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Non-inverting audio Op-Amp

Cirkit

Oct 28, 2015
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I am trying the reduce the gain of a non-inverting Op-Amp circuit used as an audio pre-Amp. I have measured the output and need to reduce the audio level by about 20dB.

How do I calculate the correct resistor value change? Is it a linear relationship or do I need to convert dB gain to voltage gain and input into this equation:

noninvrt.gif
 

duke37

Jan 9, 2011
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The circuit you show will always give gain. If you wish to reduce the signal level, get rid of the op-amp and use a potential divider.

If I remember correctly, 20dB is a factor of 100 in voltage.
 

BobK

Jan 5, 2010
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Nope, factor of 10 in voltage, 100 in power.

You post is not clear, you say you want to "reduce the gain" but you have not told us what the gain is.

Duke assumed you meant you want of gain of -20 db or 0.1. Is this correct? If you plug 0.1 into the gain formula you will find that you need a negative resistance.

Bob
 

WHONOES

May 20, 2017
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You can reduce the gain to unity by throwing away R2. If you need anything less then it will have to be a divider.
 

Cirkit

Oct 28, 2015
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R1 is currently 10k and R2 4.7k. That equates to a voltage gain of about 3. I need to reduce the output from the Op-Amp not the signal entering it.

Based on some audio measurements, I need a 15 to 20dB reduction. How does that translate to voltage gain? Do I subtract 0.1 (-20dB converted to voltage gain) from the existing voltage gain (3) or divide, then calculate the value of R1/R2 using the above formula?
 
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duke37

Jan 9, 2011
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It is still not clear what you want to do. What is the signal level of the source and its impedance? What is the required level of signal and the impedance of the load?

You want a non-inverting amplifier, why is this? An inverting amplifier can give amplification and attenuation.
 

Audioguru

Sep 24, 2016
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Are you attenuating the input so that you can feed it into the tape playback preamp? It won't work well because the tape playback preamp has frequency equalization.
 

Cirkit

Oct 28, 2015
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Are you attenuating the input so that you can feed it into the tape playback preamp? It won't work well because the tape playback preamp has frequency equalization.

No it's an intermediate stage before feeding a differential output stage for an XLR connection.
 

BobK

Jan 5, 2010
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Gains are multplied. If you have a gain of 3, and need 20db less, you multiply 3 by 0.1 to get a gain of 0.3.

You cannot get a gain of less than 1 with that configuration.

You can get a gain of 0.3 with a voltage dovider or an inverting amplifier.

Bob
 
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