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 push-pull 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
Or to the attention of Dr. Polisois
(§) Note : in a way, this corresponds to an instant, local, feed-back effect.