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Old 21st September 2012, 10:04 PM   #1
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Default Rod Elliott ESP opamp pre's question

Hi all,
I've been looking at the opamp preamplifiers over on Rod Elliott's site, namely Project 02, Project 88, and Project 97. They're all pretty easy to understand, even for a novice like myself. However, I there is one thing I don't understand, I'm hoping someone here can explain to me:

In all three of the designs, Rod has a 100k resistor to ground on the output of the preamp (parallel to the load). In Project 02 it's R2, in Project 88 it's R10 and in Project 97 it's R116. What is the purpose of this resistor? I understand that it forms a high-pass filter in conjunction with the preceeding coupling capacitors and blocks DC from reaching the input of the power amp. What I don't understand is why he wouldn't just leave the resistor off and rely on the input impedance of the power amp. What advantage is there to including that resistor as opposed to leaving it off?

Thanks for any insights!
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Old 21st September 2012, 10:28 PM   #2
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It's not part of a filter, it is simply a path for the coupling cap to charge if nothing is plugged in to it, so that when something is plugged into it you don't get a pop-thump.
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Old 21st September 2012, 10:28 PM   #3
WSJ is offline WSJ  United States
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The input resistor provides a path to ground for the op amp input bias current. It also discharges the cap in the previous stage. Without the input resistor the
op amp input bias current would eventually charge up the cap in the previous stage and the voltage on the input pin would drive the output to the rail.
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Old 21st September 2012, 10:32 PM   #4
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Quote:
Originally Posted by richie00boy View Post
It's not part of a filter, it is simply a path for the coupling cap to charge if nothing is plugged in to it, so that when something is plugged into it you don't get a pop-thump.
Thanks!
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Old 22nd September 2012, 08:52 AM   #5
AndrewT is offline AndrewT  Scotland
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If the source and the receiver are soldered together then the Grounding resistors at the output and the input become simple parallel loads that are unnecessary.
You can then delete them.
Where detachable connections are used, then it is wise to have them for the reasons stated.
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