From: henry@monod.Biol.McGill.CA (Henry Pasternack)
Subject: Re: Question on choke-input filters.
Organization: McGill University
Date: Mon, 9 Nov 1992 23:04:41 GMT

   Richard Karlquist writes:

>If the resonant frequency of the input choke and filter capacitor
>(= 1/2*pi*L*C) gets anywhere near as high as the AC frequency
>(60 Hz.), you're in serious trouble anyway.  If you have any
>design margin at all, this resonance will be way below 20 Hz.
>so it won't affect audio.

   Yes, but the current flowing in the filter choke is proportional
to the amplitude of the signal envelope, which certainly can vary at
a rate near the filter resonant frequency.  "The Radio Amateur's
Handbook" states:

   "If too high an LC filter product is used, the resonance
    may occur at the radio-telegraph keying or voice syllabic
    rate, and large voltage excursions (filter bounce) may be
    experienced at that rate."

   Note that the application referred to is radio-telephony, which
has a lower-frequency limit of approximately 300Hz.  It is mainly
this caveat in the Handbook that prompted me to post my question
in the first place.

>Yes, but only above the critical current.  This requires you to
>put a bleeder resistor on the output, unless your amplifier runs
>class A.  The bleeder power is quite large unless you use a
>"swinging choke", which is virtually impossible to obtain these
>days.  If you put a big bleeder resistor on a capacitor input
>filter, it will also improve the regulation.

   With a 400V power supply and 100mA of idle current, taking into
account output and driver stage quiescent demand, the required
minimum choke inductance is 4H.  This size choke is quite easily
obtained.  In fact, I already have two 10H chokes in my junkbox.
No bleeder resistor is required.  Naturally, the filter capacitors
should be rated for the full 1.414*RMS voltage that will be developed
if a failure causes the quiescent load to go away.

>This is a dim-bulb idea.

   Don't be a dork, Rick.

>It is far easier to get another power transformer than to find a
>suitable choke.

   Power transformers are considerably more expensive than chokes,
both on the new and surplus markets.  Plus, I already have these
parts.

>The only reason choke input filters were ever used in the first
>place was because they don't require high peak currents in the 
>rectifiers, and this was a problem for high vacuum rectifier tubes.
>Even vacuum tube buffs like you now use solid state rectifiers,
>I would hope.  Solid state rectifiers can easily handle high peak
>currents.

   I have considered this matter in some detail.  The reality is
that it is difficult to design a capacitor-input power supply for 
a tube amp because of the problem of predicting the final output
voltage under load.  Typical crummy power transformers have load
regulation on the order of 10%.  Given the poor regulation of a
capacitor-input filter, you can see a variation of, say, 100V
from no-load to full-load.  The volume manufacturer can test his
circuit and spec his parts to get just the right performance he
needs.  The hobbyist has a much harder time.  One is forced to 
design conservatively and sacrifice output power as a consequence.

   The choke-input filter is attractive because its regulation
is quite tight.  The disadvantage is that most designs will want
a second stage of filtering, and this means another heavy, bulky
filter inductor.  I suspect it is the cost of two chokes that
has made the choke-input filter less popular.  I am also wondering
if the filter bounce problem has something to do with it.

   Another advantage of the choke-input filter is that the
rectifiers conduct throughout 100% of the AC cycle.  This lowers the 
peak current that passes in the rectifiers and the transformer
secondary.  Consequently, the rectifiers and transformer run
cooler and more reliably, and more power is available from the
same sized power supply.  The huge capacitor-input filters that 
are popular in audiophile amplifiers put a tremendous strain
on rectifiers because the conduction duty cycle of the diodes is
low and peak currents are extremely high relative to the RMS
current.

-Henry