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Old 7th October 2013, 06:07 PM   #1
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Default Best LM1875 Circuit?

What is the best LM1875 circuit? Is the schematic same for tda2030? Can i swap them with no extra changes?
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Old 7th October 2013, 09:17 PM   #2
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As far as I am aware yes. There should be no changes.
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Old 7th October 2013, 09:35 PM   #3
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The test circuit in the LM1875 datasheet has a 1M resistor between the input and GND, just before the input cap. Be sure to put it because my amplifier was oscilating without that resistor ... don't ask why...
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Old 7th October 2013, 10:39 PM   #4
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The test circuit in the LM1875 datasheet has a 1M resistor between the input and GND, just before the input cap. Be sure to put it because my amplifier was oscilating without that resistor ... don't ask why...
Opamps need a DC path for input bias on inverting and non-inverting inputs, otherwise you get instability and all that goes with it.

Mike
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Old 7th October 2013, 11:12 PM   #5
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It is not for bias because it is before the decoupling cap, and furthermore 1M would be too large a resistance to provide this bias current; this is where the 22K after the decoupling cap is for.

This is not to say I can tell you where the 1M is for and how it does prevent oscillations. But, by applying some logic, it is clear that the 1M becomes relevant only at very low frequencies. So the 1M might have to do with creating a path to ground for the signal wire in case the preceding stage is also cap decoupled. I will have a good night's rest over the question how the absence of this resistor could lead to oscillations before venturing any more guesses.
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Old 8th October 2013, 03:17 AM   #6
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Minor point: "Decoupling" caps are the ones that go from power to ground (or sometimes from rail to rail). AC Coupling caps are the ones in series with the audio signal.
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Old 8th October 2013, 11:23 AM   #7
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I promise that it will be more entertaining than the datasheet sample, but I cannot promise that it will be more suitable. Anyway, I prefer it.
Its my 2007 design, when I had even less idea of what I was doing; however, after a technology update for 2013, she finally images like a champ, has even more headroom, and the blockbuster dynamics are now sheer overkill. In my opinion that thing sounds like music. It is surprising.
Click the image to open in full size.
Don't be confused--Although diodes are shown, they are actually filters (a sort of shield); therefore, the power is expected to be DC from a real power supply board. It is compatible with the prefabricated K50 boards.
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Old 8th October 2013, 11:39 AM   #8
AndrewT is offline AndrewT  Scotland
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................... however, after a technology update for 2013, she finally images like a champ, has even more headroom, and the blockbuster dynamics are now sheer overkill. In my opinion that thing sounds like music. It is surprising.............
What were the changes?
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Old 8th October 2013, 01:22 PM   #9
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Don't be confused--Although diodes are shown, they are actually filters (a sort of shield)
Hi Daniel,
I'm curious, how do those diodes "filter" DC?

Mike
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Old 8th October 2013, 04:03 PM   #10
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Originally Posted by gootee View Post
Minor point: "Decoupling" caps are the ones that go from power to ground (or sometimes from rail to rail). AC Coupling caps are the ones in series with the audio signal.
Gootee, you are so right.
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