Thought on OTL tube amps

Jallen's comment above reflects a popular myth.

OTLs can be as reliable and safe as any other amplifier. Its all in the design. The only OTL design out there that really did have issues is the one that has generated a lot of the mythology around OTLs years ago (and in the case of that particular design, generally known as a Futterman, those stories are often true). The three myths thus generated were/are speaker safety, amplifier reliability and sensitivity to load. I'm only addressing the speaker safety issue here but will cover the others if anyone is interested.

Speaker safety is a design issue and nothing else with OTLs, the same as it is with a transistor amplifier (the concern is what happens if a tube fails/shorts).

A common myth is that OTLs run the same high voltages that regular tube amps do. This is false- if you think about it, the fact that the amp has no output transformer means that the tubes have to generate voltages and currents that are normal for loudspeakers, to do this requires a tube-type that runs at much lower voltages and higher currents. For example in our amps the B+ is only about 140Volts.

All it takes is a little testing and experimentation to find the limits of any OTL design, so its easy to set up the amplifier with a scheme to protect the speaker in the event of a tube failure. Otherwise the company goes out of business. I hope this is obvious for what it is: common sense! After over 33 years of business, its safe to say that we have built more OTLs than any other manufacturer so I think we can agree that if there was a problem it would have manifested by now :)

Here is the issue with OTLs and speaker loads. There are two major concerns. The first is this:

http://www.atma-sphere.com/papers/paradigm_paper2.html

In short you want a speaker that is designed for tubes. The second is that the smaller you make the OTL, the more it will want higher impedances to really perform right.

A lot has to do with the circuit of the amp and you should check with the manufacturer on this, but in general you will find that the triode-based OTLs can driver lower impedances as their output impedances are lower. Now this brings on a discussion of loop negative feedback, which is a common method of lowering the output impedance on any amp, but here is something everyone should know:

The addition of negative feedback will not allow the amplifier to make more power into lower impedances.

This is because loop feedback really doesn't change output impedance! If it really did, feedback would give you greater power at lower impedances, and it doesn't.

There is an additional factor that comes into play here. Put plainly: There is no argument for 4 ohms or less in high end audio, if **sound quality** is your goal. No matter what kind of amplifier you have, transistor, class D, transformer-coupled tube amp, SET or OTL, the simple fact is that all of them sound better at higher impedances simply due to the fact that they will make less distortion in one way or another. In the case of transistor/class D amps, you get less odd-ordered harmonics, which BTW the ear finds to be very objectionable. In the case of all tube amps, you get less lower-ordered harmonics (2nd, 3rd, 4th) which the ear interprets as 'bloom', 'warmth', 'thick', etc. these lower orders mask detail so if they are gone the system is instantly more transparent without additional brightness (brightness is associated with the 5th, 7th and 9th harmonics).

So the bottom line is you want a higher impedance in the speaker if you can get it, all other things being equal.

Now OTLs, while having an economy of scale, do not suffer the way most other amps do when you make a bigger version. If the designer has done his homework, bigger OTLs will never loose PRat; they will be just as nimble as a smaller amplifier without loosing detail. The other advantage of OTLs is that if built right, they do not suffer crossover or notch distortion issues at the zero crossing point- the only other amps like that are SETs. The point here is that you can get a bigger amp to drive lower impedances if needed.

Now with respect to the impedance curve of the speaker: it is a common myth that OTLs need a *flat* impedance curve. That's not quite true- if the speaker impedance is too low for the amp, then yes, that's a good idea (and it usually is anyway). But otherwise, what an OTL is **really** looking for is that the entire impedance curve be high enough- often that is a lot more important than a flat impedance curve.

A good example is the Sound Lab ESL. This speaker has an impedance curve based on a capacitor and some inductors and has nothing to do with box resonance. This is why transistors have such a hard time with this speaker- its as high as 50 ohms in the bass and only 1.5 to maybe 3 ohms at 20KHz. That's quite a spread. Now a transistor amp can make lots of power in the highs, but hardly any in the bass. So they tend to be bright with no bass or just seem like they can't drive the speaker. You put an OTL on there and suddenly the speaker has bass authority and extension that the transistors could not do, and with an OTL that might be 1/3 the power of the SS amp will make the same volume.

There is an outboard device called the ZERO that is an excellent problem solver for tube amps (OTLs in particular) if you simply *have* to use a 4-ohm speaker. The most common question we get is 'doesn't that obviate the OTL quality?' and the answer is No. The ZERO loads the amp at 16 ohms and has only provisions for 4,3 and 2 ohms. The turns ratio is so low that it has bandwidth from 2Hz to 2MHz, exceeding the bandwidth of nearly any amp its hooked up to. It also does not block DC as its an autoformer. This sort of thing requires that you have an amplifier that already has a fairly low output impedance- you could never do this with a conventional output transformer!

