The yet tiniest single-sided LM3886?

[hurtz]

Hi folks,

after seeing a lot of the LM3886 amps that were assembled directly on chip, I decided to create a simple "cleaner" version on a single-sided PCB which schould make it easier for the beginner to assemble it. I would be grateful to hear your thoughts!

The PCB was designed with Designspark (tried it for the first time)

Values are as following:

VN: -25.5V negative supply voltage
VP: +25.5V positive supply voltage
Out: output to speaker
Gnd: self-explanatory...
In: input to the amp

Cz: 100nF, 100V, polyester
Cin: 4.7 uF, 50V, polyester
Rg: 22k, 1W, carbon
Rf: 22k, 0.25W, carbon
Ri: 680R, 0.25W, carbon
Rm: 10k, 0.35W, metalfilm
Rz: 2R7, 2W, carbon
Rbr: 10R, 0.25W, carbon

The values were taken from this Gaincard-clone idea:
Mick Feuerbacher Audio Projects

The PCB is 30.46 * 28.18 mm

The power supply with caps and diodes is on a seperate board.



Best regards,
hurtz

[hurtz]

And here is the PSU, I used five 35V 4700uF caps since I've got a lot of them lying around, diodes are MUR860, Transformer will be 18V, 80VA.
This should result in +-25.5V, (8 Ohm) 30W output; 15W dissipation.

I see many people are using small around 100nF caps in parallel, what are they for?

Again, any input is welcome :-)

[geraldfryjr]

Very cool! Nice work! jer

[pinkmouse]

Neat. I'd look at all those ground tracks though, a star is a much better solution.

[hurtz]

Thanks :-)

If there are no major objections/suggestions I will try this design, and if it performs well, I will publish the etchable version of the board, and by request of the PSU although I don't think many people have 5mm caps lying around :-)

Since it should stay tiny, there is no room for a ground-star :-/

cheers!

[infinia]

Originally Posted by pinkmouse

Neat. I'd look at all those ground tracks though, a star is a much better solution.

+1 (here you probably want to turn the whole layout inside out, and keep the INs and OUTs on opposite sides)
dont forget 2x Cs! they really need to be close to the chip! the value and part quality is esp important for the power amp stability and the sound. Sorry but this blows up yer small PCB...deadbug style is looking good.

[discrete]

Originally Posted by hurtz

And here is the PSU,

The power supply layout is just wrong....

Imagine the 50/60 Hz current pulses going from the diodes to the capacitors.
Now consider that the PCB tracks have some resistance, and see where you are taking the output from. Never share a track/wire that carries current pulses...

[Juergen Knoop]

since your amplifier board doesn't show any decoupling capacitors, you may get into trouble. Not necessarily, but maybe!

[hurtz]

So as I understand it the decoupling capacitors are there to provide current to chip really fast right?
Never actually thought a cable could be too slow for this :-/ but, OK.

@infinia at what would I look when selecting them? I could make a guess and say a low internal resistance, but this is as far as my wisdom carries me. Could I also use my 35V 4700uF 85°C ? There is also something called ripple current: 2710mA 120Hz, which as I understand is äquivalent to the internal resistance?

@discrete I calculated for a maximum of 4.44A (thats as much as the transformer can handle) at 35um copper thickness and 200mm track length a width of 2.348mm so it should be fine, or am I missing something? (I actually used 2mm until now, so new design is mostly 5mm which should be more than enough)


So here is revision 1.1 of the PSU and the board. as Cs I used caps of the same size as on the PSU board.

PSU: 80 x 60mm
AMP: 50 x 40mm

PSU:


Amp:

[discrete]

Originally Posted by hurtz

@discrete I calculated for a maximum of 4.44A (thats as much as the transformer can handle) at 35um copper thickness and 200mm track length a width of 2.348mm so it should be fine, or am I missing something? (I actually used 2mm until now, so new design is mostly 5mm which should be more than enough)

...more like this (please excuse the freehand doodle).

Think of it as an RCRC filter, where the tracks are really small Rs.
A star for the ground would be another improvement.