Push Pull vs OTL vs SET (Tube amps)

Discussion in 'General Audio Discussion' started by gbeast, Dec 19, 2016.

?

What is your preference?

  1. Push Pull

    2 vote(s)
    3.4%
  2. OTL

    12 vote(s)
    20.7%
  3. SET OPT

    44 vote(s)
    75.9%
  1. Jerry

    Jerry Friend

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    Yes, the bakoon has current and voltage output. I hardly ever use the voltage out.

    As for the current output, it makes hd800 bass hit like never before. Slightly warm presentation, with details and expansive stage. But i think people won't mistake it for a tube.

    Let me be clearer then. I heard that tube can give you better musicality and feel/nuances of the music than SS amp. If i have a good solid state amp, and i want to complement it with a good tube sound, would the wa6se be a good choice? How far is it behind thr ZDS in terms of musicality and technicalities for example? It is quite considerably cheaper than the Zana. If it's pretty close, then i would say it's a good buy.

    Thanks.
     
  2. skem

    skem Friend

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    I’m looking for starting guidance on a line-level tube-based I/V convertor architecture.

    In multiple places @purr1n has expressed preference for SET over PP amplifiers.

    This makes a lot of sense, and others have largely agreed.

    On there other hand...There seems to be lots of enthusiasm for shunt-regulated PP designs---which is kind of single-ended in nature, IF (and that's a big IF) I understand it correctly. To quote this link (and nice collection of designs), "In the society of tube-loving folk, we find a sub-group, and not small group, which believes that the SRPP is the only tube circuit worth considering." One asks why? Is this because these enthusiasts like nice technicalities and are less sensitive to a loss of subtlety and low-level detail. The Aikido design introduced about 16 years ago could be built for less than $400 . Reviews of those who built it were very promising. Comparisons by Steve Graham (a review here and more impressively a non-review mention here) to the $4000 Audio Research Reference 3 preamp found it didn’t have as much stage but held up very well, bested only by the AR REF3 and crazy Shindo stuff. If anybody has thoughts about this design relative to SET options, I’d welcome them. The ability to avoid expensive Iron seems itself worthwhile IF nuance is preserved. Indeed, my experience with Lundhal 1540 is that it gives up too much nuance for my tastes.

    n.b., I have also seen one person call SRPP an OTL design, but I don’t know if that’s what we normally refer to as OTL.

    Note: This line-stage will be feeding (for the foreseeable future) a very good solid-state power amp.
     
    Last edited: Jul 28, 2020
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  3. JeffYoung

    JeffYoung Friend

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    I think SRPP sounds more like SE than PP. But it's a hotly debated topic.

    Note that they also exist in the solid state world. The Aleph 3 and FirstWatt J2 are two examples.
     
    Last edited: Jul 28, 2020
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  4. skem

    skem Friend

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    A very nice article here: http://www.nutshellhifi.com/library/FindingCG.html

    (excerpt of more general interest below)


    The Sound of Different Harmonic Spectra

    As mentioned above, odd and even harmonics can be recast as asymmetric distortion and symmetric distortion, thus the very different effects seen with IM distortion tests. As D.E.L. Shorter of the BBC pointed out in the April 1950 Issue of Electrical Engineering, real music is dominated by a great many closely-spaced tones - a choir or massed violins having the most dense spectra of all. Shorter showed that with a few as three closely spaced tones, IM sum-and-difference sidebands outnumber the much simpler harmonic series. In effect, as the number of tones increase, the number of IM sidebands increase at much faster rate than simple harmonics. The boundary case is 3 tones of equal magnitude; for 2 tones, IM is about the same as harmonic distortion, for 4 tones, IM is far greater than harmonic distortion. I leave it to the imagination of the reader to figure out how many simultaneous tones are present in realmusic — a lot more than three!

    The influence of IM vs THD has additional consequences for the type of music we listen to. Jazz and folk music have sparse spectra, thus THD will play a larger role in subjective coloration. By contrast, a cappela singers, large choirs, and massed violins have very dense spectra, with many closely-spaced tones drifting in and out of phase-lock all the time. This type of music will be strongly degraded by even small amounts of IM, but not as sensitive to relatively small amounts of low-order harmonic distortion. Thus the origin of the endless audiophile wrangles that are actually based on the type of music the listener prefers.

    (Musicians can and do maintain phase-lock for a few seconds, despite the seeming impossibility of this. I found that out the hard way on my work on the Audionics Shadow Vector quad decoder. Every now and then on certain records the dynamic matrix circuits would go crazy — this turned out to be brief periods of phase-lock by the musicians. The SQ-encoded Loggins & Messina "Full Sail" album had one track with a violin in Left Back, and a harmonica in Right Back. Sounded great in stereo and on headphones, but with quad decoding, the logic detector would whirl the sound round the room as the musicians drifted in and out of phase-lock. Truly weird effect, and not apparently intended by the producer.)

    So, depending on the type of music you listen to, the spectral distribution and class of distortion (symmetric vs asymmetric) will affect the subjective tonal character. It is much more complex than the simplistic "2nd Harmonic is Always Better" guff reprinted in the popular press.

