A single-ended triode amp will always have an output transformer. The transformer is both the boon and doom of the amp- it allows the tube to drive the speaker, but the bigger you make the transformer, the more limited the bandwidth will be.
As a result, the best SETs are the smaller ones (2A3s, 45s, that sort of thing) assuming you have a speaker that is efficient enough to work with them.
OTLs have no output transformer and so don't have the power vs bandwidth issue. Instead you have the opposite problem- its harder to make a practical low-powered OTL because you have to be more careful about choosing a speaker for it.
Usually SETs are zero feedback. OTLs normally have feedback (although ours tend to use little or none). A triode zero-feedback OTL operating class A, like SETs, will generate primarily lower-ordered harmonics, with very little higher-ordered harmonics. This will give either one a relaxed quality.
The ear uses odd-ordered harmonics to tell how loud a sound is, so if those harmonics are not emphasized by the system, the system will have a sense of ease and a lack of hardness.
OTLs are normally push-pull, if built single-ended will produce very little power for the number of tubes involved! Since push-pull operation allows for even-ordered harmonic cancellation, OTLs generally lack some of the romantic lushness of SETs as they will also lack the 2nd harmonic. This is an advantage if you are looking for neutrality and transparency: anytime distortion is present, detail is obscured.
Normally push-pull transformer-coupled amps will have an increase in distortion at very low power, robbing them of that 'inner detail' detail that is part of the 'magic' that SETs are known for. OTLs, like SETs, produce less distortion as power is decreased, giving them a 'magical' quality that they share with SETs; good 'inner detail' and good low level resolution while being very smooth at the same time.
I apologize for the nutshell quality of this post- I'm glossing a lot of things over as this can be a detailed subject!
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Aarif, removing distortion never causes an amplifier to have fake neutrality.
OTLs have an economy of scale- the bigger you make them, the more efficient they can run. A big OTL that can make 200 watts will very likely be comfortable on 4 ohms, some smaller ones that make 150 watts might be able to do that also. A 30-watt OTL will not like 4 ohms much at all.
IOW the smaller you make the OTL, the higher you will want the load impedance to be. Many OTL headphone amps are designed for 32 ohms or more, plus they do not have to make more than 0.2 watts. Headphone amplifiers also are a special case because most headphones are designed to operate with amplifiers that have a much higher output impedance than their own impedance.
Without more information about your speakers, it would be hard to make a certain case for either amplifier technology, and we have to include your listening preferences somehow too. Although I have heard very good things about the Graaf, I've not actually heard it, so the following is my opinion only. I think it would have a chance at being a more neutral, musical presentation if the speaker impedance is linear and 7 or 8 ohms at least. I don't agree with some of the tube choices in that amplifier, as I feel that they tend to make the amp sound brighter, so I do have reservations at the same time.
In the end, like so many other things in audio, you will have to audition to know for sure. Remember- its all supposed to be fun :) |
The harmonic structure of a good OTL will be the 3rd harmonic, which the only harmonic that the ear considers musical and thus does not object to. SETs have that and the 2nd harmonic.
Paul, what happens with these harmonics is not that they are not audible! It is that they are not **objectionable**. When GE did their experiments in this regard, they found that people will not *object* to nearly 40% of even-ordered content. That is not the same as 'you can't hear it', you certainly can, and audiophiles have terms for the presence of even orders (just like they do for odd orders) in small amounts. From small amounts to larger amounts: Warm, lush, rich, bloom, thick, murky, woolly, muddy. FWIW these terms refer to even ordered harmonics that are in greater supply than the terms that apply to odd ordered harmonics: hard, harsh, brittle, clinical, etc., where the odd-ordered enhancement may well be less than .001%.
With regards to the phase splitter- some OTLs that is an issue, others it is not. In our amps, there is no 'phase splitter' as a stand-alone circuit- that function is integrated into the single gain stage that exists in our amps.
Most SETs have more than one stage of gain- its my contention that if they were able to have only one, they would be a lot closer to the neutrality that our amps routinely express. I've got a 45 SET and a 2A3 SET- they sound great within their dynamic limitations, but they also sound veiled compared even to amps we were making 10 years ago, despite using the best parts available. As far as I have been able to experience, transformers always limit bandwidth, add distortion and rob the music of detail.
Given the example of the speaker above, I would recommend something with more power than the Graaf, or any SET for that matter. The speaker is simply too inefficient for an SET to strut its stuff. You would really need at least 10 more db for that. There are OTLs that can easily drive that load though. IMO 15 watts would not be enough power, unless you listen near-field and at moderate volumes. |
Paulfolbrecht, since you asked- in our amps there is only one stage of gain, not two. Most SETs I've seen have either two or three. This is because there is usually some gain in the output, while in our amps there is none.
