Cap vs. Choke Input Filters (Steve Bench)

Cap vs. Choke Input Filters
Date: Mon, 19 Jul 1999 10:37:58 -0700
Subject: Lc vs other filters

Hi Ken,

On the regulation of LC vs cap input…. Generally this is true, until you get to higher voltages, then the opposite occurs, primarily due to resistance of the power transformer and filter choke (to maintain critical inductance). The crossover between the 2 depends on the transformer regulation etc, but is in the ballpark of 1250 volts (the 845 crowd).

Consider 2 circuits…..

Goal: 1250 volts. Current varies from 50 to 100 mA (slight class AB with 845) monobloc. Ripple: about 2.5V P-P. For an LC filter L critical at 50 mA is about 25H. Both full wave bridge using 1N4xxx diodes.

Circuit 1:

LC Filter. Consider a 30H and 50 uF (series 150u 500V to get 50u 1500V) The inductor has 350 ohms resistance (Scaled from Hammonds offerings on the AES website). Transformer nominal 1450V, 100 mA 15% regulation.

Results: at 50 mA you get 1380V at about 2.5V ripple, at 100 mA, 1250V at 2.5V ripple. Notice 1380V/50mA is still above critical current, for 30H.

Circuit 2:

CRC Filter. C=67u R=39 ohms C=67u Transformer nominal 992V, 146 mA 15% regulation (same power rating).

Results: at 50 mA you get 1380V at about 1.2V ripple, at 100mA, 1250V at 2.4V ripple.

As you go higher in voltage and/or lower in current the cap input is actually better!

Wierd, ain’t it?
EMailed only.
Best Regards,
Steve

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1 Comments

1. Julian, February 16, 2013:

   As a relative newcomer to tube amps about to start a construction of my own . The immediate observation with these supplies is the small values of capacitor used. Typically in the design of a power supply the ripple voltage varies with current at higher currents the capacitor gets rather large and an inductor is used for energy storage. In modern switching supplies increasing the frequency reduces the size of both. I have two points first at 60Hz and several hundred mA the size of inductor is huge . Modern capacitors continue to get better and cheaper I.e. for 470uF 450V the price is around $2 .

   If inrush is a problem there are thermistors available to put in series with the capacitor, they are energy rated based on the capacitor, making the selection easy. Furthermore during use they run warm to keep the running resistance low. During startup they are high resistance during the inrush current.

   My point is that big caps are much better and available in big sizes now with esr ratings improved several orders of magnitude over the last decade or 5.

   Food for thought!