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Illusus 25th September 2003 06:00 AM

Does this make sense?
2 Attachment(s)
I recently received my LM3875's, this is the schematic I compiled from some of the ones floating around. Will this work?

analog_sa 25th September 2003 07:44 AM


Will this work?
No. Instead of compiling try copying.

Nuuk 25th September 2003 07:45 AM

It should do. It is virtually the same circuit that I started with except that I used a 47K in place of the 18K.

Rudy 25th September 2003 07:49 AM

ehm ... this is inverted amp technologie ( :rolleyes: ) but the input is on the non-inverted input from the chip ..

so it's a no go

MarcelvdG 25th September 2003 07:50 AM

Swap the inputs, otherwise it is a power Schmitt-trigger instead of a power amplifier.

peranders 25th September 2003 09:13 AM

Illusus, wether you are skilled or not check allways the datasheet and application notes. There you have all info for using the IC. If you don't understand some parts we will happily help you.

I know my post is rather boring but National happens to have one of the best datasheets and application notes, very much worth reading.

millwood 25th September 2003 10:16 AM


Originally posted by Nuuk
It should do.

it couldn't have worked, as the inputs are swapped.

it seems that you wanted an inverting topology but with different resistor values. the resistor values will work, but the topology will not.

SY 25th September 2003 12:01 PM

Besides swapping inverting and noninverting inputs, and setting it up for large DC offset by not compensating for bias current, you've also got a situation where disconnecting the speaker could remove the input ground reference.

Bad circuit all around. Use the app notes as P-A suggests. Those National guys are pretty smart and actually know how to use their chip.

Illusus 25th September 2003 12:29 PM

Oh my... inversed input is just a typo, thanks for pointing it out, I may have followed it during construction. With that reversed, do all the values look good? :scratch:

I have read the national sheet, it's not that boring ;) . I ask because I just want to make sure everything is ok, I'm very unsure of my "skill". Someone once told me there are no stupid questions, I hope he didn't say that to make me look stupid.

The reason I decided to compile a couple of suggestions before I start is because it seems that on every "look at my igc" thread the changes I included now, are suggestions.
I have been reading entry level elctronics text books and plan on taking some night classes as I become more and more interested. I am starting to understand schematics not just copy them. I still don't understand much but I do get excited when I look at a simple cuircuit and can follow it. It feels good knowing which component does what, I do have a problem with how much it does of what it's doing (values) bear with me guys one day I may even have some noteworthy input not just annoying questions.

Nuuk 25th September 2003 12:30 PM


it couldn't have worked, as the inputs are swapped.
I didn't check the inputs but I think that was more of a typo than anything. Still, as you guys point out - it would not work that way round.

I do agree that for anyone starting out, it is best to stick with a proven circuit.

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