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Thread: What an OTL is, and why you should care

  1. #11
    VIP/Donor [WBF Founding Member] ack's Avatar
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    Quote Originally Posted by Atmasphere View Post
    In our case we don't use negative feedback, so our amps tend to be unconditionally stable- IOW will not oscillate regardless of input condition or load on the output.
    And you can still get low enough output impedance? How? By virtue of using multiple tubes in parallel? Something else?
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  2. #12
    [Industry Expert] Addicted to Best! Atmasphere's Avatar
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    ^^ The question probably should be 'how low does the output impedance of the amplifier have to be in order to get excellent performance from the speaker?'

    Obviously 4 ohm speakers are challenging for an OTL with no feedback and not a lot of power to begin with. In practice, we find that the use of feedback has no effect on the power output into a particular load impedance whatsoever. In fact it should not; if it did it would violate a fundamental rule of electricity known as Kirchoff's Law.

    Our larger OTLs (MA-1, MA-2 and MA-3) are more comfortable with 4 ohm loads. But there is more to it than that- the real question is whether the amp is behaving like a voltage source. Now it happens that not all loudspeakers are designed with a voltage source amplifier in mind, nor are all amplifiers voltage sources. A good example of an amplifier that gets a lot of rave comments, but is nowhere near a voltage source is just about any SET amplifier. They measure poorly but get nice comments, and this has been going since the SET reappeared in the marketplace in the early 1990s! One need not look much further than that to know that there is more to this than just voltage response. It turns out that most tube amplifiers without loop negative feedback will tend to operate more like a power source than a voltage source, and there are a good number of speakers designed with that sort of behavior in mind. You can read more about that at this link: http://www.atma-sphere.com/Resources...ier_Design.php

    However to answer the question generally speaking any OTL will employ paralleled power tubes to reduce its output impedance. It happens that we use triodes, which tend to be lower output impedance on their own, and the Circlotron output circuit helps too, as the output impedance tends to be about 1/2 that of a totem pole circuit employing the same type and number of power tubes.

    A certain amount of pragmatism must be employed when solving any engineering problem. IMO being pragmatic is important in audio- the less made up stories and assumptions one carries as a designer, the more likely a simple solution may present itself. As an example I like to point to the issue of lower impedance loudspeakers in general. Whether OTLs even exist is unimportant when we examine the low impedance issue- that is to say certain things come to light very quickly. First of all, all amplifiers, tube or solid state and class D, have higher distortion when driving 4 ohms as opposed to 8 ohms. Its easy to see the in the specs. Its also easy to hear- because the types of distortion that typically appear are higher ordered harmonics and intermodulations, both of which are audible to the human ear in small amounts.

    Since this is very much the case one might ask why there are so many 4 ohm speakers, and if you look at the history of audio the reasons why go back decades, to a horsepower race that looked good on paper several decades ago ('look- if you cut the impedance in half, the power of the amp is doubled!!') but in practice was only to the benefit of those that placed a higher value on the specs than they do on the sound actually emerging from the speaker itself. These days we have become so used to this that quite literally we don't think about it.

    And this is what I mean by being pragmatic. Not only do amplifier distortion specs suffer, but also the speaker cable itself becomes much more critical, as it becomes a significant portion of the source impedance driving the loudspeaker. By comparison, 16 ohm speakers are relatively non-critical of speaker cables. I'm old enough to remember the old days when a length of zip cord from the hardware store did the job, but back in those days I had 16 ohm speakers. That was part of the reason I got away with it. With a 4 ohm speaker, the cable should not really exceed 4-5 feet for best results and attention should also be paid to the overall gauge, purity of materials and also geometry, all of which affect the characteristic impedance of the cable (which affects to some degree the efficiency of the cable at delivering the signal to the speaker).

    Amplifiers of course make less distortion into the higher impedances; quite literally if you are a speaker designer and want your speaker to sound smoother and more detailed at the same time, raising the impedance while maintaining other parameters equal will do the trick, regardless of the amplifier used. Now if we are talking 8 or 16 ohms, all of a sudden OTLs are quite practical as long as the speaker does not also have excessive inefficiency issues. Something I often tell people because it is a simple truth is this:

    "If you are interested in sound quality above all else, your amplifier investment dollar will be best served by a loudspeaker that is at least 8 ohms or more. If sound pressure is your goal, then you have a weak (3 db) but valid argument for a 4 ohm speaker, if you have a solid state amplifier."

    Now a side benefit of higher impedances is that generally speaking, tube amps (OTL or not) will put out more power into a higher impedance. Not only that, but quite often they run cooler and draw less power from the wall because the output section is operating more efficiently- less of the power made by the amp is being converted to heat (if only in the OPT). Its something to think about. This is why I don't think there is a good argument for lower impedance speakers in high end audio regardless of the amplifier used. The harder you make your amp work, the more distorted its output will be and that translates directly to how close to the music the end result will be.
    Atma-Sphere Music Systems, Inc. (http://www.atma-sphere.com)

  3. #13
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    Well put and explained Ralph---Yes indeed I was first made aware of your excellent Amps driving my good friend David Magnan's Stacked Quads some years ago

    My interest in OTL's furthered by another old pal Jon Syder--who was Harvey Rosenbergs right hand man .

