Stereo Vacuum Tube Amplifiers

Output Transformers

        In our experience making a good audio transformer is a black art; the reason being that its output has to energize a transducer which in turn, after flying through an uncertain medium, has to impinge on & so activate an incredible frequency analyzer, the human ear; it has to generate that elusive quality named Presence.*  To further complicate matters, there are two such analyzers per member of the audience & two or more operating transducers in the area to create stereo & surround effects.  All these factors, plus room acoustics, are continuously interacting; gauging the end result is like writing the Farmer's Almanac, predicting the weather one year in advance.  

        Now the device has to handle a very complex signal at high efficiency: an AC component of 1000:1 frequency range & power, a DC component for single ended amplifiers & also coming from the transducer or its frequency filters, a combination variable, resistive-resonant inductive component that can feed back into the signal; don't forget the fact that a transformer is a two way device: at any time it can either step up or down.  To minimize the latter, the dynamic output impedance of the amplifier has to be kept well below the nominal output impedance determined by the transformer turns ratio; this is achieved by introducing negative feedback, tapping voltage off the secondary (Williamson) or having a tertiary winding connected to the cathodes of the output tubes (Mc Intosh).  The electric current flowing through the device is also fluctuating widely.  The AC component can swing between zero & the maximum power rating of the amplifier.  In a pure class 'A' single ended output stage, the DC component can vary by 45 % before unacceptable distortion sets in.  The core of the device has to be large enough to transfer the maximum rated AC power within the near linear part (the middle section) of the magnetization/demagnetization Nyquist diagram; a larger transformer for the task always yields benefits, within limits; if made too large, eddy currents & ohmic winding resistance will soon overcome all advantages.  The transformer we have produced for our single ended, 'Vacuum Tube Amplifier' & 'Tetrode Tube Amp.' is a well designed, good size, 5W hi-fidelity unit.

        It is vitally important in audio transformer design to locate the windings as close as possible to each other; the square wave response (or response to an impulse-like signal) of the device depends on it.  Stray capacitance between the windings - which increases with this type of construction - can be neglected, because it is only effective at the upper end of the audio spectrum; the overtones thus generated will lie well beyond the audio range; of course, a low pass filter must be included in the negative feedback loop to preclude any adverse affect on the amplifier - failure to introduce such a filter is a fault we have encountered in many designs both vacuum tube & solid state - interleaving ** is a means of achieving that.  Our output transformer (common to both assemblies) is adequately interleaved & sufficient in every respect for the purpose for which it has been designed; traditional techniques have been adopted in its construction because we believe such 'alchemy' effectuates optimal results; it can only be out-performed by substituting 'Radiometal 36' as the core material, a rare & costly alloy yielding an outstanding degree of audio excellence in units rated up to about 20 watts, & which in addition allows for heavier wire gauges to be selected for a given core size owing to its inherently elevated permeability - we have found its properties to be thoroughly impressive. 

      * A quality in reproduced sound that gives the listener the impression that the recorded performance is taking place in their presence. 


    ** Select 'O. T. Interleaving' for details.