Which pre & post regulator decoupling caps? - diyAudio
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Old 28th March 2010, 06:16 PM   #1
nad is offline nad  United Kingdom
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Default Which pre & post regulator decoupling caps?

Hello folks,


Would anyone wish to answer my question? Which decoupling capacitors do you recommend fro pre and post regulators?

I've read that Panasonic FC (pre) and Rubycon ZA (post) are the best to go for.

Anyone agree/disagree or offer an alternative?




Thanks.
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Old 28th March 2010, 10:48 PM   #2
nad is offline nad  United Kingdom
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C'mon folks,

I know it's not homebrew cabinets or cloned amps, but surely someone out there can post their opinion on this? There seems to be a hell of alot of questions/threads going without a single reply here, what's up with everyone? It never used to be this way.
Seems folk can't be bothered to reply to little questions anymore, it used to be informative forum but now it's just a case of 'will anyone reply?'

Anyhow, thanks.
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Old 29th March 2010, 12:34 AM   #3
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I have seen it posted several times that, at least before the regulators, the circuits prefer 'slower' caps. I suppose other parts of the circuit come into play, but there are technical explanations that are on here if you search hard enough.

I did not have great luck using Panasonic FMs before a set of regulators in my preamp. Sounded tinny to me.

One commercial example would be my 20 year old Harman/Kardon CD player. When I popped the top on it, the original caps were standard, 85C Matsushita (Panasonic) before the regs and finer, audio-grade caps were used in later parts of the circuitry.

Search threads for LM317, etc. to find different implementations and caps/sizes used.
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Old 29th March 2010, 01:04 AM   #4
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Quote:
Originally Posted by nad View Post
C'mon folks,
Seems folk can't be bothered to reply to little questions anymore, it used to be informative forum but now it's just a case of 'will anyone reply?'
So you have waited a whooping 4 1/2 hours? Good grief!

Right after the rectifiers is wise to use caps with "non so small" ESR. After the regulators, on the board is where low ESR counts and needs to be.
And more important, stay with the maximum voltage of the capacitors close to the voltage on the board, don't go higher just "to be safe" - ERS goes down with the maximum voltage.

Last edited by SoNic_real_one; 29th March 2010 at 01:07 AM.
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Old 29th March 2010, 02:50 AM   #5
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Quote:
Originally Posted by SoNic_real_one View Post
And more important, stay with the maximum voltage of the capacitors close to the voltage on the board, don't go higher just "to be safe" - ERS goes down with the maximum voltage.
I think folks have found the opposite to be true, i.e. that a somewhat higher value cap provides a better resulting power supply.
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Old 29th March 2010, 03:14 AM   #6
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It totally depends on what regulator IC's you are using.
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Old 29th March 2010, 03:18 PM   #7
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Only way to tell is to measure the output impedance of the regulator with various caps.
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Old 29th March 2010, 06:44 PM   #8
Gopher is offline Gopher  United Kingdom
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It all depends - my advice is forget boutique caps after the reg. Use medium esr types and draw 300ma + standing current from the reg (for the LM317 at least). I'll tell you why later if you really want to know. The other important stuff is to precede the reg with a really low cutoff (i.e. a few Hertz) RC filter using an ultra-low ESR cap and to use a high value ultra-low low ESR cap (200-1000uF) for Cadj. The other things you see like using external Vrefs to replace the reg's internal one amount to burnishing the turd for 3 pin regs. You may as well move to Jung types at that stage.

Personally I wouldn't use 3 pin regs for anything except digital circuits and CD servo duties. Critical audio amplification and DAC circuits need Jung type regs.

Last edited by Gopher; 29th March 2010 at 07:03 PM.
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Old 30th March 2010, 01:19 PM   #9
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For the Jung-Didden Super Regulators a ceramic or film bypass across C5 may lead to oscillation. These gentlemen measured the regulator impedance down to micro-Ohms. You'll find that careful layout, star grounding, no daisy chaining of supply rails will work more to your advantage than boutique capacitors.
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Old 30th March 2010, 02:42 PM   #10
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Quote:
Originally Posted by Gopher View Post
It all depends - my advice is forget boutique caps after the reg. Use medium esr types and draw 300ma + standing current from the reg (for the LM317 at least). I'll tell you why later if you really want to know.
Its no secret - the output impedance graphs in the LM317 datasheet are shown for 500mA output current. Take a peek at the LM317L datasheet to see what happens at 40mA- not a pretty sight.

Quote:
The other important stuff is to precede the reg with a really low cutoff (i.e. a few Hertz) RC filter using an ultra-low ESR cap
Impractical and unnecessarily expensive. To get to a few Hz with 300mA+ running through you'd end up with of the order of a 10,000uF cap at this point - such large values don't come at all cheap in ultra-low ESR. By all means use a nice big cap but just a standard one and parallel it with something that's still going to give a low impedance above 100kHz - like a high enough voltage high value SMT ceramic.

Quote:
... and to use a high value ultra-low low ESR cap (200-1000uF) for Cadj.
What's the point of an ultra low ESR cap here? I can't see how its going to give bang for the buck in the reference leg where the source impedance its seeing is at least 100R. Low ESR caps are best suited to low impedance circuits.

Quote:
The other things you see like using external Vrefs to replace the reg's internal one amount to burnishing the turd for 3 pin regs. You may as well move to Jung types at that stage.
That stage I'd say comes before the time you start shelling out for ultra-low ESR caps of thousands of microfarads at high voltages
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