    Keep up the great work!

    BruceD

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    I was looking for Atma-sphere thread and found this old post. However, I still need to get some tips on how to preserve the 6as7G tubes.

    Is it better to leave them on (filament heating) or I should just turn them off? When not listening, I will not engage the B+ voltage.

    I read somewhere, in order to prolong the life span of 6as7G, it's better to leave them ON and when we replaced these tubes, we need to have a 72 hours of preconditioning session which I have diligently followed.

    Thanks for the advice in advance.

    Quote Originally Posted by Atmasphere View Post
    In audio, the acronym OTL has been around since the 1950s. It refers to a tube amplifier usually, one without an output transformer.

    In the world of audio, tubes are often docked for poor bandwidth and high distortion by the advocates of transistors. But it turns out that a lot of that distortion can be traced to the presence of an output transformer, which also limits bandwidth. IOW if you can get rid of the transformer, you have the possibility of reducing distortion and increasing bandwidth.

    This is indeed a fact, not just a possibility, but like anything else in life comes at a price. In designing such an amplifier, it is rapidly seen that only a few types of power tubes are suitable for OTL service. Such tubes usually have high plate current capacity, reduced plate resistance and are fairly capable of operating with good linearity at lower plate voltages (some examples are the 6AS7G and variants, the 6C33, EL509, EL519/PL519, along with some out of production types like the 6LF6, 6336 and 7241). Of course, the output transformer serves to match the high impedance of the typical power tube to the low impedance of a loudspeaker, and removing it, even with the high current tube types mentioned, usually means that the resulting amplifier is going to have a fairly high output impedance when compared to a transformer-coupled amplifier. For this reason many OTLs often employ high amounts of global negative feedback, in order to allow the amplifier to operate more as a voltage source which is typically required in the operation of many loudspeakers (although this is not by any means a universal truth- more on that later).

    There are two main types of OTLs in service today. The Futterman (named for Julius Futterman) circuit and its variants represents one approach; the primary identifier being that the output section uses a 'totem pole' approach, wherein the output circuit is push-pull, but one tube is driving the speaker with its plate connection, while the other uses its cathode. This approach means that the output impedance varies depending on whether the amp is pushing or pulling so to speak, and this usually requires a fair amount of feedback to linearize the circuit. The second type of circuit employs an output circuit known as a Circlotron, wherein there are two banks of power tubes balanced against two power supply banks of equal polarity in a bridge circuit. This approach has a lower output impedance than a totem pole and lower distortion as it is a true symmetrical push-pull circuit (and is more expensive to build due to the need for two separate power supplies).

    Both circuits have special driver circuit requirements as usually the grids of such power tubes (and bear in mind, usually there are a number of power tubes in parallel to increase current capacity of the output circuit) have a considerable amount of capacitance and in the case of the Futterman, also have unequal drive requirements (to deal with the fact that the output section is asymmetrical).


    If a similar amount of feedback is used in these circuits, the distortion will be similar or lower than is seen in most traditional transistor circuits. Bandwidth will also be considerably wider; its no problem at all to have full power bandwidth from as little as 1 Hz to well over 500KHz. Usually bandwidth has to be limited to prevent problems with RFI and oscillation, not unlike a solid state amplifier. So in this regard, OTLs are quite successful!

    More in succeeding posts...

  5. #15
    [Industry Expert] Addicted to Best! Atmasphere's Avatar
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    IMO/IME, there really isn't much benefit to running the filaments when the amp is not in use, unless you are doing so to retain some of the warmed up character of the the (since the driver circuit is fully active if the filaments are lit). If you are looking for longest tube life, just warm the amp up at least a minute before applying B+.

  6. #16
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    Quote Originally Posted by Atmasphere View Post
    IMO/IME, there really isn't much benefit to running the filaments when the amp is not in use, unless you are doing so to retain some of the warmed up character of the the (since the driver circuit is fully active if the filaments are lit). If you are looking for longest tube life, just warm the amp up at least a minute before applying B+.
    Do you have any customers running your amp on Martin logan CLX?
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  7. #17
    [Industry Expert] Addicted to Best! Atmasphere's Avatar
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    Yes. They are using an outboard device called the ZERO (see www.zeroimpedance.com).
    ML speakers are built with a low impedance in an attempt to make them more compatible with solid state amps. The problem is that ESLs in general are incompatible with solid state unless a bit of current feedback is employed in the amp. This is because the impedance curve of an ESL is not also a map of its efficiency, unlike a box speaker.

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