Tube Amp Questions
You didn't specify a budget but given the ARC option you mentioned, I assume you're looking for something in the $2K price range. With that in mind, there are lots of options out there if you're willing to go used, maybe even an older Krell or Mark Levinson, something that can handle a 2 ohm load without breaking a sweat. How's your current 80wpc Denon managing such demanding speakers? |
@kalali Well, the Denon doubles to 160 watts @ 4 ohms, so it works ok. Truth be told, I got the speakers at such a good price locally that I'm starting with them as the building block and going from there. The ARC VS110 I offered on went to someone else, so...in hind sight, it was prob for the better as the majority of feedback I've gotten from here as well as from friends is that SS is a better option for Thiels. You're right: I'm shopping in the $2K - $3K range. There are two options that are catching my eye right now, a Krell KST-100 & a Pass Labs x150. Both are Class A for some of their range, then switching to Class A/B. My concern now is my ARC LS-7 line stage. How do I know if it will "match" these (or any other) amplifiers? Arvin |
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 |
@almarg Hello Al! Thanks for both of your posts...incredibly useful info!!! If I may, one more question for you: As I'm so new to the whole idea of component matching, using the example of mismatched impedances between preamp & amp, what does the result "sound" like? In other words, how do I know by listening that my equipment just isn't "right" for each other? Thanks for any additional insight you may be able to share! Arvin |
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 |
