lm1875 board design

[BionicSniper]

Well I want a smaller amplifier with better quality than what I am using right now (pioneer vsx-516). But being in college money is a problem... Any way here is a design for the amp portion of the system I'm working on. This pcb's will be hand fabed by me (1oz copper and I can do solder mask and all that fun stuff). I can do 2 layer as well but it is somewhat of a pain.

I used the circuit from nationals data sheet.



and then here is my layout thus far. (ignore that the silkscreen has the polarity of the power inputs switched, I just fixed it)




the large caps are nichicon audio caps and then c3 is a nichicon regular cap. Does anyone have recommendations for c1? Right now it is just a 805 smd ceramic cap (all the small power ones are but they need to stay as that as they are .1uf decoupling caps). And then c4 leads to audio ground is it ok if that one is just a ceramic smd as it is not directly in the audio path? My only other concern was the audio ground plane being so close to the negative power feed.

Volume control will be done on a seprate board before the audio gets to the amp board if I decide to do volume in the amp. I'm working on the PSU board right now but it may take till tomorrow for me to finish the first design

[justblair]

Interested in what you have, but I can't see your design.

I am not a yahoo member..

Photobucket lets you link to images so they are available to all...

JB

[BionicSniper]

there we go fixed it. used the wrong link...

[justblair]

The SMD caps that filter your power lines could be closer to the power supply pins on the amp.

Are you going to connect the audio ground and power ground off the board?

Your Audio signal is forms a plane? I have not seen this done before, planes have either been power planes or GND planes in esigns I have seen elsewhere.

EDIT No They dont.. My idiocy shows through at every opportunity!

Neat design though

Blair

[justblair]

Should the speaker gnd not be attached to PWR Ground?

On the gain clones that I have built, and i am sure that this was based on builds I have seen on other sites, Power ground and signal ground are either connected by thin wire link or via a resistor/cap combo. But the speaker return attaches to Power Ground directly.

My memory is bad, so hopefully someone else can confirm this?

[kaos]

Do you plan on starring the ground back at the power supply?? If so, I’d keep the speaker return ground separate from the input/feedback ground and return it to the star point. Also, it looks like the values of R1 & R2 are reversed on the schematic.

[BionicSniper]

well I have two separate grounds on that board. There is a ground for the power system and a ground for the - audio. All of the grounds from the amp that are not directly power related feed into the ground plane on the board which also connects the audio (-) in and out. I made the power ground as short as possible and it rests between the - and + power in.

So the audio ground feeds from the audio in - and audio out - and the power ground goes to the 0 on the psu board (basically the system ground)

Here it is with the filter caps closer (whops forget to connect c2 to ground)



I have to head to some airforce ROTC meetings so I will be able to update these with any suggestions in about 2 hours. THANKS A TON GUYS!

[justblair]

Yes I can see that you have seperated audio from power grounds on the board.

However it is more normal to seperate the audio input ground to the power/speaker return ground. They will need to be attached to eaxh other somewhere, but that can be at the ground star. Keeps the signal ground clean if you like. On the output side, the gnd will be sinking more current hence its connection to power ground.

Alternately you can return the speaker return to the star point seperately of the power ground. More cable, but it will work.

If I am speaking out my hat here, I am sure that someone else will step in...

[kaos]

Sorry, I wasn't clear. What I was trying to say was that I'd split the ground (circled in yellow) and return the output ground to the supply separately from the input ground. (similar to what justblair just said)

[Minion]

You might want to put the speaker output somewere else , like to the side or front of the LM1875... I say this because with the heatsink on the LM1875 it is going to be sitting on top of the speaker outputs which will make shorts to the heatsink more likeley .....
The heatsink will also be sitting on top of R3 and R4 and possibly R5 and C4 , If you are going to be securing the heatsink to the PCB you will possibly short these components unless there is some clearance between the heatsink and components and PCB.....


Cheers