Tuesday, 23 April 2019

Field Coil Power Supply 2

The beginning of a well engineered field coil power supply steps around the audio fashionable - both the vintage Tungar types and the commercial and cheaply made capacitor loaded variants. Experimentation and experience reveal that a split bobbin AC transformer, Schottky rectification with an input choke load lead the way to best performance. A tuned reverse recovery spike filter (snubber) and a 220nF cap to soften dynamics in the choke improve performance further. Rod Coleman’s well designed and evolved constant current sources on the output ensure a constant and stable magnetic field for the voice coil. Not as poetic as museum caliber vintage but performance offsets the collectable caché.

Split bobbin AC transformer prevents coupling line noise to the secondary. 

Schottky diodes into a choke load follow as an excellent and well documented rectification solution:

"... a properly designed choke-based supply's ripple is a perfect sine wave, with no high frequency noise.

Silicon diode types except Schottkys emit a burst of ultrasonic noise as they turn off. This noise can go forward into the load circuit and back into the AC line where it can also excite transformer resonance. The "slow" diodes still have this ultrasonic noise. Schottkys are the only type which do not. Schottkys also have half the voltage drop of other diode types and are usually faster. 

... With the choke there is no steep high current pulse, so no disadvantage to Schottky diodes. You get the advantage of no ultrasonic noise, lower voltage drop (so lower power consumption in the diode) and no big massive current pulses."  - J. Swenson

The superior voltage stability and regulation of an ICPS (input choke power supply) is well documented.

In summary there is nothing rare or esoteric used to achieve better sonic result only sound attention to what the energy is doing and what devices work best regardless of mythology or cost.

It could be said that building these in the springtime of the French countryside adds a certain "Je ne sais quoi" or that the local wines mellow the impression...

... but, in fact, it can be discerned quite easily without augmentation as the magnetic field is more stable, with ample, dynamic and clean power, and this provides a smoother, more nuanced presentation. 

It was then left to be discovered if two could be fit into one…

And so it was done - two into one.

Half the chassis + twice the time = same cost.

Provide the voltage and current requirements and we can build one for your field coils - vintage or new. Just send us an email.


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  2. Thank you for your positive feedback.


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