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Old 3rd October 2011, 09:35 AM   #1
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Default Simple BUF634 buffer schematic?

I'd like to try a simple BUF634 buffered passive preamp. Does anyone have a link to a good schematic?
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Old 3rd October 2011, 03:11 PM   #2
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Well, if it's buffered it's not really a passive preamp.

You probably mean with no gain, right?
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Old 3rd October 2011, 03:59 PM   #3
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Correct :-)
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Old 3rd October 2011, 04:22 PM   #4
AndrewT is offline AndrewT  Scotland
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Forum Search gave 92 replies for this past year on buf634.

What more do you expect?
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Old 3rd October 2011, 04:44 PM   #5
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I did search and found no shortage of schemes using a BUF634 with an opamp, but none using one without (i.e. as a zero feedback simple buffer). I may have missed some info but did search.....
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Old 3rd October 2011, 11:56 PM   #6
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You don't need to use it inside an op amp circuit, but the offset voltage is fairly large.
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Old 4th October 2011, 12:54 AM   #7
BrianL is offline BrianL  United States
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I'm not sure what you are looking for. Look at Figure 2 on the datasheet; that's pretty much what you've gotta do. I'd add a PP cap in parallel with each of the electrolytic caps shown - 10-100nF. I'd make the input resistor (Rs) something like 1k; I'd probably add an output series resistor of 1-10 Ohms; As mentioned, the DC offset is pretty significant; that calls for some sort of servo or output coupling cap which seems to me a significant compromise compared to wrapping an op-amp around it.

Thermal issues and appropriate heat sinking should be considered.
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Old 4th October 2011, 01:31 AM   #8
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I've tried BUF634 open loop right after a rail splitter and the dc offset wasn't all that bad, around 5mV I think and the output impedance was somewhat high, so might not really need an output resistor depending on what you are driving.
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Old 4th October 2011, 03:55 AM   #9
gootee is offline gootee  United States
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Quote:
Originally Posted by carlmart View Post
Well, if it's buffered it's not really a passive preamp.

You probably mean with no gain, right?
NO gain?! Well even ALL passive preamps MUST have GAIN. But it would have to be a gain of between zero and something less than one, for a passive preamp (i.e. a variable attenuator). <grin> Sorry, couldn't resist.
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Old 4th October 2011, 05:23 AM   #10
BrianL is offline BrianL  United States
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The output resistor is to help stabilize against reactive loading. Curiously, the output impedance is not specified, but taking the slope of the upper-left graph on pg 6 of the datasheet, it appears to be <10 Ohms.

Correction: I'd make Rs more like 50 Ohms.
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