Question about OPT impedance in RDH4

[Tony Bones]

I don't think there are errors in RDH4 (not that I know of) but I do think that Crowhurst did not write clearly. Look at the length of this thread. All trying to figure out three equations.

The subject at hand (the load on a PP pair, not Crowhurst) is subtle but it's not impossible. There's no question in my mind that the authors of RDH4 understood it well. In fact, they apparently understood it so well that they felt it was enough to just write down a few equations along with a couple of comments and expect readers to understand it too. Well, obviously it didn't work.

If you want to understand PP circuits - not just equations - then it's necessary to develop two skills: (1) think in terms of current, not just voltage, and (2) separate the AC (signal) component from the DC component. Try to think about just the signal part alone and realize that the contributions of the DC are not relevant. In a class A tube, the signal current goes positive and negative around the DC idle current. In a PP pair, when one goes positive the other goes negative exactly the same amount (assuming perfectly matched and perfectly linear tubes, which I won't repeat unnecessarily.)

Now comes the fun part. The AC circuit has just three components in a single loop: the two tubes connected at their cathodes and the OT primary between their anodes. From the perspective of the OT, the (AC) plate voltages of the two tubes add, but it's the same (AC) current through both tubes. Think about this for a while. When it feels comfortable enough, ask yourself how the two tubes share the load presented by the OT.

[jazbo8]

In fact, they apparently understood it so well that they felt it was enough to just write down a few equations along with a couple of comments and expect readers to understand it too. Well, obviously it didn't work.

Since you have not read RDH4 recently, it's worth noting that the three equations quoted/referenced by the OP merely sets the stage for a far more in depth discussion on the push-pull operation, so in fact Langford-Smith has covered the subject extremely well as usual.

[Alan0354]

Please also read Chapter 13, sections (ii) - (v). That's why I asked you earlier to draw out the composite load line, if you have a different interpretation of the push-pull operation, then it would not look like the example given in Fig. 13.33 of RDH4.

I just finished (ii), I want to go through (iii) and (iv) before (v) because it's about distortion where I have the most questions in how to lower distortion. I've gone through (ii) up and down, in and out.

My question is on just one equation of R"_L=1/2 R_L. I understand the other two. I did not seen any relevance in (ii). (ii) only talked about drawing the loadline according to R'_L=1/4 R_L in equation (8) in page 576. This I agree totally. But in page 572, it claims R'_L=1/4 R_L ONLY applies to when one tube is removed OR in cutoff ( as if one half of the primary is open).

So where is R"_L=1/2 R_L come from?

I went on Amazon to look for Langford-Smith http://www.amazon.com/s/ref=nb_sb_no...Langford-Smith. Which one is it?

Thanks

[Jazz P Bass]

For those of you that want to play along at home, here is an archived copy, broken down by chapter.
Enjoy.
RDH4 mirror

[Alan0354]

RDH4 is like text books back in the older days, very condensed, no nonsense. You really have to read it over and over.....and over. Then chew on it and read. But hey, that's the best I've seen so far. I have read (ii) like 6 times, deriving all the equation step by step, still digesting it.

For those that prefer to download the RDH4 in one shot, I have the link in post 4 also.

[Dave H]

So where is R"_L=1/2 R_L come from?

If you are referring to RDH4 then the answer is out of thin air. They think it's obvious and therefore doesn't require an explanation (This is why some of us are saying it is badly written). The real reason RL'' is 1/2RL and not 1/4RL is because this is the situation where the other tube is making an equal contribution to driving the load. You can't just look at one tube and the turns ratio it is driving in isolation.

[Alan0354]

If you are referring to RDH4 then the answer is out of thin air. They think it's obvious and therefore doesn't require an explanation (This is why some of us are saying it is badly written). The real reason RL'' is 1/2RL and not 1/4RL is because this is the situation where the other tube is making an equal contribution to driving the load. You can't just look at one tube and the turns ratio it is driving in isolation.

Not true, If you read p 576, he used R'_L =R_L/4 for load line all the way for Class A where both sides are on.

[J M Fahey]

...

[Alan0354]

I read (ii) again, actually equation (11) in page 577 comes close.

(11)=> r_ab=R'_L(1+r_p1/r_p2) , where r_ab is the impedance load V1 see with V2 there and speaker load.

At quiescent point, r_p1=r_p2 so at quiescent, R"_L=2R'_L=1/2R_L. But this is only at one point as r_p1 and r_p2 change with current through the tubes.

[Justin Thomas]

Still room on the flexible love chair in the lobby, Juan...

Seriously, though, as a beginner who sucks at math, I got hold of RDH<3> and find it much more comprehensible. Much that was implied or assumed in 4 was spelled out better in the earlier editions. 3 is also about 600 pages shorter...

Justin

[Tony Bones]

Look. I'm sorry for crapping up this thread. I mistakenly thought that there was a in interest in discussing class A PP. In fact, the real interest is in discussing what some dead engineer was thinking when he wrote a few lines in a text book. That is an entirely legitimate topic (I'm not being sarcastic!) It's just not a topic I'm interested in.

Again, I appologize for crapping up what was otherwise a good thread.

