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Test & Technic Valve Headphone/Integrated Amplifier

while developing radios and gap whose properties have to a single-ended circuitry to com- cracks the 100 kilohertz HF li-
amplifiers around this new com- be balanced out painstakingly pensate the magnetic preload mit hands down. This may also
ponent. The fundamentally sim- between all parameters. In a of the output transformer, the- be credited to a sophisticated
ple structure of a dual or triple nutshell: a transformer with an reby making an air gap obsole- driver circuit which, just like
stage SE amp dates from this air gap is not exactly the dream te. The result is a very much the actual power stage, can draw
era. And it is – or more exactly: of an audio designer. There are more compact transformer with on fully stabilised power sup-
has basically remained – un- indeed several methods of get- far better properties in every plies and makes use of only a
changed since then. And from ting rid of the anode current, respect. It goes without saying slight dose of negative feed-
the outset it has railed against but usually they only cast out that Octave doesn’t shout it back.
a seemingly unsolvable problem the devil by Beelzebub (cue: from the rooftops how this is
(more details on this in our info parallel feed). exactly done. Yet the result The enclosure format is not
box further below). We’re tal- doesn’t only speak for itself in a design quirk, but rather due to
king about the inevitable pre- Gapless transformer terms of sound. For instance, a construction which uses all
load and magnetisation of the With his “magnetic-field com- the V16 boasts a frequency re- kinds of tricks to bring down the
output transformer in the SE pensated output transformer“ sponse which traditional single- signal-to-noise ratio to much
amp due to the anode current Andreas Hofmann has now suc- enders can only dream about, lower than a sensational minus
flowing through the primary ceeded in squaring the circle or sort of. There’s virtually no roll- 100 decibels. This has such a
winding. Until now this requi- more precisely: in closing the off in the deep bass range, while positive effect on both the head-
red a transformer with an air magnetic circuit. He conceived the therefore super fast amp phone and the loudspeaker ope-

Transformer, core material, saturation and air gap

In a valve amp the output Luftspalt material. To reduce eddy should be adequately sized to offer
Air gap
transformer matches the current losses, very thin, enough headroom. For limit fre-
high valve impedance of stacked metal sheets quencies as low as possible the
several kiloohms “down“ to isolated from each other are transformer must also have a
the low speaker impedance. used as core material. One sufficient number of windings on the
Only that way the valve amp of the classic designs is the primary side to ensure the inductivi-
E-I-BLECH
can actually drive the E-I core E-I core (see diagram) which ty is high enough. Hereby the
speakers. Moreover, the exists in various sizes, magnetic load of the core consi-
transformer, whose primary and depending on its performance. derably goes up with a dropping
secondary winding are not in a High-grade audio output transfor- limit frequency: if, for example, 20
galvanic connection, keeps the mers usually employ more specific instead of only 50 hertz shall be
operating voltage away from the sheet laminations fitted with nested transmitted, this results in a doubled
load. In the output transformer (a copper wire windings for enhanced or tripled magnetic load. Yet the
mains transformer is also an output properties. Whereas in a push-pull upper limit frequency largely
transformer for a fixed frequency) amplifier the inverse anode currents depends on the winding’s capa-
the power – in the form of alterna- cancel each other, the single-ended citance values, which are reduced
ting current – “flows“ from the output transformer is primarily by the nesting. However, the highest
primary winding through the core traversed by the anode current feasible degree of nesting is not an
material into the secondary winding. which causes a pre-magnetisation exclusive quality criterion for output
The magnetically soft core material, that must be counteracted by an air transformers, either; they should
a silicon-iron alloy, accomplishes gap in the iron core. This will in turn rather be seen as an “artistic
this by a reversal of magnetisation increase the magnetic impedance in synthesis“ and, in view of the amp’s
(passing through the hysteresis the magnetic circuit and is rather extremely modest output power, can
curve). “Magnetically soft“ means undesirable. In order not to drive the prove to be very large, very heavy
that in an electrical field the material core material into saturation by the and very expensive in single-enders,
constitutes a magnet. The power to direct current already, there must be but without equally impressing by
be transmitted meets its natural limit enough “room“ for a further drive their specs. So a normal single-en-
in the maximum possible magnetic with alternating current. So for ded output transformer is just
flux density in the iron core, which is achieving usable frequency re- nothing more than a bulky compro-
roughly 1.8 tesla in a modern core sponses, the transformer core mise …

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