The technical key to integrating a tube preamp (high output impedance typically) with any SS amp including Class D is the impedance matching. Some basic class D amps have only 10 kohm input impedance, which is not a good match. 40kohm input impedance on the amp or higher. Input impedance of my Bel Canto ref1000m amps is 100kohm. Bingo! Prior ref1000 model (lower cost) was only 10kohm, same as input impedance of the Icepower Class d module used.
Audio Research VSi75 - End of Tube Life? KT120 vs KT150?
I have recently bought a second hand ARC VSi75 amp. It came with both the original KT120s, which have about 250 hours of use, and some after-market KT150s, which have about 1800 hours of use.
With the KT150s, the sound is quite hard, cold sounding, tonally a little bleached, but with great dynamics, extended bass and treble, and much more three dimensional. KT150s are claimed to have a tube life of about 3000 hours, so these should be not much past half their life. There is no distortion or noise.
The KT120s sound warm, tonally rich, colorful and much more musical. But they also don't have the dynamics or frequency extension. Nevertheless I much prefer this sound.
Does it sound like my KT150s are at the end of their life after 1800 hours? This cold, steely, colourless sound does not match what I read about these tubes, but it also doesn't sound like the symptoms of normal tube aging.
A quad of KT150s is reasonably expensive. Is what I am hearing just the normal difference between KT120s and KT150s? If so, it is probably not worth the cost of buying another set of KT150s to find out.
With the KT150s, the sound is quite hard, cold sounding, tonally a little bleached, but with great dynamics, extended bass and treble, and much more three dimensional. KT150s are claimed to have a tube life of about 3000 hours, so these should be not much past half their life. There is no distortion or noise.
The KT120s sound warm, tonally rich, colorful and much more musical. But they also don't have the dynamics or frequency extension. Nevertheless I much prefer this sound.
Does it sound like my KT150s are at the end of their life after 1800 hours? This cold, steely, colourless sound does not match what I read about these tubes, but it also doesn't sound like the symptoms of normal tube aging.
A quad of KT150s is reasonably expensive. Is what I am hearing just the normal difference between KT120s and KT150s? If so, it is probably not worth the cost of buying another set of KT150s to find out.
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@mapman my tube preamp is 1 volt output line stage = 20 dB, and input impedance on my Cary SA-200.2 solid state amplifier was 22K Ohms RCA-unbalanced. This got very LOUD at 10-11’oclock, starting at 7’oclock on the volume control. Had plenty of gain there. Agree, maybe 10k ohms is not enough on the amp side depending on the preamp. Will keep that in mind if I test some class D amps later. Thx. |
There was a discussion a few years ago at psaudio with folks and matching different preamps, output and input impedance matching, other considerations in addition to voltage. This is what i was referring to as well. https://www.psaudio.com/pauls-posts/mix-and-match-impedance/ No issue with lower oi or ii on my tube preamp to SS, not quite 1/10th or 10x but still ok. The KT120s or KT150 based amps no issue being driven, good matching helps for sure. Nothing today the KT120s still sounding nice with more burn-in time :)
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- 22 posts total