interesting OPT winding comments
- From: Bret Ludwig <bretldwig@xxxxxxxxx>
- Date: Sun, 29 Jul 2007 13:07:49 -0700
http://www.diyaudio.com/forums/showthread/t-1933.html
diyAudio.com diyAudio Forums Archive > Top > Amplifiers > Tubes
DIY output transformer info site - Click HERE for Original Thread
smoking-amp
Found this site with good info on winding output transformers:
http://www.geofex.com/Article_Folde...des/xformer.htm
Noticed one error in a formula they have for computing turns:
N=(E*10**8)/(kfbA) A (area) should be in sq. cm not sq. inches.
Site doesn't mention toroidal xfrmr winding techniques though. But the
key to high performing toroids is just using back and forth dithering
of the core while winding so as to make winding layers chevron shaped
and cover near 360 degrees in one sweep around core per winding.
(don't overlap ends) (use good insulation between layers and core too)
This essentially amounts to micro-sectioning a winding. Interleaving
does not benefit in toroid case (since leakage inductance is already
minimal) and only makes distributed capacitance worse. I have some
cheap (surplus) Avel-Lindberg power xfmrs that appear to have been
wound this way. They have a flat frequency response to 150KHz and
resonance at 450KHz!!! This is far better than most "Audio" output
transformers! I measured a Hammond 1650T (a low turns ratio model)
trannie to compare and it is flat to 30KHz and resonant at 50KHz. Main
drawback of toroid outputs is that the DC current balance must be
strictly maintained to prevent saturation problems.
See my earlier posting on how to extend any transformer's phase linear
bandwidth up to Mhz range.
smoking-amp
Just a comment on E-I transformer laminations.
Most info I have seen on DIY output transformers have mentioned
"scrapless" or "no waist" laminations. These are proportioned in such
a way as to waist the least amount of steel sheet when punched out and
are the most commonly available laminations. They are widely used in
power transformers due to their economy.
They are NOT however the best laminations for audio transformers. The
famous Acrosound and Dynaco transformers used long E laminations.
These have longer length to the three legs of the E lam. The reasons
they are better for audio are twofold: The grain oriented steel used
can have a larger proportion of the E lamination magnetic field
correctly oriented to the grain orientation in operation. The leakage
inductance of the windings is reduced as the coil is made longer. (See
page 97 of "High Fidelity Sound Engineering" by Norman Crowhurst,
Newnes, 1961)
The reason being that the leakage pathes become long thin loops
through the winding cross-section which have nearly the same path
length as the desired flux thru the steel core. Only if the leakage
path lengths are significantly shorter than the steel can they
overcome the much higher Mu of the iron. The conventional "Scrapless"
laminations give a near square winding cross-section with near
circular leakage pathes, N. G.
It is a simple extrapolation to take this long E enhancement idea to
the extreme and one ends up with the toroid configuration where all
flux pathes have to go around the toroid and there is no path length
advantage whatever for the leakage flux. (Assuming the windings cover
near 360 degrees.) Toroids also maximaize the steel grain orientation
in the direction of the magnetic field 100%. Not surprising that
toroids have greatly improved bandwidth and better magnetic
properties. Their lack of any lamination end gaps distributed around
the path causes the effective permiability to be greater so close
attention to DC balance is a must! (ie. have to use a DC bias servo on
output tubes to zero out DC current imbalance or else adjust every
day!)
Since toroids are so difficult to wind without automatic equipment, I
suggest using four I laminations in a square configuration with
windings on all four legs as a simple DIY alternative. Solves the DC
balance sensitivity issue too.
Better yet, see my other posting on how to use a cheap 60 Hz power
transformer and a ferrite toroid with low turns to get near MHz phase
linear response bandwidth.
alex278
quote:
Originally posted by smoking-amp
But the key to high performing toroids is just using back and forth
dithering of the core while winding so as to make winding layers
chevron shaped and cover near 360 degrees in one sweep around core per
winding. .
