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Old 6th December 2008, 05:02 AM   #1
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Default LM3875 as a voltage buffer?

Hi everyone,

I need a voltage buffer to match impedances in a summing buss that I am building. Can the LM3875 be configured to work as a voltage buffer with a gain of -1? My power supply is a bipolar 24V. I would be very grateful for a circuit. I'm unsure about what Rf/Ri ratio would be optimal for this.

I need to buffer a 50k or 100k pot. The pot atteunates the output of a John Hardy 990 discrete opamp configured a a summing amplifier. The output of the 990 feeds the pot which in turn feeds a cinemag output transformer. I can't really use a regular buffer chip because the 990 has a 28dBu headroom which means the 990 could easily output 19V signals. Therefore I need a buffer that can handle input voltages of about 19V (or 28dBu of headroom). The output transformer is a Cinemag CMOB 1 with a 680 ohm input impedance and a 1:1 turns ratio.

alternately do you folks out there have suggestions for another chip that would work better?

Thanks in advance!

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Old 6th December 2008, 09:58 AM   #2
AndrewT is offline AndrewT  Scotland
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Join Date: Jul 2004
Location: Scottish Borders
a discrete opamp follower by an IC buffer?
Use any of the discrete FET buffers that abound here. Select one that can run on >=+-30Vdc rails. Start with Pass.
regards Andrew T.
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Old 6th December 2008, 04:00 PM   #3
Gopher is offline Gopher  United Kingdom
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Why on earth would you want to use the LM3875?

Number 1 - it's not unity gain stable so it will make an excellent RF oscillator.

Number 2 - there are far better OP-AMPS with far lower distortion you could use.
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Old 6th December 2008, 06:56 PM   #4
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Location: Los Angeles
Yes, I think a discrete buffer is the right way to go. Could you guys please post links to threads that discuss discrete buffers? I couldn't find any. I'm basically a tube guy ; ) this is really my first solid state project and its for a friend!
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Old 7th December 2008, 04:41 PM   #5
ratza is offline ratza  Romania
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wrong post, please delete.
Any solution is a compromise.
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