Chris,
at that low frequency, indeed you can make the individual amps for 100
ohm and put them in parallel, or for 25 ohm and put them in series.
There should be no phasing problems, at such a low frequency. But you
have to make sure that both amplifiers have extremely similar responses,
in terms of gain curve mainly.
The advantage of using a combiner is that it isolates one amp from the
other, so the system is highly tolerant to differences between the
individual amps, down to the point where one amp module can completely
fail, and the other will continue limping along. Without a combiner, the
failure of one module would probably cause the demise of the other too.
I have been giving some thought to your transformer. First, I suggest
NOT buying that giant toroid. Bigger isn't always better. A huge core
has a lot of ferrite, that causes losses! That huge toroid has an
enormous space for winding, which you would never take advantage of. A
smaller core in a better design can provide far better performance. So,
let's start optimizing this design:
The first step is turning it into an autotransformer. The schematic you
linked shows a conventional transformer, with separate primary and
secondary, and both of them having one end grounded. That's very
inefficient! There is no reason at all to use separate primary and
secondary windings, if anyway they will be connected together! So, for a
start I would change this into an autotransformer: 7 turns total, with a
tap at 5 turns - or multiples of that, depending on the core used. This
eliminates 36% of the wire, and strongly improves coupling, without any
ill effect at all. Also transformer action is now required for only 571W
instead of the full 2000W, which allows using a far smaller core, and
far less total copper.
In that autotransformer, the current coming from your amplifier enters
the transformer through the tap, and splits in two parts: 5/7ths of the
current flows "up" through the 2 turns and into the load, while 2/7ths
flows "down" through the 5 turns and returns via ground. The voltage
applied to the 5 turns induces 2/5ths of that voltage in the 2 turns. So
the output voltage is 7/5ths of the input voltage, while the output
current is 5/7ths of the input current, the transformer works at only
2/7ths of the total power (571W), and at the output you still get the
full 2000W.
There is an old adage: Engineering is a combination of material and
brains. The more you use of one, the less you need of the other.
Okay. Now lets try to come up with a good transformer for those 571W. I
will write here as I attempt to design it, so you can learn how to do it.
A good core shape is an RM or a pot core. They have bobbins (easy to
wind), round center legs (even more easy to wind), a much shorter path
length than a toroid, and they are available in suitable materials. The
catch is that they aren't very large. So, let's take the largest RM core
offered by FairRite, and see very dumbly how it works out.
This core is available both in the 95 and the 98 materials. They are
quite similar, but I like 95 better because of its flatter loss versus
temperature curve. So, the chosen core would be the 6295420121.
First let's find out how many turns we need. 2000W on 50 ohm is 316V.
This core has a cross sectional area of 1.95cm². Its volume is 14.36cm³.
How much power can we make it dissipate? That's a decision one has to
take. I would say, 2W is fine for continuous use, some more is
acceptable for intermittent use. So, at 2W to be on the safe side,
139mW/cm³ loss is acceptable. Looking at the material loss chart given
by Fair-Rite, an acceptable maximum flux density at 136kHz seems to be
0.11T.
Now we can use equation 4 on my page
http://ludens.cl/Electron/Magnet.html
to calculate the required number of turns:
316V /4.44 / .000195m² / 136000Hz / 0.11T = 24.4 turns
That's the minimal requirement. Since we need multiples of 7 turns,
let's use 28. So the recipe is 28 turns total, with a tap at 20 turns.
We don't need to make the 28 turns of the same wire, since the top 8
turns carry more than twice as much current as the lower 20 turns. To
evenly distribute losses, it's better to distribute copper cross-section
according to actual current flow.
Also it's hard or impossible to wind very thick, stiff wire on such a
bobbin, and on top of that thick wire suffers badly from skin effect. It
follows that you should wind this transformer with several strands of
thinner wire. That invites using a single size of wire, but using more
strands for the 8 turn winding than for the 20 turns.
For best coupling it would also be optimal to interleave primary and
secondary layers. In this case you could first wind a layer with 10
turns, then one layer with 8 turns, then a third layer with 10 turns,
and interconnect the three layers properly so that the 8 turn layer ends
up at one end of the other 20 turns. This scheme is still reasonably
easy to do, but doesn't allow us to use optimal copper cross sections
for each winding... Anyway, let's try a modification of it:
The winding space on the bobbin is roughly 18*8mm. We can start from
enamelled wire of roughly 0.7mm diameter (AWG #22), and wind 10 turns
with two strands side-by-side. That's a total width of 14mm, which
should fit in the 18mm bobbin space despite some slight kinks and
imprecisions. Try to keep the winding centered, leaving some empty space
at each side, because this reduces the risk of flashover between layers.
Then wind two or three layers of Mylar or Kapton tape, cut just a tad
wider than the bobbin, so that it seals well against the bobbin sides.
Then comes the 8 turn winding. For simplicity let's use the same wire,
but 4 strands instead of 2. Wind 4 turns, with the 4 strands nicely
side-by-side. That will use up most of the bobbin width. Then wind one
or two layers of Mylar or Kapton tape, threading the four wires through
it, and then wind the other 4 turns. So this is a double-layer winding,
with both ends coming out of the bobbin on the same side.
Now wind another two or three turns of Kapton or Mylar tape, and then
wind the topping layer of 10 turns of 2 strands of wire. Finish with
another few layers of tape.
The whole thing should be only around 5mm tall, fitting comfortably in
that bobbin.
Now the windings have to be interconnected. A lot of wires will be
sticking out of the bobbin... The two 10-turn windings have one end on
each side of the bobbin, while the 8-turn winding has both ends on the
same side. First thing is to take one end of one 10-turn winding, and
the opposite end of the other 10 turn winding, and join them. This can
be done on top of the Mylar tape, at a place of the bobbin that will end
up in one of the core's openings. Make the connection nice and short.
Then the still free end of a 10-turn winding on the same side of the
bobbin where both 8-turn ends come out, has to be joined to the CORRECT
end of the 8 turn winding. The correct one is the ENDING, not the
BEGINNING, assuming that you wound everything in the same direction. So,
join that, fit the core (you can tape it together for now, later glue or
clamp it), and the transformer is ready for testing.
I would expect this to work pretty well, although it's not entirely
optimized. We could have used more strands of a thinner wire, and
interleave 3 primary with 2 secondary layers, for example. Anyway it's
MUCH better than winding separate primary and secondary on a stack of
large toroids, let alone a giant and non-optimally shaped toroid!
Manfred
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