Jon Yaeger wrote:

Assuming that a tube amplifier can be built with a balanced output
(e.g. the Zechendorff circuit), what are the pros and cons of using an
autoformer to match output impedance vs. a standard transformer?

It would seem offhand that an autoformer could have sonic advantages
(e.g lower losses, no coupling degradation, etc.). Would it cost less
to manufacture? And finally, would a toroid be a suitable
implementation (I am aware of toroid problems, especially with
saturation, when used for standard output coupling).

TIA,

Jon

An auto transformer is useful to match awkward speaker
impedances to impedance sensitive amps.
To get a good match of a 2 ohm speaker, to say an amp which runs best
with 8 ohms, a 2:1 step down ratio is required.
The impedance ratio is 4:1, so you get 8 ohms to 2 ohms.

To achive this transformation, with a typical low power tube amp,
which puts out only say 20 watts into 8 ohms, one could use a
toroidal power tranny, of around 300VA, with 2 x 40 volt windings
in series, and leave the mains primary unterminated.
Such a tranny will have a B of perhaps 1.2 Tesla at 40v at 50 Hz, for
one 40v winding,
but at 20 watts into 8 ohms the voltage is only 12.6 vrms, so
with this voltage applied to two "40" volt windings,
the B at 50 Hz is only 0.189 Tesla, and at 12.5 Hz its still
only 0.756 Hz, and so still well away from saturation.
The BW is usually 100 kHz so the toroidal isn't a bad device to
couple speakers with, to get an impedance match at low losses,
and wide BW.

Various taps down the one winding can be placed to get
a wide range of impedance matches, so driving a woofer, midrange, or a
tweeter
at any selected voltage level can be done can be done without using
series
resistances, to achieve well equalised levels,
and the output impedance at the taps is always lower than the amplifier
impedance,
unless one tries to step up the output voltage, which imho, is
pointless.

SO, those who do have 4 or lower Z speakers could easily
make a match to tube amps best suited to 16 ohms, such as Quad II,
even when set on the amp for 8 ohms.

There are plenty of tube amps which have the poor method
of impedance matches done by simply having a single secondary OPT
winding
for 16 ohms, with tap at 0.7 times the total turns for 8 ohms, and a CT
for 4 ohms, which means that half the sec is not carrying speaker
current
when 4 ohms is used, and HF response is poorer.

The auto tranny at the speaker allows the amp to enjoy the 16 ohms it
prefers,
and less OPT winding losses. The winding losses in the toroidal
auto tranny are minor.
The turns per volt required for a mains 300 VA tranny could be about
2 turns per volt, so 80 turns are required fore 40v at mains F.
160 total winding turns would be fine for a 20 watt amp.
Its easy to place 160 turns of wire by hand around a toroidal core,
using a shuttle made from a piece of old plastic tube, or broom handle
with notches.
Use about 1.3 mm dia wire, and wind the right length for say 1/2 the
winding on the shuttle, then wind it off the shuttle onto the toroid
and set the wires straight, tightly, and true, and neatly.
Tape insulation is easy to apply in lengths.
I have re-wound complete toroidal power
transformers like this, and of course it took ages to do the
thin wire high voltage turns, since there are so many turns.
The low voltage heater windings were dead easy to do, once I
made a sort of clamp to hold the partially wound tranny as I placed on
the turns.

Using auto trannies to have the primary in series with a speaker
secondary
doesn't convey a huge advantage.
But in a PP amp, one could deliberately
split the primary into cathode and plate windings, a'la Quad acoustical,

and have each half cathode winding in series with a grounded and centre
tapped
speaker secondary, perhaps with other symetically placed taps to allow
impedance matchings. The feedback effect is enhance this way.
The Quad OPTs don't allow for such elegant use of the speaker sec,
since it isn't a centre tapped winding.

The text books, including RDH4, will explain
the winding losses with two winding isolation trannies,
and with auto trannies, which are more efficient,
especially with a 2:1 step down transformer.


Where there is a will, there is a way.

Patrick Turner.