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Impedance Select Switch?
Impedance Select Switch?
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Old 7th May 2018, 12:12 AM   #1
BillyKalfus is offline BillyKalfus  United States
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Default Impedance Select Switch?

I'm designing a headphone amplifier with an OPA1622 output stage. I want it to be capable of being used with 32 ohm and 600 ohm headphones, so I figured I would put a selection switch in the feedback path allowing me to change the gain of the amp for the expected impedance. In the schematic below, this switch would break the connection between R8 and R6. Note that R7 and R10 are the two sides of a volume control pot.

However, I'm beginning to wonder if this is overkill or could cause problems. It could be easier just to always use higher gain, but this would mean that the the volume pot wouldn't be able to travel the whole distance for lower impedances.

I wanted to get some feedback from those more experienced than me in this area. Is this overkill? Is there a better way to implement this? Any feedback would be greatly appreciated!

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Old 7th May 2018, 05:55 PM   #2
PRR is offline PRR  United States
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Response is reasonably linear-in-dB.

Input impedance falls-off to as low as 3K.
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Old 7th May 2018, 06:16 PM   #3
BillyKalfus is offline BillyKalfus  United States
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What are R37 and R38 in your graph?

>>Response is reasonably linear-in-dB.
This is just the Baxandall volume control, which is meant to accomplish this.

>>Input impedance falls-off to as low as 3K.
This circuit is being fed by another op amp, the OPA1612, which at the frequencies I will be using it at has an output impedance of ~5 ohms.

So what does this mean then for an output gain select switch?
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Old 7th May 2018, 11:47 PM   #4
PRR is offline PRR  United States
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Quote:
Originally Posted by BillyKalfus View Post
What are R37 and R38 in your graph?
This circuit is being fed by another op amp, the OPA1612, which at the frequencies I will be using it at has an output impedance of ~5 ohms.
Tiny sense resistors to compute input impedance.

At first glance I thought Zin went to zero. Then realized limited gain made it not so bad. You did not show a driver so I assumed this came from a jack. Growing up with 50K preamps I fret. But in today's world 3K is not a tough load.
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