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, 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? __________________ Bill Fitzpatrick, the grumpiest person on the face of the planet. We may run out of food, we may run out of energy, we may run out of breathable air but we'll never run out of idiots. So, eat the idiots, burn their bones and be grateful for less hot air. ________________ Not of the body and certainly out of my mind. Pass the mugwump ji . . er . . juice. Nikko Pre-Amp, GAS Son, GAS Grandson, Mirage Electronic Crossover, Marantz CD63SE, Satellites and 2 12" woofers/ch. Not all DIY but what the hell. Rap sucks big time. Mingus rules.

19th January 2004, 10:51 PM	#8
SY diyAudio Moderator Join Date: Oct 2002 Location: Austin, TX	Rather than dropping things with an output series resistor, why not use a 10:1 voltage divider at the input? __________________ "It is not seemly, after wiping your nose, to spread out your handkerchief and peer into it as if pearls and rubies might have fallen out of your head." - Erasmus

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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, 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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