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merlinx76 5th March 2005 04:37 PM

LM4780 power supply
I plan to build BrianGT's LM4780 kit but with a snubberized supply and a 19-0-19 traffo.

Would 2 power supply boards be recommended?

If I use only 1 supply board will a single bridge with 4 MUR860's(because my tranformer is center-tapped) be enough for 90W x 2 into 4 ohm speakers?

doggy 17th September 2005 04:52 PM

blown fuses
I put the 4780 brian gt kit together. The transformer is a plitron with dual secondaries. I left the diode bridge and the amp board together. I got 30 volts dc at v- pg- pg+ v+ The fuses keep blowing; up to 4 amp so far. The chip is mounted directly to the heat sink but the heat sink is isolated from the ground. What size of fuse do I need? Is there a schematic for wiring up the 8 diodes with the red yellow, blue grey secondaries? Red yellow serves one diode rect. and blue grey the other .

wes-ninja250 17th September 2005 05:08 PM

Are you using slow-blo fuses? If not, that's your fuse problem right there. Otherwise, try a 5A. :)


doggy 17th September 2005 05:23 PM

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No just regular automotive fuses. Who would carry slow blow fuses?

wes-ninja250 17th September 2005 08:57 PM

Well, 20 years ago Radio Shack carried 'em...

If you can't find them locally, Digikey ( has them. Small orders cost $5 to ship, larger orders have free shipping.

Suggested Part Numbers --

3AG (1.25 x .25"):

Metric (5x20mm):

...The ones starting with F are worth about a buck each and are sold in groups of five. The ones starting with W are about fifty cents each and are sold individually. Hint: Order spares.


doggy 17th September 2005 11:44 PM

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Thanks for the help. I got some 5amp slo blow at Active Electronics Vancouver. What a hell of a time I had to get it to work!! I finally had to have each primary feeding 1/2 of both sides of the rectifier boards.

The amp sounds great and so quiet, no on off pop, nothing! It is also bringing some new appreciation for some speakers I have.

Just for fun I by-passed one board only with some oil caps. Will plan to listen and evaluate.

jackinnj 18th September 2005 01:20 AM

my LM4780 supply:

here's the schematic:

doggy 24th September 2005 04:45 PM

2 Attachment(s)
Good looking power supply! I guess it's up to me tolearn more about power supplies; imho it makes a big difference in the amp performance.

In the interim I did this: added nichicon, g-luxon 4700uf directly to the boards bypassed with .001uf non polar snubber 10r+100nf metal poly cap. Sounds good.

doggy 24th September 2005 10:01 PM

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One channel did not have the same clarity and bass authority. What I did was use nichicon 4700uf on one side and g-luxon 4700uf on the other all 50 v. I finally tried swithching all I could to try and discover what the difference was. The side with the g-luxon was better all around, Went and changed out the nichicons and put in the g-luxons. Result; these unknown to me luxons are way better imho. I've used nichicons before but never g-luxon.

a nice find,
cheers doggy

Russ White 24th September 2005 10:34 PM


Originally posted by jackinnj
my LM4780 supply:

here's the schematic:


Thats a nice supply. :)

Now those are snubbers. :up:


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