Could you explain on that, please? I don't know what you mean by
'dithering the core while winding'.
smoking-amp
The toroid core gets a winding put on in one rotational pass around
the core. Since the number of turns will usually be far greater than
what would fit on a single layer, the core rotation during winding
gets a small but frequent rotary back and forth (triangle wave)
superimposed on the single rotation pass (ending up with something
more like a sawtooth with leading slopes longer than trailing slopes).
This makes the turns stack up in chevron or V shaped layers (looking
at both sides of the toroid, slanted layers if looking at just one
side), giving the winding vertical thickness from the core to
accomodate the total turns.
Since the turns getting stacked up in chevrons are near to each other
electrically, they have relatively little voltage difference between
them. This reduces the effective distributed capacitance of the
overall winding. Essentially producing a "micro sectioning" of the
winding.
Other windings get put on top with the same method, with ample
insulation between windings to minimize inter winding capacitance. No
need to interleave windings, since all magnetic flux is constrained to
the core by symmetry, hence minimal leakage inductance already. All
windings should cover near 360 degrees around the toroid.
(Leakage flux involves flux pathes that are shorter than the core
material length and symmetric with respect to the winding, and there
are next to none with toroidal symmetry. A square-ish or round winding
cross section is the worst for leakage, since the leakage pathes make
loops in the winding cross section that are shorter than the magn.
material path. A long thin winding cross section is best, toroids
doing especially well, long E laminations giving a practical
compromise. )
Don
tubelab.com
quote:
Better yet, see my other posting on how to use a cheap 60 Hz power
transformer and a ferrite toroid with low turns to get near MHz phase
linear response bandwidth.
OK, I give up, please give me another clue, the search engine could
not find it.
I did some experimenting with series connecting a conventional OPT
with an air core transformer from an old HP wide range audio
oscillator. I could never get the response flat.
smoking-amp
Hi George,
Yup, these threads were from way back. And it IS just the series
transformer connection scheme with low pass and hi pass filters shown
in the RDH4 (page 888). Here is the thread:
http://www.diyaudio.com/forums/show...12534#post12534
Problem may be that the scheme requires a good broadband inductor for
the LP filter. Maybe have to make the LowPass inductor a series
connection of an E-I core inductor and a ferrite inductor too.
Don
smoking-amp
Check out this thread too. Won't work for SE very easily though.
http://www.diyaudio.com/forums/show...6852#post626852
I just read thru this thread again myself, forgot all about it in the
trauma of moving this last Fall. Peter M.'s suggestion of using
permalloy wire for wrapping should work well for a SE style xfmr since
it gives a distributed air gap.
I also had worked out the kinks from the scheme to get rid of the
distributed capacitance from bifilar windings used in it. Uses the
same technique of distributed common mode chokes used in transmission
line xfmrs. I forgot all about it! I was going to post on this. The
trauma from moving was like a nuclear blast that wiped out my memory.
Don
Yvesm
Hi,
Nothing 'bout toro?ds here, but it may save hours of tidy hand
computing.
http://www.dissident-audio.com/OPT_da/Page.html
Yves.
alex278
quote:
Originally posted by smoking-amp
The toroid core gets a winding put on in one rotational pass around
the core. Since the number of turns will usually be far greater than
what would fit on a single layer, the core rotation during winding
gets a small but frequent rotary back and forth (triangle wave)
superimposed on the single rotation pass (ending up with something
more like a sawtooth with leading slopes longer than trailing slopes).
This makes the turns stack up in chevron or V shaped layers (looking
at both sides of the toroid, slanted layers if looking at just one
side), giving the winding vertical thickness from the core to
accomodate the total turns.
Don
Ah, you mean progressive winding. OK.
.
- Prev by Date: Re: Here's a site with definite ideas on which is the best "valve".
- Next by Date: Another DIY opt site
- Previous by thread: WTB Fisher X-100 output transformer
- Next by thread: Re: interesting OPT winding comments
- Index(es):
Relevant Pages
|
