Ralph, thanks very much for the highly informative post.
I have one further question. In this thread, and I believe in another current thread, you alluded to PRaT as something that can be designed for, within the constraints of certain tradeoffs that depend on the type of amplifier. Could you elaborate on what technical factors and design characteristics affect the PRaT of an amplifier? That's something which has never been clear to me. Thanks!
Best regards,
-- Al |
10-27-15: Mapman
Chalice Audio Grail amps are interesting to me. I didn't realize a SET amp could produce 50 w/ch into 8 ohm. How do they do that? Mapman, it uses a parallel pair of type 845 tubes, which I believe were originally developed for transmitting applications, and have much greater power capability than the 300B, 45, and 2A3 tubes you are probably more used to seeing in SET amplifiers. There are a number of other 845-based SETs out there. The 211 is another such high powered tube that is sometimes used in SETs. Both types require about 32 watts just to heat the filaments of each tube(!), and run at very high B+ voltages (I believe often in the vicinity of 1000 volts), as well as handling substantial amounts of current. All of which presents much greater challenge when it comes to design of the output and power transformers than in the case of the lower powered SETs. BTW, A'gon member Ait has described in past threads how he designed and built for himself a SET providing 200 watts per channel, which utilizes an 833C transmitting triode operated at 2500 volts(!), and having output transformers weighing 62 pounds each. Not an OTL of course, but quite a remarkable tour de force! Psag, thanks for the reference to the Berning Quadrature Z, which I hadn't been aware of previously. Quite an innovative and remarkable design, providing 200 watts of tube power per 32 pound monoblock. But of course it is neither a SET nor an OTL, although its output transformer operates at low RF frequencies rather than at audio frequencies. Best regards, -- Al |
Hi Psag,
I had looked at the writeup at his site, and also at the patent which it references. There is a transformer between the outputs of the power tubes and the output of the amplifier, along with some solid state switching devices. As I read the writeup, what he said is that there is no AUDIO output transformer, but "the high-voltage, low-current tube impedance-plane is re-mapped to the high-current speaker impedance-plane through a special transformer at a constant RF carrier frequency of 250 kHz."
So while his very innovative approach avoids the use of an AUDIO output transformer, the design is not Output Transformerless (OTL).
Best regards,
-- Al
|
Ralph, yes I had looked at the patent, and I completely agree with your comments.
Tubegroover, thanks for the suggestion! I'll be sure to check out those discussions. Prior to this thread I had no awareness of this amp, as I mentioned earlier, so I'm sure I will find it to be of particular interest.
Best regards,
-- Al
|
11-25-15: Psag
... That is why Berning refers to his amp as an OTL. Those who say otherwise are obviously invested in saying otherwise. Certainly not true in my case, apart perhaps from whatever negligibly small interest I might have in not having my original statement that it is not an OTL contradicted. Also, Ralph makes a good point that the term "OTL" is normally used just in the context of tube amplifiers, since the vast majority of solid state amps don't have output transformers or autoformers anyway. And since the Berning's output stage utilizes solid state switching devices, it arguably does not even fall within that context. Regards, -- Al |
Using the Zero nullify an OTL status.You already get rid of transformer
then put new Transformer?Whats happening? going in circle? Xonex77, your question is a logical one, but there is a good answer to it. From the FAQ at the website for the Zero: What is the difference between the ZEROs autoformer and a typical tube amplifier's transformer?
The ZEROs have a few advantages over "typical" tube amp transformers:
A)
The music comes out on the same winding wire that it goes in on so the
music does not have to pass from a primary winding to a secondary
winding.
B) There is no DC current to contend with. When a
transformer is made to accommodate a DC field, its audio transparency is
compromised.
C) The impedance ratio is very small (16 ohms to 4
ohms, compared to a few thousand ohms to 4 ohms). This simply means that
it is much easier to achieve things like, frequency response extremes,
than with a "typical" tube amp transformer. The ZEROs sport a frequency
response of 2 Hz to 2 MHz. All transformers (and autoformers) are not created equal! Regards, -- Al |
By the way, all of my amps are FCL (flux-capacitor-less). I suspect that they'll perform best with contemporary music :-) Chris (Cal3713), thanks for the nice comment. Best regards, -- Al |
Ralph (Atmasphere), I think that when Xonex77 refers to "increasing the impedance of a speaker" in the second sentence of his latest post, he is referring to increasing the load impedance seen by the amplifier, for a given speaker. But more significantly, in the first sentence of that post he appears to be envisioning that a Zero, when inserted between an amp and a speaker, would act as an inductor (rather than acting as an autoformer), with much of its inductance placed in parallel with the speaker's impedance. Resulting in a lower overall impedance, as seen by the amp, than if no Zero were present. Which as you'll certainly agree is not correct.
Best regards,
-- Al
|
Realdeal, as I indicated in my response to the very similar comment that was made by Xonex77, that is simply not correct. A suitably designed autoformer can transform voltages and impedances no differently than a two-winding transformer can. Although there are obviously other differences between the capabilities of the two kinds of devices, most notably the obvious fact that an autoformer can’t provide electrical isolation.
You (and Xonex77) may want to read up on autoformers a bit more. You might also want to reflect upon the various McIntosh solid state amplifiers which use autoformers at their outputs. Which by presenting the output stages with the same load impedance when a 2 ohm speaker is connected to the 2 ohm tap as when a 4 ohm speaker is connected to the 4 ohm tap as when an 8 ohm speaker is connected to the 8 ohm tap, enable those amplifiers to have identical power ratings into 2, 4, and 8 ohms.
Regards,
-- Al
|
Xonex77, as someone with multiple degrees in electrical engineering and
multiple decades of experience designing and managing design of analog
and digital circuits for advanced defense electronics, I can assure you
that I am well aware that placing an impedance in parallel with some
other impedance (of similar type, i.e., not an L in parallel with a C) results in the total impedance being less than either of
the two individual impedances, at a given frequency. The point,
though, is that you and Realdeal appear to be envisioning autoformers as
behaving as if they were inductors, and they are not inductors. While
autoformers (as well as transformers) possess some degree of parasitic
(undesired) inductance (and also resistance), they are different
animals. And as I indicated, an autoformer can transform voltages and
impedances just as a transformer can. See the section in this Wikipedia writeup on the equivalent circuit of a real-world transformer, and this Wikipedia writeup on autoformers. Some brief excerpts from the latter writeup: ...
The voltage and current ratio of autotransformers can be formulated the
same as other two-winding transformers.... As in a two-winding
transformer, the ratio of secondary to primary voltages is equal to the
ratio of the number of turns of the winding they connect to.... In audio
applications, tapped autotransformers are used to adapt speakers to
constant-voltage audio distribution systems, and FOR IMPEDANCE MATCHING
[emphasis added] such as between a low-impedance microphone and a
high-impedance amplifier input. Surely you don’t deny that a
transformer can transform impedances (in proportion to the square of
the turns ratio)? Well, an autoformer ("auto-transformer") can also. Regards, -- Al |
Upon re-reading my previous post, to be more precise it occurs to me that I should re-word the second paragraph as follows:
The point, though, is that you and Realdeal appear to be envisioning (incorrectly) that autoformers behave as if they are inductors, and nothing more. But while autoformers operate based on inductive principles, they behave differently than inductors, assuming that a load is present. Just as the primary winding of a two-winding transformer behaves differently than an inductor, assuming that a load is present. And as I indicated, an autoformer can transform voltages and impedances just as a transformer can.
Regards,
-- Al
|
