90db 4 ohm with tubes?

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

Hi Bruce,

That all sounds generally correct, but I'll add some qualifications:

1)Re the last paragraph, yes a 1.2 db fluctuation is pretty minor in comparison to room effects, and in your case I suspect is nothing to worry about. More generally, however, and particularly where the fluctuation may be significantly greater (as it would be in the OP's proposed configuration), I would keep in mind that a given variation in measured frequency response at the listening position, caused by room effects, may be subjectively much less objectionable than an identical frequency response variation caused by impedance incompatibilities in the system. At least in the situation where the in-room response is measured in the traditional simplistic ways that do not take arrival times into account.

The reason for that is the ear's ability to discriminate between "first arrival" sounds and reflected sounds that arrive subsequently. See this Wikipedia writeup on the Haas and Precedence Effects.

2)Re the ARC spec you quoted for "output regulation," it may help to clarify matters if I describe how I would calculate the approximate variation in db that would correspond to a damping factor of 8, and a load variation between 8 ohms and an open circuit (i.e., infinity ohms). On the other hand it may just confuse matters further, but I'll do it anyway :-)

(a)First, see this Wikipedia writeup on the Voltage Divider Effect.

(b)In the first figure, consider Vin to be the voltage the amp is "trying" to put out at some instant of time (i.e., the voltage it would output under open circuit/no load conditions (hypothetically speaking, as of course a tube amp that has an output transformer should not be run unloaded)).

(c)To keep things simple(!), let's assume that the speaker impedance and the output impedance of the amp are purely resistive.

(d)Think of Z1 as the output impedance of the amp, and Z2 as the impedance of the speaker.

(e)For an 8 ohm tap, or for an amplifier having no output taps, damping factor is normally defined as output impedance divided into 8 ohms. So the damping factor of 8 for the VS-115's 8 ohm tap means that the output impedance is about 8/8 = 1 ohm (much lower than for the OP's amp, and in fact lower than for the majority of tube amps).

(f)Referring to the Wikipedia page on the voltage divider effect, under no load conditions Z2 is infinite, no current will flow through Z1 (because a complete circuit is not present if Z2 is infinite), therefore the voltage drop across Z1 will be zero, and therefore Vout = Vin.

(g)If Z2 were 8 ohms, based on the voltage divider effect Vout = (Vin) x (8/(8+1)) = 0.888Vin

(h)Voltage ratios are converted to db as 20 times the logarithm of the voltage ratio.

20log(0.888) = -1.02 db.

So the output variation from an 8 ohm load to an open circuit, assuming a damping factor of 8 and based on the oversimplified assumption that the speaker impedance is purely resistive, would be 1.02 db, a little less than the stated figure.

That value would be somewhat worse, of course, from 4 ohms to an open circuit, if the 8 ohm tap were used. Using the 4 ohm tap on the other hand, which probably has an output impedance that is around half the output impedance of the 8 ohm tap, would of course reduce the variation significantly.

Finally, although you most likely have this in mind, it should be noted that a variation of 1.2 db is only half as much as +/- 1.2 db.

Best regards,
-- Al

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

I'm driving them with a much lower power 30w output Mastersound Compact 845 int amp and the sound is great, to my ears :)

Al, the amp will be fine on that speaker if it employs negative feedback of sufficient amount. At that point it will behave as a voltage source as far as the 28 ohm peak is concerned.