    Preference for spectral distribution plays a major role in the "tone color" of an otherwise flat-response amplifier. Thinking about "spectral tone color" in a more sophisticated way shows just how far off-course we have drifted in The Age of Digital.


    Device, Topology, and Harmonic Spectra

    All of the foregoing applies to triodes — conventional RC-coupled, transformer, choke-loaded, SRPP, and active-load circuits such as mu-followers. It does not apply to: cascode-connected triodes, pentode, bipolar transistor, or MOSFET’s. This second group of devices do not have the simple square-law transfer characteristic of triodes; instead they have a much more complex exponential curve, and that translates into a much greater proportion of upper harmonics.

    When you compare device specifications, take a close look at the ratio of 2nd to 3rd harmonic distortion for a basic single-ended circuit. Low-distortion triodes (6J5, 6C5, 6SN7, 6CG7, the new JJ ECC99, and direct-heated types) have much lower 3rd harmonic; for devices in the second group, the 3rd harmonic will equal or exceed the 2nd harmonic. Medium-to-high distortion triodes (12AU7, 6DJ8) fall between the two groups. (This is why the 6DJ8 is known for a "high-definition" transistor-like sound - the distortion spectra isn’t that different!)

    Sometimes people get a little confused at the differences and similarities of SRPP, mu-followers, cascode, and pentode. The important distinction is to find what’s driving the upper grid, which behaves in the same way as the screen grid in a pentode. At audio frequencies, if the upper grid tracks the voltage swing on the lower plate, the composite device will behave like a triode. If the upper grid is AC-coupled to ground, then the composite device will behave like a pentode (or beam tetrode). If the upper (or screen) grid is connected to small fraction of the voltage swing, this is called ultra-linear operation, with distortion characteristic partway between triode and pentode operation.

    Triode operation is characterized by a low plate resistance (Rp), low to moderate gain (mu), medium to high Miller capacitance, and low distortion with rapid falloff of upper harmonics. Comparisons of triodes to other classes of device reveals that they have lowest distortion of any amplifying device ever made.

    Pentode (or cascode) operation is characterized by very high plate resistance, medium to very high gain (depending on the impedance of the load), very low Miller capacitance, and medium to high distortion with a large proportion of upper harmonics. From this it can be seen that pentodes (or cascodes) are best suited for very low-level amplification and RF frequencies, where distortion is less important than noise and high-frequency amplification.

    With the advent of Class A transistor amps, followed by the vacuum-tube revival in the late Eighties (thank you, Glass Audio), device linearity is once again starting to be seen as important, especially with the revival of direct-heated triode amplifiers. What has gone unnoticed in the uproar over so-called "high-distortion" SE direct-heated triode amplifiers is that the output-tube distortion is actually 3 times lower than the next-best devices, triode-connected pentodes. (Don’t think so? Read "Vacuum Tube Valley" magazine, which shows just this result for 300B’s, 6L6’s, EL34’s, and 6550’s.)

    With power devices, you don’t get much choice about loading; to deliver power into the speaker, you must use a transformer, and all that does is translate volts into amps. Any attempt to raise the load impedance seen by the power tube plate inevitably decreases power, so most triode amplifier designers choose primary impedances between 3 and 6 times the Rp of the power tube. This gives a reasonable compromise of power, low distortion, and adequate damping factor for the speaker. (The damping factor seen by the speaker is close to the ratio between Rp and the primary impedance.)

    For driver circuits, there are the options of active loads (mu-follower), transformer coupling (providing very high impedances in the audio range), or direct-coupling to the power-tube grid (assuming bias-stability problems can be dealt with). Triodes and pentode/cascode/transistor circuits have very different responses to increased load impedances.

    For triodes, there is a moderate increase in gain, a large decrease in distortion, and the possibility of even greater decreases in upper-harmonic distortion. In effect, if the load impedance is an impedance 10 or more times the Rp of the driver tube, the triode can be persuaded to behave as a near-ideal triode. (In practice, distortion in the mu-follower can interact with the distortion in the driver tube, resulting in a more complex transfer curve.)

    For pentodes, cascodes, and transistor drivers, raising the load impedance results in very high gain, a possible increase in distortion, and a possible increase in upper-harmonic content. This is a very different picture than triodes; however, if the amplifier has feedback, the increased gain can be used to increase the feedback factor. The increase in feedback greatly reduces the lower harmonics (2nd and 3rd), but as mentioned earlier in the Crowhurst article, does nothing to reduce the upper harmonics. Increased feedback also leads to sharper clipping, which decreases the subjective sense of dynamic range.

    So if you were to compare 2 transformer-coupled low-mu triodes to a single pentode or a transistor with active loads, the overall gain and raw THD might be similar, but the proportion of upper harmonics will almost certainly be much greater with the high-gain, high-feedback circuit. The old Brook ads are right: low-mu triodes throughout are the way to go, even if it takes a few more devices to do the job.
     
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