90db 4 ohm with tubes?

As a follow-up to my and Marakanetz' earlier posts, I found the following statements in John Atkinson's report on his measurements of the V12, which appears to be generally similar to the V12R aside from differences in the input section:

... allowing for some variation in the calculated output impedance with the level and load impedance used, we're looking at source impedances of 5.5 ohms (8 ohm tap) and 2.8 ohms (4 ohm tap) in triode mode. While these are both high, they are exceeded by the ultralinear figures: 12 and 6 ohms, respectively! Fortunately, the impedances don't change much across the audioband, but, as figs.1-3 reveal, there will be a large and audible change in frequency response depending on which loudspeaker is used with the Cary and which transformer tap and mode of operation are used....

... I was concerned by the highish levels of HF intermodulation and the very high source impedances, particularly in ultralinear mode. Unless you really need the extra 3dB of dynamic range you get from ultralinear operation compared with triode, and the extra 3dB you also get from the amplifier's 8 ohm tap compared with the 4 ohm tap, my advice is to use the CAD-280SA in triode mode with the speakers hooked up to its 4 ohm outputs.

Caution would seem to be in order here. I would not advise pairing such an amplifier with a speaker having such wide variation of impedance as a function of frequency, without first carefully researching the results others have obtained using those speakers with the same or similar amplifiers, AND carefully auditioning the speaker with that specific amplifier.

Regards,
-- Al

Phooey I say, If the amp has a substrantaial power transformer (it does) and good output transformers (it does) then you can play you music through those soeakers. You will have to decide if you like the sound however-you may or may not- but I say it won't be the amps fault. I have lower wattage tube amps driving speakers that like to dip south of 4 ohms which sound absolutely fabulous. I am certain the math would say no... but my ears say yes,

Dr. M, the concern is NOT the speaker's 4 ohm or whatever nominal impedance. The concern is with the wide VARIATION of that impedance as a function of frequency (from about 4 ohms to about 28 ohms according to the measurements of the particular speaker that were cited earlier, with the 28 ohm peak occurring in a critical part of the spectrum), and the INTERACTION of that VARIATION with the amplifier's high output impedance.

Yes, if the sonic results turn out to be poor "it won't be the amp's fault." It won't be the speaker's fault either. It will be the result of a mismatch.

Regards,
-- Al

Well, if it's worth anything, the amp is very powerful, and yes it does have 4ohm taps. Good comments, all. Interesting. Any other thoughts?

Dr. Mechans, I have beaten this subject to death with Al and Ralph (Atmasphere). Check my threads. Al is 120% correct.

The ability of a tube amp to drive a speaker with wide impedance fluctuations with tight output regulation is a function of the amp's output impedance rather than the muscle of the power or output trannies.

My amp is an ARC VS-115 which Soundstage bench tested to have an output impedance of 1.1 ohms off the 8 ohm tap and approximately 50 to 60 percent of that off the 4 ohm tap. As a result, Soundstage measured output regulations to be +/- 1 db off the 8 ohm tap. Possibly half that off the 4 ohm tap.

Coincidentally, when looking over the VS-115 stats off the ARC web site, output regulation was reported to be 1.2 db off the 8 ohm tap, obviously quite close to the Soundstage bench tests. The likely reason my amp's output impedance is so low is because ARC uses about 12 db of NF.

I think Al might concur with the surmise that if an amp's output impedance is higher, say 5.5 ohms, its output regulation would be much higher in dbs as a function of the speaker's impedance variations.

If you or others are interested in this topic, pull my threads and you'll see what I went through to get this far along. I remember many of the techies like Al and Ralph wrote some years back that the design of an amp necessitates trade-offs and compromises. Although NF has been pooh-poohed for various reasons, without using some NF, my output impedance would not be as low as it is, and correlatively my amp's DF would be much lower, and so forth and so on.

Cheers and thanks again Al.

Bruce

Thanks again Al.