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-   -   Smoothing capacitors for LM3875 power supply (http://www.diyaudio.com/forums/chip-amps/208729-smoothing-capacitors-lm3875-power-supply.html)

maurycy 13th March 2012 07:07 PM

Smoothing capacitors for LM3875 power supply
 
I am rebuilding my LM3875 chipamp (new case, adding second transformer and ps board). Currently, the power supply board (based on CarlosFM unregulated PS schematic) has two 10uF 50V smoothing capacitors. Since I am rebuilding the amp, should I upgrade the caps to a higher value?

The setup is run by 21V-0-21V 80VA center tap trafo per channel. Speakers are 8Ohms with sensitivity of about 86db. I also have 6Ohm speakers with sensitivity of 94db but I know that LM3875 does not play well with less than 8Ohm speakers.

Thanks.

Puffin 13th March 2012 08:58 PM

From memory Carlos's schematic uses 2 or 3 sets of 10,000uf caps?

EDIT - here it is.

http://i73.photobucket.com/albums/i2...nsex/GCPSU.png

maurycy 13th March 2012 09:34 PM

The power supply is from BrianGT (chipamp.com) kit. It says that it is based on unregulated CarlosFM power supply but it only has two 10uF caps. The PS has only full wave rectifier and smoothing caps. It is not snubberized.

Minion 13th March 2012 10:17 PM

10uF in the whole power supply ?? I don"t see how it is possible and still sound good , You would think there would be a lot of ripple , My latest amp used 20,000uf per rail (40,000uF total) for a single lm3886 chip ......

maurycy 14th March 2012 12:11 AM

This is for the older Rev. C of BrianGT's (chipamp) LM3875 kit. Yes, there are only two 10uF caps. There are 1500uF 50V caps too but they are on the amp board. I can't find the schematic, but looking at the PCB it looks like it is simple full wave rectifier with those 10uF smoothing caps. The PS just changes AC to DC.

So if I were to replace those 10uF caps with bigger ones, what value should I go for?

Thanks.

ChristianThomas 14th March 2012 12:40 AM

Go for something like 10,000uF on each rail.

Minion 14th March 2012 02:09 AM

Quote:

Originally Posted by maurycy (Post 2944959)
The PS just changes AC to DC.

So if I were to replace those 10uF caps with bigger ones, what value should I go for?

Thanks.

The PSU does more than that , a psu has to also filter out the ripple from the AC , It also has to store enough energy to supply enough current for loud transients in the amp , and the capacitors together with the impedance of the power supply form a filter that affect the audio ..... Most poeple claim that the more capacitance in the PSU the better the low frequency responce of the amp ......

:)

Redshift187 14th March 2012 03:12 AM

With such a small VA transformer, there wouldn't be a lot of point going for too high a value, but I would go for at least 4700uF per rail.

Puffin 14th March 2012 07:06 AM

180va tranny at least.

maurycy 14th March 2012 12:27 PM

Thanks for replies.

I will try 4700uF on each rail and see how the amp performs. I think I may have those in my parts bin.

As to the trafo, I have one 80VA for each chip. It should run just fine. Before, I have used only one trafo like that for both chips and never had any problems with it. Not everybody pushes their amps to the limits. :) My main rig is 2x16W Marantz receiver which has plenty of power for my type of listening.


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