LM3875 PCB single sided

Bedi · 17th January 2010, 11:52 AM

hi guys,

please check my one sided LM3875 pcb layout, thanks

schematic is from NS non inverted

AndrewT · 17th January 2010, 03:13 PM

could you number the component locations and post the schematic with matching numbers.

Bedi · 17th January 2010, 07:58 PM

Here it is

ChristianThomas · 18th January 2010, 02:30 AM

Hi Bedi

It appears that it will work, from not too close scrutiny late at night. But I think the layout could be improved a fair bit.

I would try to keep the power side away from the signal side, and since you don't appear to be concerned about where the heat sink is going I would put all the power components on the back and the small signal components on the front. And get those capacitors close to the chip. Personally I would run twisted wires to the base of the chip and put the caps just outside those. Have them come down vertically onto the board if possible or, if horizontal, from directly behind - maybe even going underneath the caps. I'd then have the output coming out on the same side, leaving the front clear for low level signals. For these I'd have two tracks coming straight out, with the resistors a bit further away. Then you can have your feedback resistor going across from pin to pin. almost right under the chip. Also , the last thing you want is your loudspeaker cable going right past your feedback resistor.

I think there is a MISTAKE here. R2 should be after the 220R. It will still work but you won't get your 22k loading.

One other thing, do you really need the gain to be so high. Most power amps are around x 20 (26dB) and you have chosen x33. If you need it then fine, but the distortion will be higher because you will have lost bandwidth and the feedback won't be doing its best. These chips can sound grainy at times and one shouldn't encourage it. If you discover you really don't need the gain then it might be worth changing R3 from 680R to 1k1 or a bit above. You may hear a striking difference.

Good luck. (Check your pins, because I haven't)

All the best

Christian

AndrewT · 18th January 2010, 10:18 PM

you have omitted the decoupling caps and the RF filter you have adopted a DC coupled design and signal ground is connected direct to power ground.

This all invites disaster.

Bedi · 19th January 2010, 08:49 AM

But I have a pair of amplifiers in these plates, and everything works.Why disaster? DC offset is 35mV (when input signal is open to 200mV). Of course,I would like to do best, but only on single sided PCB.BrianGT made a great double sided PCB, but is it possible to do the same thing on single sided PCB? Any suggestions
regards,
Bedi

17th January 2010, 03:13 PM	#2
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	could you number the component locations and post the schematic with matching numbers. __________________ regards Andrew T.

18th January 2010, 10:18 PM	#5
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	you have omitted the decoupling caps and the RF filter you have adopted a DC coupled design and signal ground is connected direct to power ground. This all invites disaster. __________________ regards Andrew T.

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18th January 2010, 02:30 AM	#4
ChristianThomas diyAudio Member Join Date: Feb 2006 Location: On a hill, in a wooden shack, next to the woods, in Somerset.	Hi Bedi It appears that it will work, from not too close scrutiny late at night. But I think the layout could be improved a fair bit. I would try to keep the power side away from the signal side, and since you don't appear to be concerned about where the heat sink is going I would put all the power components on the back and the small signal components on the front. And get those capacitors close to the chip. Personally I would run twisted wires to the base of the chip and put the caps just outside those. Have them come down vertically onto the board if possible or, if horizontal, from directly behind - maybe even going underneath the caps. I'd then have the output coming out on the same side, leaving the front clear for low level signals. For these I'd have two tracks coming straight out, with the resistors a bit further away. Then you can have your feedback resistor going across from pin to pin. almost right under the chip. Also , the last thing you want is your loudspeaker cable going right past your feedback resistor. I think there is a MISTAKE here. R2 should be after the 220R. It will still work but you won't get your 22k loading. One other thing, do you really need the gain to be so high. Most power amps are around x 20 (26dB) and you have chosen x33. If you need it then fine, but the distortion will be higher because you will have lost bandwidth and the feedback won't be doing its best. These chips can sound grainy at times and one shouldn't encourage it. If you discover you really don't need the gain then it might be worth changing R3 from 680R to 1k1 or a bit above. You may hear a striking difference. Good luck. (Check your pins, because I haven't) All the best Christian

19th January 2010, 08:49 AM	#6
Bedi diyAudio Member Join Date: Oct 2009	But I have a pair of amplifiers in these plates, and everything works.Why disaster? DC offset is 35mV (when input signal is open to 200mV). Of course,I would like to do best, but only on single sided PCB.BrianGT made a great double sided PCB, but is it possible to do the same thing on single sided PCB? Any suggestions regards, Bedi

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