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Preamp upgrade help
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Old 4th November 2017, 03:07 AM   #1
jpmullins1 is offline jpmullins1
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Join Date: Nov 2017
Default Preamp upgrade help

I am trying to implement some improvements published by a member named Plexus to a Parasound PPH-100 phono preamp (Plexus Parasound PPH-100 Modifications). I am having trouble understanding one of the instructions in the writeup: "the final gain stage, now using a FET opamp, needs to have it's non-inverting input, which was normally to ground, be impedance matched with the inverting input. ... so i added an 18k resistor in to ground."

I could use help with clarification on which pin of the op amps is the non-inverting input that needs the resistor to ground. Much thanks in advance for any available assistance!
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Old 4th November 2017, 10:26 AM   #2
AndrewT is offline AndrewT  Scotland
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Join Date: Jul 2004
Location: Scottish Borders
the pin numbering in the sch is wrong.
opamp a shows out=6, -IN=2, +IN=3
opamp b shows out=1, -IN=2, +IN=3
Look at a dual opamp (opa2134)
opamp a out=1, -IN=2, +IN=3
opamp b out=7, -IN=6, +IN=5
Presumably with these dual opamp a is used in one channel and b is used in the other channel?

Check to see what pins are used for which channels and which is used as signal input and which is grounded.

In any case -IN are always=2&5 and +IN are always=3&6
+IN is non-inverting input
-IN is inverting input
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regards Andrew T.
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Old 5th November 2017, 10:30 AM   #3
epicyclic is offline epicyclic  United Kingdom
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The pins you want are pins 3 and 5 on the OPA2604 dual opamp .


Andrew the circuit has single opamps in the inputs of the 2 channels but a sharing dual on the outputs .
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Old 5th November 2017, 09:32 PM   #4
jpmullins1 is offline jpmullins1
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Join Date: Nov 2017
Thank you both for the replies, that clears things up. I'm rusty on electronics projects, but I should have figured out "+" = non-inverting. I appreciate it.
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