Download DCMB needs good partners : "The special Low flux

THE SPECIAL LOW FLUX , SELF COMPENSATING OUTPUT
TRANSFORMERS FOR SINGLE ENDED AUDIO AMPLIFIERS
Ari Polisois system.
First a short resumé of the state of the art situation.
The description is also intended for unskilled audiophiles.
Almost every valve amplifier ( except the OTL – Output transformerless) require an OPT ( an
output transformer), to match the high impedance and generally high voltage of the power
valve(s) to the lower impedance ( and lower voltage but higher current requirement) of the
loud speakers.
The biggest family of audio amplifiers uses the so-called push-pull technology, requiring
one or more pairs of output valves, delivering opposed phases power signals to the output
transformer. The latter uses two primary windings, one for each alternative phase. The
secondary, connected to the speaker(s) combines these opposed signals into just one, powerful
sound.
Another topology is named “single ended” because there is only one valve (or sometimes
more, but simply connected in parallel, to increase the power output) at the end of the power
stage, with a single primary winding inducing a lower voltage but higher current in the
secondary winding.
In the push-pull output transformer, the power valves’ idle direct current crosses the two
halves of the primary winding in opposed directions, thus cancelling any magnetic core
saturation due to the above d.c. flow. The Single ended transformer’s primary winding is
crossed, instead, in just one way and, therefore, the coil, made of several hundreds of turns,
easily produces a high magnetisation level. As a result, several “remedies” must be applied,
such as creating an air gap in the core path, to reduce the flux. As a consequence of the latter
step, the primary's coil number of turns must be increased and usually a larger size core must
be chosen, leading to a significant bulkiness of the size of the transformer. The power
handling limit, due to the early saturation of the core produced by the idle d.c. is quickly
reached forcing a compromise : 50% of the core capacity is used for DC and 50% for the
alternating current signal. If the SE amplifier is designed for a consistent output power ( over
50 Watts, true ones and not “musical”, inflated by 50 or 100% more) the output transformer
becomes unreasonably big.
For your guidance, a push-pull 50W transformer would weight about 5-6 kgs. A Single ended
50 W transformer close to 20, if it has to deliver a wide enough frequency range.
I have been able to break this limit with my novel system.
I started, like many others, by compensating the DC with an opposed coil, that will be referred
to as “tertiary”. However, the same coil, crossed by the Output Valve’s variable plate current
( or anode current) of the modulated signal , whilst neutralising the unwanted magnetisation in
the core, caused by the idle direct current, ALSO OPPOSES TO ANY ALTERNATING
CURRENT and , simply, kills the variable waves. Result : no signal is induced in the
secondary and the loudspeakers are silent, even if a strong signal is generated by the Valve.
The solution I found restores the AC with a loss restricted to 5 -10 % . Moreover there is
practically no limit to the power output watts that can be handled, except, obviously, the
increase of size due to thicker copper wire and iron core size.
By just looking at the output transformer, the push-pull topology is still very interesting, but it
must be stated that the Single ended topology leads, according to many audiophiles, to a more
pleasant sound and a much simpler circuit layout, with half of the valves required by a pushpull amplifier delivering the same power and with far less components. Easier and faster to
assemble, a single ended amplifier, with my transformer, can become a real challenger in
terms of cost of the parts and labour, not mentioning the nobler sound.
I have designed and tested so far several dozens of different types Self-offsetting Lo-flux
OPTs ( the novel type described above) , built by a precision winding work shop, A2B
Electronic, the audio division of A.B.B., a company founded in 1974 with a lot of experience
in the manufacture of Transformers ( every kind ) , power supplies and other industrial
electro-magnetic applications. The novel OPTs have been fitted to Single ended amplifiers
and laboratory tested with very satisfactory results.
Their sonic performances have been appreciated by many skilled audiophiles and electronic
engineers, impressed by the clarity of the sound.
You are invited to enquire for the price and specs of the output transformer you need to fit to a
high end amplifier you want to build or if you intend to replace a non satisfactory standard
transformer.
Bear in mind the following comments a "Maestro" in the design of audio transformers, as well
as a reputed sound recording engineer, has expressed, openly and honestly, in his lifestyle
way, after comparing his high end push pull amplifier to my SE DCMB amplifier using the
OPT-OPUS-20-800 :a) the use of a special SE design, direct coupled, with a minimum of components,
optimises the amplifier design as an "extreme short direct path" of superb SE
amplification.
b) The output transformer is auto compensating for the direct current flowing through it.
You never need any gap adjustment or tuning. The direct current is auto compensated
to a flux close to zero inside the core. This means that ANY tube at ANY quiescent
current will be adjusted automatically to the output transformer.
c) As opposed to the standard SE transformers, where HALF of the magnetic headroom
is used to deal with the direct current, in this novel design THE FULL MAGNETIC
HEADROOM inside the core is available. In standard SE output transformers only
half of the magnetic flux is available.
d) Inside the standard SE transformers, there is a direct relationship between magnetic
flux and current applied through the output tube, in such a way that half of the
magnetic headroom available is fully used. In this invention, the magnetic AC-flux
( full headroom available) is again compensated to almost zero, meaning that inside
the core, AT VERY HIGH POWER LEVELS, the magnetic flux inside the core will
be limited (§). For this reason the novel OPT is called "low flux, auto compensated,
SE gapless output transformer".
Well, quite gratifying.
From the author:
Ari Polisois
[email protected]
Or [email protected] to the attention of Dr. Polisois
(§) Note : in a way, this corresponds to an instant, local, feed-back effect.