Quieting a Switching PSU? or easier to replace it with a linear PSU? - Page 2 - diyAudio
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Old 27th October 2009, 11:41 PM   #11
Pano is offline Pano  United States
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Jack, what do you make of the LM317 output impedance and ripple rejection plots?
Effective enough for SMPS noise? It's not great at higher audio frequencies, but may be good enough. 30-40dB clean up is nothing to sneeze at.
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Old 28th October 2009, 02:16 PM   #12
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Quote:
Originally Posted by panomaniac View Post
Jack, what do you make of the LM317 output impedance and ripple rejection plots?
Effective enough for SMPS noise? It's not great at higher audio frequencies, but may be good enough. 30-40dB clean up is nothing to sneeze at.
I go with the work that WJ and Jan did for Audio Amateur in 1995 --

How about the Wenzel "clean-up" circuit?
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Old 28th October 2009, 03:37 PM   #13
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The HF noise from SMPS can pass right through 3-pin linear regs, which even a little RC in front can help prevent. Here's a little example I did some time:

Using 3-pin regulators off-piste: part 2
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Old 28th October 2009, 11:48 PM   #14
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Very nice Martin! Just what I wanted to see. =)

Jack, what's the Wenzel circuit?
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Old 29th October 2009, 04:57 AM   #15
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here's a link to the wenzel circuit... looks like a slightly more complex adaptation of a gyrator circuit (like in Martin's article) to me:
Finesse Voltage Regulator Noise!

also, for other's benefit, a link about gyrator circuit (a possible substitute for a large inductor):
Gyrator - Wikipedia, the free encyclopedia
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Old 29th October 2009, 07:23 AM   #16
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Hey guys,
so, after looking into this for a bit, I've come up with a draft plan.
I am going to do a combination of things cause i'm trying not only to clean up the power, but hoping to get from +/-12v rails to +/-18v to get more headroom for the opa2134 opamps.

I've attached a schematic of the first phase, which is an excerpt from the lm 117 datasheet.

First is a 'lower noise' adjustable voltage boost/regulator with an LM350 or 338 for the positive rail because the psu is rated at 3.5amps on the positive rail. I'll also build a parallel circuit out of the negative counter part (the lm 337), which is only 0.5 amp. The datasheet recommends using solid tantalum caps, so i'll probably stick with that. Only change I'll make is to take Martin's advice to put a small choke before the regulator (probably this 10h from mouser 542-PM124SH-100M-RC) and maybe after?

So, those two will boost my +/-12v rails to +/-18v (should be about that if I set the pot to around 3.3k). Not sure which is the best kind of pot to use for this application though, if you have any thoughts.

Then, I intend to put some sort of low pass filter (around 75 hz) - a pi filter on each line cap-choke-cap, with 2200uf panasonic FM or FC caps and 2.2mh chokes would work, but those chokes are a touch pricey and hard to get given the amperage i'm looking for. Alternately, I could use the gyrator circuit suggested by Martin - the Wenzel one is just a bit involved for me. Another alternate is swiping some chokes from some dead mother boards or old power supplies. : )

Martin (or anyone), could you recommend - for the gyrator circuit - a different NPN than the BC547B for higher amperage? ie 3.5amp or so. Also, would a gyrator circuit for the negative voltage rail look the same or be with a different transistor? I didn't do the frequency calculations (cause i don't know how) but I assume your values would be fine for my purpose: 10k resistor, 3.3f cap (what kind of cap do you suggest?)

Thank you in advance for any thoughts,
Adam
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Old 29th October 2009, 12:51 PM   #17
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If you are going to make your own switcher from scratch -- consider using the Linear Tech low noise devices -- these alter the slew rate which drops the noise by many 10's of dB's -- you trade off slew rate for a bit of heating.
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Old 29th October 2009, 12:58 PM   #18
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AT77 - what i call a 'gyrator' isn't really: its simply an RC filter with a pass transistor (emitter-follower) added

Scaling the idea up or higher currents is easy enough - use a darlington, Sziklai pair or FET for the pass element. Here's a good article by Rod Elliott that outlines some of the considerations:

Capacitance Multiplier Power Supply Filter
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Old 29th October 2009, 04:41 PM   #19
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Hey Martin,
his final design looks cool and to be a fairly easy build - nice. I couldn't gather anything about voltage drop in the circuit as it is (besides changing the value of the 12k resistor). I assume there is less than 0.5v drop if any. Just concerned because my application of it is post regulator for opamps, not in a non-regulated ps for a power amp. Also considering using the same circuit for the 3.3v and 5v rails (digital and logic power).

Aside from that, how about doing your simple circuit but using a KSD1691 for the positive rail and KSB1151 for the negative rail? or TIP110/TIP115?
And perhaps, for greater capacitance, changing the values of the resistor and cap.
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Last edited by at77; 29th October 2009 at 04:48 PM.
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Old 5th March 2010, 03:46 PM   #20
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Ok, this probably has a simple explanation but I can't see it! I'm trying to get about 200v dc for a valve preamp. I have 2 small transformers back to back giving me 177vac. This goes accross a diode rectifier bridge and should give me about 238vdc (177x1.4). So why do I end up with 160vdc. I've never seen this before!
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