).

It’s a primary reason I have suggested Pass Labs Class A solid state amplification (and gently joined by @almarg).

In addition, Ralph has explained in many threads that while tube amps usually have greater **total** harmonic distortion (THD) than solid state amps, they usually have lower amounts of the components of the distortion to which our hearing mechanisms are most sensitive, and which are most objectionable from a subjective standpoint. Namely certain higher order odd numbered components, such as the 7th harmonic. Also, solid state amps typically (but not always) require greater amounts of feedback in the design than tube amps. Depending on the specific design that can often contribute to transient intermodulation distortion (TIM), which can be particularly objectionable and which is not normally specified or measured. And as far as I am aware a standard for how to measure TIM does not exist.

I almost always agree with the insights Ralph provides here, and that certainly applies in this case. At the same time, though, it seems evident that both kinds of amplification may or may not provide fine results depending on the specific amplifier and on how it synergizes with the particular speaker. So my philosophy when it comes to audio is to focus on the specifics of the particular equipment that is involved, rather than on generalities which usually hinge on matters of degree. And the main reason I leaned toward suggesting solid state amplification earlier in this thread is that in the majority of cases small two-way speakers are designed with the expectation that they will be driven with solid state amplification. Which as a result of its near zero output impedance (accomplished with the help of feedback in most cases) maintains essentially constant output voltage into varying impedances. And hence delivers more power into lower impedances than into higher impedances, as long as the amp is operated within its capabilities. I also suggested that among tube amps those having low output impedances (for a tube amp) are likely to be most suitable.

Based on the first of Ralph’s posts dated 2-26-2020 in this thread, though, it sounds like the Salk WOW1 (and other Salk speakers), like the Daedalus speakers I use, tend to be more versatile with respect to the choice of solid state vs. tubes than many others.

Best regards,
-- Al

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

Regarding the PrimaLuna EVO 400, I don’t know about that specific model but Stereophile’s measurements of several other PrimaLuna amps reveal them to have very high output impedances, even for tube amps. That stands a good chance of resulting in tonal imbalances when paired with many small two-way speakers. As well as with some larger speakers, depending on how their impedance varies as a function of frequency, and especially if the speaker was designed with the expectation that it would be driven with solid state amplification.

Regarding VTL, over the years a number of members have reported in threads here that they had customer service experiences with VTL that were, shall we say, less than ideal.

To add to Tvad’s comments about VAC amplifiers, with which as a former VAC owner I entirely agree, a nice thing about them is that they usually include a 2 ohm (or "1-2 ohm") output tap, in addition to the usual 4 and 8 ohm taps. The lower the impedance designation of a given output tap the lower the corresponding output impedance will be, and that additional tap adds significantly to the amp’s versatility with respect to speaker matching. The VAC models that were mentioned above include that third tap.

Regarding ...

At what *starting* level of db sensitivity does the choice of quality all-tube amplification open up? 88db? 92 db? more? (I suppose I’m not thinking of the tiny-watt tube systems, but things that are up to 35-40 wpc)

... Answers to that question will vary considerably among different listeners. And IMO a major reason for the variability is that it depends on the dynamic range of the kinds of recordings that are listened to (i.e., the **difference** in volume between the loudest notes and the softest notes). As I think I mentioned earlier in one of your threads there are some classical symphonic recordings in my collection that have been engineered with minimal or no dynamic compression which can reach brief dynamic peaks of 100 to 105 db at my 12 foot listening distance, while being listened to at average levels in the 70s, and with the softest notes being in the low 50s. And lest that amazing 50+ db dynamic range be questioned I’ll add that I have verified those dynamic ranges on a few such recordings by examining their waveforms on a computer, using a professional audio editing program.

Tvad’s suggestion of 90 db or more as the answer to your question is certainly reasonable. Beyond that the only things I can suggest are to try plugging various numbers into the SPL calculator I linked to earlier, and preferably to obtain an SPL meter which provides fast response time and "c-weighting." I use an older Radio Shack model 33-2055 digital SPL meter, which may be findable at eBay.

Finally, as a Daedalus owner I certainly agree with Tvad’s comments regarding them. However all of their models are floorstanding, and their prices may be discouraging. There is a used Daedalus Athena for sale here presently for $6K; with the possible exception of some particularly old models I doubt that other models are likely to appear for sale for much less than that.

Best regards,
-- Al

From a specification standpoint I don’t see any issues pairing the Quicksilver Line Stage and 60 watt mono amps with low efficiency speakers such as the Salk WOW1, provided that peak SPLs in the mid to upper 90s at the listening position are sufficient for your purposes.

