Author Topic: PreenFM Noise issue; whining, tone, high pitch  (Read 232 times)


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PreenFM Noise issue; whining, tone, high pitch
« on: July 23, 2018, 08:04:17 AM »

Just writing for the community about the last discovery about The noise.

In short.

The noise in the audio path is coming from the OLED screen inductor.
The whole issue is happening because the inductor that is used on the OLED screen is unshielded and radiates EMF (electromagnetic field). One can google more info about the inductor EMF if needed.
Also what doesnt help is that inductor is located exactly on the other side of the Opamp gainstage and its components. This all together equals an audioble noise.
Gainstage picks up inductor EMF vibration. Frequency is the same.

This is a spectrometer screenshot. The conditions for dB measurements are relative. Signal has been run through a MIC preamp with full gain.
Condtions are exactly the same for all the measurements, so you can compare.

PreenFM 2 with Yellow OLED screen. This is what the peak looks like when the noise is present in the audio signal.
dB scale measurement is only for comparsion. Its peak is around 13650 Hz = 13,650 kHz

Test 1:
Aluminium shield between the OLED screen and main board. 0,75mm thickness, no ground.

Test 2:
Shielded SMD inductor instead of the original open inductor. Observe the shift in the frequency.

Test 3:
Shielded SMD inductor plus Aluminium shield. Building instructions in next post.
« Last Edit: August 31, 2018, 11:32:23 AM by served »


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Re: PreenFM Noise issue; whining, tone, high pitch
« Reply #1 on: August 31, 2018, 11:16:23 AM »
Here is how we build our units to achieve the Shield + Inductor shielding solution against the High-pitch noise.
This is the maximum that can be achieved according to our todays knowledge.

Rev3 shield and Inductor mod is done on all the assembled PreenFM units including the Eurorack build.

Possible inductors that we use are from Mouser:
(I would suggest using the first one.)

The work involves de-soldering 1 SMD inductor and re-soldering a new one.
While doing this. You must keep the PCB as clean as possible. If you have not worked with SMD components before be very careful!
It can be done with a regular Solderingiron. Tweezers can also help.

Moddification looks like this:

The original inductor located L1

New Shielded Inductor. Orientation is not important.

Aluminium shield fixed to the screen.

« Last Edit: August 31, 2018, 11:27:32 AM by served »