Tweakers' Asylum

Is there merit in using both together, or does it "depend"?

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Christopher Witmer

In Reply to: RE: I'll gave you some quick info posted by Russ57 on September 18, 2008 at 05:10:19

Thanks for the quick run-down. I will follow the leads that have been provided so far. A quick question, if you don't mind: Is there merit in using both caps and a choke together, or does it "depend"?

With gratitude,

-- Chris

Follow Ups:

In general, you always would - Russ57 08:35:48 09/18/08 (4)
You can use a cap input filter followed by a choke. This is called a "PI" filter and is very common. It always helps to have multiple stages of filtering. A choke (or inductor) is called "L", a resistor "R" and a cap is "C". A LCLCRC filter is nice design. A CLCRC is okay. If you have a cap input filter the worst way is to use a huge first cap and too few sections such as CRC (okay for a solid state amp perhaps).

In a nutshell, the choke input eliminates brief high current charging and gives 100% diode conduction. The downside is lower output voltage and increased cost. Some might claim it isn't as fast responding. Still it is the favorite of diehard tubeheads. Just be sure to buy a choke rated for input duty as they aren't all the same.

Russ
- Speed of chokes versus caps - Christopher Witmer 15:45:57 09/18/08 (3)
  Thanks for your very helpful reply. Another Q based on my total lack of experience in this area: you said that some people would claim that chokes are not as fast as caps, but I was under the impression that operation of chokes is considerably faster than that of caps . . . do you have any idea how I might have gotten that (wrong?) impression? Is this a controversial point or is it clear-cut? (Or perhaps it depends on what one means by "speed" in this case -- perhaps the "speed" of a single component versus the "speed" of an entire complex system)
  
  Thanks again for any light that you or anyone else might be able to shed on this.
  
  -- Chris
  - "Speed" of chokes and caps - Al Sekela 10:18:49 09/19/08 (1)
    The output section of the filter should be a capacitor, which supplies the instantaneous current demanded by the load. A choke input, if used, addresses the rectifier commutation problem I discussed above, and any chokes used in the filter attenuate supply noise. As far as the load is concerned, the characteristics of these elements determine the "speed" of the filter.
    
    Chokes and capacitors used in power supply filters all have finite bandwidths. This is because chokes have some stray, or parasitic, winding capacitance, and capacitors have some parasitic inductance, which are unavoidable consequences of how they are built. Based on the theory of L-C circuits (see link), this means chokes look like inductors from DC up to their self-resonant frequencies, and like capacitors above those frequencies. Similarly, capacitors look like capacitors from DC up to their self-resonant frequencies, and like inductors above that frequency. Because the choke inductances and filter capacitances are large, these self-resonant frequencies can be quite low: well into the audio band.
    
    A filter capacitor only looks like a reservoir (low impedance) to events that do not exceed the self-resonant frequency of the capacitor with the event bandwidth. A filter choke only looks like a barrier (high impedance) to events that do not exceed the choke self-resonant frequency with the event bandwidth. This is why power supply filters are poor obstacles for high frequency noise (another reason why the diode commutation noise issue is so important) and poor reservoirs for demanding loads.
    
    Your application (load) has some power supply rejection capability. If it is poor, then your power supply filter needs to help by having a wider reservoir bandwidth, i.e. be a "faster" circuit. Similarly, your AC mains and power transformer/rectifier setup has some noise level and bandwidth. If the noise is a problem for your application, then you need "faster" filter circuits to help attenuate the noise.
    
    Since components with higher values are "slower" as well as more expensive, designing a power supply filter is not simply a matter of throwing money at the problem by specifying the biggest components you can get. Russ57 is correct that you do need to understand the requirements of the load and to model the response.
    - Play-Hookey series on basic AC circuits (Open in New Window)
    - We could write a book:) - Russ57 11:57:39 09/19/08 (0)
      Funny how the simple stuff isn't so simple! I often find guys would like to take a short cut and just be told what to do. Sadly it doesn't work for audio. Morgan Jone's books are worth having and he impressed on me the need to always make a list.
      
      If it wasn't clear, by response speed I was talking about ringing when subjected to stepped loads.
      
      Russ
      P.S. I tend to be a little "old school" when answering such questions on a public forum. There are several new ways of thinking out there such as having a high impedance part as the last element to force local current return. A CCS is a good example. I won't touch the low-everything stuff that is still raging on tube diy:)
  - This is where you have to model - Russ57 07:12:13 09/19/08 (0)
    Everything is a mix of compromises and trade offs. I understand that you are trying to get a handle on what the parts do. That is smart. Just put at least equal attention into what you need the power supply to "do". What might be right for a class AB1 push pull amp might not be best for a SET amp or a preamp. Some circuits have good PSSR and don't require the same level of filtering.
    
    So you have to decide on the circuit first based upon what needs to be accomplished. Then you look at what that circuit needs from a power supply. Then you consider things like how much room can the power supply take up and what part of the total cost of the amp.
    
    There is no such thing as the "best" and there is no "one size fits all " approach. If I were looking for such a thing I'd go with a large henry, low DCR, choke input supply. I like something at least 10 henry and under 50 ohms. I would use a tube rectifier (damper diodes). I would stay with a strict class A1 design. I would use motor run caps (or film caps). I'd go with a LCLCRC filter. This assumes we are talking about a power amp.
    
    Russ

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