The high damping factor (20) of the amp, which corresponds to a very low output impedance for a tube amp, seems fine for use with such speakers with respect to the tonal effects I referred to earlier that can result from amp/speaker impedance interactions.

If the speakers were much more efficient (e.g. in the mid to upper 90s) I would be a bit concerned that the high gain of the preamp (18 db) might limit the useful range of the volume control to very low settings. But that seems very unlikely with 84 db speakers.

I have no experience with Quicksilver products, so I can’t comment on their sonics. But the impression I’ve gained from comments by others that I’ve seen here and elsewhere over the years has been very favorable.

As Tvad said, "go for it!"

Best regards,
-- Al

Caution: This is a long post :)

Here are some comments concerning amplification that I would add to this excellent discussion, which also have potential implications regarding speaker selection:

First, small box-type two-way speakers (such as those the OP is particularly interested in) are very often designed such that their impedance in the bass and mid-bass regions is significantly lower than their impedance at higher frequencies. The intent being that when such a speaker is driven with a solid state amp the near zero output impedance of nearly all solid state amps will result in greater amounts of power being delivered at low frequencies than at high frequencies (for a given input voltage to the amp), thereby augmenting what would otherwise be a relatively weak response at low frequencies.

Since the output impedances of tube amps vary widely, what that means is that if a tube amp is to be selected for use with such a speaker it should have relatively low output impedance (for a tube amp). And since amplifier output impedance is inversely proportional to damping factor, damping factor should be relatively high (for a tube amp). Otherwise weak bass and over-emphasis of higher frequencies would be the likely result with many and perhaps most such speakers.

If a tube amp is chosen for use with such a speaker I would suggest a damping factor of at least 8, and preferably more. Most or all of the Quicksilver amps that have been discussed meet that criterion. Other tube amps having relatively low output impedances/high damping factors include those made by Audio Research, Music Reference, and McIntosh.

Second, I’ll mention that a bit more than a year ago I purchased a Pass XA25 from Mark at Reno HiFi, and I could not be more delighted with the amp as well as with the purchase experience. It replaced a somewhat older but very highly regarded tube amp, a VAC Renaissance 70/70 MkIII, which I had used for the previous seven years. (That amp cost more than $14K when it was new, ca. 2000). With my particular speakers (Daedalus Ulysses), at least, it comes remarkably close to the VAC with respect to dimensionality, imaging, liquidity, and other traditional fortes of high quality tube amps. It’s a bit less rich sounding than the VAC, but I interpret that as an increase in accuracy, which is fine as far as I am concerned. A caveat, however, is that my speakers are rated at 97.5 db and have an extremely flat and benign impedance curve.

The INT-25 integrated amp of course combines a very similar power amp with a line-stage preamp.

Also, per Stereophile’s measurements the maximum power capabilities of the XA25, the INT-25, and the INT-60 are all within about 1.2 db of each other, into both 4 and 8 ohms. I suspect Mark at Reno will tell you that the XA25 and INT-25 sound better when operating within their “class A envelope” than when asked to provide an amount of power which causes them to transition to AB. The class A envelope for those amps is spec’d as 50 watts peak into 2, 4, or 8 ohms. (That number would be significantly less on the more usual rms basis). However the INT-60 is described as being primarily a class AB amplifier anyway, which leaves class A at 30 watts. (It isn’t clear if that is peak or rms). So all three amps seem roughly comparable in those respects.

Finally, per the following calculator which I referenced in one of your other threads …

https://myhometheater.homestead.com/splcalculator.html

… Two speakers having 85 db/1 watt/1 meter efficiency, if driven with 25 watts and with a bit of “room gain” factored in, can produce an SPL of 95.3 db at a 10 foot listening distance. 130 watts (Stereophile’s measurement of the XA25’s capability into 4 ohms, which is a bit higher than that of the INT-25 and INT-60) would produce 102.5 db. Both those numbers are probably sufficient for most users on most or all of their recordings, but will be marginal **at best** for some listeners on some of their recordings, particularly those having very wide dynamic range (such as some classical symphonic recordings which have been engineered with minimal or no dynamic compression).

All of which, IMO, reinforces the notion of considering higher efficiency speakers. Especially given that the OP already has a sub, which would help to compensate for the tradeoff that often exists between the efficiency and the deep bass extension of a speaker, for speakers that are of similar physical size.

Best regards,
--Al