Sure. The issue is this: there are human hearing perceptual rules and there are the specs that we see on paper, and surprisingly they have very little to do with each other. So if you design your amp to look good on paper, it at best will sound like a good hifi.

What we really want is for it to sound like music. For that, we have to get the circuitry/stereo to obey as many of these perceptual rules as we know about. This often results in amps and speakers that don't measure so well. For example, the human ear uses the 5th, 7th and 9th harmonics to determine the volume or sound pressure of a sound. Electronics have the ability to distort these harmonics (enhance them); this results in such electronics sounding artificially louder than they really are. We see this all the time- 90% of the time if an audiophile's wife is screaming them to 'turn that down!!' its likely because these loudness cues exist in un-natural abundance.

You can design to minimize the distortion of these harmonics. The first step is to not use loop feedback, as it is known to **enhance** such harmonics. Of course, then to get rid of distortions you have to use every design trick in the book to try and not make distortion.

IOW what we want to do is engage the human limbic system rather than the cerebral cortex. So in addition to distortion issues, the circuit or system has to be fast enough; if too slow the processing of the sound moves to the cerebral cortex. IOW the experience of the sound becomes intellectual instead of toe-tapping.

This is a sort of basic introduction but I think is also the easiest to understand. Obviously without feedback the THD of the amplifier tends to go up, but THD is a description of a settled-out sine wave and really says nothing about the behavior of the amplifier with a constantly-changing non-repetitive waveform, which is what nearly all music is. The evidence right now is that negative feedback actually *increases* distortion with non-repetitive waveforms- quite the opposite of its intentions.

If you look at this from the field of Chaos Theory, what we find is that the formula that describes feedback in an amplifier is nearly the same as we see in Chaotic systems- and not what we want for reproduction. Chaos Theory suggests that amps with feedback exhibit a chaotic behavior which will include harmonics of the fundamental waveforms up to and exceeding the 81st harmonic! I find it interesting that as the science of math evolves, it is opening doorways to improvements in audio that many people have thought were far too cut and dried for there to be any serious evolution.

negative feedback....to control speakers even better.

Negative feedback cannot be used to control speakers. It can only be used to control amplifiers. The idea that it helps with speaker control is part of the myth of damping factor, a subject that has its own thread on this forum right now.

This myth has heavy subscription by the Voltage Paradigm camp- see the link I provided in my second post to this thread. It is true that higher speeds in the amplifier will reduce odd-ordered harmonic generation when feedback is applied, and it is also true that the amount of this is variable from amplifier to amplifier, and does account for some of the differences that we hear.

Most OTL manufacturers use feedback. I think that we are one of the very few that do not. It is true that this limits the speakers that work with our amps, but it is not a matter of speaker control- usually its a matter of tonal issues related to incorrect voltage response for that speaker. Our philosophy is that if the speaker requires feedback for the amp to work with the speaker, there will be no way that speaker will ever sound like real music as the amplification will be inherently incapable of the task. It might sound like a good hifi, but that is not our goal.

There is no question that this philosophy has been a major marketing problem for the company, and it has contributed to the idea that OTLs are load sensitive (again, see the link I provided earlier). But if you want it to sound real that's what you have to do as its all about human perceptual rules. In the end, this means that there are dozens of speakers that work with our amps rather than hundreds.

Its very difficult to compete with OTL in harmonic richness for any solid state amplifier. I admit it. We are doing our best but its difficult.

I went down a similar path 25 years ago. It got a lot easier when we got rid of the feedback. All I can suggest to anyone who has not tried the same is, make sure all your ducks are in a row- run the most linear circuit you can and put it on a speaker that allows you to hear the results without tonal issues. Only then can you see if you really want to go back.

OTLs don't run hot unless, like any other amplifier, they are biased to do so. Heat and good sound happen to go hand in hand; sure, you can get an amplifier that does not make a lot of heat to sound OK but again its the sort of thing that once you try using a little more bias and thus making more heat, you may not want to go back.

Afc, getting rid of the output transformer has nothing to do with hum.

There are several advantages to getting rid of the transformer: wider bandwidth and lower distortion. One advantage that most people don't think about is the fact that because the output transformer steps voltage down, in an OTL that translates to less gain needed. This is why its possible to build an OTL that has only one stage of gain.

The claims of longer tube life with sweep tubes may be true, but not particularly because of the voltages used. The voltages used in an OTL are so low that even conventional audio tubes would have a long life if that tube could work in an OTL.

Zombie thread come to eat your brains!

Futterman himself seemed to do a pretty good job at keeping the amps together according to one of our customers, although HP related an event in TAS that suggested that they could fail spectacularly.

I heard rumors about the NYAL stuff being not as stable but never knew anyone that actually had experienced a problem themselves. During the 1990s, that rumor was so prevalent that we came to call it the 'Futterman legacy' since it was our single biggest marketing problem. In essence, because we made an OTL people assumed it was unreliable. The simple fact that we don't have to ship amps back to be repaired if a tube fails is why we are still around after 39 years.

