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In Reply to: RE: schematic posted by Tre' on June 28, 2008 at 20:03:43
So, if it is SE, I pick a SE transformer.
Why don't you just build a PP output stage? You can still have a SE driver stage and introduce some of those "SE magic" into the amp?
Follow Ups:
I already have a PP Class A 300b amp with a single ended driver.
I like the idea of an output transformer without a gap in the core forcing more winds to get the inductance back up causing more shunt capacitance thus losing highs....etc...
Plus, I already have some good PP transformers and good single ended transformers are expensive and I'm cheap. :-)
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
It would be very interesting to compare this approach with either:
a) well-executed parafeed, or
b) a SE OPT with minimal air-gap due to utilizing current-cancellation in the secondary.
"a SE OPT with minimal air-gap due to utilizing current-cancellation in the secondary. "
Can you describe this circuit for me?
Thanks, Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
A tertiary winding provides DC [usually from a filament supply] to the secondary to 'cancel out' a major portion of the DC on the primary.
That's a neat idea.Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Wouldn't you get hysteresis losses just like in a push-pull amplifier?
Yes, but there are hysteresis losses in SE also.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
I'm not really a magnetics guy, but I believe there are going to be hysteresis losses in just about all applications where there is current in a core.
There should be none in an SET wherein the tube is drawing current through the core of the device- this on account of the fact that the magnetic field never gets reversed. Losses only occur when the magnetic field gets reversed, as in a push-pull amplifier. This is one reason that SETs can have lower distortion than push-pull amplifiers at low power.
"Losses only occur when the magnetic field gets reversed, as in a push-pull amplifier"Not true. Losses occur when the magnetic field is decreased. Reversing may cause proportionally more losses but Hysteresis loss is a function of changing magnetic fields.
"We start with an unmagnetized sample at the origin (P1) where both field strength and flux density are zero. The field strength is increased in the positive direction and the flux begins to grow along the dotted path until we reach P2. This is called the initial magnetization curve.
If the field strength is now relaxed then some curious behavior occurs. Instead of retracing the initial magnetization curve the flux falls more slowly."
So you see we have hysteresis loss without reversing the field.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
I concede that the magnetics are not completely linear and exhibit losses as described. However, it is also true that in reversing the field, there is extra energy required, right at the zero crossing, in order to reverse the field. Correct me if I am wrong, but in the circuit you have above, it appears that within a certain range you have opposing magnetic fields which cancel, just like in push-pull. This says to me that a 'push-pull style' non-linearity is introduced at the zero crossing. Am I missing something?
hey-Hey!!!,
The shape of the curve containing a sine-wave shaped signal is about the same. Start from a DC idle point, or a zero DC idle point and take the second cycle under examination. It will be a material-dependant 'S' shape, give or take. Mu-metal will be thinner than M4, but with a peak roughly 1/3 the flux density for example.
Now the current cancelling SE will start from a zero flux point, rather like a current balanced PP. One can gap for a smaller DC tolerance and cancell only part of the power tube's standing current too. Or put a small gap in a PP OPT's stack. Given the benefits I've heard from gapping small signal Iron like grid chokes I am looking forward to having Heyboer build me some PP ls to run gapped cores with. Likely to be made with 49% Ni so that I can gap to the same initial permeability of M4 SuPerOrthoSil...:)
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
No, you're not missing anything now.The hysteresis loss is probably greater once you reverse the field.
But even when you don't reverse the field you still have hysteresis losses.
What I'm trying to get away from is the lack of frequency extension caused by the gap in a SE transformer.
P.S. It's interesting how many SET'ers are going parafeed. With parafeed one would have the same zero crossing losses as with push pull.
I think it's all about trade offs.Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
hey-Hey!!!,
I don't think the gap is hurting the frequency extension. Other methods used to deliver enough inductance at LF mean more winding capacitance and leakage L( more turns ). Your trade-off's yes?
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
A well designed and constructed SE transformer should have plenty enough bandwidth to cover the audible spectrum. However depending on your desired impedance, power rating, and materials construction, it may cost $$$.
Donald North
and you create a bigger one, seems a very true axiom.
Have you built this circuit yet? I'd be interested in seeing how it turns out.
Why do you assume that the one is bigger than the other.I would think it's the other way around.
Solve one big problem create one small problem.
BTW I think I'll do parafeed instead.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Bandwidth is certainly a serious issue and I am a big fan of wide bandwidth.
My understanding is that some of the popularity of SETs comes from good 'inner detail', which to my understanding comes from really low distortion at lower power levels, something that push-pull can't do so well; though if you make a smaller P-P amp, I'm sure it helps :)
P-P always had pretty good bandwidth though. So- do you expect this to be a bit of crossover between worlds?
Yes. I guess that's what I'm hoping for.
We'll see.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
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