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Old 18th January 2004, 11:52 PM   #1
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Default Unity Gain Operation (or low gain)

Hi,

I am thinking about using LM1875 or equivalent power opamps to drive Grado headphones. I am hoping that it might well be operating in Class-A at ear-splitting volume.

The main concern is that it might blow the cans. But what if I operated the opamp at fairly low gain such as 1 ~ 3. I found a following schematics in LM675 specs.

I am wondering if I can use this with LM1875. Also, I am wondering if I were to attack another resistor to the feedback resistor to fix up gain other than 1.

Thanks,

Tomo
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Old 18th January 2004, 11:55 PM   #2
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Oh,

And, I would be appreciated if you can tell me how this is proper when the chip is not unity gain stable?

Seems like snubber inserted between the inputs ... am I wrong?

Tomo
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Old 19th January 2004, 12:31 AM   #3
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For 8 Ohm headphones, put a 330 Ohm resistor in series with the headphone.
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Old 19th January 2004, 12:49 AM   #4
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I'm wondering why it's OK to put a series resistor in line with 8 ohm headphones. Won't this screw up the damping?
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Old 19th January 2004, 01:18 AM   #5
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1) Per the datasheet, the minimum gain for LM1875 & LM3886 is 10. Then the fastest and easiest way to drop power is to use resistor in series with headphones. For 8 ohms, use 330.
There may be a more "correct" or eligant way, I don't know of one.

1) It may "technically" screw up the damping.

2) Try it, and let us know if it does.
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Old 19th January 2004, 03:41 PM   #6
GregGC is offline GregGC  Canada
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Default Re: Unity Gain Operation (or low gain)

Quote:
Originally posted by Tomo
Hi,

I am thinking about using LM1875 or equivalent power opamps to drive Grado headphones. I am hoping that it might well be operating in Class-A at ear-splitting volume.

The main concern is that it might blow the cans. But what if I operated the opamp at fairly low gain such as 1 ~ 3. I found a following schematics in LM675 specs.

I am wondering if I can use this with LM1875. Also, I am wondering if I were to attack another resistor to the feedback resistor to fix up gain other than 1.

Thanks,

Tomo

It should work. You can add a res between -In and GND to change the gain. R2/R1 should be bigger than 10 (so that LM1875 is stable). The cap, use someting like 1uF.
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Old 19th January 2004, 09:39 PM   #7
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Hi,

I don't want to be limited by the guidelines. Besides according to this schematics, you can have it operating at UNITY gain.

I want to know how the snubber connected across the inputs will bring stability to opamp which isn't unity gain stable.

Tomo
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Old 19th January 2004, 09:51 PM   #8
SY is offline SY  United States
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Rather than dropping things with an output series resistor, why not use a 10:1 voltage divider at the input?
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Old 19th January 2004, 09:53 PM   #9
GregGC is offline GregGC  Canada
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Quote:
Originally posted by Tomo
Hi,

I want to know how the snubber connected across the inputs will bring stability to opamp which isn't unity gain stable.

Tomo
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Old 19th January 2004, 11:13 PM   #10
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Hi,

Thank you Greg!

I found the section. Time to study it in close details.

Tomo
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