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Old 12th September 2006, 03:09 PM   #11
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Default Old uA7805 v. "newer" LM317

leadbelly: " ... Those older fixed regulators are noisier because they use a cheaper voltage reference. Why do you think you see so much use of the LM317 on this forum? ..."

I am sure you are right on here ... the uA78xx and uA79xx series of fixed voltage 3-pin chip regulators have been around for decades. In fact they were originally a "flagship" product of the original Fairchild company in the late 1970's, possibly earlier.

Modern iterations of the -78xx/-79xx series from TI, National and others do have a "noisey" voltage reference, only good out to a couple of decimal places. But that is the reason I usually use a tantlum capacitor on the output leg = to "tame" this schrew and make it work very well for audio work.

We are talking here of a simple substitute for an existing power supply wall wart and not the ultimate heavily filtered supply of a higher performance output amplifier. The Squeeze Box 2 & 3 does have very well regulating internal filters = good enough to "tame" the wall wart for most consumers. When the Squeeze's internal filters are coupled to a 78xx based linear supply (with a tantlum snubbing cap in place), the Squeeze Box power bus will be very well mannered indeed = with a PS noise floor well below 100 db in virtually every situation. (The Squeezer is a "pure" digital device after all = no need to get too picky here = we only need to be helpful to the DAC's downstream.)

Granted the LM317 from National (and TI and others) is an almost perfect solution for this type of job and may be nominally as good as or possibly a little better than the -78xx, the truth is that these two options are so close in specs and results ... as to be splitting hairs very, very finely in this case.

I have used both and agree with your recommendations but I know that either would do the job very nicely. Truth is one, paths are many ... I say, pick 'em ... the end results will be so similar as to indistingishable, the -78xx circuit being the less complicated choice.

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Old 12th September 2006, 05:45 PM   #12
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Quote:
Originally posted by mrjam


From ST Micro datasheets:
1) LM317 - noise .003% Vout - BW 10Hz-10kHz
2) 78M05 - noise 40uV - BW 10Hz-100kHz

So the output noise with 5V output is:
1) 1.5uV/sqrHz
2) 126nV/sqrHz
Apples and oranges.
1) "M" = 0.5A
2) Is he going to buy an ST Micro brand 78xx or some cheap noisy clone? (What are the odds?)
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Old 12th September 2006, 06:21 PM   #13
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Default noisey?

mrjam: " ... The reason isn't noise ..."

wine&dine: " ... 2) Is he going to buy an ST Micro brand 78xx or some cheap noisy clone? (What are the odds?) ..."

Odds are good = cheap knockoffs are too common.

BUT with the inclusion of a decent snubbing cap of the plastic or tantlum variety on the output and input legs, the differences between the LM317 and the -78xx disappear.

... and that's why God gave us osciloscopes.

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Old 13th September 2006, 12:08 AM   #14
gsuser is offline gsuser  France
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Many thanks for all these replies, not sure I understand everything... In fact, I checked the Squeezebox, and it constantly needs 1100 mA (with wifi and display on). That's why I thought about 2.5A for this power supply.
The "positive-voltage regulators" shown by FastEddy will be too difficult for me to built (first DIY work )
I think I can handle with leadbelly's link to "enhanced regulator Altus Audio" but what regulator should I use ? And what values for the other components ? I don't know what kind of transformer will be the best one (toroidal or other) Are 9v 25VA right ?
Thanks again for your help !
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Old 13th September 2006, 02:23 PM   #15
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Default what's the brick say?

So, what exactly is written on the existing wall wart (brick) ... 9 VDC ?? 5 DVC ???

What exactly is the output of the existing wall wart ?? ... 25 watts?? 15 watts?? 10 watts??

Knowing this, it is quite easy to add extra filtration to your existing wall wart without resorting to building a complete power supply from scratch ....

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Old 13th September 2006, 03:15 PM   #16
gsuser is offline gsuser  France
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Quote:
So, what exactly is written on the existing wall wart (brick)
It's a very small and light brick 5v 2A made in china
(I have a bigger one for my cell phone )
I asked for the 9v 25VA for the "enhanced regulator Altus Audio transformer".
The Squeezebox needs 1.1 A.
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Old 13th September 2006, 03:32 PM   #17
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Default Re: what's the brick say?

Quote:
Originally posted by FastEddy
Knowing this, it is quite easy to add extra filtration to your existing wall wart without resorting to building a complete power supply from scratch ....
No! Power supplies are not that simple. If you add extra capacitance, you raise the output impedance, possibly making it sound worse. If you don't understand the theory, you should be building to schematics.
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Old 13th September 2006, 03:55 PM   #18
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Default Obviously a switching supply.

Obviously a switching supply.

My favorite inline filter for this type is two inductors, one in each line (+ & -) with a 1.0 uF. Poly or MTF Capacitor (to 35 Volt rating) in between the two cheap inductors ...

-/\/\/\------------------ (+ 5)
...........=|=
---------------/\/\/\ --- (- 0)

Add line connectors to suit ...

I'll do a decent graphic if anyone is interested.

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Old 13th September 2006, 04:05 PM   #19
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Default increases impedence?

Yes, you are right, it does ... and that is exactly what you want from a power supply's output filters = increased impedence to any frequency above 1 Hertz ! === Increasing impedence to noise and hum.

Note that the use of inductors and/or capacitors does not "raise the output impedence" of the output DC, direct current/voltage.

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Old 13th September 2006, 06:16 PM   #20
gsuser is offline gsuser  France
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I really don't want to add filtration to my switched psu, the only projet for it is the garbage can ! Iwant to build a new linear one.
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