Balanced Line Attenuator
In a home recording environment equipment often doesn’t come in a large variety because of either limited budget or limited space… or both. I am presenting here an balanced attenuator which comes in between fixed gain preamp and recording device/soundcard.
The purpose of this device is to adjust the level and monitor it to prevent saturation of the next stage in the recording chain. It uses a L-pad followed by a balanced buffer stage. The attenuation steps are 0dB, -3dB, -6dB, -9dB and -12dB.
In the schematics above J7 will be used later on future revisions to indicate symmetry in the balanced signal.
J3 will connect the attenuation selector.
There are certain modifications i did the initial schematic tho. R7, R8 must be of greater value. At least 100k. With 100k you will get a -0.8dB signal at 0 dB setting.
The circuit is pretty straightforward, you can use any quad opamp chip for U3 as long at it operates from a 9V single supply. I do recommend a FET input opamp if R7 and R8 >= 100k as they tend to have lower noise than bipolar with high input impedance.
U2 i used a LM324 as it allows operation at 5V single supply. U2:C is used to bias the ADC inputs at 2.5V.
I will explain how to rectify the signal with a PIC in a future project but you will find the hex file for this one at the end of this article.
For more detail: Balanced Line Attenuator
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