Unity Gain Operation (or low gain)

Tomo · 19th January 2004, 12:52 AM

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 · 19th January 2004, 12:55 AM

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, 01:31 AM

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

Bill Fitzpatrick · 19th January 2004, 01:49 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, 02:18 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, 04:41 PM

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, 10:39 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, 10:51 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, 10:53 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

limiting the HF in GC

Tomo · 20th January 2004, 12:13 AM

Hi,

Thank you Greg!

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

Tomo

19th January 2004, 10:51 PM	#8
SY diyAudio Moderator Join Date: Oct 2002 Location: Austin, TX Blog Entries: 1	Rather than dropping things with an output series resistor, why not use a 10:1 voltage divider at the input? __________________ "...we stumble and get up, we are sad, confident, insecure, feel loneliness and joy and love. There is nothing more; but I want nothing more.” - Christopher Hitchens 1949-2011

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19th January 2004, 12:55 AM	#2
Tomo diyAudio Member Join Date: Jun 2002	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

19th January 2004, 01:31 AM	#3
crown300 diyAudio Member Join Date: Oct 2002 Location: pittsboro, NC	For 8 Ohm headphones, put a 330 Ohm resistor in series with the headphone.

19th January 2004, 01:49 AM	#4
Bill Fitzpatrick diyAudio Member Join Date: Jun 2001 Location: Eugene, OR	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?

19th January 2004, 02:18 AM	#5
crown300 diyAudio Member Join Date: Oct 2002 Location: pittsboro, NC	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.

19th January 2004, 10:39 PM	#7
Tomo diyAudio Member Join Date: Jun 2002	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

20th January 2004, 12:13 AM	#10
Tomo diyAudio Member Join Date: Jun 2002	Hi, Thank you Greg! I found the section. Time to study it in close details. Tomo

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