Hello, new to the board :) / High frequency parasite? [workaround found]

[clément]

Ah nice! Thanks for the example. It's indeed really different compared to mine, not sure which one I prefer though
I just did the same test (a few tones then the noise louder):
https://www.dropbox.com/s/i60emmqf7vzya1f/PreenFM2%20OLED%2015.5kHz.wav?dl=0

As I said earlier, I absolutely don't mind about the general background noise which is absolutely more than fine. The problem I have here is the peak at around 15.5 kHz that is driving me nuts. Each time I open the channel on my mixer I hear this "whistle" first. I suppose I'm a bit over-sensitive there…

EDIT: a link to a picture from Ableton's Spectrum Analyzer
https://www.dropbox.com/s/f7mr6ejufo7i5x4/15.5kHz%20peak.tiff?dl=0

[clément]

I think I'm getting closer… i noticed that the sound on the Left and Right output is different.

- Right output has the high frequency but is also sort of distorted
- Left output gives cleaner sound, less distortion, but more high frequency parasite

Could it be a problem around one of the DACs then? Or the filter with the last chip?

[Xavier]

Are the volume the same on both output when you center the sound ?

I noticed that after removing the 2 buttom screws of the OLED, if i push the OLED closer to the PCB the high frequency peak is increased, if i pull it it decrease.

Do you have any pins you didn't cut that would come close to the OLED and capture the noise ?

[clément]

This is actually my next step! I didn't cut really short the legs of the components so next time I open it I'll make it as short as possible and see if this might not be an electro-magnetic / proximity perturbation.

By the way I did my first rehearsal today with the PreenFM, it was such a pleasure, I think some of my bandmates will be tempted as well

[toneburst]

@Xavier is there anything that could be slipped between the bottom of the OLED and the PCB, to insulate from the noise a little? A sheet of foil-backed cardboard, perhaps?
This may be a very silly question, as I don't know anything about electronics, but I have seem similar shielding around the power-supply of my Poly-800.

a|x

[Mmarsh]

here is the MI topic, very informative: http://mutable-instruments.net/forum/discussion/3455/oled-noise/p1

[Xavier]

That's very interesting but the noise  with the OLED on the shruthi does not seem to be the same.

There's no "white" noise added in the preen, what shows up in voxengo SPAN and that Clément hears is a 15.6Khz sin.
I don't know if this peak is louder on clément's preenFM2 or if he's particularely sensitive at this frequency.

[clément]

Yes I found several topics on the MI forums, but the problems seem to differ from one persone to the other (some talk about a general noise, others about a high freq like mine).
I updated my post above with a picture of the spectrum of my background noise (I thought I had attached it before but I'm not sure if it was successfully embed from Dropbox).

The safest solution might be going back to LCD, it's a pity because it looks great and, who knows, maybe it's my precise OLED that is acting in this way and an other one would not… On the MI forum, most people who had issues or who are more sensitive to those issues on the MI forums went with this solution.

The thing I'm completely sure now is that it's mainly a proximity thing: I breadboarded the screen outside of the PCB and the issue is almost 100% gone.

[Xavier]

The thing I'm completely sure now is that it's mainly a proximity thing: I breadboarded the screen outside of the PCB and the issue is almost 100% gone.

Yes that's what i think too.
And i just tried to insert  a flat piece of metal wrapped in a plastic bag, between the OLED and the PCB.
The peak diminishes (-8db / -10db)
Maybe that's the way to investigate.

[matrix12x]

I also thought making a shield for it would be a good idea. too bad there was no way to enclose/encase the rear of the OLED.

[clément]

Xavier! The metal ruler worked!!!
First I put some electric tape on the PCB and on the back off the screen.
Then I moved around with the ruler as I could, looking for a magic spot and as you said there's a place that is killing the problem straight. I mean it's still a bit there but so quiet.

It is, more or less, around the Positive solder points of the Right Output. Which is also the area where, what a surprise, there is the coil on the OLED

[Xavier]

Great news,
Thanks for the update.
If one day you open your preenfm one more time and if you have the occasion to take a picture, don't hesitate 

I'll change this thread title adding that there is a workaround.

[clément]

Well right now it's pretty ugly inside, tape everywhere, an old key to do the shielding… 
I will try to come up with an idea to make some kind of a Faraday Cage around the coil. Basically this parasite is really to be treated like a sound wave. I have the feeling that if you build a wall around it doesn't get through.

[clément]

Here it is, the shitty DIY Shield to protect from the coil on the OLED.

Cutting some stripboard to make a box


Soldering the first edges


Finishing and putting as much solder as I could


Testing


Adding aluminium tape and a magnet (this is pure mojo)




The results are incredible!


Edited by Xavier : I attached the pictures bellow so that they never disapear. 

[funkyfli]

Maybe a new product?  Patent it!
So, the noise is a proximity issue.  Interesting.