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Old 26th June 2006, 08:09 AM   #1
pwan is offline pwan  United Kingdom
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Default Carlos's Low value res config with 3875

Hi,

The original Carlos's proposal on the low value res config (2K / 100 Ohm for feedback res) is to LM3886. I wonder if anyone ever try this with LM3875 (whose current output is much lower than 3886 in Carlos's design) ?

Thanks.
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Old 26th June 2006, 08:32 AM   #2
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If you have different resistor values you will also get different properties like noise and output offset. To me this seems to be a bit below optimum.
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Old 26th June 2006, 09:37 AM   #3
sangram is offline sangram  India
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I tried with 4766 and it worked fine (4.7K/220). DC-offset is very low (less than 5mV per channel).

I don't like the sound, but it could be any number of things. I suspect it's the chip that doesn't sound good, but I'm going to experiment with different values and then with different resistors before I blame the chip, the resistors, or the values.

Just FYI, I liked the LM4780 best when connected with high value resistors (220K/10K) but in that case even the resistors used were different and the offset was too high. I don't believe in resistor sound too much, but knowing the horrible components we get in our country, I'm not ruling anything out...
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Old 26th June 2006, 11:30 AM   #4
frank1 is offline frank1  United Kingdom
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Quote:
Originally posted by sangram
I tried with 4766 and it worked fine (4.7K/220). DC-offset is very low (less than 5mV per channel).

I don't like the sound, but it could be any number of things. I suspect it's the chip that doesn't sound good, but I'm going to experiment with different values and then with different resistors before I blame the chip, the resistors, or the values.

Just FYI, I liked the LM4780 best when connected with high value resistors (220K/10K) but in that case even the resistors used were different and the offset was too high. I don't believe in resistor sound too much, but knowing the horrible components we get in our country, I'm not ruling anything out...
How about measuring the performance so you can quantify "doesn't sound good"?
The PC sound card is a very powerful tool for audio testing. There are some very good audio spectrum analyser tools available (free). Some even contain a tone generator.
Personally, I use TrueRTA (there is a free but limited version) for testing my amps.

What's audible should also be measurable!

Frank
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Old 26th June 2006, 11:54 AM   #5
pwan is offline pwan  United Kingdom
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Quote:
Originally posted by peranders
If you have different resistor values you will also get different properties like noise and output offset. To me this seems to be a bit below optimum.
Perander,

What do you mean "To me this seems to be a bit below optimum."?

PS: Thanks frank, sangram.
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Old 26th June 2006, 12:23 PM   #6
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Quote:
Originally posted by pwan



What do you mean "To me this seems to be a bit below optimum."?


Maybe that noise will be too low
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Old 26th June 2006, 12:27 PM   #7
sangram is offline sangram  India
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Quote:
Originally posted by frank1


What's audible should also be measurable!

Frank

I agree. I actually have a full suite of test tools downloaded some time ago, including a distortion analyser, tone generator, etc. My recording software has a spectrum analyser built in as well, and I have Speaker workshop as well which works well.

Never got around to using any of it though...

Maybe I'm just lazy!!!

Edit: I think what P-A was saying is that the resistor values need to be higher than 2.2K/100 ohm. I guess I agree.
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Old 27th June 2006, 11:34 AM   #8
pwan is offline pwan  United Kingdom
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OK I will try it soon.

Thanks all for your comments.
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