But I have no idea where the rumor got started. I do know that Fourier, who went out of business in the late 1990s, didn't help, but their amps had engineering flaws that had nothing to do with them being OTLs!!

Harvey Rosenburg (founder of the NYAL) was adamant that the amps had no reliability problems, even years after the company failed. But he had a lot of ego tied up in them and was never the most reliable witness so I regard his comments as a bit of a red herring.

So I really can't confirm or deny that rumor; sorry I can't shed more light on it.

If you want power, quite often "paralleling tubes is the only option" regardless of OTL or not.

The fact that the Berning amplifier (while an outstanding and very innovative design) does not fall within the definition of an OTL is something that has been put to bed many times. While I respect David as one of the top designers in the audio world, I do find it inexplicable that his website makes this claim.

OTLs by definition do not have an output transformer; that much should be pretty clear. 'OTL' refers simply to 'Output TransformerLess. Over time (the last 50 or 60 years) the acronym generally has referred to a *tube* amplifier without an output transformer; in that regard having semiconductors in the output section would not qualify the amplifier as an OTL. And of course the Berning amps all include semiconductors as an essential part of the output circuit.

I read the patent many years ago. I suspect Al has too.

The transformer is impedance matching. It is air core, as it is operating at RF frequencies to modulate the output of a switching power supply. The output of the switching supply then has its RF component filtered out and the result is the audio signal. So the output transformer and the switching supply work in tandem to do the impedance conversion.

A conventional output transformer does impedance matching as well only it does it at audio frequencies.

Either way there is a transformer that represents the load to the output tube or tubes. It can only be called an OTL when a transformer can no longer be called a transformer. Since these terms have been with us a long time and will continue to be with us for decades to come, the result is that the acronym 'OTL' is misapplied to this amp.

I use the Classic Audio Loudspeakers model T-3.3 at home. It goes down to 20Hz. A set of our M-60s has no troubles driving them to very high volume levels with very satisfying bass (shakes the house). M-60s are full power down to 2Hz. 

Like **any** other amp, if you want to hear what its really capable of, its a good idea to put it on a speaker to which it is well-suited.

If the Berning folks might be stretching it a bit to call their amp an OTL, what about companies, like Tenor (I believe), that put out a solid state amp that they described as OTL?  This was absolutely true, but, aside from a few solid state amps like models produced by McIntosh, all solid state amps are OTLs (low output impedance of transistors makes transformers unnecessary).

I remember when Allied used to write up the receivers in their catalog back in the early 70s. They referred to their circuits as ITL/OTL and maybe with good reason at the time. A lot of solid state amps back in those days employed input transformers as phase splitters to the output section!

These days though OTL generally refers to a tube amp, not a solid state amp as its no big shakes with solid state in that regard.

The ZERO will not kill dynamics. The ZERO is a problem solver, used when someone wants to drive a speaker that is lower impedance, perhaps too low for the OTL to do on its own. All you have to do if you don't want to use the ZEROs is get a speaker of higher impedance (8 ohms minimum usually works with our stuff) or get a bigger OTL.

The ZEROs usually get used with our smaller OTLs, an example being our M-60s used with Magnaplanar 3.7s or the like. The result of using the ZEROs is that the output power of the amp is doubled (from 40 watts to 80 watts), the distortion is reduced and the tubes run cooler as the power they are making is dissipated in the load rather than in the output section. MA-1 amplifiers drive the Magnaplanars just fine without the ZEROs.

The ZERO would not be possible if the amp did not already have a lower output impedance and did not make any DC voltage at its output. The result is that the ZERO then has actually greater bandwidth than the amps- going from 2Hz to 2MHz. So it does not limit bandwidth of the amp. The resulting combination is impressive- a tube power amp with full power from 2Hz to about 100KHz or so, which can drive 2 ohms or more. You can't do that with a regular tube amp!

How do you categorize amplifier with an output transformer?OTL?Are we going in circle?or we might just go back to SE or PP.The mean reason why these amp were design is that they will work without OPT.

Again, what we found after getting into the business is that some people wanted to use our amps with really low impedance speakers. The combination was non-functional. Like many tube amps, OTLs don't like low impedance loads. So what to do? If the customer really wants to use the OTL with that kind of speaker then you make it possible by using a set of ZEROs.

In some cases its just the woofer section that might have the low impedance. In such cases you can install the ZEROs so the amp is driving the mids and highs directly while using the ZEROs to deal with the lower impedance of the woofer array.

The ZERO is intended as a problem solver plain and simple.

It not exactly clear what you are talking about, but if I read this right you are incorrect about increasing the impedance of a speaker. One way to do it is to put drivers in series rather than parallel; of course that is a design issue.

There are a good number of 16 ohm speakers made these days, on account of the fact that 16 ohms works extremely well with tubes. The speakers we use at our shop are 16 ohms. They have two woofers which are in series.