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Old 5th June 2008, 09:45 AM   #501
AndrewT is offline AndrewT  Scotland
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Hi,
it does both.
the extra R creates an RC filter that removes some of the ripple on the first cap.
The rectifier sees one cap to charge and a resistor in series with another cap. The rectifier thus sees a smaller load and supplies less current.

But,
the first cap suffers more ripple current than when the R is omitted. It must be selected to be able to survive this demanding position.

The last set of caps (after the R) supplies almost all of the transient current demand of the following stages. These will determine the sound quality and bass/mid performance. Size them accordingly.
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Old 5th June 2008, 11:36 AM   #502
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Quote:
Originally posted by AndrewT
Hi,
it does both.
the extra R creates an RC filter that removes some of the ripple on the first cap.
The rectifier sees one cap to charge and a resistor in series with another cap. The rectifier thus sees a smaller load and supplies less current.

But,
the first cap suffers more ripple current than when the R is omitted. It must be selected to be able to survive this demanding position.

The last set of caps (after the R) supplies almost all of the transient current demand of the following stages. These will determine the sound quality and bass/mid performance. Size them accordingly.

Andrew if i remove the R, i dont will loose power that are disipated in this R. You think that is better to use that R? I was thinking about use an Soft Start or something like that instead that R.
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Old 5th June 2008, 11:53 AM   #503
AndrewT is offline AndrewT  Scotland
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soft start is a different animal.
Soft start is to limit the start up current of a large transformer.
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Old 24th September 2008, 11:07 AM   #504
sasmit is offline sasmit  India
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can we build a dual regulated supply using a center tapped transformer.. and a single bridge rectifier...using LM338 ? It seems possible ...can anyone give a valid reason or something I am missing out , both the secondaries are anyways going to be connected
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Old 24th September 2008, 01:48 PM   #505
AndrewT is offline AndrewT  Scotland
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If you use a +ve regulator along with a -ve regulator you can use either a a Dual secondary or a centre tapped secondary.

If you intend using two +ve regulators for a dual polarity supply then you must use a dual secondary along with dual bridge rectifiers.
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Old 24th September 2008, 02:22 PM   #506
Ted205 is offline Ted205  United Kingdom
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how about a tracking preregulator added into the mix:
Click the image to open in full size.
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Old 24th September 2008, 02:36 PM   #507
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by Ted205
how about a tracking preregulator added into the mix:

if reg1 needs diode protection then why does reg2 not require it?
Remember to install reg2 the right way round.

Don't let the charging pulses through C1 contaminate the clean ground of reg1.

C2 makes a big difference to the performance of reg1. Is 10uF big enough?
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Old 24th September 2008, 04:19 PM   #508
Ted205 is offline Ted205  United Kingdom
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c2 - 10uf is plucked from the datasheet.

confused by
"Don't let the charging pulses through C1 contaminate the clean ground of reg1."
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Old 24th September 2008, 06:10 PM   #509
AndrewT is offline AndrewT  Scotland
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c2 can benefit from being ten to 20 times larger.

The pulses charging the smoothing cap are connected to the clean ground and will pulse this ground giving a pulsing output from the regulator which is referenced to that now contaminated clean ground.
The smoothing cap must be connected to the output not strung though the clean ground.
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Old 24th September 2008, 06:33 PM   #510
paulb is offline paulb  Canada
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Quote:
Originally posted by AndrewT
if reg1 needs diode protection then why does reg2 not require it?
The top protection diode is because of the output capacitor, guarding against current coming back through the reg when the input is shorted. Maybe it should go back to the input.
The other is for the capacitor on the adj terminal.
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