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In Reply to: Re: My super cheap semi-diy PC desktop system:) posted by Ugly on February 8, 2007 at 14:56:55:
What size cap and inductor would you reccomend if i descide to go with a Pi filter?? I have alot of various capacitors laying around so it may be less expensive then a buildng some sort of LDO regulator circuit.
Follow Ups:
I am so far removed from my engineering math skills I won't even try to go there now. However IMO this is one design where it will be very easy to go overboard with good success. The ouput cap should be as big as bloody well possible to keep ripple as low as you can. Inductor size depends on power draw but I'm thinking 10mH should get you in the ballpark but bigger will always be better here. The input cap will likely be the power supply output filter cap which should be fine.Did I say that output cap size being big here will be important? because it will.
Hmm im think ive already done this actually if the the first cap is the output cap in the power supply. I have a choke inline with the 12v line and a 10000uf + .1uf from v+ to ground right before the amp. The only other thing i was thinking of adding is an rc network from +v to ground maybe a 1ohm in series with a .47uf cap from +v to ground to lower the indctance of the big cap a bit. I think building a voltage regulator may beyond my skill level. My only real expereince with regulators is the simple 78xx/79xx and 317/337 types.
10KuF isn't going to support much of an inductor at these power levels but then again you are presumably really only worried about fairly high freq stuff here so maybe a smaller inductor is in order. 100uH ish? In general the more inductance you add the worse the regulation will get so there is definitely a negative downside to this. However this exact negative effect can be overcome to an extent by bulking up on the amt bypass capacitance. It's a bit of a game to land everything right where you want it. It really is nice to have an oscilloscope to do this kind of development. Do you have access to one?Bypass the amps using normal good bypassing technique, ie short leads on all caps jammed right up against the power pins of the amp as close as is humanly possible and higher SRF (self resonant frequency) like a ceramic or polypropylene cap gets higher priority for the really tight to the power pin positions compared to lower SRF parts like the big aluminum electrolytic which can be farther away. BTW after the amp has been properly bypassed you may find no sonic benefit to adding the L in the pi or tyhe R of the RC. The better you can lay all the parts out in theory the more this is true.
In my opinion filtering is a bandaid and cascading filters would only make the downsides worse. But in the end you may want to cascade a pi with an RC to see how that sounds but again I'm betting that with the right pi or RC there is no need for rc after the pi or visa versa since in essence you are lowering the load/line regulation each filter stage you add.
BUT
You might want to try comparing the sonic benefits of an RC filter to the pi filter individually. The RC may actually end up the more reasonable design for this app!?!?! I've never designed for this exact problem so everything I've said so far is a big guess.
Hey thanks for all the help, I'm not sure you understood what I meant about the RC network. This would be a 1ohm resister in series with a .47uf capacitor, this is placed from V+ to ground after the large capacitor to help bring down the inductance, I got this idea from tnt audio Ill give you the link. I'm not sure if it will help with the type of power supply im using, but its worth a shot. I'm still getting a sort of high pitched buzzing type sound from the speakers when no music is playing and I put my ear close to the speaker. Its quite low and you don't hear it when the music is playing, but id like to get rid of it if I can. It doesn't get louder when I turn the volume up it always stays the same level???
I have a bunch of parts laying around not being used that i can experiment with. I have a few more of the 10000uf/63v BHX Aerovox capacitors that I used, I can try adding another for a total of 20000uf and see how it works out, I'm not to sure the PC power supply will like that on startup though, but its worth a shot. I also have a bag of capacitors with lots of smaller Panasonic FC, Cerafine and Oscons. I could try building a bank of smaller capacitors instead of one or two 10000uf. Ive found that a few smaller caps paralleled sounds better then a single large cap, so its worth a shot. Im really doing this mostly for the learning experience and fun, I wasn't even expecting it to sound as good as it did:) Thanks alot for your help.
Frank
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