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Old 4th January 2011, 12:00 AM   #1
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Default Power supply question

Hello folks,

This is my first post, although I have been slowly researching and building an EL34-based PP hi-fi amp for a couple of years now (I think the end is near).

My immediate questions have to do with the term "low impedance power supply", which I have heard many times, but don't really understand. So here goes:

1. What exactly does it imply (in newbie terms please)?

2. Why is it supposed to be better?

3. How is it achieved (bigger/smaller caps, bigger/smaller choke, bigger/smaller decoupling resistors, etc.)?

BTW, my current PS design is just your basic SS rectified, C-L-C-R-C-R-C.

Thanks in advance for your help.

Dave
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Old 4th January 2011, 01:16 AM   #2
ArtG is offline ArtG  United States
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Dave Gillespie published an article in audioXpress that covers your question quite well. The examples that he shows, while suggesting a solution to another problem, should provide a good, clear answer to your question. Fortunately, this is available online. Go to the link below and download the .pdf: Gillespie2544.pdf


audioXpress - Articles and Addenda
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Old 4th January 2011, 04:33 AM   #3
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Quote:
Originally Posted by EL34Dave View Post

1. What exactly does it imply (in newbie terms please)?
Basically it means that it can pump out lots of current easily, which put another way means that the voltage won't sag much when the current demand goes up. The above implies a low resistance (in DC terms) or a low impedance (in AC terms at various freqs). If you think about it, the 120V wall socket has low impedance as it can source up to 15 or 20 amps until the breaker trips. The service wires connected to your house have even lower impedance since they can source boatloads of current from the giant transformer down the street.

Quote:
Originally Posted by EL34Dave View Post


2. Why is it supposed to be better?
If your PS had zero impedance then you could pull any amount of current out of it and the voltage wouldn't sag at all. It would be "infinitely stiff". For transients like drum whacks, etc you want the power supply to be able to instantaneously supply the current at your B+ voltage. If it has high impedance, it's harder for the amp circuitry to pull current from it, resulting in the voltage sagging.

Quote:
Originally Posted by EL34Dave View Post

3. How is it achieved (bigger/smaller caps, bigger/smaller choke, bigger/smaller decoupling resistors, etc.)?

BTW, my current PS design is just your basic SS rectified, C-L-C-R-C-R-C.
Using higher current rated chokes and transformers will lower the impedance, as will having less series resistance in the PS. The downsides to this are that less series R means more ripple voltage (possibly manifesting itself as hum), and higher voltage than your target B+ with lightly loaded transformers. Low ESR rated caps are also a good idea.

In general, SS rectified supplies have lower impedance, since tubes have higher forward voltage drop than SS diodes, and tubes also require a minimum transformer impedance to prevent arc-over on start-up. Tubes are also limited in the size of the first cap that they can tolerate.

Using a large value, low ESR final cap will also help stiffen the supply for transients that are of short duration, since most of the time the amp circuitry is pulling current from that last cap unless long duration transients are encountered, where the components upstream of the final cap must be able to also provide the current.

So, as with many things, it's a trade off between impedance and smooth filtered DC.

Consider downloading Duncan Amps free power supply designer II (PSUDII) and modeling your supply; it's very easy to use. Change the load to constant current from resistive, then apply a step change to the current after a few seconds to determine the voltage sag.

PSUD2

Last edited by boywonder; 4th January 2011 at 04:43 AM.
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Old 4th January 2011, 01:47 PM   #4
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my thinking is that it has less to do with a limitless current draw and rather a low insertion loss current draw, what ever that draw is.

it means using high quality inductors with low DCR (tipically higher guage) and high quality caps with low esr (tipically film or if electrolytic, bypassed with film) in the power suppy vs using inductors with high DCR and caps with high esr.

If you use tube rectification, it introduces limits on the amount of capacitance.

if you don't use a low impedeance power supply, you can mitigate by using caps along the power supply path inbetween componets that have high dcr or esr.
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Old 4th January 2011, 10:28 PM   #5
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Thanks for your help, guys.

Dave
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Old 5th January 2011, 02:25 AM   #6
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To be clear only the first cap after the rectifier is limited in a tube rectified supply. The later caps that supply the bulk of the current can be quite large.
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