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Old 18th June 2007, 03:42 PM   #1
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Default choke loading vs output transformer

Hello,

I've made some experiments on various output stage in class A mosfet source follower and want to increase efficiency......... I am very interested in the inductance idea to create a current source and to approch (theorically at least) 50% efficiency. But the drawback is that for a good bass response, the inductance must have a value of min 100mH.

-For the best sound, it should be air core, so a lot of winding so a big dcr so capacitor to remove dc on loudspeaker, so qualities of sound deacrease because of capacitor....

-Compromise on inductance is to use iron-inductor, but there is still DC (ok not that much) on speaker. So the same capacitor.

-Transformer???? because it will have the same air-gap, but not any more capacitor.

I'm very not transformer expertjavascript:smilie(''), that's why I ask you if you think that a good output transformer with 1:1 ratio and with a primarie that can handle something like 3,5 ADC (plus the swing) for a 50W class A can be of good quality.

Do the bifilar winding technic can be use here (ratio 1:1) and what are drawbacks of this?
What is the best magnetic circuit? EI, C, U

Thanks to all transformer experts...............
Hugues
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Old 18th June 2007, 10:08 PM   #2
v-bro is offline v-bro  Netherlands
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I don't think you want to have DC on your transformer windings either....
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Old 18th June 2007, 11:14 PM   #3
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Old 18th June 2007, 11:29 PM   #4
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I don't think you want to have DC on your transformer windings either....
v-bro,
I think this DC have the same effect for an inductor or a transformer, same air-gap, same primary copper diameter...

Quote:
http://www.diyaudio.com/forums/show...6338&highlight=
darkfenriz
I had already red this thread with a lot of interest, but for me the DC offset is not acceptable, so it's choke+output cap or transformer.

Thanks
Hugues
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Old 18th June 2007, 11:40 PM   #5
v-bro is offline v-bro  Netherlands
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Correct me if I'm wrong, but isn't a choke a series component and a transformer in parallel? If you mean you want to place these components on an amplifier output a transformer could heat up when too much DC current is present...just like a voicecoil....
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Old 19th June 2007, 12:09 AM   #6
Eva is offline Eva  Spain
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A coupling capacitor for the speaker is not required in choke loaded output stages. Also, a gapped transformer functions much in the same way as an inudctor, it's actually a coupled inductor.
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Old 19th June 2007, 05:58 PM   #7
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A coupling capacitor for the speaker is not required in choke loaded output stages. Also, a gapped transformer functions much in the same way as an inudctor, it's actually a coupled inductor.
I agree with the fact that a transformer can be seen as a coupled inductor... but I don't see how to couple the loudspeaker without capacitor on a choke loaded output stage, the dcr of the coil must be 0 for that (supraconductor??????)
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Old 19th June 2007, 06:02 PM   #8
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No transformer expert to answer my question???
If this transformer is doable, where to find information for the calculation???? What kind of magnetic circuit, C, EI??? Which magnetic material??? which size??? which magnet wire diameter??? bifilar winding???

Thanks
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Old 19th June 2007, 08:52 PM   #9
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Hi hugobors

sorry you'll have to use the same equations the rest of us have to.

the turns ratio of a transformer

N/V = 1/(4.44FAB)

where F=lowest frequency of operation, A= sectional area of core; B=max. magnetic field

N=turns V=volts

You can use this equation for ordinary transformers which operate without a D.C. current in the core (A.C. or split primaries like old valve amps)

Note that inductance does not come into this equation, only whether the core saturates or not for the given frequency response.
Often old design manuals said "use a coil with an inducatnce of XYZ" which is usually irrelevant - once the transformer works for a given F , A, B you can check the inductance but usually is much higher than would seem to be necessary.


In any case, the inductance
L=u.u0*A*N*N/lm

where u=relative permeability of the magnetic material, u0=permeability of free space A=area of core section N=turns and lm the magnetic path length.

The tricky part is if you want a single-ended coil carrying a (large) current. You have to take care that the magnetic core does not saturate or you won't have the inductance you expect.

Then you need to know the magnetic field
B=u.u0.H
H=the magnetisation = amps x turns / path length
H=N.i/lm

then you need to modify the inductance equation to allow for an air gap which reduces the magnetic field:

L(with air gap)=u0*A*N*N/(lm/u+g)

g=air gap (total in core).

You will find that ordinary iron cores with B(max)=1.5 T (for example) can only run at about 0.6T with a d.c. bias to remain anything like linear. Then you find you need a huge gap, in turn this means you need a huge iron core ...

Having got a core size you need to work out the turns you need, then based on the size what wire you can fit into the bobbin, and then calculate whether the wire can carry the current and its D.C> resistance. A typical design cycle goes like this-

start small
increase air gap to reduce B
increase size because can't get the inductance ...
increase size because the wire won't fit ...


why bother?

but if you insist, a push-pull arrangement is simpler and for higher powers uses a smaller core than an air-gapped one. Then you have to keep the two halves exactly balanced to cancel any D.C. components, which is another issue ...


cheers
John
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