What's the greatest bargain in SET these days?

I haven't taken the time to read most of this thread, but I believe I can provide some insight into the points raised in Rebbi's post just above.

As you no doubt realize, global feedback affects the characteristics of an amplifier in a number of different ways. Everything else being equal, an increase in global feedback will reduce gain (as you observed), output impedance, and total harmonic distortion. It will, to some degree that may or may not be objectionable, increase transient intermodulation distortion, and certain odd-order components of the harmonic distortion to which the ear is particularly sensitive.

I suspect that the reason for the changed perspective you described was not due to the effects of feedback on the sonics of the amplifier in itself, but rather to the interaction of the amplifier's differing output impedances, at the different feedback settings, with the impedance characteristics of the particular speakers. Many speakers, including many two-way monitors such as your Reference 3A MM de Capo iA, have substantial impedance rises occurring somewhere in the mid-range. For example, see this impedance plot for the de Capo i, which rises to more than 10 ohms at around 2.5 kHz, while being close to 6 ohms in the mid-bass and much of the treble region.

Given that kind of speaker impedance characteristic, and everything else being equal, higher amplifier output impedance (resulting from less feedback) will result in at least a slight increase in emphasis of frequencies around 2.5 kHz, relative to higher and lower frequencies. While lower amplifier output impedance (resulting from more feedback) will not produce that emphasis, at least to the same degree. And from a subjective standpoint greater emphasis of the mid-range and lower treble is typically perceived as a more "forward" presentation.

Regarding your concluding question, about how your observations may extrapolate to a SET having no global feedback, I would be hesitant to draw any generalizations. Different zero feedback SET designs may vary dramatically in their output impedances (as well as in many other ways, of course), and will vary in the interactions of those impedances with the impedance characteristics of whatever speakers are being used.

In an ideal world an amplifier would be able to achieve zero distortion (of all kinds) using no feedback, and it would have an output impedance whose match to the impedance characteristics of the speaker is consistent with the overall voicing of the speaker that was intended by the designer. But of course in the real world tradeoffs always come into play, and the optimal amount of feedback will inevitably be dependent on some combination of the particular amplifier, speaker, system, room, recording, and listener.

On another note, I've been remiss in not yet extending congratulations to Brownsfan on his retirement. It's certainly proven to be a great institution in my (and my wife's) experience :-) And of course best of luck with the new locale.

Best regards,
-- Al

Like Tubegroover I don't want to rain on anyone's parade, but I thought I would point out what appear to be highly credible measurements of the Reference 3A MM de Capo BE, performed by the National Research Council of Canada. They indicate a measured sensitivity of 86.7 db/2.83 volts/1 meter. As can be seen in the first graph, the sensitivity is in the vicinity of 92 db at a few specific frequencies, but is considerably lower at a great majority of frequencies.

Also, the impedance curve shown near the bottom of that page indicates that their impedance is about 6 ohms throughout much of the mid-bass region, where lots of energy is often required. 2.83 volts into 6 ohms is 1.33 watts, which is 1.2 db greater than 1 watt. So on a per watt basis, the 86.7 db figure is arguably optimistic by 1.2 db, meaning that 85.5 db/1 watt/1 meter is probably all that can be counted on with a lot of music.

Something to consider, despite the comments by the esteemed designers mentioned above. Regards,

-- Al

Rebbi, regarding the Dynamo's lack of input switching, I would suggest that you consider using it in conjunction with Decware's $329 passive switchbox. Both my technical instincts and my extensive experience with a lesser switchbox some years ago tell me that doing so would result in zero sonic compromise, at least in anything less than a vastly more expensive system.

And for the additional interconnects that would be required, consider Mogami 2534-based cable from Markertek, in whatever length is necessary. I and a number of other A'goners have used it with excellent results. Mogami 2534 is also the de facto cable of choice in the majority of studio and other professional applications, when terminated in a balanced configuration.

Regards,
-- Al

Again, I don't like to rain on anyone's parade, but fwiw:

The PX-25 is rated at either 5 or 6 watts, depending on what specs are looked at. The Art Audio Jota, another SET amp, is rated at 20 watts or more, depending on tube choice. Here is a comment that was posted elsewhere a few years ago by one of our particularly knowledgeable and experienced members, Jazdoc, who uses Daedalus Ulysses speakers (which I also happen to have), which are MUCH more efficient than the de Capos:

I tried my Ulysses with the high current Art Audio Jota. It was great at low volume but audibly ran out of gas a mid-high listening volumes.

