How much (capacitance) is too much? - diyAudio
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Old 31st March 2011, 10:15 PM   #1
dobias is offline dobias  United States
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Default How much (capacitance) is too much?

How much capacitance is proper?
I'm a rank amateur to electronics. I'm collecting all of the parts to assemble an SET amp with a separate PS. I'm using an LC filter after the rectifier tubes & from there providing separate LC's to each channel (a la Lundahl).
I've read everything I can find about sizing of the LC components & have wound up with two ranges of cap sizes. The RCA handbook has led me to use a 4uf cap after the first choke of 4.5H & 25uf caps after the two channel's separate 4.5H chokes. The power output is 350vdc & 250mA.
The other indicated sizing is for 110uf after the first choke & 230uf after the other two. The attitude with these higher caps is "all you can afford".
I'm sure that too much capacitance must have bad effects such as ringing.
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Old 31st March 2011, 10:27 PM   #2
DF96 is offline DF96  England
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If the first (reservoir) capacitor is too large then the charging pulses become large and narrow. The result can be buzz due to induction and ground currents, and excessive transformer heating. A valve rectifier may have a maximum cap specified in the data sheet.

After that the smoothing caps do not matter too much. One proviso: the caps can resonate with the smoothing choke to create subsonic ringing.

Note that old designs use small caps and big chokes, because that was the best fit with the technology available in the 1950/60s. Modern practice uses bigger caps and smaller chokes, as big caps are now available in reasonable sizes and prices.
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Old 31st March 2011, 10:36 PM   #3
dobias is offline dobias  United States
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Default How much (capacitance) is too much?

Thanks for your response.
Yes, the info for the 5R4GYA rectifiers shows 4uf max for the first cap, but that is for a CLC filter. I think I used the RCA handbook's instructions & charts correctly for sizing the 4uf & 25uf caps after the chokes. Yes, that was the old RCA. Anything new online scoffs at LC filters except for Lundahl where I gather that "more is better". I already had the three 4.5H chokes (the one after the rectifier is huge) & that's what I wanted to include.

Last edited by dobias; 31st March 2011 at 10:38 PM.
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Old 31st March 2011, 10:57 PM   #4
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LC filters in linear power supplies are kinda rare these days. Any reason why you don't just use the CLC filter suggested by the SR4GYA tube data? Anyway, I believe the capacitance formula is C = 1/2pi1400fc where C is farads and fc is your cutoff freq (60 or 120?). The 1400 is your load resistance.
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Old 31st March 2011, 11:08 PM   #5
DF96 is offline DF96  England
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The cutoff frequency needs to be much lower than the line frequency, otherwise you get very little smoothing. In addition, that formula applies to an RC filter not CLC.
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Old 1st April 2011, 12:38 AM   #6
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Quote:
Originally Posted by dobias View Post
Thanks for your response.
the old RCA. Anything new online scoffs at LC filters

Nothing wrong with LC filters if the load is fairly constant as an SET amplifier would be. Industrial 3 phase power supplies mainly use LC or just L because they give good power factor and transformer utilisation as well as working well with phase controlled rectifiers. Consider a voltage regulator as power supply rejection is very poor with single ended designs
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Old 1st April 2011, 01:00 AM   #7
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The cutoff freq needs to be way lower than line freq? Huh?

Ahh... set fc an octave below or so for lowpass filter. OK.
But he's not using CLC. The formula I gave was for LC.

Last edited by sofaspud; 1st April 2011 at 01:06 AM. Reason: clarification
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Old 1st April 2011, 02:43 AM   #8
6L6 is offline 6L6  United States
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In a CLC filter with a tube rectifier, the first cap can be too big -- use the suggested value in the tube databook, and use a 630v (or better) film cap. They seem to be very durable in that position.
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Old 1st April 2011, 04:05 AM   #9
dobias is offline dobias  United States
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Everything I've read has convinced me to go with choke input. The advance of cheap capacitors has made the CLC filter the popular way to go. The expense of the larger chokes in LC filters make them unpopular.
Unless I can get verification that larger caps are beneficial, I'll have to stay with the modest sizing I worked out using the old RCA handbooks. Even the formulas found on the internet lean heavily towards the more popular CLC & ignore the LC types. Lundahl Transformers appears to have decided that the LCLC is superior.
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Old 1st April 2011, 01:32 PM   #10
DF96 is offline DF96  England
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Quote:
Originally Posted by sofaspud
The formula I gave was for LC.
Are you sure? L does not appear in it. The formula you gave was for a CR filter.

Quote:
Originally Posted by dobias
Everything I've read has convinced me to go with choke input.
With choke input the problems with peak charging current created by a big first cap no longer apply. Bigger caps are then better, from a ripple point of view. Subsonic resonance still needs thinking about.
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