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In Reply to: OTL vs. Class D? posted by ltman on December 31, 2023 at 10:09:03:
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).
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Follow Ups
- RE: OTL vs. Class D? Not as simple as that! - Ralph 09:53:46 01/03/24 (1)
- RE: OTL vs. Class D? Not as simple as that! - ltman 15:10:38 01/12/24 (0)
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