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In Reply to: Purpose of these Caps? posted by Triode_Kingdom on August 27, 2016 at 23:42:32:
The caps are what a guy on the joenet calls "bridging caps" and this circuit can be looked at at a number of different ways depending on the size of the caps and the load on the driver stage.
This implementation essentially puts two coupling circuits in parallel and which one gets "used" is based on the impedance of the coupling circuit.
The cap coupled impedance is dominated by the impedance of the coupling cap and the magnetic coupled circuit is dominated by the copper resistances of the windings. This means that the cap coupled part has varying behavior with frequency and the magnetic circuit varies with load current. If the cap is very small (0.01ยต) and the load is very light this approach simply serves to transition from a magnetic couple to a cap coupled circuit at some high frequency and since cap coupled circuits don't show the squirrelly behavior above 20Khz that transformers do, it is an easy way to get a nice 10Khz square wave out of a mediocre transformer.
At the other end of the spectrum when this approach is used with a heavy load, the frequency where the Z of the cap is = to the series DCR of the windings marks the transition from magnetic to cap coupled behavior. This comes with one extra set of issues and that is the DCR of the magnetic circuit causes more loss of amplitiude in the magnetic circuit than the capacitive circuit so you see a couple of dB increase in amplitude as you transition to the cap coupled circuit. This transition typically occurs in the audio band unless you use really big or really small caps.
In the case of the fisher they use big electrolytic caps and probably rely on some A2 operation which fits into everything I say above since the large load of an A2 grid would cause a pretty sincere shift of frequency response with a small cap and the HF behavior is probably pretty miserable if the cap is removed altogether.
dave
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- RE: Purpose of these Caps? - dave slagle 06:29:44 08/28/16 (0)
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