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-   -   Unity Gain Operation (or low gain) (http://www.diyaudio.com/forums/chip-amps/26330-unity-gain-operation-low-gain.html)

Tomo 18th January 2004 11:52 PM

Unity Gain Operation (or low gain)
 
2 Attachment(s)
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

Tomo 18th January 2004 11:55 PM

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

crown300 19th January 2004 12:31 AM

For 8 Ohm headphones, put a 330 Ohm resistor in series with the headphone.

Bill Fitzpatrick 19th January 2004 12:49 AM

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?

crown300 19th January 2004 01:18 AM

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.

GregGC 19th January 2004 03:41 PM

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.

Tomo 19th January 2004 09:39 PM

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

SY 19th January 2004 09:51 PM

Rather than dropping things with an output series resistor, why not use a 10:1 voltage divider at the input?

GregGC 19th January 2004 09:53 PM

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

http://www.diyaudio.com/forums/showt...threadid=25515

Tomo 19th January 2004 11:13 PM

Hi,

Thank you Greg!

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

Tomo


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