I have nothing but the greatest respect for the credibility and opinions of both Charles and Jazdoc. I have no experience with Art Audio products, and I have no idea how to reconcile these seemingly conflicting inputs. But I thought I should point this out.

Best regards,
-- Al

Ralph/Atmasphere has made the point in a number of past threads that a major reason SETs are commonly perceived as having especially good dynamics relates to how their distortion varies as a function of signal level.

With most push-pull amps, once output level falls below some number of watts (often measured in single digits, I believe), distortion will rise as the level decreases further.

With SETs, on the other hand, distortion will continue to progressively decrease as output level decreases, until it becomes vanishingly small at very low output levels. While distortion will progressively increase as the output level approaches the amplifier's maximum power capability. Since as Ralph has said our hearing mechanisms utilize certain distortion components as loudness cues, that kind of variation of distortion as a function of signal level will maximize the perceived contrast between high volume notes and low volume notes. Also, the vanishingly small distortion at low signal levels will enhance resolution of low level detail.

Makes sense to me. Best regards,
-- Al

11-25-14: Larryi
If the De Capo is easy to drive, that is actually more important than the efficiency rating. I have heard low wattage tube amps driving the very easy load 15 ohm Rogers LS3/5a speaker that is extremely inefficient (low 80's for efficiency) and the combination worked well even in a big room. On the other hand, the Wilson WP7 has a mid 90's efficiency, but even 40-50 watt tube amps would find that speaker to be a beast because of its very challenging load characteristics.

Excellent comments as always, Larry, as are the responses from the others. The impedance characteristics of the de Capo (magnitude and phase) are shown in the graphs near the bottom of this page, which I had linked to earlier in the thread. They strike me as being indicative of an easy load, the worst case probably being the combination of a 7 ohm impedance and a phase angle of around -32 degrees which occurs just in the immediate vicinity of 100 Hz.

My Ulysses, btw, have extremely benign impedance characteristics, although their nominal impedance is 6 ohms. That may have been a little less true, though, in the version of the Ulysses which existed at the time Jazdoc left the comment I quoted.

Best regards,
-- Al

11-25-14: Darkmoebius
Almarg, I think this Positive Feedback review excerpt could help explain why Jazdoc's PX-25 "ran out of gas at mid-high listening levels" with his Daedalus Ulysses....

Thanks very much for the reference, Darkmoebius. Note, though, that it was not the PX-25 which Jazdoc tried with his Ulysses. It was the "high current" version of the Jota, which is rated at 24 watts.

Best regards,
-- Al

Rebbi, thanks for calling attention to the Dynamo review. I haven't received my copy of that issue yet, but I'm particularly struck by the mention of the amp's high output impedance. A 10.2 ohm output impedance/source impedance on the 8 ohm tap will cause the resulting sonics to vary DRAMATICALLY as a function of the impedance vs. frequency characteristics of the particular speaker that is being used. The most neutral behavior being likely to occur with speakers having impedances that are both relatively flat as a function of frequency and relatively high. That is true of most SETs, but to a considerably greater degree than usual in this case due to the uncommonly high output impedance.

The 4 ohm tap will have a lower output impedance, most likely around half of that value, or 5.1 ohms, which is still quite high.

I assume, btw, that since those numbers aren't specified at the Coincident website that they were told to him by Mr. Blume, as TAS (in contrast to Stereophile) doesn't usually perform such measurements.

On another note, like the others I am very much enjoying following your extremely well done blog.

Best regards,
-- Al

P.S: My post just above went up within a few seconds of the one just above it by Tubegroover, and was composed and submitted before seeing his. Great minds think alike!

Best regards,
-- Al

01-06-15: Charles1dad
I wonder what accounts for such a high output impedance of the Dynamo. The Frankenstein has an output impedance of 1.8 ohms, quite a difference.

Good question, Charles. I don't know the answer, of course, but if I were to hazard a guess, and assuming that the 10.2 ohm figure is correct, I would say that the need to minimize the size and cost of the output transformers probably has something to do with it.

Best regards,
-- Al

01-06-15: Charles1dad
I suppose it's possible to have good quality transformers that suffer unusually high output impedance.

Agreed, Charles. My comment should not be taken as implying that the output transformers are not "good," or are not capable of providing fine performance when paired with suitable speakers. My comment was speculation as to the reason, or one of the reasons, for the high output impedance. Nothing more, nothing less.

Best regards,
-- Al