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Old 8th October 2006, 03:06 AM   #1
Poseidon's Voice
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Join Date: Jul 2004
Location: North Carolina
Default LM3875 power supply questions...

Hi fellas,

1) Has anyone experimented with different types of diodes in the power supply? I.e. is there a difference between the MUR860 vs. others like IR Hexfred and Schottky (like from the Parts Connexion) ?

2) What is the minimum current and voltage requirement for the diodes, I'm planning on using 22+22 V/400VA transformers per channel.

3) Is there an audible difference between different toroidal transformer types, like Avel Lindberg vs. Plitron vs. Antek (E-bay) ?

Sorry for the newbie questions.

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Old 8th October 2006, 05:42 AM   #2
lineup is offline lineup  Sweden
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Join Date: Dec 2005
Location: the north

for them feeling in-secure about themselves
and those who are not knowing too much about
audio reality - like noo-bies / new-bies

such can feel a little better if using all kinds of re-inforcements
and audio-phile tricks in a power chip amp
It is in a way like buying yourself A BIGGER CAR, and feel a little less small

Who would like to drive a Volvo, go shopping for food,
when you can do it in a Ferrari or Porsche.
Will it bring home your food better .. hardly

If you read or even better search the official document on LM3875
written after extensive tests at National Laboratories performed by people
with more adequate techniqual education, than you and me, and certainly than all
amateur experts and noo-bies at any audio forum

If you search LM3875 datasheet
and find they recommend any fancy diodes
other than
normal silicon rectifier diodes,
like them diodes we use, we who know,
and in this group I include legends like Nelson Pass

My guess him at http://www.passlabs.com/ has bought 100-1000 normal high current for every exotic high price diode he has bought

Myself use
4 x 1N4007, 1 ampere,for preamps and lower current circuits
4 x 1N5406, 3 ampere, in the class of LM3875 max volt supply

As this is an repetitive AVERAGE 3 A, shorter currents can be higher .. maybe 5-6 A
And by that level, you would already have LM3875 SpIKe protection
having cut off LM3875 output,
which will not produce very nice SOUND.

I think you may go for 4 x diodes, that can take 5-6 ampere,
normal low price slow diodes.

me, lineup
say this above, because I do not like when sometimes people low on money
are fooled by those really rich guys and other wannabee experts
and have to waste money, they may have spent better
onto something more vital and important than a hobby

lineup - trying to spotlight a little bit into one of them dark corners of diy audio
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Old 8th October 2006, 06:38 AM   #3
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Location: Toronto, Canada
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Why the MUR860 diode?
“Do something really well. See how much time it takes. It might be a product, a work of art, who knows? Then give it away cheaply, just because you feel that it should not cost so much, even if it took a lot of time and expensive materials to make it.” - JC
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Old 11th October 2006, 05:42 PM   #4
mctylr is offline mctylr  Canada
diyAudio Member
Join Date: Nov 2005
Default Learning some basics

I think you can learn to answer most of these questions yourself with a little easy to understand knowledge about electronics.

1) Learn Ohm's Law. Don't try building anything without knowing it.

V = IR or I = V/R or R = V/I where V is voltage in Volts, I is current in Amps, and R is resistance in Ohms.
P = VI or P = (I^2)*R or P = (V^2)/R where P is power in Watts

2) Know what a diode does to AC current; it rectifies the current allowing the positive voltage to pass, see http://www.ibiblio.org/kuphaldt/elec...mi/SEMI_3.html for more details about diodes and rectifiers.


The AC current is at 50/60 Hz from your AC mains, even after going through the step-down transformer to reduce the AC voltage.

Any silicon diode can handle the switching speed (1 / 60Hz = 0.016 seconds, or 160 ms) so you just need one that meets the forward and reverse voltage specifications, and current of your application.

Based on what you've specifed (22+22 Volts from a 400 VA transformer) you should look for diodes (or a bridge rectifier module) that can handle Maximum repetitive reverse voltage and Maximum forward voltage of more than 50V (round up from 22+22V=44V), I would suggest 60-75V to include a safety margin. It should be able to handle surges of ~10A, so I tend to overspec for a safety margin of average current of 12A.

Here's a bridge rectifier that can handle 100V and avg. 12A, http://www.fairchildsemi.com/pf/GB/GBPC1201.html

I don't know of any audible differences between transformer types or manufacturers. Torridal create less electro-magentic interference*, but the square-ish designs tend to be cheaper for equalvant VA rating.

*) http://www.ibiblio.org/kuphaldt/elec...l#xtocid179314

I hope that helps at least a bit to make things clearer.
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