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Chip Amps Amplifiers based on integrated circuits

An open source layout for LM3886?
An open source layout for LM3886?
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Old 4th May 2018, 09:48 AM   #51
00940 is offline 00940  Belgium
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@bozoc: sadly that heatsink didn't fit. Not wide enough for the diode bridge
@Lingwendil: yes, that's worth a try. On 2 layers boards, their cost of shipping is actually a bit higher than smartproto, so the final price is about the same.

Anyway, here are two auxiliary pcb. One is the PS (60*100mm), one is a simple DC protect (60*40mm). Eagle files in the zip. There's a small error to correct on the ps, the top molex connector should of course be labeled v- and not v+.
Attached Images
File Type: png lm3886-ps-sch.PNG (9.7 KB, 1194 views)
File Type: png lm3886-ps.PNG (38.9 KB, 1162 views)
File Type: png protect-sch.png (12.4 KB, 1167 views)
File Type: png protect.png (45.3 KB, 1164 views)
Attached Files
File Type: zip lm3886-aux.zip (180.0 KB, 65 views)
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Old 4th May 2018, 10:27 AM   #52
00940 is offline 00940  Belgium
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And the bom for the PS. Are not included the screws, spacers, etc. Are also missing the resistors for the snubber. The common 10nf/150nf caps are included.
Attached Files
File Type: pdf bom-ps.pdf (108.4 KB, 143 views)
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Old 4th May 2018, 04:33 PM   #53
bozoc is offline bozoc  Croatia
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Quote:
Originally Posted by 00940 View Post
@bozoc: sadly that heatsink didn't fit. Not wide enough for the diode bridge
I see now, I used this guy, GBU1007 is compatible with the Ohmite heatsink.

I like the DC protect board.
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Old 4th May 2018, 05:06 PM   #54
00940 is offline 00940  Belgium
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The DC protect board is mostly coming from the discussions here and the detector is a simplified version from ESP protection project. The added led indicates when the protection triggers.

It works well in sims and I've used that detector before so I know it works well. This implementation is pretty bare with no mute at turn-on, turn-off but the lm3886 is well behaved under that respect. It's made to be powered from the positive power rail.
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Old 5th May 2018, 08:45 AM   #55
Mark Whitney is offline Mark Whitney  Netherlands
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Here is a link to updatemydynaco.com, it shows why an HBR works and how to implement it. As pointed out before, just place a wire if you don't think you need it.

A layout is always a compromise. I connect the feedback resistor close to pin3, therefore it is difficult to extend the AGND under the resistors. What is the effect of this?

If we look at the loop area of pin 9 and 10 to PGND (feedback+input). Placing the same plane under these circuits will effectively reduce loop area.
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Old 5th May 2018, 04:06 PM   #56
00940 is offline 00940  Belgium
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Sorry for the long answer but I'm trying to understand how this goes...

Quote:
Originally Posted by Mark Whitney View Post
Here is a link to updatemydynaco.com, it shows why an HBR works and how to implement it.
In plain terms, you'd rather have the feedback cap ground pin return to the input signal groundplane ? I did reproduce the sim from akitika + Tom's sims on current returns and it's indeed the best solution.

Quote:
A layout is always a compromise. I connect the feedback resistor close to pin3, therefore it is difficult to extend the AGND under the resistors. What is the effect of this?
I'm not quite clear on what you call AGND. Is it the input gnd plane ?

Ok, here's my understanding of the problem.

Let's consider the loops formed by the feedback resistors. Basically, the feedback network is a divider at the output of the amp. There's a first current loop formed by the feedback resistor and pins 3-9. Gnd aren't included into that loop. So it's only a matter of keeping the loop small, to avoid to pick up radiated crap. It's also a matter of capacitive coupling to the gndplane under it. It's best not to run dirty high current tracks under it. And it's finally a matter of parasitic capacitance at pin 9. For that loop, the compromise is whether to go searching for an optimal sense point, at the risk of picking more radiated noise.

My take on this is that as long as the sense track (from the feedback resistor to the amp output) stays close to the output pour (which is low impedance) to minimize loop area and is not too contaminated by capacitive coupling from the power ground plane (hence a thin track), we're good. That's why I followed Tom's suggestion on this. Btw, that's apparently how it is done on the lm3886dr.

Then we have a second loop, with more current. It's the loop going from the output of the amp, through the two feedback resistors, the feedback cap, the ground loop breaker resistor, power gnd, the power supply, the supply pins of the lm3886. If we want to minimize the loop area, we actually have to run the feedback resistors over the power ground plane, not the input one. While the input gnd is tied to the loop, no current from the loop has to run through it.

The compromise here is in between loop area, which would require the power gndplane to run under the feedback network and capacitive coupling of noise. With through holes resistors and short tracks, probably no big deal. Another aspect of compromise is how we're using the output connector as sense gnd so the layout has to conform to that.

Quote:
If we look at the loop area of pin 9 and 10 to PGND (feedback+input). Placing the same plane under these circuits will effectively reduce loop area.
By pgnd, you mean the signal/input groundplane or the power gndlplane ?

BTW, could we agree on a common terminology ? Let's say there a signal groundplane (sgnd) and a power groundplane (pgnd) on this pcb. AGND sounds too much like analog ground.
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Old 5th May 2018, 05:16 PM   #57
Mark Whitney is offline Mark Whitney  Netherlands
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There is a loop from Pin9 - resistor - GND - resistor pin10.

There is some good data to be found in this post:
LM3886 PCB vs Point-to-Point (with data)

You can see that the layout where the input and feedback grounds are connected together has the lowest THD+N

LM3886_P2P_Layout_INpFBlongGND.png

Only I replace the 6CM lead with a 1ohm resistor.
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Old 5th May 2018, 08:26 PM   #58
00940 is offline 00940  Belgium
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Well, there are two issues here. One is how things are joined and the impact of common tracks' impedance. No objection on that, the feedback cap has to go to the input gndplane. The other thing, less clear, is about loop area and radiations pickup. Groundplanes are relevant to both issues as provide a low impedance connection but also allow to reduce some loops.

So, here's an attempt to satisfy all the compromises. Yeah, I wish... As before, the zip is for the eagle files.

The feedback cap now returns to the input gnd. The feedback resistor going to gnd is over the split between the signal and the power gndplanes. If you consider the loop mentioned in the post above, it's quite tight.

The two gndplanes are joined near the output connectors by a resistor. It can be jumpered in case of a mono amp.

Better now ?
Attached Images
File Type: png lm3886th.png (62.8 KB, 1126 views)
File Type: png lm3886th-bot.png (32.1 KB, 349 views)
File Type: png lm3886th-top.png (35.3 KB, 230 views)
Attached Files
File Type: zip lm3886.zip (71.8 KB, 154 views)
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Old 5th May 2018, 08:29 PM   #59
nigelwright7557 is offline nigelwright7557  United Kingdom
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For my TDa7294 pcb I went for a copper pour for the low level audio ground.
I starred the power supply ground and Zobel ground connection to the zero volt pcb input pin.
This way I don't have any real current going through the ground plane and modulating it.
The smoothing capacitors are important too. The input from diodes are from one side only of the smoothing caps. And the output from smoothing goes to TDA7294.

The results were pretty much zero noise and hum.
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Old 6th May 2018, 04:29 AM   #60
bozoc is offline bozoc  Croatia
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Interesting, you switched feedback resistor to Signal GND. Are there any measurements on that? I think I read somewhere that you can get lower THD with LM3886 and feedback cap grounded to PWR GND.
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