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

[clément]

Hello everyone,

My name is Clément, I'm from France but relocated for a few years in Brussels.
I put my PreenFM2 (got the metal box kit) yesterday and could not stop playing with it since! I can not say how much I am pleased and impressed by everything. The design of the case if wonderful it's amazing to have such a nice piece of equipment kinda half DIY. The sound is great, the navigation easy and the program on the computer makes it even easier to create sounds, even for a total FM noob like me


Since I completely fell in love after such a short time of using it, I also have to report something that is bugging me, and I haven't seen this problem reported so far…

I have a high frequency parasite, kinda like this more or less 15kHz old TVs were producing. It's not that loud, be enough to drive me crazy and not to use it on stage.

I tried all kinds of plugging, with & without USB, power adapter, and even a 9V battery, so I'm 99% sure it comes from the PreenFM.

It's here always at the same volume wether I turn the Vol knob up or down, so I would guess it's not exactly on the signal path, but more in the Ground?


Has anyone experienced this? Any idea what could be the source of this parasite?



And here's a picture of my build, just for the pleasure

[funkyfli]

The 'parasite' is more than likely from the OLED.  They are noisy little b*ggers. On my synth, the noise is bad at the top 5% of the volume pot but much lower below.  It is low enough that it does not bother me at all.  They do like to infect the ground with digital noise.  Maybe, a bad solder joint?

Martin.

[clément]

Thanks Martin, I think the OLED seems to be exactly the problem. I just checked on google and the first answer is from… the Shruthi community! I have never noticed this on mine because I don't have an OLED screen it seems (I have to say the screen of the PreenFM looks so nice!). Anyways, some people have been struggling with the same issue, there's even someone who measured it around 15kHz so I guess I'm getting close. I can confirm that in the last 5% of the Volume course it gets worse, but still, to me it's not usable like this on a long term.

Although I was sure of my soldering I soldered again everything with no result. If I'm patient enough I might take the screen off and take a listen…

To be continued!



EDIT:
OLED removed => no more noise!
Now I have to find a way get rid of it because I do need the screen

[funkyfli]

I just checked as a matter of interest.
With a simple monophonic sine wave at -6 dB.  The regular noise is at -67 dB.  Which seems very reasonable for a synth.  The noise at the 5% end is about -54 dB. I think that might be to do with the limits on the pot.

Martin.

[Xavier]

Hi Clément,

I'm glad you like your new FM synth. 
I can hear a high pitch noise parasite if my headset volume amplifer is set the the MAX.... which kill my hears if i play sound with pfm2 volume more the 50%.

For the story, the first idea was to put a blue OLED screen, but the noise it added made it unusable.
The yellow one is hardly hearable on reasonable volume on the preenfm i have.

You can try to manually add (make pins touch the PCB no need to solder) some electrolyte capacitor in parallel to the existing 220uF ones.
Le me know if you find anything,

Thanks Martin for the details, nice to see you onboard

Xavier

[Mmarsh]

Seems to me there was a thread on the Mutable forum regarding a fix for the OLED noise involving soldering a cap across a pair of pins on the screen.  Don't remember the details...

[funkyfli]

Hmm.  I have an ugly blue Newhaven one.

[Xavier]

Hmm.  I have an ugly blue Newhaven one.

  And it's usable ?
My 2 newhaven blue OLEDs add a huge background noise and not only in the high frequencies.

And right, i just noticed voxengo SPAN shows a peak at around 17Khz with the yellow one. But i can't hear it. My hears must be too old 
I can lower it a little (-3db) by adding a big electrolyte capacitor between GND and 5V at the bottom left of the PCB.

Let me know if you find something better,

Xavier

[clément]

Hello everyone,


First of all thanks a lot for taking the time to reply!

Unfortunately so far my research wasn't very successful… I tried to put a big electrolytic cap across the spot you talked about Xavier but the reduction is not enough for me I'm afraid. Didn't get great success around the 220uF neither.

Mmarsh: indeed I found a topic in the Mutable community, the idea was to put a cap at the spot Xavier was referring to, but straight on the OLED pins, which I tried also.

Martin: I suppose I'm a bit over-sensitive in this frequency range, but I'm sure my synth is somehow faulty, I'm going to double check again the soldering because I'm usually not really difficult with background noise.


I did find one solution that works 100%: plug the PreenFM through a passive D.I. with GND Lift. Like this I have a regular background noise, there might be this frequency a bit stronger (when you know it), but it's more than reasonably quiet!


EDIT: I forgot, does anyone know of a compatible non-OLED screen?

[Xavier]

Hi Clément,

There's no LCD screen compatible with this OLED one.
LCD are much thicker.

But read your PM

Xavier

[clément]

Hey Xavier, thanks for the reply, I'll go to my PMs

[funkyfli]

I wonder if individual OLEDs have different noise levels?

[Xavier]

If you actually can use your BLUE oled display, i would answer yes.

But from my experience, my 2 blue displays are unusable.
The 5 or 6 yellow ones i tried were all silent.

Xavier

[funkyfli]

Maybe I'm too forgiving 

I'll upload an audio test...

[funkyfli]

My analyser tells me the offending frequencies on the blue OLED are 2-3kHz!

https://www.dropbox.com/s/3i55fexekwdrzuw/preen%20test.wav?dl=0
Wav file below... regular noise, sine waves, full volume noise.

Not terrible noise, but now you've got me thinking I should upgrade to yellow.  Though, I do like the red, white and blue colour scheme. 'Vive la Francais!' 

Martin.