Audio Attenuator PCB

Overview

Primarily was interested in designing a custom PCB for the first time. This is a pretty basic circuit that attenuates or reduces the volume. Reason for a static value vs one of those inline volume control knobs is the cheap ones tend to introduce static in to audio when adjusting the dial. Also stereo separation can sometimes start to offset where one side is louder than the other.

This is primarily designed to handle the scenario where you have line level output and you need to feed it in to an input that takes mic level audio. Using the given values will give a 10:1 reduction in power which corresponds to 20 db reduction in audio. This is the least it will reduce the audio. In reality the output impedance also plays a role. For example if you have an output impedance of 1000 Ohm, that adds to the 1k Ohm resistor meaning it becomes 2k and 100. Which gives you about a 30 db reduction in audio. In the situation of a really hot line level audio needing to feed a mic input that still works fine. If you're interested in learning more this has a pretty good summary of what's all happening.

Requirements

Building it

• R1 and R2 are the larger of the two resistor values. Recommend 1k ohm 1/4 watt 1% and then match them to find two as close together as possible
• R3 and R4 are the smaller of the two resistor values. Recommend 100 ohm 1/4 watt 1% and then match them to find two as close together as possible
• J1 and J2 are based off the audio jacks I found on amazon. Take note that a few have same footprint but pins are arranged differently. Should have one in middle towards front and 4 along back. Only the two outer pins are used. Below are links to actual component used including the alternate 4 pin and 3 pin versions
  • CUI SJ1-3525N - 5 pin version. The extra pins are commonly used amps and such where if nothing is plugged in to jack then audio will pass through and come out via RCA jacks or something but then when you plug headphones in it will disconnect the path to RCA jacks and send audio to headphones. Gives more flexibility to buy some extra to use in other circuits and only a couple cents more.