Tube Amp Questions

The majority of tube-based preamps, including the LS-7, have coupling capacitors at their outputs. The impedance presented by a capacitor rises as frequency decreases, which can result in the output impedance of the preamp being much higher at deep bass frequencies than its specified value, the specified value usually being based on the output impedance at a mid-range frequency such as 1 kHz.

So if the input impedance of the amp is not high enough, and keep in mind that for the LS-7 ARC recommends 20K as a minimum with 60K being preferred, the interaction of that input impedance with the rise in the preamp’s output impedance at low frequencies will result in weak bass. Depending on how the impedances of the specific designs vary at other frequencies there might also be adverse sonic effects at higher frequencies, but their sonic character won’t have much if any predictability since those effects will be dependent on the specific designs.

The commonly cited rule of thumb to assure impedance compatibility for a line-level interface is a 10x minimum ratio between the input impedance of the component receiving the signal and the output impedance of the component providing the signal. However, what is often overlooked is that the 10x ratio should be applied at the frequency within the 20Hz to 20kHz audible frequency range at which output impedance is highest. And as I said manufacturer specs commonly just indicate a nominal output impedance that is presumably at 1kHz or some other mid-range frequency.

If the preamp or other component providing the signal has been reviewed by Stereophile, John Atkinson’s measurements usually indicate the worst case output impedance, i.e., the highest output impedance at any audible frequency. If only a nominal output impedance is known, though, IMO a minimum ratio of at least 50x, and ideally 75x, should be used, especially if the preamp may have a coupling capacitor at its output as most tube preamps and at least a few solid state preamps do.

All of which is not to say that there will necessarily be a problem if those ratios aren't satisfied; as I indicated it would depend on how the impedances of the specific designs vary as a function of frequency.  But it is to say that satisfying those ratios assures that there won't be a problem.

Regards,
-- Al

Arvin, according to its manual the Pass X150 has a specified balanced input impedance of 22K, with no spec indicated for its unbalanced input impedance. The unbalanced input of course is what you would be using with your LS-7. Most likely the unbalanced input impedance is much lower than 22K, quite possibly 11K. Per my earlier comments even 22K would at best be marginal for use with your LS-7, and 11K would be completely unacceptable. So I would not recommend that amp.

On the other hand, I looked at Stereophile’s measurements of the KST-100, and I see no compatibility issues whatsoever. The only concern would be the condition of the specific amplifier you may purchase, given its age.

Good luck. Regards,
-- Al

My suspicion is that the VS110, as well as many other tube amps, would be a poor match for the Thiel CS3.7. From Stereophile’s measurements of the speaker:

...the CS3.7’s impedance remains between 2 and 3 ohms over much of the audioband (fig.1), and that there is a demanding combination of 3.8 ohms and –40° capacitive phase angle at 60Hz. Thiel specifies the impedance being nominally 4 ohms, with a minimum of 2.8 ohms. I actually found the minimum impedance to be 2.4 ohms at 125Hz. The difference between 2.8 and 2.4 ohms is academic, either mandating use of an amplifier that has no problem delivering high currents.

Figure 1 at that link, which depicts the speaker’s impedance magnitude and phase graphically, further reinforces my belief that the CS3.7 is not a tube-friendly speaker.

Also, as stated in Stereophile’s measurements of the VS110:

The output impedance from the 4 ohm tap was a fairly high 0.72 ohm across most of the audioband, this rising to 1.3 ohms at 20kHz. The equivalent figures from the 8 ohm tap were 0.93 ohm and 1.9 ohms, respectively. As a result, the modification of the amplifier’s frequency response due to the Ohm’s Law interaction between its output impedance and the way in which a speaker’s impedance changes with frequency will be quite large.

So IMO, generally speaking, a solid state amp would stand a much greater chance of being an optimal match for that speaker. Although in choosing one keep in mind that its input impedance should conform to ARC's load recommendation for the LS7:

Recommended load 60K ohms and 100pF (20K ohms minimum and 1000pF maximum)

(The pF part won’t usually be a problem, unless the interconnect cables are unusually long. But many and probably most solid state amps will have input impedances below 60K ohms, although I wouldn’t worry about it if the input impedance is around 30K or more).

It may be worthwhile for you to search Audiogon’s "Virtual Systems" section to see what amplifiers others are using with the CS3.7 or similar Thiel speakers.

Good luck. Regards,
-- Al