LM 1875 questions

[MJS]

Hi again, Sorry again should have looked at the photo as that makes it clearer.

This uses a bit of a brute force method, using alot of copper to try to reduce the impedance between the different parts. However I think it would be better to return the input ground and the power ground to the center point on the main smoothing caps and create a star with a single connection to the thick trace down to the local decoupling caps. This would define the current paths and prevent the output currents sharing a path with the input voltage referance.

Also without the rectifyer bridge on the circuit you could cause problems with the rectifying currents comming to the PCB if the cables arn't kept short and tightly wound together and balanced in length. I would put the rectifier on the PCB so I could balance the current paths and try to shorten the distance from the bulk decouplers to the amplifer. The supply could be fused before the rectifier or not fused at all, the amplfer is protected to some extent byt the sPike protection so fusing shouldn't be needed, this should make it possible to get the parts much closer together.

Having said that I am sure it will work, these amps are fairly forgiving if you have good local decoupling so most solutions will work.

Regards,
Andrew

Well, I only can say thath it sounds very good with any kind of noise

I accept thath the fuses is only a solution for to put the power suply in the same PCB

But I have seen many others PCB´s with a plane ground

With the LM3886 I had all the problems with the grounds untill i make this (Please see my new post,) http://www.diyaudio.com/forums/showthread.php?s=&threadid=118174

but never I had a problem with the distance of componentes or put or no the 0,1 uF decoupling caps



Regards

 

[gfiandy]

Hi HPL,

Have you made this PCB yet, becuase if you have not you should remove the unconnected copper plane areas. They form a parasitic capacitance between the input and output and can reduce the stability margin.

If you have then try the amp out it may work fine but if it does not then use a scaple to peel these bits of copper off.

Hi MLS,

I did not mean to offend you, I am sure your design works well and sounds very good. However it may be possible to further improve on it.

Regards,
Andrew

 

[gfiandy]

Paswa,

Yes the bulk decoupling capacitors (the larger ones) can be placed much further away from the amplifier if required. Th bridge rectifier should be placed near to the large capacitors and the paths from the tansformer power connector should go straight tot he bridge rectifier.

Do you plan to have one PSU (power supply) for all the amplifers or seperate PSU for each amplfer?

Regards,
Andrew

 

[Paswa]

Do you plan to have one PSU (power supply) for all the amplifers or seperate PSU for each amplfer?

Yes, I'm planning only one PSU for the six ics. the ics will be five LM1875 and one LM3876. I'm planning to replace the two filter caps that I placed near the IC and to place it in the power supply section. I will use a 10,000 mfd capacitor for each rail.

 

[gfiandy]

Hi, I would recomend having both the 10,000uF and the local caps as this will improve stability paticularly in to low impedance loads.

If you use a bridge rectifier for the main capacitors the voltage to the amp will be DC (with ripple) so you don't need any diodes on the amp PCBs.

Regards,
Andrew

 

[gootee]

Yes, I'm planning only one PSU for the six ics. the ics will be five LM1875 and one LM3876. I'm planning to replace the two filter caps that I placed near the IC and to place it in the power supply section. I will use a 10,000 mfd capacitor for each rail.

Hi Paswa,

If you look at all of the many, many other chipamp designs, you will see that basically every one of them has relatively large electrolytic capacitors near the chipamp pins, in addition to the even larger smoothing caps in the power supply. This is usually even true when the power supply is on the same PCB as the amp.

How did you decide that you could eliminate those local capacitors??

Without them, the parasitic inductances and resistances of the wires and PCB traces between the power supply and the chipamps' power pins will make it impossible for the chipamps to get large surges of current, fast-enough. Each chipamp needs its own local reservoir of quickly-available energy, if you want good performance.

P.S. You might want more than 10000uF for your main smoothing capacitors. There is some "rule of thumb", for estimating that; something like 2200uF per amp of current.

Here is a link to the basics needed for linear power supply design:

http://www.zen22142.zen.co.uk/Design/dcpsu.htm

 

[Paswa]

Gootee, The ps link very useful
THANK YOU ALL

 

[MJS]

Hi HPL,


Hi MLS,

I did not mean to offend you, I am sure your design works well and sounds very good. However it may be possible to further improve on it.

Regards,
Andrew

I´m not offended



I´m sure that is possible to make it better, but when I do it I don´t know how

Best Regards

Miguel

 

[eketehe]

fyi

i have built lm1875 amp powered only by 3A 18-0-18 ( +/- 24vdc ) of square transformator per chanel with 4 x 2200uf/35V in paralel locted in separated p.s. box.

sounds very good, but after read this discusion i'm interest to put the bigger trafos to find the better hearing.
i'm about to rebuilt my 2.1, i'm insist to have the subwoofer, to awake the contrabass / 5th of bases sounds tht poor with lm1875.

i'll check the sound and may inform you the result.