3886 PCB critique

[rayban68]

Hello all, could you please take a look at my first ever pcb design? I check the online pcb trace width calculator, the 70mil traces that I have are good for 4.5 amps at 15 degree temperature rise, which is plenty for this amp I think. Anyways, please let me know if I need to make any changes before I send it off.

Thanks muchly

[rayban68]

And, of course, the schematic. straight off the appnote, and very similar to some that were posted on here. I could have used one of the boards that were posted on here, but I wanted the joy of creating my own

[bigparsnip]

why is there a ten ohm resistor on the output?

And you could proably make you ground tracks a lot shorter if you rotate C6 90 degrees couter clockwise and move things in a little closer to the chip.

Also, for the input traces, and the mute circuit, I would use thinner tracks as this should allow you to route things a bit more easily as well.

[rayban68]

Thanks for the reply bigparsnip, i will shrink those traces down a bit. Regarding the resistor on the output, that is the resistor-inductor output that is shown on the datasheet and this is nationals rational for it:

(Note 17)
Provides high impedance at high frequencies so that R may decouple a highly capacitive load and reduce the Q of the series resonant circuit due to capacitive load. Also provides a low impedance at low frequencies to short out R and pass audio signals to the load.

And further down in the application information section:

REACTIVE LOADING

It is hard for most power amplifiers to drive highly capacitive
loads very effectively and normally results in oscillations or
ringing on the square wave response. If the output of the
LM3886 is connected directly to a capacitor with no series
resistance, the square wave response will exhibit ringing if
the capacitance is greater than about 0.2 µF. If highly capacitive
loads are expected due to long speaker cables, a
method commonly employed to protect amplifiers from low
impedances at high frequencies is to couple to the load
through a 10Ω resistor in parallel with a 0.7 µH inductor. The
inductor-resistor combination as shown in the Typical Application
Circuit isolates the feedback amplifier from the
load by providing high output impedance at high frequencies
thus allowing the 10Ω resistor to decouple the capacitive
load and reduce the Q of the series resonant circuit. The LR
combination also provides low output impedance at low
frequencies thus shorting out the 10Ω resistor and allowing
the amplifier to drive the series RC load (large capacitive
load due to long speaker cables) directly.

[bigparsnip]

Ah right, I didn't notice the L1 in the resistor name ther, so I ssumed it was simply a resistor and not an inductor. In that case, will you be soldering the wire for the inductor directly onto the resistor leads, or would it be easier to add a couple of pads near each end of the resistor to allow for it to be mounted directly on the PCB?

[rayban68]

I will be wrapping the 10 ohm resistor with wire to create the inductor and soldering the wire directly onto the resistor.

[digi01]

I think you should minimization the pcb overall arrangement and prevent the copper trace in the "T" types.


-digi

[rayban68]

Ok I'll have to sit down tommorow (watching the Canucks game tonite, and Calgarys goin down!!!!) and do these changes. Out of curiosity what do "T" shaped traces do? is it difficult for manufacturing?

Thanks

[B.VDBOS]

C2 and C4 should be as close to the chip as possable

Also if you want to, you could remove the NC pins from chip, just give them a wiggle and they fall off

Its may be better to have a star ground ie. one trace for every ground all joining at a common piont