Tube Amps Watts vs SS amp

Soundermn 5-10-2019
His dealer brought over a Primaluna HP Integrated to listen with his [AR9] speakers.

It was not a good match. I don’t know the technical reason - it was definitely plenty of output power. But the bass was kind of flabby and weak, and overall the speakers sounded flat and uninviting. I was shocked. This amp has never sounded so poorly with any speakers I’ve heard.

I would expect that a major reason for that is the relatively high output impedance (and correspondingly, the relatively low damping factor) of that amp. Some PrimaLuna amps have output impedances that are much higher than average even for a tube amp. But even if (as appears to be the case) the amp in question was the PrimaLuna DiaLogue Premium HP model that was reviewed by Stereophile here, and even though JA noted in the measurement section of the review that "these impedances are significantly lower than with other PrimaLuna amplifiers I have measured," the amp’s damping factor (corresponding to the measured output impedances divided into the 4 or 8 ohm rating of the corresponding tap) is just slightly greater than 3!

In the first of my posts dated 5-8-2019 in this thread, in which as you also did I tried to focus on the OPs issue rather than on generalities about tubes vs. solid state, I had suggested to him that although I couldn’t find an impedance curve for the AR9 (which probably would have allowed a more precise judgment to be made), if he were to go to a tube amp it should be one having a damping factor of at least 8.

Regards,

-- Al

In a nutshell since your speakers are 4 ohms the efficiency is 3dB less than the stated 87dB.

Hi Ralph (Atmasphere),

In this particular case the 87 db spec is stated to be based on an input of 1 watt, rather than on an input of 2.83 volts. Also, as I mentioned in my previous post the brochure for the speaker states that an input of 8 watts results in an SPL of 96 db at 1 meter, which is consistent with 87 db/1 watt/1 meter. So if those numbers are accurate the efficiency is 87 db, rather than 84 db.  

Best regards,

-- Al

@bestbaker, as mentioned earlier the speakers are spec’d at 87 db/1 watt/1 meter, with an impedance of 4 ohms nominal and 3.2 ohms minimum. A brochure I found at HiFiEngine.com indicates that an input of 8 watts will produce 96 db at 1 meter, which is precisely consistent with the 87 db/1 watt number. I couldn’t find an impedance curve for the speaker, though, which would indicate how its impedance varies over the frequency range.

**If** the 87 db and 96 db specs are accurate (and speaker efficiency and sensitivity specs are often optimistic by a few db) it can be calculated that at a listening distance of say 12 feet in a medium to moderately large size room two such speakers driven by say 50 watts will be capable of generating an SPL at the listening position that is in the mid to upper 90s. That will be enough volume for most listeners with most or all of their recordings, but it will certainly not be enough for some listeners with some recordings. Especially recordings having particularly wide dynamic range (i.e., a large **difference** in volume between the loudest notes and the softest notes), such as many well engineered minimally compressed classical symphonic recordings.

What may be a more significant concern, though, (which Millercarbon alluded to in his post early in this thread) is impedance compatibility. The speakers were presumably designed with the expectation that they would usually be driven with solid state amplification, and solid state amplifiers almost always have output impedances that are a tiny fraction of an ohm. While the output impedance of tube amps varies widely, and can be anywhere from somewhat less than an ohm to several ohms or more. If the impedance of the speaker varies significantly over the frequency range (and it probably does, although as I mentioned I couldn’t find an impedance curve for it), the interaction of that relatively high amplifier output impedance with the speaker’s impedance variations is likely to have unpredictable tonal consequences that were not intended by the designer.

To minimize that you would want to choose a tube amp having relatively low output impedance, which correspondingly means a damping factor which is relatively high (for a tube amp). I would certainly not feel comfortable choosing an amp having a damping factor of less than say 8, and many and probably most tube amps do not meet that requirement.

Makers of tube amps having relatively low output impedance/high damping factors include Audio Research, Music Reference, McIntosh, and possibly various VAC models which provide a 2 ohm output tap. However finding an amp which provides adequate power, low output impedance/relatively high damping factor, high quality sonics, and meets your stated budget, is likely to be a challenge. Personally I would follow the advice of those who recommended staying with solid state.

Regards,

-- Al