A series R is needed at IN-1 for the 220pF to act as a filter.
More importantly, the NFB loop to the negative input is connected wrongly. See the TI app note, p.2.
http://www.ti.com/lit/ds/symlink/lm1875.pdf
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that i plan to bring to the main ground point, the point where the chassis is grounded. the mains socket earth, speaker return, pcb ground points, all of these will be bolted to the chassis.
Seems an unnecessarily large loop for the speaker return current to traverse - ideally the speaker return would be at the point where the 0V reference is for the power entering the circuit. Which is the trafo CT.
the trafo ct will also go to the chassis ground point. Whats a good grounding scheme here. Whats a good alternative point for bringing the speaker return.
As per my plan , the chassis earth point will have these - both channel pcb ground point, earth point from mains, trafo ct point, speaker returns.
Since the source of the current to the speaker is the reservoir caps, the ground return current should return to the caps too.
added the ground point on the pcb for speaker return, next to the speaker out. Attaching the image, please let me know if its ok as I dont have a cap reservoir point as i am using a ground plane for the supply
I plan to take a print out in eagle (save to pdf and print) for toner transfer pcb, do I need to check the mirror flag while saving to pdf.
I plan to take a print out in eagle (save to pdf and print) for toner transfer pcb, do I need to check the mirror flag while saving to pdf.
Attachments
ok.
do I need both input and fb np caps. If I have an input cap, there should be no dc to amplify anyway, right?
Both capacitors should be NP. They can see either polarity. Many people feel that NP sound better anyway, and the cost is similar, so there's no downside.
There is lots and lots and lots that could be improved in your pcb.
Turn cap C8 90 degrees.
Turn cap C6 90 degrees.
Turn caps C13 and C14 90 degrees.
Turn resistors R8 and R4 90 degrees and get caps C7 and C3 as close to them as possible and jump all wires with them to get direct access to the GND.
Turn fuses 90 degrees to get shorter power tracks for the chip.
CLEAN your input and feedback circuit!!! Make it in as tiny area as possible.
Think how you could make all the pcb traces as short as possible.
For example: Turn R6 90 degrees, move R2 parallelly underneeth it including the cap C1.
You are also injecting interference from your PSU with the C7 directly to your input circuit - why?
If I were you, I would start the whole design from the beginning. Organice the circuit in different areas. Like; input circuit, feedback circuit, output circuit and power circuit. Draw everything separately and do not even attempt to combine them all to one before you are absolutely sure you are not able to make them any SMALLER any longer.
You might also want to reconsider if all the components are absolytely necessary? Do they actually make the circuit perform better when realised on the pcb??? Or, do they make your pcb so complex that the amplifier performance will actually suffer from them?
Turn cap C8 90 degrees.
Turn cap C6 90 degrees.
Turn caps C13 and C14 90 degrees.
Turn resistors R8 and R4 90 degrees and get caps C7 and C3 as close to them as possible and jump all wires with them to get direct access to the GND.
Turn fuses 90 degrees to get shorter power tracks for the chip.
CLEAN your input and feedback circuit!!! Make it in as tiny area as possible.
Think how you could make all the pcb traces as short as possible.
For example: Turn R6 90 degrees, move R2 parallelly underneeth it including the cap C1.
You are also injecting interference from your PSU with the C7 directly to your input circuit - why?
If I were you, I would start the whole design from the beginning. Organice the circuit in different areas. Like; input circuit, feedback circuit, output circuit and power circuit. Draw everything separately and do not even attempt to combine them all to one before you are absolutely sure you are not able to make them any SMALLER any longer.
You might also want to reconsider if all the components are absolytely necessary? Do they actually make the circuit perform better when realised on the pcb??? Or, do they make your pcb so complex that the amplifier performance will actually suffer from them?
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There's common impedance now between that take-off for the speaker return and the noisy ground currents through the local decouplers. I suggest having a real star point or at least returning those decoupling currents to 0V prior to the take-off for the speaker return.
Same applies for your signal ground, that shares common impedance with noisy decoupling currents.
Id start from zero becasue this designs makes no sense at all. The circuit is all over the place. It should be very compact both, supply and input part. supply rails close together, positive and negative inpputs too and two different ground points for output and input..
And why to waste space with those diodes when you can use bridge diode?
And why to waste space with those diodes when you can use bridge diode?
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