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Old 10th October 2002, 11:12 PM   #1
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Default LM3875 Inverted Layout Comments Please

Can anybody please post some feedback on this layout based on Thorsten's LM3875 inverted schematic posted here:

http://home.attbi.com/~greggbaker/invertedLM3875.gif

I would really appreciate comments on what should be the proper grounding layout....

The jpg is not to scale. The actual size is 92.5mmx40.5mm

Thanks in advance.
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Old 11th October 2002, 01:27 PM   #2
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Default pad separation

it would appear that the pad separation is a little close on a couple of the LM3875 pins. As some of the pins are not connected to anything I just snip them off -- they really aren't needed for support. I have enclosed an updated snapshot of the PCB design which I use. The connector is a PC-power type. The image below is a little pixilated due to resizing, for a better image go to www.tech-diy.com/LM3875.gif
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Old 11th October 2002, 03:17 PM   #3
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Default Pins

Quote:
Originally posted by jackinnj
it would appear that the pad separation is a little close on a couple of the LM3875 pins. As some of the pins are not connected to anything I just snip them off -- they really aren't needed for support.
In fact pins 2, 5, 6, 9, 10 and 11 are not connected on the 3875, so I think they can be snipped off.

One thing I was not aware of on the 3886 I was oing to use, is that the mute has to be conneced through a resistor and not left open, as I thought it should be.

Jackinnj: what program did you use for your pcb? and where are the resistors?

Ppereira: have you shown your pcb design at the gainclone site?


Carlos
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Old 11th October 2002, 03:23 PM   #4
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Default Assembly

One thing I forgot to say is that if you snip the pins off, you should provide some way to secure the pcb and heatsink together. And only solder the pins after pcb, heatsink and chip are all bolted to their respective surfaces.

This is also valid if you leave all the pins.

If you don't take such precautions you may have future problems. Intermittents probably, which are the worst.


Carlos
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Old 11th October 2002, 03:53 PM   #5
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Carlmart -- I use Ultiboard -- the 700 pin version (if anyone want's to know how to bootleg more pins just create two circuits being careful of the board layout -- then merge the two text files containing the plotter code).

The feedback resistor and compensation network are on the bottom side of the board --
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Old 11th October 2002, 05:13 PM   #6
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Carlos,

I posted it here first because there is a wider variety of experienced people in this forum. My problems so far with Opamp based amplifiers revolve around grounding noise. I was hoping that somebody could pinpoint where I am making the mistakes that I am sure I am making

Jackinnj,

Thanks for your comment. I will remove the unused pins. Nice layout. It's based on the same schematic as mine, right?
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Old 11th October 2002, 06:18 PM   #7
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Quote:
Originally posted by jackinnj
Carlmart -- I use Ultiboard -- the 700 pin version (if anyone want's to know how to bootleg more pins just create two circuits being careful of the board layout -- then merge the two text files containing the plotter code).

The feedback resistor and compensation network are on the bottom side of the board --

you bastich! right now i'm limited to 100 with labcenter electronics ARES lite....700 to me would be more than i could use
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Old 11th October 2002, 06:46 PM   #8
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Default yupp, same schematic

I don't think that you need to use an expensive cap in the compensation loop -- I got mine from Mouser. the THD measures around 0.01% to 0.04% -- not bad for a couple bucks.
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Old 12th October 2002, 12:44 AM   #9
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How does your setup sound? Do you hear any noise (static) coming from the speakers when idle? That's my problem right now using a non-inverted "gainclonish" schematic... I assume it's a grounding problem because the noise is slightly louder when I turn the volume all the way down... (dact stepped attenuator 20k)

I am using the auricaps because I just happen to have a few laying around...
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Old 12th October 2002, 01:27 AM   #10
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Default A big power opamp, huh?

Interesting thread.

I hope this question isn't too naive (I'm into tubes, not SS), but has anyone tried what most folks first do to get better sound out of an opamp: biasing the output into class A?

As the supply rails are loosely regulated, a true CCS should be used and not a simple resistor to the negative rail.

The LM3875 is called a "quasi-class AB" amp, so I hope that it will take pulling into a little deeper class A.

Any opinions on what current values to try?

Thanks all,
Steve Jones
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