Please Check this LM3886 Schematic

[mymindinside]

Hi all,
this is the first time i've used EAGLE to make a schematic. Its based on the Nat Semi Typical Application data sheet plus what i've seen on the internet. can you please check it for any stupid connection/ placement problems as well as suggest any alternate values for the components.
the Nat Semi data sheet has a switch on the mute pin, is this to mute the amp while its running? i thought the function of the mute was to protect the amp during start up...should i include this switch (labelled S1 on the data sheet)?.

in particular i'm confused about the value of the big caps. i've seen values from 220uf to 4700uf what does this value depend on?
Thanks in advance for any help...

love,light and peace
mymindinside

[mymindinside]

sorry folks forgot to attach the schematic. this is a jpeg i can also post the .sch file

thanks again

love,light and peace
mymindinside

[markiemrboo]

The output Zobel filter (Rz and C8) return shouldn't be going to input ground, but instead power ground. I'm not sure whether it matters if the mute capacitor (C7) goes to input ground. My latest layout has it going to power ground though.

I guess it depends on how far away the main power supply filter caps are from the chip, or if you're using big filter caps at all (some claim that big caps ruin the treble). I've never used anything bigger than 100uF and 100nF bypass caps close to the chip, but then my main filter caps are only about 2 inches away from the chips anyway....

As for the mute switch... I guess that just depends on whether you want to mute it or not Without the switch the main purpose of mute seems generally to be to stop any horrible shutdown / startup noises.

[mymindinside]

Thanks markiemrboo,
My PSU will be in the same case as the amp but not sure on the layout yet. i was thinking of using this kind of snubberized PSU http://dogbreath.de/Chipamps/SnubberPSU/SnubberPSU.html
what do you recommend, if the main psu caps are close to the chips should i go with a smaller cap on the chip?
do the values and connections look ok except for what you pointed out?
thanks for the help.

love,light and peace
farhad

[markiemrboo]

Originally posted by mymindinside
Thanks markiemrboo,
My PSU will be in the same case as the amp but not sure on the layout yet. i was thinking of using this kind of snubberized PSU http://dogbreath.de/Chipamps/SnubberPSU/SnubberPSU.html
what do you recommend, if the main psu caps are close to the chips should i go with a smaller cap on the chip?
do the values and connections look ok except for what you pointed out?
thanks for the help.

love,light and peace
farhad

Hi again.

I just had another look and I think that R3 should also be before R2 (straight after C5), so it would look something like...

PHP Code:

in + --- C ------- R --- to opamp input
             |
            R
             |
         to input ground 


The gain seems a bit high at ... 33? Most of the 'gainclones' I have seen are more around the 20 mark, which seems to work quite nicely. To get a gain of 23 you can change R5 to 1k. Doing this will make the feedback high pass filter at 3.4Hz with the 47uF cap, which is better.

The input filter (1uF and 22k) works out to be 7.2Hz. From what I know, which admittedly isn't all that much, this can present a slight phase shift all the way up to about 72Hz. It probably wouldn't hurt to increase the 1uF to 2uF or 3uF-ish. 2uF (you could use two 1uF's in parallel) and 22k would work out at 3.6Hz, which is better.

I think most people here try to aim for about 1-2Hz. If you want to get this then you could change C6 to 100uF (assuming you change R5 to 1k), which gives 1.6Hz, and C5 to 3uF / 4uF, which would give about 2Hz. Alternatively you could increase R3 to something like 47k and then just 2uF would give you 1.7Hz. The potential rise in DC offset at the output shouldn't be an issue as you have a feedback cap

All the other values look OK to me....


As for the caps near the chip.... there seems to be some different views on this and I am not 100% sure what's "correct". I have never used over 100uF and 100nF close to the chips pins... even with the main PSU caps about maybe 20/30cm away (the amp I am using right now actually). It doesn't seem to be causing me any problems!

[mymindinside]

Thanks again,
all the formulae are greek to me, so its great that you helped out with the values. i will be using a passive pre stage so i thought a high gain would be good, i can change it to 23 and then if its too low i will swap in the resistors to increase it.

love, light and peace.
mymindinside

[markiemrboo]

Originally posted by mymindinside
Thanks again,
all the formulae are greek to me, so its great that you helped out with the values. i will be using a passive pre stage so i thought a high gain would be good, i can change it to 23 and then if its too low i will swap in the resistors to increase it.

love, light and peace.
mymindinside

Sounds fair enough

The gain you need will depend on what your source puts out really then. I'm using a computer and sound card (2Vrms output?) and a gain of 23 does just fine. I have actually lowered the master volume on the computer as about 1.2Vrms will get me to full power i.e 1.2 x 23 = 27Vrms output

If you do find you need the gain then by all means swap the 680R back. I would probably leave the feedback cap at 100uF ish / a little bigger than 47uF in either case though.

[mymindinside]

Hullo all,
i finalized my board, followed the reccomendations on the values
i was doing the board layout and i have another question. In my schematic i had 2 star grounds, one for signal and one power ground.how does one make star grounds on a board? when you hit the ratsnest command to clean up the lines it routes the signal through the other components and there is no 'star' point. i was assuming that it was important to have all the components to physically be connected by pcb traces which converged at one point. is this correct? what to do?
any other suggestions/tips on designing a board for a first timer?

[gootee]

First of all, don't ever use an auto-router. Route it all manually. Manual pcb routing is fun, anyway.

And yes, it sounds like you are correct about the basic idea of star grounding.

- Tom Gootee

http://www.fullnet.com/~tomg/index.html

[Nordic]

And save when you are done.... was working on something till 1 this morning... left everything open to continue this morning... and the damn PC, must have done a security update and rebooted overnight, greeting me with the blue welcomming screen

Once you have your schematic done, click switch to board...

Then you get the board with a ll the components on one side, and yellow (unrouted) connections....


Move your components into place...

Now hit the ratsnest button, and it will recalculate the unrouted lines to the closest points... sometimes you need to move two components close to each other, do the ratsnest , and then move them , to keep your preffered orientation...

then use the little red line icon in the row just above the text and tool buttons...

Now click on the yellow trace you want to route and draw its path out.... you can select diffirent corner types by rightclicking as you go on, or use the icons that will apear in the top menu bar...

The little yellowish line next to the red one (buttons) will undo a routing and change it back to a yellow line...

Right at the bottom of the side menu, above the the ellow icon with the exclamation mark, is alittle icon, that looks (to me) like a wallplug in a little circle, (just under the big X ratsnest button)...

This will check if your layout corresponds with your schematic...

Most common warnings are about forgeeting those little green dots on junctions in the schematic. but it gives you the coordintes of the errors, so it is easy to fix.