[Alan0354]

Look. I'm sorry for crapping up this thread. I mistakenly thought that there was a in interest in discussing class A PP. In fact, the real interest is in discussing what some dead engineer was thinking when he wrote a few lines in a text book. That is an entirely legitimate topic (I'm not being sarcastic!) It's just not a topic I'm interested in.

Again, I appologize for crapping up what was otherwise a good thread.

this is the most detail book I've seen that explains when a lot of other books regurgitate the result but never border to explain why. Like why for triode, the optimal primary impedance should be 2ra of the tube, like why in pp, even harmonics cancelled and odd harmonics added etc.

I believe in order to deeply understand and design amps, this will give more insight than just remember the guide lines. I think it's worth chewing on this slowly. Just because I asked, read and post on this thread since yesterday, I feel I learn a lot already.

[Alan0354]

Still room on the flexible love chair in the lobby, Juan...

Seriously, though, as a beginner who sucks at math, I got hold of RDH<3> and find it much more comprehensible. Much that was implied or assumed in 4 was spelled out better in the earlier editions. 3 is also about 600 pages shorter...

Justin

This is JMHO

There are very few older books that spell out things clearly. That's why if you look at calculus, physics type of books used, they are all newly written in a much more detail with examples for discussion. But I'm afraid this is it for the very few tube enthusiast like us. Like any advanced book, if one can go through 3 or more pages a day, that's a good day. Going through the formulas is always tedious and painful. But that's the only way. Since I started this thread from over a day ago, I only gone through from page 571 to 577 . If I can get the answer, it will be a fantastic day for me.

[Alan0354]

I read (ii) again, actually equation (11) in page 577 comes close.

(11)=> r_ab=R'_L(1+r_p1/r_p2) , where r_ab is the impedance load V1 see with V2 there and speaker load.

At quiescent point, r_p1=r_p2 so at quiescent, R"_L=2R'_L=1/2R_L. But this is only at one point as r_p1 and r_p2 change with current through the tubes.

I actually snoozed on this after I posted. Something still bordering me in Fig. 13.35 that derived (11). For normal case where the two voltage sources of V1 and V2 is totally uncorrelated, I agree with this finding. BUT we know the two sources are equal and opposite, the stuffs I posted in post #35 comes into question. I know that was a wild guess, but I have basis for that post. This is one that is kind of similar to what I am trying to say:

For MOSFET, the input capacitance is very high, even small ones are like 200pF from gate to source. So on the first pass, it's not go to use as a source follower to buffer a common cathode triode stage with high output impedance. But in fact it is very good for that. The reason is the gain of the of the source follower is about 0.95 ( just work with me!!!). Because the source swing with the gate, the input capacitance is reduced to

C_in = C_gs(1 - A_{source follower}) = 200(1-0.95) = 10pF.

The loading is reduced by 20 times!!!

This logic should work for this case also as shown in the formulas in post #35 where there is no loading of the even harmonics, and loading only on the odd harmonics.

[J M Fahey]

There are very few older books that spell out things clearly.

Well, I have an opinion on that.

Fact is, these are University textbooks.

You do not get straight to them, by any means.

Electronics is just one specialized branch of Physics.

To be more precise, a subsection of the wider area covering Electricity and Magnetism.

So in the normal course of studies, (I won't use school level labels because they differ all over the World, but use age, each one imagine what grade they mean in their own Country),you approach that gradually.

You start with Math taught between 7 to 13 y.o. (approximately) and elements of Physics (may be called Natural Sciences but it's the same) between 11 and 13 y.o. , then higer level Math between 14 and 18 y.o. plus now properly labelled Physics and divided in various branches, (Mechanics, Optics, Electromagnetism, etc.) between 16 and 18 y.o.

And then it's University level, with "Physics 101" .... which properly should be called "Physics 501" or so, you already have some years of background.

And RDH4 should probably be, say, "Physics 801" or something.
And it also carries years of Math background.

So a textbook writer will not waste time re-explaining what for students at that level should be well known, so they concentrate on the "new" knowledge and don't waste time.

So getting straight into RDH4 is fascinating but can lead to lots of misunderstandings.

Personally, if there's something I don't understand,there or anywhere else, rather than blame the book I blame myself, try to "go backwards" , find the sources , understand them and then come back.

FWIW tons of people , both in Forums or personally, ask me advice on what they should study "to design amplifiers".

Most get dissappointed by my answer

Which is: go to the Library and get a high school Physics book, what is taught around 17/18 y.o. in your Country, the book or chapter dealing with "Electricity and Magnetism".

No need for Calculus, most can be understood at an Algebraic Math level (the 4 basic operations, roots and squares, and notions of logarithms) .

The few wo do come back still disappointed: "those books don't mention tubes, transistors, loudspeakers, etc"

No, not directly, they "just" teach: voltage, current, power, battery, generator, resistor, capacitor, conductor, insulator, switch, potentiometer, transformer, ionization, frequency, magnet, electromagnet, galvanometer, voltmeter, lamp, resistance, impedance, transductor, resistance variation with temperature, electromagnetic wave, etc. , all with suggested practical experiments so you see what happens.

If you understand and can smoothly use that, tubes, resistors, diodes, amplifiers. speakers. pickups, shielding, etc. , all you use in Audio Electronics is quickly understood, in a jiffy.

The old trick of starting a house by its foundations