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Chip Amps Amplifiers based on integrated circuits

Single supply LM3886 layout
Single supply LM3886 layout
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Old 3rd December 2017, 10:20 PM   #1
00940 is offline 00940  Belgium
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Default Single supply LM3886 layout

There are times when a single rail amp has its uses. Here's an attempt for an amp running on +48V (from an smps for example). That's pretty much a datasheet implementation.

A big concern was the thump on start up with an output cap. Hence the output relay for muting at start.

What do you think, any errors, improvements to be pointed out ?

The board is 8*5 cm big.
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Old 4th December 2017, 05:42 PM   #2
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Slight improvements in grounding.
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Old 7th December 2017, 10:04 AM   #3
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Well, I guess there's no obvious error...

Time to order.
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Old 8th December 2017, 04:50 AM   #4
tomchr is offline tomchr  Canada
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You might want to discharge C16 faster when the supply collapses on turn-off.

It also looks like you're running the signal ground through the high-current portions of the ground plane. I suggest finding a quieter route for that.

I suggest revisiting the decoupling on the LM3886 as well.

Tom
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Old 8th December 2017, 06:33 AM   #5
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Thanks for weighting in. I must say though that I don't get all of your comments.

- on the discharging of the c16 cap, I see the point.
- on the grounding: all signal ground points have their own groundplane (at the right of the board, in blue). This plane connect to the power ground at the output connector. What else should I take care of? The layout in the first post didn't have that plane though.
- I'm ok with revisiting the decoupling... What would you suggest to improve? Is it a question of values or layout?

Thanks for any further help.

Edit: for discharging c16 would adding a diode from the positive terminal of c16 to the 48v rail do the trick? That and replacing r7 by a 4.7v zener to protect the mosfet.
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Old 8th December 2017, 06:58 AM   #6
blu_glo is offline blu_glo  England
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Comments rather than criticisms further to the above about the layout....

R4 could be better off connected to P on the relay (leave O disconnected). This ensures the C10 negative plate never has chance to charge to any DC voltage.
R4 at 330R also needs to be rated 1W at very least for worse case dissipation.
Using 1K instead allows you to use normal 0.6W/MRS25 resistor and should be perfectly adequate.

R12 and 13 should be reasonably substantial metal or carbon (does not matter in this position) film 2W or more whatever you can fit.
If anything goes unstable you want them to have at least a chance of coping and living to tell the tale!
Whatever you do; do NOT use wirewound in R12 position, not even the "non-inductive" type!

Mount R4, 12 and 13 about 3-5mm off the PCB to allow air to circulate and so they don't scorch the board if they get hot.

Don't think you need both C3 and C4. C4 alone should suffice otherwise keep an eye on your required treble response....

Very best regards,
Simon

Last edited by blu_glo; 8th December 2017 at 07:16 AM.
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Old 8th December 2017, 07:06 AM   #7
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"BP" by C6 means "Bipolar"? there is no need in single supply circuit for bipolar at this point. Take advantage of a good quality polarised electrolytic at this point.
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Old 8th December 2017, 07:29 AM   #8
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You're right on c6 of course.

Indeed, r4-12-13 are intended to be 1w.

On r4, it might be a concern as it is if one plugged in a speaker with the amp already powered on. The reason I did it that way is that I will be using the amp in a powered speaker. I need to check again the delays to see if I can go up to 1k. What I could also do is have a low value r4 for startup and add a high value resistor before the relay to keep the negative terminal of the output cap at ground.

Thanks for the comments, that helps :-)
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Old 8th December 2017, 07:41 AM   #9
blu_glo is offline blu_glo  England
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I'd be surprised R4 constantly present would have any effect on an active speaker (other than shunt the input impedance slightly) which active speaker is it?
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Old 8th December 2017, 07:43 AM   #10
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!! the voltage across C6 is also 1/2 the supply voltage! 16V not enough - recommend 50V or 63V for the supplies you suggested!
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