90db 4 ohm with tubes?

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.

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,

Al,

Just a quick follow-up to your last post. Copied below is an excerpt from the ARC web site that specifies the VS-115's output regulation:

"OUTPUT REGULATION: Approximately 1.2dB 8 ohm load to open circuit (Damping factor approximately 8)."

I assume that the reference to "open circuit" permits the inference that the amp can manage wide impedance fluctations ranging from 4 ohms to 28 ohms within a relatively narrow band, +/- 1.2 db. As I said before, when Stereophile and Soundstage tested the output regulation of the Ref 150 and VS-115 using a simulated speaker load having wide impedance variations, the output regulation results were quite similar.

I suspect that even a solid state amp that uses NF will likely compensate for speaker impedance varations too. Of course, as you said, the designer's intentions count for a lot.

As a layperson, I would describe ARC's output regulation control as not perfect; but just adding a little "flavor" to the acoustic presentation. I surmise, a room's sonic ambience will add much more "flavor" to the acoustic presentation than a 1.2db output fluctation. Just a guess.

BIF

Thanks again as usual Al. As I said, it took a while for your tech explanations to finally sink in, but I think I now have the gist of it. As it turned out, the S8s (v3) changed the cross-over a bit from the v2. Paradigm told me that the max impedance of the v3 is 21-22 ohms at the 2K Hz peak as compared to 28 ohm for the v2. So the v3 impedance range is 4 ohms to 21 ohms; not as crazy as before.

New item: My brother gave me a gift of his old vinyl collection from the 70s and 80s. And I'm having a ball listening to some really good stuff, like the Beatles, Grateful Dead, Moody Blues, Billy Joel, Johnny's Dance Band, the Pretenders, etc. Even better, the LPs are in pristine condition.

Glad I sorted that tech stuff out -- with your help of course. NF aside, the ARC gear sounds mighty sweet.

Cheers and many thanks.

BIF