Crossover distortion, FWIW, is not a function of the 'phase splitter', it is a function of the push-pull operation of the output section. A number of crossover issues stem from output transformers, and some of them stem from class of operation. In a class A push-pull amp, carefully designed, you will not get any crossover distortion at all.
You are right in that if there are crossover issues, they can often appear at low power only, masking low level detail. This is something that simply does not happen in our amps, and it is easy to hear how they have plenty of low level detail, actually more than SETs running at similar power levels do.
You are also right in that harmonic spectra is not the defining issue regarding sound of OTLs and SETs. I should point out here though that most OTLs employ negative feedback, often fairly large amounts, while hardly any SETs use feedback. Our OTLs use little (1 or 2db) or none, so in a way they tend to have more in common with SETs sonically than they they do with a lot of push-pull amps.
Larryi, just for the record, an OTL that has a direct-coupled output does not have to have either a coupling cap or a servo circuit. They can be built to be so stable that the DC Offset can always be very small and only need adjustment occasionally, once every 3-6 months.
The multiple power tube issue is shared by SETs with multiple tubes, Push-pull with multiple tubes and of course transistor amps with multiple output devices. In all cases, it is possible to build the amplifier so the multiple output devices are not a defining characteristic. |
Paul, I like the type 45 tube a lot too. Not many speakers that will work with a 45 SET though. I am using a set of Coral Betas, which are fine as long as you don't expect any volume.
Larryi, We've been doing this over 30 years now and all our amps have had a direct coupled output. We've seen many tubes fail in that time. So far, we can still count the number of times that a power tube failure damaged a driver on one hand. In all the cases but one, alternative power tubes had been installed in the amp. So it appears that the amp is quite safe. All of our amps except the big 500-watt unit do not use a servo circuit (we installed one in the big amp, but for convenience only). They are quite stable, and simply don't need a servo. The key is controlling the power tubes, something that you would **think** would be common sense, but it is an issue in a lot of designs. That is one of the reasons why our amps have been so reliable over the years (the other being that the amp does not need negative feedback to work). |
The Merlin needs a little more power than most SETs can produce, if you really want to hear what the speaker or the SET is capable of. With that speaker, hands down an OTL will do better. |
Martykl, one thing about SETs that makes them quite different from OTLs is the way their distortion signature interacts with the human ear/brain system. SETs tend to have very low distortion at low power, but on transients, where the power occurs, the distortion can be quite a lot higher (10% at full power is quite common).
Thus there is high distortion on transients, and while normally the odd-ordered harmonics are kept at vanishingly low levels, at high power they have an effect. Of course, they are masked by the presence of even-ordered harmonics, but despite that the brain reacts to their presence anyway.
That reaction is a sense that the amplifier somehow is a lot more dynamic and can play louder than it should be able to given its actual power output. You commented about that quality in describing your amplifier above.
OTOH if the amplifier actually has the power to make real spls, and if the distortion is kept low, then the result is that the system is can be more relaxed- that the volume you pick is not based on perceived 'loudness'. IMO, a good system will not have a sense of volume or loudness- it should act that way real music does in that regard. |
Marty, I understand what you are saying. I want the amp to play what is good in a recording, and with the poor recordings, not get upset by artifacts in the recordings. In other words, forgiving? -but without giving up anything to resolution. Its a fine line to tread.
Bobby P. has been making certain over the years that Merlins are tube and particularly OTL friendly. OTLs and Merlins are a very common and successful combination! |
Hi Marty, most people with VSMs usually go with the M-60. They have enough power to really make the speaker sing. |
Everyone says they have the best. In the English language, the meaning of the word requires that there can be only one. All the others are then 'better' but not 'best'.
This has been argued a lot elsewhere on this site. Any consensus on the idea of 'best in my experience'?? instead of merely 'best'? |
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The Audio Note AN-e LX HE is a fairly benign load and is no problem for an OTL of sufficient power- our customers use our M-60s on that speaker. The speaker definitely benefits from the greater power!
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I would say that a well designed SET 2A3/300B has the warm sound than also well designed an OTL having the same output wattage. There really aren't any OTLs that make the 2-7 watts as suggested by the above post. OTLs become less practical at lower power levels- driving the real world impedances that one might do with a transformer-coupled amp gets harder. That is one reason OTLs tend to have a lot more power than SETs, and is why our smallest amp makes 30 watts into an 8 ohm load. To make the amp much smaller than that becomes impractical. However I have found that the OTLs tend to make less of what I have come to call 'loudness cues', IOW higher ordered harmonic distortions (which show up in most SETs when driven over about 20-25% of full power). The result is that the OTL won't sound as loud (even though it is probably louder) and so invites you to use the power it has. This can be done comfortably. The reason SETs sound so dynamic is due to distortion and how that interacts with the ear, not actual volume! |
