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MIDIbox FM background noise [solved]


Schrabikus
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Hello, friends!

Can you please listen to attached mp3 and tell, is it normal, or something wrong?

Noise remains when i disconnect LCD and CS from the core.

So far i can not exclude PSU (it's one from dvd player +12v/-12v/+5v) but noise lasts about half a second after switching PSU off.

Cable from core to OPL3 board is about 12cm, soldered directly to core.

Thanks in advance.

And the OPL3 +5v led lights with only J2 connected.

MBFM_noise.mp3

Edited by Schrabikus
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It sounds like the noise is coming from the LED switching matrix, this could be the result of a certain coupling between the 5V and +/-12V voltage domain.

Try the following:

- ensure that the ground of the audio jacks + the OPL3 board is directly connected to the ground of the PSU (and not to the core module)

- add 100 uF caps between ground an the -12V and +12V supply input of the MBHP_OPL3 module (take care for the polarity!)

- if this still doesn't help, add 10 Ohm resistors in serial between the +12V and -12V output of your PSU and the appr. MBHP_OPL3 inputs, this helps to decouple the noise as well!

Best Regards, Thorsten.

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Have no words to thank you, Thorsten!!!

With 100 uf caps at OPL3 board the noise decreased by half (MBFM_noise_capacitors.mp3)

Than i've changed 47 uf PSU caps with 200 uf and added 2x10 ohm resistor

Aaaand the noise disappeared completely!!!!! (MBFM_noise_capacitors_resistors.mp3) :sorcerer:

conclusion: switching PSU from dvd player can be used in audio apps!

MBFM_noise_capacitors.mp3

MBFM_noise_capacitors_resistors.mp3

Edited by Schrabikus
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  • 1 year later...

I just had this problem, an idle noise in the background, using a TRACO TXL 035-1212D - a quite high end PSU for this application in fairness.

 

Cured it totally by moving the sleeve of the audio outs to the - on the PSU rather then the one on the Core as i was doing previously..

 

No need for caps or whatnot, this is a total solution!

Edited by mrspring
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