I'm trying to use the mic input on my iPhone to record line level audio.
I have made a cable with an attenuator as shown in the picture below.
Recording from my macbook pro works like a charm (3,5mm headphone jack).
Recording from my DJ mixer (stereo rca), or UR22 audio interface (1/4" jack). I get a rather loud buzzing sound. audio clip
This is my first time building something like this myself, I'm probably making an obvious mistake, would be nice if someone could help me out.
Update: turns out it was due to a faulty jack, not making proper ground connection on the TRS side.
to anyone wanting to to do the same, use the schematic, it works! but switch R1 and R2 for something lighter if you dont need heavy attenuation.
Best Answer
Audacity says your hum is 50Hz and -37dB which is quite high.
The cause is due to ground currents and high impedance between the two power sources from Y filter ground currents in the DJ mixer which is AC powered or powered by different outlets.
Assuming the power cords are far from the audio cables, then the common mode hum can be reduced by two methods;
a) Reducing the differential ground impedance between DJ case or jack shield and Iphone ground input (DCin) or by connecting both to earth ground.
b) raising the common mode (CM) impedance of both signal and ground from the DJ Mixer using an "Audio isolator" which the RCA pugs and jacks with a large CM choke or DIY and make it with the cables cut into a large audio CM choke. This effectively balances the signal source.
b) is preferred to isolate the iPhone from earth ground for leakage current ( which feels like a buzz) when holding the phone and touching another earth gnd or bare feet on non-insulated concrete or grass.
To further explain this, I found this video; https://www.youtube.com/watch?v=l4famaQmWnA