90db 4 ohm with tubes?

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.

I'd agree the problem here is a speaker with wide load impedance fluctuations coupled to a power amp with high output impedance. My stipulation of good transformer and power supply isn't the solution in this case.NFB is an option for lowering the amp's output impedance but IMO introduces more harm overall than good. Widely fluctuating speaker loads just seems like a bad design choice. Why not just design a speaker with a flatter impedance curve? This would allow more amplifier choice and flexibility.
Regards,

Charles...., you are spot on correct. But to satisfy your own curiosity, check the published bench test reports on a number of the "Big Boy" speakers like Revel Salon 2 and Magico. They have some pretty rough impedance curves and the manufacturers recommend high current - high power amps (i.e., SS) which usually have extremely low output impedances.

It is my anecdotal understanding that there are a few speakers that have relatively flat impedance curves that would make them tube friendly. Your point is really a "bitch" issue that the big tube amp manufacturers need to take up with the big speaker manufacturers.

In my case, I like the sound of my ARC amp, NF and all. Fortunately, through dumb luck, I only recently figured out that my amp can drive my speakers without adding too much "acoustic flavor."

Cheers,

Bifwynne,
A good friend of mine recently bought a pair of the Rockport Altair speakers. They are a nominal 4 ohm load but are driven effortlessly by tube amplifiers and sound wonderful and very natural. For this reason I'll assume they must have a relatively smooth impedance curve and thus aren't restricted to high current SS amplifiers in order to be driven. I wish other designers would follow Andy Payor's example. If it's going to be a lower nominal lmpedance, strive to keep it reasonably flat.
Regards,