Why are they building -15v regs - diyAudio
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Old 7th December 2008, 09:27 PM   #1
RCruz is offline RCruz  Switzerland
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Default Why are they building -15v regs

Hi

I am studying the layout for a +-15v PSU for my new pre amp.

Untill now, I needed to use dedicated +15 and -15v regs like 317 / 337 with the known shortcomings.

Following this article: http://www.pedjarogic.com/gc/supplies.htm I understand that a simple topology adjustment enables me to use a positive reg for the negative rail. Quote"Negative regulators with necessary current ratings are neither easy to find nor are cheap these days, but if each secondary has its own rectifier, the positive regulators can be used both for positive and negative rails."

As I am not at ease regarding these matters, I need your advise on the possible shortcomings of such a design.

Why is it not used more frequently ?

Do the (-) regs shortcomings justify the savings by using only one rectifier bridge ?

Regards

Ricardo
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Old 8th December 2008, 10:43 PM   #2
dtproff is offline dtproff  United States
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OK... I'll way in on this. Please take what I say with a grain of salt and remember all advice is free so you get what you pay for...

In general, you can adjust your topology to drive from a single rail. The problem comes when you need to operate at either extreme of the amplifier limit. Meaning anytime you approach ground or the Vcc rail. Remember, all circuits need headroom to operate near the rails. That is what makes +/- voltages so easy to work with. Your signals are alrady working around the midpoint (GND) so everthing stays nice and semetric.

If I am wringing the last few pennies from a design, the negative rail is the last thing I drop because biasing everything to the correct point and making sure everything has the proper headroom can be a pain. Besides that, the parts that operate well near the rails are relatively expensive (rail-rail op amps) as opposed to using generic LM339/LM393 type parts.

I hope that helps...

Tony
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Old 9th December 2008, 05:46 PM   #3
Eva is offline Eva  Spain
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This schematic uses LM338 which is a 5A positive floating regulator. It's true that 5A negative regulators are hard to find, but do you really need that much current? For a preamplifier?

Consider the LM317/LM337 pair, which is rated at 1.5A and widely available.

Only special regulators, like high current, high voltage, micropower or low dropout ones, are hard to find in its negative version.
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Old 9th December 2008, 07:42 PM   #4
RCruz is offline RCruz  Switzerland
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Thank you Eva

Apart from the benefits of using two bridges, if I build a +-15v psu using two LM317 , should I expect any improvements regarding LM317 / 337 setup ?

Have positive regs better specs than negative ones ?

I sometimes experience instabilities while using 337 regs.

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Ricardo
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Old 9th December 2008, 08:03 PM   #5
RCruz is offline RCruz  Switzerland
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Quote:
Originally posted by dtproff
That is what makes +/- voltages so easy to work with. Your signals are alrady working around the midpoint (GND) so everthing stays nice and semetric.

Thank you Tony

I agree that I must use two rails, I also know that two bridges are better than one but I wonder if LM317 equals LM337 in performance ?

Regards

Ricardo
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Old 9th December 2008, 08:14 PM   #6
dtproff is offline dtproff  United States
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Eva is absolutely correct. 1.5A should be fine, and they will work as well.

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Old 9th December 2008, 08:16 PM   #7
dtproff is offline dtproff  United States
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I would like to apologize for my confusion on the negative rail issue. Thanks for the correction Eva. For whatever reason I was thinking he wanted to get rid of the negative rail completely.

Somedays it really sucks to get old AND feable minded.

Tony
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Old 9th December 2008, 09:48 PM   #8
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Somewhere in Walt Jung's articles in AudioXpress magazine. He writes the the LM317 is better than than the LM337 and suggests going the dual bridge route.

It's in one of the Walt Jung pdf's.

http://www.audioxpress.com/magsdirx/...enda/index.htm
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Old 10th December 2008, 12:07 PM   #9
AndrewT is offline AndrewT  Scotland
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Hi,
even those that design and build discrete regulators sometimes use two positive regulators rather than design a + & - pair.
The only down side to this twin positive regulator is the requirement for dual secondaries and dual bridge rectifiers.
Most will accept the extra cost because of the technical benefit accruing from +ve regulators.
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Old 10th December 2008, 12:19 PM   #10
kevinkr is offline kevinkr  United States
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Quote:
Originally posted by AndrewT
Hi,
even those that design and build discrete regulators sometimes use two positive regulators rather than design a + & - pair.
The only down side to this twin positive regulator is the requirement for dual secondaries and dual bridge rectifiers.
Most will accept the extra cost because of the technical benefit accruing from +ve regulators.

Exactly what I do with my bi-polar regulators, and for diy it has the additional benefit of not requiring the debug of two new and potentially unstable designs. (Mine are a mix of discrete and op-amps somewhat along the line of the Jung/Sulzer super regulators.)
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