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Critique my PCB! Headphone amplifier
Critique my PCB! Headphone amplifier
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Old 28th November 2017, 11:38 AM   #11
Mark Whitney is offline Mark Whitney  Netherlands
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The current loops to the capacitors from the PSU and the current loop returning from the output are passing through the same part of the ground plane. Post 7 left channel shows some of the power loops and the right channel the output loop. By moving the power connection and capacitors to one side of the IC and the output to the other you remove the output voltage error caused by the power current.

The ground plane on the output side of the IC is a good place to connect the SGND to the (power/output)GND.

Fixed the SGND error in the schematic.
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Last edited by Mark Whitney; 28th November 2017 at 11:48 AM.
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Old 28th November 2017, 03:45 PM   #12
JPS64 is online now JPS64  Germany
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Just my 2 cents.

A lot to do...

JP
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Old 28th November 2017, 04:37 PM   #13
OlegSh is offline OlegSh
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Critique my PCB! Headphone amplifier
I would move power entry connector right behind the output connector such that their GND pins are inline with each other, otherwise there is a chance that some of the supply and output return currents will flow through the signal GND in the middle.

Oleg
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Old 28th November 2017, 04:46 PM   #14
JPS64 is online now JPS64  Germany
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+1 @OlegSh
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Old 28th November 2017, 05:17 PM   #15
abza is offline abza  South Africa
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Woah ho ho, thanks @JPS64! I'll take a look a little later this evening, but first glance looks awesome.

A lot to be learned here. I can immediately see what @OlegSh was referring to when insisting that the traces try not intersect the ground cuts - you've made it look easy

Can anyone recommend a good tutorial/place to learn about how to become comfortable following current paths, detecting loops etc? Must admit I have a hard time visualising this, where others seem to spot these issues immediately.
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Old 28th November 2017, 07:02 PM   #16
OlegSh is offline OlegSh
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Critique my PCB! Headphone amplifier
Yeah, the layout by JPS64 is really great and gives very informative visual impression!

As for the return paths and such, the rule is very simple, if there is a trace where current flows "forward" then there should be a closely coupled trace where the current can flow "backward". If the return trace takes a detour (or GND plane under the "forward" trace has a slot forcing the return current to deviate from its ideal path) this will immediately increase the loop area created by the "forward" and "backward" current paths and introduce extra parasitic inductance. If such loop is connected to the high impedance node it can act as aerial picking up all kinds of noise from the surrounding. It can also radiate noise if high (supply or output) "pulsing" currents flow along this loop and induce noise into other sensitive circuits around. If excessive loop area is present around sensitive nodes it can severely degrade circuit performance and sometimes impair stability, for example when decoupling capacitors are placed too far from the supply pins of high speed op-amp. For more info have a look at this tutorial. It deals with mixed analog/digital designs but has some useful info with figures. There are many other tutorias, just google for "PCB layout" in combination with "grounding".

Regards,
Oleg

Last edited by OlegSh; 28th November 2017 at 07:08 PM.
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Old 30th November 2017, 08:02 AM   #17
abza is offline abza  South Africa
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Right, next version!

Changes:
  1. Pretty much everything - components remain the same (as I have them in my parts bin) but layout is much more clearly defined between signal and power sections
  2. No intersection of power traces, power traces remain above the GND plane
  3. Signal traces remain over the signal ground as far as possible
  4. As @Mark Whitney suggested, SGND and GND connect on the schematic
  5. As @OlegSh suggested, SGND and GND connect right at the output and power terminals (a thin ground path connecting them)
  6. Lots learned from @JPS64's layout, particularly with regards to using a small trace within the GND plane for sections where routing top traces would be impossible. I managed to get away with just one (top left 100uF capacitor)
  7. Power rails now each originate from the terminal, rather than daisy-chaining or branching off.

I know there is a lot of dead space to be removed - the layout can be significantly tightened up - but is this an improvement with regards to the trace and ground layout?

Thanks everyone!
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Old 30th November 2017, 01:49 PM   #18
00940 is online now 00940  Belgium
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It's getting better, however your feedback lines are unnecessarily long.

R1l and R1R should go to the middle of the pcb, under and above the output resistors; that would allow you to shorten tracks significantly.

Ideally, the R0 resistors should also go to the middle, in order to sense the output ground. Speaking of the output gnd, there is no reason to link it to power ground through a thin track. Actually the thicker the better there. Just make the two groundplanes overlap at the right end of the sgnd groundplane

I don't get why your 680p CIN2 capacitors are that big compared to the much bigger CIN1 caps ?

I see you plan on using 22uF x7r caps. Beware that x7r caps usually have strongly reduced capacitance vs DC voltage bias. A typical 22uF/25V murata 1210 X7R cap will only have 7.3uF at 15VDC. That's actually less than a 10uF/50V cap from the same serie at 15Vdc. Murata (and other manufacturers) have simulation tools on their website to help you determine what you actually get.
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Old 30th November 2017, 06:42 PM   #19
OlegSh is offline OlegSh
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Critique my PCB! Headphone amplifier
I'm with Ben.

Keep in mind that the feedback network should be very tight and RG0 should be connected closer to the output since you want the feedback to correct the error at the output not the input. The RG1 should also go between the op-amp and the buffers to minimize the feedback loop area. Take your time. Analyze what you see and try again. Also make those huge RF filter caps at the inputs small in size since these huge caps have such a high parasitic inductance that they will not work effectively as RF filters. Also turn the buffers by 90 degree as JP did in post #12, it will help you routing the power supply and signal traces more conveniently. If you'll do everything right you will not have to introduce many GND plane cuts to prevent mixing signal and supply return paths.

Regards,
Oleg

Last edited by OlegSh; 30th November 2017 at 06:48 PM.
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Old 30th November 2017, 06:46 PM   #20
00940 is online now 00940  Belgium
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oops, please indeed replace r1 by rg1 and r0 by rg0 in my previous post.
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