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Analog Line Level Preamplifiers , Passive Pre-amps, Crossovers, etc.

What does my potentiometer do?
What does my potentiometer do?
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Old 3rd April 2020, 04:38 AM   #1
TomC1970 is offline TomC1970  United States
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Default What does my potentiometer do?

First off...yes...I'm a *******. "I know just enough to be dangerous" would be an overstatement. If I put a pot at the output of my opamp circuit...do I have a gain control? Or a volume control? Or a variable output impedance control? Thank you all for your patience. ---Tom
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Old 3rd April 2020, 09:28 AM   #2
jaddie is offline jaddie
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You have all of them, though a volume control is a specific function with a purpose and the others are generic

What are you trying to achieve?.
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Old 3rd April 2020, 09:36 AM   #3
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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You will have a volume control if the wiper is the output.
It will not adjust the gain but may have an effect on the impedance feeding the next stage.
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Old 3rd April 2020, 09:54 AM   #4
jaddie is offline jaddie
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Quote:
Originally Posted by JonSnell Electronic View Post
You will have a volume control if the wiper is the output.
It will not adjust the gain but may have an effect on the impedance feeding the next stage.
Connected that way it will adjust the total system gain by applying varying degrees of attenuation.

The output impedance will vary with wiper position.
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Old 3rd April 2020, 01:26 PM   #5
Mark Tillotson is offline Mark Tillotson
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If the pot's a 1M ohm one, its also a notable Johnson noise generator. If its 2k its nice and quiet and within the capabilities of most opamps to drive without extra distortion.


If the next stage is bipolar you need a coupling capacitor in series with the wiper output to prevent loud crackles.
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Old 3rd April 2020, 02:15 PM   #6
TomC1970 is offline TomC1970  United States
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Thank you all. The concept in mind is a preamp with 2 outputs....with a switch to choose the output. Output one is a single opamp unity gain "buffer" circuit.....output 2 sends the signal through a second opamp to serve as a "driver" for going to tubes and transformers. The pot in mind would be at the output of the second stage. ---Tom
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Old 4th April 2020, 03:59 AM   #7
jaddie is offline jaddie
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Quote:
Originally Posted by TomC1970 View Post
Thank you all. The concept in mind is a preamp with 2 outputs....with a switch to choose the output. Output one is a single opamp unity gain "buffer" circuit.....output 2 sends the signal through a second opamp to serve as a "driver" for going to tubes and transformers. The pot in mind would be at the output of the second stage. ---Tom
Put the buffer(s) after the pot.

There's no need for a special driver for going to tubes and transformers if the output buffer is correctly designed. This is a world of low source Z drivers and high to very-high Z inputs, so it doesn't take much.

You might want to make sure your opamp buffer is unity-gain stable. Some aren't, some need compensation. Or run a bit of gain. And a small build-out resistor on the output...stuff like that.
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Old 4th April 2020, 05:53 AM   #8
TomC1970 is offline TomC1970  United States
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Thanks for the advice and info guys!
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