5% THD to .000 THD SOUND

Gdnrbob & Bombaywalla, thanks!

I am in essential agreement with most of what has been said above. I would commend the following thread to the OP and anyone else who may be interested, in which I, Atmasphere, and others posted on the subject of amplifier specs in general and THD in particular:

https://forum.audiogon.com/discussions/amplifier-specs-does-they-matter

One of the basic points I stated in that thread was precisely what Bombaywalla said above: Specs and measurements, including THD, can often be useful in RULING OUT potential candidates for purchase from consideration, in some cases by identifying specs that may be TOO good, and thereby may signify the likelihood of undesirable design tradeoffs such as excessive use of feedback.

Personally, regarding the specific amplifiers that were cited in the OP, if I were amplifier shopping I would probably not include either one on my short list, regardless of what the reviews may have said and even if cost were no object. An amp having a 5% THD spec, no matter how euphonic and pleasing it may sound on initial audition, would over the long term cause me to unduly focus during my listening sessions on how untrue the amp is being to the recording, rather than simply enjoying the music. And an amp having vanishingly small THD would over the long term cause me to unduly focus during my listening sessions on whether it might be introducing excessive brightness, sloppy transients, and other effects that can result from heavy-handed application of feedback, or from other techniques that may have been used to achieve those numbers.

As I see it there are more than enough well regarded amps to consider, in all power ranges, that have specs that are neither "good" nor "bad" to a degree that would raise concern. Given that, I would see little point in pursuing ones having specs like these.

Regards,
-- Al

Charles, you raise a good question about the 5% THD number. I looked into it a little, and as you can see here, near the bottom of the page, that is what the manufacturer specifies. And that set of specs is quoted in various reviews.

As I’m sure you realize, the Kagura is a SET amplifier utilizing a parallel pair of 211 tubes. In looking further, I found datasheets for some incarnations of the 211 stating operating conditions in class A audio frequency applications that include an output power (per tube) of typically 12 watts **at 5% second harmonic distortion.** See the following:

http://www.tubeampdoctor.com/images/File/ER-211%20Datasheet.pdf

http://www.diyparadiso.com/datasheets/211-vt4c.jpg

I’m just speculating, of course, but perhaps the manufacturer wanted to release specs prior to concluding developmental testing and/or evaluation of a meaningful number of production units, and therefore simply chose a very conservative number that they could feel completely confident would be met. And perhaps they chose that number simply based on what appears in the 211 datasheet.

On the other hand, though, the 5% number in the datasheets is just second harmonic distortion, so presumably a corresponding spec for THD would be at least slightly worse. And the fact that the Kagura’s 50 watt power capability spec reflects 25 watts per tube, rather than the "12 watts typical" indicated in the datasheets, would also seem to suggest that the 5% THD spec may not be all that conservative.

Regarding your and Inna’s philosophy that "you just have to actually listen to components and decide based on what you hear," I of course agree (aside perhaps for the word "just"). However there is only so much equipment that any individual will have an opportunity to hear, i.e., that can be put on his or her short list. Given that, my own philosophy is why take chances by including on a short list components for which there is cause for concern, whether it be due to specs, published measurements, widespread controversy about its sonics, excessive criticality of system matching or setup, manufacturer reputation for customer service and support, or any other such factor. Especially if the short list indeed needs to be "short" in a particular case, or if an audition needs to be relatively limited in scope or duration.

Simply put, my preference is to play it safe when it comes to deciding what to short list. But that is just my own preference, and I don’t consider it to be any more valid or less valid than anyone else’s approach.

Best regards,
-- Al

Bwaslo 8-1-2016 1:36 am edt
The percent distortion spec, as usually given, should be read for the POWER level that produces it (i.e., to indicate what the manufacturer assumes is the maximum power the amp might do).

This is an excellent point, among other excellent points in Bwaslo's post.  In looking further at the descriptions of various Kondo amplifiers I see that **all** of the SET and parallel SET models listed at their site (Kagura, Ongaku, Souga), and also the Gakuon II which is not listed as a current model, are indicated as having 5% THD at their specified maximum power ratings.  Presumably if their max power ratings had been specified less aggressively (i.e., somewhat lower), the specified THD would have been significantly lower.

Interestingly, though, their Overture integrated amp, which is a push-pull EL34 design, has its max power spec based on 1% THD.  Just guessing, but perhaps their thinking is that differences in how THD is distributed among the various harmonics when the two kinds of designs (DHT SET vs. push-pull pentode) are operated near their limits would result in say 2% THD in the push-pull EL34 design being subjectively more objectionable than 5% in the DHT SET designs.

On the other hand, though, as Atmasphere has pointed out on numerous occasions, for best sonics a SET should not be called upon to provide more than a small fraction of its rated power, as in contrast to push-pull designs the distortion produced by a SET becomes vanishingly small at low power levels.

All things considered, for an application requiring just a small fraction of the Kagura's rated power I'll retract my previous statement about ruling it out from consideration on the basis of the 5% number.  Although I would feel more comfortable in doing so if comprehensive measurements (such as John Atkinson provides in Stereophile) were available.

Best regards,
-- Al

Thanks once again, Bwaslo.  And thanks Charles & Derrick for the nice comments.  For the record, the Kagura uses 3 db of feedback, which while of course not being an insignificant amount is much less than what is employed in many other designs, tube-based as well as solid state.  From its description:

Kagura’s pure silver coiled output transformer handles output impedances of 4, 8 and 16Ω. By selecting positions at the speaker impedance switch and change relevant jumper plates, speaker impedance switchover is done with NFB level always set to our designed level –3dB.

Best regards,
-- Al