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Old 22nd May 2017, 02:28 PM   #1
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Default Voltage Gain in LME49610 Reference Schematic

I am using PCM1794A with the following output stages (from datasheet, with 2Vrms out):

Click the image to open in full size.

I want to use LME49610 as my buffer to my headphone, with the reference schematic in the datasheet:

Click the image to open in full size.

As seen from the LME49610 reference schematic, the first stage (U1/U2) is configured as non-inverting amplifier at gain=2, as well as DC servo. Then the feedback path of the non-inverting amplifier of the first stage is then fed into the LME49610 to increase output current capability.

However, the output after the two stages (I/V+Differential Amp.) in the PCM1794A schematic is already 2Vrms (which is enough for headphone), and I don't think I should amplify it again (by gain=2) as shown in first stage of the LME49610 datasheet as 4Vrms might be too high for headphone (is it?).

So, here is my question:

1. What should I do? Should I modify R1 and R3, R2 and R4 to achieve the gain of 1? Any other better option?
2. Is it 4Vrms too much for a headphone (assume I have the current capability by using the buffer amp)?

Thanks.

Last edited by hopkinskong; Yesterday at 10:46 AM.
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Old Yesterday, 09:30 AM   #2
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What happened to my post... Why the contents are gone?
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Old Yesterday, 02:55 PM   #3
AndrewT is offline AndrewT  Scotland
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You only have a history of two posts from yesterday and today. Looks like you have done a lot of reading for the last three years.
I suspect you are still under Moderation until you prove to be a real Member rather than some computer trying to spam us.

Take the output at R7 from the upper diagram and using the lower diagram for U1a & U3 tack on the Buffer and feedback into pin2 of U3 of upper diagram.
If you need the DC servo then add in U1b as well.
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Old Yesterday, 03:44 PM   #4
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Quote:
Originally Posted by AndrewT View Post
You only have a history of two posts from yesterday and today. Looks like you have done a lot of reading for the last three years.
I suspect you are still under Moderation until you prove to be a real Member rather than some computer trying to spam us.

Take the output at R7 from the upper diagram and using the lower diagram for U1a & U3 tack on the Buffer and feedback into pin2 of U3 of upper diagram.
If you need the DC servo then add in U1b as well.
Yes, I suppose my post is under moderation as I have edited my post.

The issue is in the lower diagram, U1a is configured as a non-inverting amplifier with the gain of 2 (1+(R3/R1) = 2), the output of the first diagram is already 2Vrms, and if I amplify the 2Vrms with the gain of 2 in U1a, my signal voltage will be at 4Vrms, and I think it is too much for a headphone.
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Old Yesterday, 04:42 PM   #5
rayma is offline rayma  United States
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Quote:
Originally Posted by hopkinskong View Post
1. What should I do? Should I modify R1 and R3, R2 and R4 to achieve the gain of 1?
2. Is it 4Vrms too much for a headphone?
The volume control can reduce the level down to zero.
There's a 100 Ohm resistor in series with the output. Which headphones do you use?
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Old Yesterday, 05:21 PM   #6
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Quote:
Originally Posted by rayma View Post
The volume control can reduce the level down to zero.
There's a 100 Ohm resistor in series with the output. Which headphones do you use?
Yes, volume control can reduce the level down to zero, but it can still output 4Vrms at full volume.

My headphone is max 100mW with 6Ω impedance, an in-ear headphone. What I want is I can have a quite universal earphone driver to drive up to 36Ω.
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