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Old 5th February 2007, 08:24 PM   #1
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Default LM3886 - P19, Any comment?

The PCB was made in Eagle 4.13, so if someone wants .pcb file let me know
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Old 5th February 2007, 09:04 PM   #2
Nordic is offline Nordic  South Africa
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care to post the schematic or source?
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Old 5th February 2007, 09:18 PM   #3
sek is offline sek  Germany
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Hi,

Just as a sidenote to the topic: Why is it that numerous people use their eCAD software for making boards without starting from the schematic?

It defeats most of the purposes of, e.g., Eagle. You would be quicker with a drawing program like Illustrator - or even Paint - if You consider the learning curve. Some of the features You're really missing:

- abstraction between component functionality and physical package (packages can be exchanged while always maintaining electrical correctness),

- electrical "rule checking" (schematics can be tested for plausibility and corrected independent of board layout),

- physical "rule checking" (board layout can be adjusted independent of electrical schema),

- maintainability (schematic or board can evolve and become improved while never risking errors in other parts of the respective other files),

just to name a few... Besides that, discussing it on a forum would be a lot easier (if we had Your schematic that explicitly matches Your board) as Nordic already said.

Just my two cents, have fun!
Sebastian.

PS: I understand that P19 is ESP's Project 19, a Chipamp that Rod Elliot made a couple of years ago, inspired by Siegfried Linkwitz's excellent amplifier project.

But still, how should we check your board against Rod's schematic. That's what Your computer is for...

edit: PS
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Old 5th February 2007, 11:49 PM   #4
jpg is offline jpg  Germany
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Quote:
numerous people use their eCAD software for making boards without starting from the schematic
Maybe roly3055 does have a schematic and just didn't post a screenshot?
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Old 6th February 2007, 01:51 AM   #5
Leolabs is offline Leolabs  Malaysia
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Quote:
Why is it that numerous people use their eCAD software for making boards without starting from the schematic?
Because the schematic is so simple or common.
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Old 6th February 2007, 10:02 AM   #6
sek is offline sek  Germany
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It's not about complexity.

My point is: would the original poster have used a schematic in Eagle, his question could narrow down to layout beauty doubts. Electrical correctness is hard to verify on a board by looking at it. Before the question wether the board is good comes the question: Was the well-known schematic applied correctly.

Tools are there to rule out this uncertainty, and they are free and in the hands of the poster.
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Old 6th February 2007, 10:20 AM   #7
sek is offline sek  Germany
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Hi roly,

to contribute to your question: Your power rail traces have 'regular' with, which is too narrow in this case. Make them as wide as possible, there's no need to keep a ground plane as far as to the chip.

Eagle has 'faston' connectors (which you seem to try to resemble with jumpers) in the library 'con-rib'. The 6.3mm vertical connector is named 'ST6,3', for example.

Try experimenting with larger filter capacitor packages or wider footprints (e.g. 7.5mm or 10mm), which are also available form many parts suppliers. Also, those caps look a little small for the purpose, but you might of course go with the 1000uF approach.

You also micht want to have a read about snubbed supplies for chipamps (just do a search for snubberized gain clones). You can increase filter capacitance while improving the sound, according to many DIYers.

What does LSP2 do? It might sit in an uncomfortable place. I won't stress that I knew if I had a schematic.

Cheers,
Sebastian.
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Old 6th February 2007, 12:33 PM   #8
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Default Schematic

The schematic is Elliott Sound P115
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Old 6th February 2007, 01:18 PM   #9
BWRX is offline BWRX  United States
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As sek suggested, use fast on tabs or through holes for wire connections. You can make your own, use the ones in the library, or find a nice terminal block footprint. I prefer to have the option to use terminal blocks for power, ground, and speaker connections.

Thicker power and speaker traces would be good (also as sek suggested).

I would suggest moving your volume pot in front of the opamp input buffer along with the coupling cap. This has a number of advantages. Now you can get rid of R6, C1, and C4 and change R9 to a 1kohm resistor. It will allow the low output impedance of the buffer to directly drive the input of the 3886 with better results. It also means you can use a smaller value coupling cap due to the higher input impedance of the buffer. You can now change the layout a bit to make the signal trace much shorter and have a more direct route from IC1 to the 3886 input pin. Also, add a ground connection alongside the +/- supply connections for the buffer supply.
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Old 6th February 2007, 01:34 PM   #10
sek is offline sek  Germany
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Allright, LSP2 is signal ground.

You include a buffer and a pot connection with your chip amp. That's not Project 19. Which is why I rumbled in the first place.

Cool idea with the jumpers to set the input stage gain.

You won't need two DC blocking caps around the volume potentiometer, one at the input side of the amp section (as in P19) will be sufficient.

220uF for supply buffering is definitely too little for a 24V supply and 4-16Ohm speakers, except you use separate supply regulation/filters (which P19 requires). Other than that, it works good to have both filter caps at the same side of the chip.

As you have the buffer/preamp onboard, I would recommend a star grounding scheme instead of a ground plane. You have a ground loop around your preamp section, and the copper that connects signal ground to power ground also shares current flow with the poweramp section.

Also, you don't seem to connect the preamp supply's ground onboard - which actually requires another ground star in the supply...

You left out the fuses. That's good as long as you're okay with that and take care of safety in the main power supply.

Increasing your R9 to 3k3 will increase input impedance and thus shift input bias a little. This could result in DC output, but you could give it a try like this and experiment with the value.

Your drawup of the jumpers and connectors looks a little odd! Did you hardwire the TL's supply by hand? Look at Eagle's 'invoke' command, either in the toolbar or as a console command. It lets you place the opamp's power pins independent of the amp symbol. You didn't do that (or at least I can't seem to see it), which is why you had to be creative with the circuitry around J4/J5.

Last, not least: I already pointed out the too thin traces for power and the missing scheme for ground. This would impact sound quality and fun the most, everything else I mention is peanuts compared to the power and ground layout and wiring problems you could have.

What project would you like to use it in?

Cheers,
Sebastian.

EDIT: Uh, Oh! I take it back. Brian's suggestion with the pot will influence the sound the most! (He was quicker typing than me ) But everything else is still important...
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