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Old 28th January 2013, 12:04 PM   #1
Loren42 is offline Loren42  United States
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Default Input Impedance Question

Hi!

I need a input line level buffer stage with an input impedance of approximately 470K.

The easiest way to do this is with a non inverting unity gain stage as shown below. Note, the device that drives this is already DC decoupled with a cap.

I would like to use something like the OPA2134 dual op amp to drive a DRV134. I think this circuit will do the trick, but I just wanted to be sure there wasn't any hidden snakes laying in the grass.

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Old 28th January 2013, 12:16 PM   #2
Mooly is offline Mooly  United Kingdom
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It's fine although you don't need R5 (and R1). Depending on what the input capacitance of the DVR134 is, then it might be good practice to add a 68 ohm or thereabouts in series with the output. And do you need a cap on the input ? That's source dependent of course but again, good practice.
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Old 28th January 2013, 12:23 PM   #3
Loren42 is offline Loren42  United States
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Originally Posted by Mooly View Post
It's fine although you don't need R5 (and R1). Depending on what the input capacitance of the DVR134 is, then it might be good practice to add a 68 ohm or thereabouts in series with the output. And do you need a cap on the input ? That's source dependent of course but again, good practice.
Thanks.

R1 was just put there for a load for the sim. Adding a series 68Ω resistor is a good idea. I'll add that one.

R5 can be deleted if it is not needed.

Yes, the source is fully decoupled with its own cap, so I did not want to add another cap in the signal path.
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Old 28th January 2013, 12:30 PM   #4
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Did you check for input bias on that opamp? Input bias current through the DC input R can cause appreciable output offset. Especially when you use input capacitor: the +input bias current will flow through 470k and the -input bias current through zero.
BTW, why the need for 470k Zin? That's unusual in audio.

jan
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Old 28th January 2013, 12:31 PM   #5
Mooly is offline Mooly  United Kingdom
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I can't really see any other problems. Just remember to decouple the opamp with something like a 0.1uf and 10 ohm in series and connect those across pins 4 and 8.
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Old 28th January 2013, 12:33 PM   #6
Mooly is offline Mooly  United Kingdom
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Hi Jan,
I think an OPA2134 was mentioned. FET input.
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Old 28th January 2013, 12:39 PM   #7
Loren42 is offline Loren42  United States
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Quote:
Originally Posted by jan.didden View Post
Did you check for input bias on that opamp? Input bias current through the DC input R can cause appreciable output offset. Especially when you use input capacitor: the +input bias current will flow through 470k and the -input bias current through zero.
BTW, why the need for 470k Zin? That's unusual in audio.

jan
The op amp is actually a OPA2134 with a FET input. Is bias still a concern?

The input impedance is driven by an old PAS 3 tube preamp and the output impedance for that preamp is required to be 470KΩ or nasty things happen to the frequency response (ask me how I know).

This circuit is just a quick & dirty fix until I complete my own DIY preamp.
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Old 28th January 2013, 12:42 PM   #8
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No FET input will probably be OK.
The LT16whatever in the schematic has max 2.2uA input bias and across 470k that's a whopping 1V DC output or so.

jan
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Old 28th January 2013, 12:47 PM   #9
Loren42 is offline Loren42  United States
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Originally Posted by jan.didden View Post
No FET input will probably be OK.
The LT16whatever in the schematic has max 2.2uA input bias and across 470k that's a whopping 1V DC output or so.

jan
Yes, I should not have used that part for the schematic. LTspice did not have a OPA2134 part and I just stuck something in there as a symbol.

My bad.
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Old 28th January 2013, 01:05 PM   #10
Mooly is offline Mooly  United Kingdom
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If your feeding this from a tube amp then you must be certain that not only is the tube amps output cap OK but also that it has a high value resistor to ground (even if its 10meg) on the output. If not and its floating then the cap could charge to some high voltage and this would zap the IC as soon as you connected.
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