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Has anyone meda a longer term comparison between their high-end OTL and newer similiar quality Class D amps?
I've tried a few chi-fi Class Ds and they sound amazing for the $$, but just can't provide the "in-the-room" realism that the OTL does.
I am driving Ref3A DeCapo I speakers with the OTL.
Unless I eventually move North, I just can't justify continuing with my Transcendent Sound stereo T-8LN. Too many months of 100f+ months (not days!) here in Sin City, Nevada. I have a friend that pays $700/month in the summer for AC. I just don't want to do that for many reasons...
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This is interesting. I am getting too old to worry much about the future, for myself but do worry a bit about what will happen to my speakers when my NYAL OTL3's are no longer repairable. It has been a long time since I tried, but when I first got by Stax F83's, absolutely nothing except these amplifiers sounded good. I still find the combination very convincing musically, These speakers have very high impedance in the midrange and are very inefficient. I use modest subwoofers for the bass.
I wonder if Ralph's class D amps have any chance of driving them well. How would they compare the the Saunders' amps? (Are the latter also class D?)
I guess there is also the Berning amps, but these are way way expensive.
The high impedance of some ESLs challenge most solid state amps to make power since their output power usually is cut in half as impedance is doubled.
The class D drives some ESLs, like the Quad ESL57, quite well. Others like the Sound Lab it can drive well but has limited power. You'd just have to try it and see.
Let's be clear about something: there are many ways to make amps that run class A, AB or class D. As a result you can't make generalities.
There are also a good number of ways to make an OTL, and OTLs can run class A, A2, AB1 or AB2.
The best way to answer this question is "It depends."
Before the question can really be answered you have to know why it is that some amps sound better, sound more musical, than others.
That has entirely to do with distortion and to a limited degree, frequency response. The 'sonic signature' any amplifier has is usually its distortion. This means that the more musical amplifiers out there tend to have a distortion signature that is mostly lower ordered harmonics (the 2nd and 3rd) as opposed to the higher orders, which are known for harshness and brightness. If the amp has enough 2nd and 3rd harmonic content to mask the higher orders present, the amp can have a smooth presentation.
There is a bit more to it than that. The other thing that really helps any amplifier sound musical is distortion vs frequency being a ruler flat line across the audio band. The reason this is so also has to so with higher ordered harmonics; if distortion begins to rise at a certain frequency, perhaps 1KHz, at 7KHz the 7th harmonic can be considerably more prominent than one might expect from the THD numbers. This is because the distortion will likely rise on a 6dB/octave slope.
The mechanism for distortion to rise with frequency is the use of feedback, combined with a circuit design that lacks the Gain Bandwidth Product to support both the feedback and the overall gain of the amp across the audio band.
What this boils down to is that if the amp has insufficient GBP, at some point in the audio band the feedback won't be supported so it will decrease on a 6dB slope. So distortion will rise on a 6dB slope. As frequency is increased, its likely that the 6dB slope will become more pronounced. So if this happens at 1KHz, you can see that at 7KHz (which is at the top of the band of frequencies to which the ear is most sensitive) the harmonic will be stronger than you might otherwise expect. If its not masked, that 7th harmonic will cause harshness and brightness.
There are a lot of amps including tube amps that don't have enough GBP to support the feedback they have.
In addition, feedback is applied to an earlier stage of the amp, usually the input stage. In a tube amp, such as yours, this might be the cathode of the input tube. The thing is, that tube isn't linear and so the feedback signal is distorted before it can do its job of mixing with the incoming audio (so as to be a correction signal). Instead because its been distorted, higher ordered harmonics are generated (as well as some IMD), which can be audible.
This is why we have not used feedback in our OTL designs for the most part, so as to avoid the distortion vs frequency problem.
So in a class D amplifier, high values of GBP are very easy to obtain. This allows the design to support very high amounts of feedback while maintaining distortion vs frequency ruler flat across the audio band. In addition, the feedback signal can be received properly without being distorted on the way. So both issues that have been problems in the prior art can be solved.
So then is comes down to the overall distortion signature. It is possible for a class D to have a distortion signature that is mostly 2nd and 3rd harmonics- that is how it is in our class D amp. As a result it sound like other amps that have this sort of distortion signature: tube amps. But because there is less distortion overall, the tendency is for it to also be more transparent.
As you know, OTLs have ruled the roost as far as transparency is concerned. But sooner or later, technology being what it is, something will come along that can do that job just as well. I think that tech (class D) is here, although its not correct to say that any class D amp will do the job. As I mentioned opening up this post, there are vast differences in amplifiers that all use the same class of operation and class D is no exception. I think I've heard more variation in class D designs than I have in tube amps overall!
I've been playing class D amps in my system the last 2 1/2 years and don't miss the OTLs at all- I get better transparency, the same smoothness in the mids and highs and no worries in the bass. After running all day the amps are cold to the touch. My speakers are 16 Ohms (ideal for OTLs).
thank you Sir for taking the time to make your very informative post!
I think you want someone to tell you that the Class D amp is just as good sounding as the OTLs. On a high-ish impedance speaker, I am not sure you will find such a person. You probably will have to decide between reducing your electricity bill vs the best possible SQ. Or move North. Montana is nice, they say.
I've been trying out Class D amps for the past 15 years or so. I do expect at some point - maybe soon - that Class D will be recognized as tough competition for the better tube amps.
But maybe not yet.
And yep, if I moved back to Beantown - or even S.F. which is where I lived when I bought the OTL) - I would have a lot more months in the year to enjoy my T8-LN.
Probably will have to sell it though. Just getting impossible to justify considering the energy and climate situations...
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