tpa3255evm spacesaver mod

[robinlawrie]

ok so to spur some advice/discussion, im doing some pretty pictures!


attached here is the basic wiring layout inside the amp, before grounding/shelding is considered.

ive simplified it somewhat (only one channel per amp is shown, and im ignoring the other inputs/outputs of the minidsp)

assume all signal and power would be twisted pairs.

 

[robinlawrie]

now here is my proposed shielding/grounding setup.

signal cables are two core with a shield.

as per this guide:

Grounding and Shielding for your DIY Audio Projects

im using the input rca jack as a common grounding point. rca jacks are isolated from case.

all signal grounds are connected together at one end (of stereo pair) and shield is connected to the same point, again, only at one end.

i might possibly need some kind of isolation for the chassis ground ( ive seen people mentioning capacitor/resistor, bridge rectifier etc) but i still need to get my head round that.


anyone care to point out any grave errors? this subject seems a minefield.

EDIT: just noticed i connected the shields and grounds on the rear rca inputs together at both ends. an artworking error. they would not be connected at the minidsp end for those cables.

 

[robinlawrie]

one element i forgot to include in drawing, is that the 3255 chips on the amp board are earthed to the chassis through the heatsink.

no idea if this is a problem.

also note, there is an aluminium partition/wall around the smps, which is, obviously electrically part of the chassis.

 

[Puffin]

I refer to the ground loop thread and the link you posted. You will need to identify the part of that long spiel where it says "the ground return mains wire of the (smps) psu. this should be connected at the signal ground"

Is SMPS a split rail supply with 0 grd ref?

 

[robinlawrie]

smps is single rail 48v

 

[Puffin]

So has +/- supply. Would you connect minus rail to grd?

 

[robinlawrie]

no? unless i should.

See the diagram a couple of posts back.. thats my plan, basically.

 

[Puffin]

Well unless my eyes are deceiving me I can't see that 48v is connected to grd? If you can refer me to the part in that text it should say pwr grd 0v is connected to star grd

 

[robinlawrie]

possibly i am confused between pwr gnd on the 48v side, and the mains gnd (earth) connection.

in that guide, paragraph 6 states:

" At the input jack ground I connect a single wire from the power supply ground." i assumed that referred to the earth connection on the mains side of the psu.

 

[mboxler]

Not sure if you were going to use SE inputs, but if so I found this interesting...

http://www.ti.com/lit/an/slaa719/slaa719.pdf

I couldn't if/how to do this to the EVM.

Mike

 

[robinlawrie]

that is interesting. - another TI document i hadn't seen.

anyway i assume this is already dealt with in the evm board, and refers to the internal pcb layout.


ive no idea if i need to run grounding leads from the evm board groundplane to the grounding point in my amp. ?

 

[mboxler]

Not sure what, if anything, can be done. I think R38, C70, and C72 are the VMid components, but can't tell if they connect to the ground plane or have separate traces. Also not sure if the GND via on the bottom of the board is the board star ground.

Sorry if I muddied up the conversation.

 

[doctormord]

Just leave as is, it is working best like it is.

 

[robinlawrie]

im not intending to fiddle with the boards at all. i was concerned with the overall wiring of my project, see the sketches a page or two back. see any major errors?

 

[doctormord]

Nope, but BT module might need an additional isolated supply.

 

[robinlawrie]

well handily the psu has two isolated aux supplies, so maybe ill run the minidsp from one, and the bt module from the other.


i am concerned about the 3255 chips being basically connected to the chassis through the heatsink.

Should i place a silicone heatpad between the heatsink and the heatpipe block (connected to chip via heatpipe and another block) ? at that point its a large surface area so shouldnt impede heatflow too much.

 

[doctormord]

Yes, i'd do so.

 

[emedia4u]

Space saving

Here is an idea for space saving.

 

[robinlawrie]

ha.. thanks for the suggestion.. i avoided that since i read that the big capacitors have to be as close to the amp as possible.. anyway, im still neck deep in my project, but i received my heatsinking components today, so this is the first time ive mounted the amps together.

the block there is connected to the heatsink with 4x 45watt heatpipes.

i dont think you can get much more "spacesaver" with these boards.

 

[doctormord]

ha.. thanks for the suggestion.. i avoided that since i read that the big capacitors have to be as close to the amp as possible..

True story for holding THD down.