Simple BUF634 buffer schematic?

sharpi31 · 3rd October 2011, 09:35 AM

I'd like to try a simple BUF634 buffered passive preamp. Does anyone have a link to a good schematic?

carlmart · 3rd October 2011, 03:11 PM

Well, if it's buffered it's not really a passive preamp.

You probably mean with no gain, right?

sharpi31 · 3rd October 2011, 03:59 PM

Correct :-)

AndrewT · 3rd October 2011, 04:22 PM

Forum Search gave 92 replies for this past year on buf634.

What more do you expect?

sharpi31 · 3rd October 2011, 04:44 PM

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.....

sawreyrw · 3rd October 2011, 11:56 PM

You don't need to use it inside an op amp circuit, but the offset voltage is fairly large.

BrianL · 4th October 2011, 12:54 AM

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.

nereis · 4th October 2011, 01:31 AM

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.

gootee · 4th October 2011, 03:55 AM

Quote:

Originally Posted by carlmart

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.

BrianL · 4th October 2011, 05:23 AM

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.

3rd October 2011, 09:35 AM	#1
sharpi31 diyAudio Member Join Date: Oct 2005	Simple BUF634 buffer schematic? I'd like to try a simple BUF634 buffered passive preamp. Does anyone have a link to a good schematic?

3rd October 2011, 04:22 PM	#4
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	Forum Search gave 92 replies for this past year on buf634. What more do you expect? __________________ regards Andrew T.

4th October 2011, 12:54 AM	#7
BrianL diyAudio Member Join Date: Mar 2002 Location: USA	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. __________________ bel

4th October 2011, 05:23 AM	#10
BrianL diyAudio Member Join Date: Mar 2002 Location: USA	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. __________________ bel

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3rd October 2011, 03:11 PM	#2
carlmart diyAudio Member Join Date: Mar 2001 Location: Brazil	Well, if it's buffered it's not really a passive preamp. You probably mean with no gain, right?

3rd October 2011, 03:59 PM	#3
sharpi31 diyAudio Member Join Date: Oct 2005	Correct :-)

3rd October 2011, 04:44 PM	#5
sharpi31 diyAudio Member Join Date: Oct 2005	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.....

3rd October 2011, 11:56 PM	#6
sawreyrw diyAudio Member Join Date: Apr 2006 Location: Minnesota	You don't need to use it inside an op amp circuit, but the offset voltage is fairly large.

4th October 2011, 01:31 AM	#8
nereis diyAudio Member Join Date: Jun 2011	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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