90db 4 ohm with tubes?

I know on some Cary amps, they allow you to switch feedback on and off. I don't remember if the V12 was one of them.

Hi Ralph,

The manual for the V12, which appears to be very similar to the OP's V12R except for differences in the input section, indicates that it is a zero feedback design. (The manual for the V12R is silent on that question, though). Also, JA's output impedance measurements of the V12, and his measurements and statements about the sensitivity of its frequency response to load impedance, seem very consistent with the use of little or no feedback.

Best regards,
-- Al

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.

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

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

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

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

Bruce, thanks very much for your comments. Charles and Hifitime as well. Here are my comments on some of what has been said:

06-10-13: Bifwynne
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.

Yes. Of course, if the impedance of the speaker reaches low values at some frequencies muscle, good trannies, and good power supplies are ALSO likely to be necessary.

06-10-13: Bifwynne
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.

Yes, absolutely.

06-10-13: Bifwynne
It is my anecdotal understanding that there are a few speakers that have relatively flat impedance curves that would make them tube friendly.

Certainly. My Daedalus speakers are one example. I believe that the Coincident speakers, one of which Charles uses, are another. I'm sure many other examples could be cited.

I would add that widely varying impedance does not NECESSARILY mean that the speaker won't work well with an amp having high output impedance. For example, electrostatic speakers commonly have very high impedances at low frequencies, which descend to very low values in the upper treble region. Check out this curve for the classic Quad ESL, which, like many electrostatics, works well with tubes. Or, among dynamic speakers, check out this curve for the Harbeth M40.1, which a lot of people use with tubes with excellent results.

As Ralph (Atmasphere) has said, and this also addresses the point in Hifitime's first post above, it depends on the intentions of the designer, and tonal balance problems usually result when the amp and speaker are not of the same paradigm.

And in the case of a speaker having an impedance that is near 4 ohms across much of the spectrum, but rises to 28 ohms in the upper mid-range, it would seem to be a good bet that it was not designed to sound its best with an amplifier whose output impedance is even higher than that of most tube amps.

Regards,
-- Al

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,

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,

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,

With my other post, I'm thinking what an amp may do when it just has to power that certain frequency (or frequency range), that has a high or low impedance variation, in comparison the frequencies that are more linear as far as impedance goes. It may be more noticeable during a solo performance too. When the amp has to power other frequencies at the same time, the power to that frequency with a different impedance can be more stable is what I'm thinking.

That speaker mentioned above with the 28 ohm peak, who does know how it will sound when something, maybe even more-so, a solo performance that may have a lot of music at the frequency itself a lot of the time. It may do okay with one amp, have problems on the next. That's one major thing I like when they do reviews, is the part of measuring something for possible problems to be aware of.

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.

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?

I think you will still have issues with frequency response variations due to impedance variations, if you run a large solid state amp. If you run a speaker with a nominal 8 ohm impedance that rises to 16 Ohms, and drops to 4 ohms at times, I'm guessing that may cause up to a 6 db variation in levels at different frequencies, with an amp that doubles its power every time the impedance is halved.

For example, if an 8 Ohm speaker rises to 16 ohms at roughly 12 kHz, the amp (large solid state) will only put out half of the power at the time, causing about a 3 db drop in sound level, at that frequency and time. If the load drops to 4 ohms at 1 kHz, the power will double at that time, giving it roughly a 3 db level increase during that drop to 4 ohms, due to the amp doubling its power at that moment. That's why I'm guessing up to a 6 db variation in this case, if the amp doubles its power that way. Even more of a change in frequency response variations yet, if the speaker impedance varies more than what I just mentioned.

So a potent solid state amp that doubles its power when the impedance is halved, may be less neutral sounding then an amp that can't double its power. I can see this happening. So basically, its still about how each amp reacts to certain loads demanded by various speakers, solid state, or tube.

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

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,

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

looks like the sound will sacrifice tonal balance.
upper mids emphisis will create fatigue.

Assuming you are referring to the current Diamond DMD version, the impedance curve shown in the review Samhar referred to indicates an impedance rise to around 28 ohms in the upper mid-range, some smaller impedance peaks in the deep bass region, and, according to the text of the review, 4 ohm impedance minima with "overall measured impedance [of] 6.5 ohms, so this is nominally a 6 ohm loudspeaker."

The rise to around 28 ohms in the upper mid-range will result with your amplifier in somewhat greater emphasis of those frequencies than would occur with nearly all solid state amplifiers, and also with some tube amplifiers (those having lower output impedance than I believe yours does). There will also be a bit more emphasis of frequencies in the vicinity of the two impedance peaks in the bass region.

I don't know whether or not with those particular speakers and your particular amplifier that would be good, bad, or something that comes down to a matter of taste and room-matching. But it's something to be aware of.

Regards,
-- Al

Your amp should have no problem driving those speakers. Not only that, you can probably get good results running you amp in triode mode. If you have a good local dealer, they will almost certainly let you bring your amp into the store and try it first.

I would say in general tube amplifiers respond favorably to higher load
impedance and potentially sound their best under these conditions.
However if the tube amplifier has good quality transformers, stiff power
supply and 4 ohm taps they can work well with 4 ohm speakers.

I heard the 4 ohm 89db Dynaudio Evidence Platinum driven by the Octave
push pull amplifier and the sound was just excellent.
I've heard the 4 ohm 91db Rockport Altair driven by the VAC Phi 300.1 (PP)
amps and also driven by the Absolare Passion PSET amp and these
produced much better sound than the MSB solid state amp on this fabulous
speaker.If your Cary amp has high caliber transformers and power supply
you'll be fine.

Regards,

I have the Dyn C1's (85db @ 4 ohm) and have driven them with an Octave V40se - V70se and V110. No problems at all.

Heard them w VTL ST85.
Good enough. Some cabinet resonance or was it port sound?
Much easier than the 718s
But low mids seem a bit bloated.
Highs are smooth, almost too smooth.

Sorry I've forgotten how to link !!!

http://www.hi-fiworld.co.uk/index.php/loudspeakers/65-reviews/511-usher-dancer-mini-two.html?start=3