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Old 25th August 2014, 09:30 AM   #1
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Default Capacitor size after zero-feedback PS

I am kindly asking for advice in determining the optimal capacitor size after the power supply regulator.
I have built a 12V 2A external power supply based on Superteddyreg. Before the regulator, I have 2 x 6800uF bypassed with 0,1uF ceramic + 22nF film capacitor so this side of the regulator is well fed. At the output of the regulator there is 10uF capacitor bypassed with 0,1uF ceramic capacitor (part of Superteddy design). The power supply connects to DAC via a 1m unshielded cable. As an additional info, Superteddyreg has no VFB or CFB, so it is essentially a voltage follower.
My questions are:
1. Should I put additional capacitor, or a bank of capacitors, after the regulator and, if yes, what would be the appropriate size in this configuration?
2. Also, if 1. is yes, should the additional caps and bypassing be put inside DAC, rather than 1m away from it?
3. Finally, does each 6800uF capacitor before regulator requires bypassing, or is one pair of 0,1uF+22nF enough for both as they are connected in parallel?

Thank you in advance.

Last edited by Willi Studer; 25th August 2014 at 09:34 AM.
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Old 25th August 2014, 09:34 AM   #2
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Quote:
Originally Posted by Willi Studer View Post
I am kindly asking for advice in determining the optimal capacitor size after the power supply regulator.
I have built a 12V 2A external power supply based on Superteddyreg. Before the regulator, I have 2 x 6800uF bypassed with 0,1uF ceramic + 22nF film capacitor so this side of the regulator is well fed. At the output of the regulator there is 10uF capacitor bypassed with 0,1uF ceramic capacitor (part of Superteddy design). The power supply connects to DAC via a 1m unshielded cable.
My questions are:
1. Should I put additional capacitor, or a bank of capacitors, after the regulator and, if yes, what would be the appropriate size in this configuration?
2. Also, if 1. is yes, should the additional caps and bypassing be put inside DAC, rather than 1m away from it?
As an additional info, Superteddyreg has no VFB or CFB, so it is essentially a voltage follower.
3. Finally, does each 6800uF capacitor before regulator requires bypassing, or is one pair of 0,1uF+22nF enough for both as they are connected in parallel?

Thank you in advance.
Not familiar with the superteddy, and the required capacitance would depend on the output impedance of the reg. Do you have a schematic?

Anyway, additional capacitance is most effective where you want the clean supply - and that's as close to the DAC cicuitry as you can get it.

Personally I don't see any need for bypassing before the reg, although before long you will get lots of posts of people telling you that a xyz bypass before the reg opens up the sound stage soooo wide

Jan
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Old 25th August 2014, 09:37 AM   #3
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Hello Jan and thanks for your response. I am enclosing the STR schematics. To answer to your question, it does not have a very low output impedance.
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Old 25th August 2014, 10:42 AM   #4
AndrewT is offline AndrewT  Scotland
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Look at the decoupling fitted on any modern digital PCB.
There are dozens of small electrolytics and just as many smd X7R
In addition there are snubbers (R+C) on every PSU.

It's the decoupling that is important when signals change currents quickly. (the bits turning off and on in ns, or maybe ps, in very fast stages).
The PSU is usually too far away to significantly influence the fast transients.
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Old 25th August 2014, 11:05 AM   #5
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AndrewT, thank you for your response. At this stage I wouldn't tamper with decoupling inside DAC and would like to focus on power supply delivering clean DC. Superteddyreg is already very quiet, all I need is to figure out the size and the placement of capacitors after the regulator, having in mind that there is already 10uF at the regulator output, that the designed current draw is 2A and that the DAC unit is 1m away from the external PSU. Thanks again.
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Old 25th August 2014, 11:34 AM   #6
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As Andrew implied, extra cap after the reg doesn't do much for a DAC, especially with 1 meter (gasp!) of umbilical.
I assume that the 10uF there is for stability (yes emitter followers can also oscillate), so that's taken care of.

Jan
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Old 25th August 2014, 12:07 PM   #7
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But there is also the high output impedance of the power supply in play. Should I then omit any large cap at the output of regulator, take the regulated DC straight via umbilical cord to DAC and place a cap there? If so, which value would you suggest? Sorry if I keep reiterating questions.
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Old 25th August 2014, 12:14 PM   #8
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Quote:
Originally Posted by jan.didden View Post
As Andrew implied, extra cap after the reg doesn't do much for a DAC, especially with 1 meter (gasp!) of umbilical.
I assume that the 10uF there is for stability (yes emitter followers can also oscillate), so that's taken care of.

Jan
Very important note. You should also not use too much capacitance because of this, you can damp the circuit too. Emitter followers have a range of acceptable capacitance on the output that is usually quite narrow. OP, are you getting a lot of ripple or voltage sag under load?
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Old 25th August 2014, 12:17 PM   #9
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To be honest, I haven't measured the ripple as I have relied on STR's own specs which mention sub-mV ripple under full 2A load.
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Old 25th August 2014, 12:41 PM   #10
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Quote:
Originally Posted by Willi Studer View Post
But there is also the high output impedance of the power supply in play. Should I then omit any large cap at the output of regulator, take the regulated DC straight via umbilical cord to DAC and place a cap there? If so, which value would you suggest? Sorry if I keep reiterating questions.
Do you have any figures for that Zout?

Jan
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