MIDIbox FM background noise [solved]

MIDIbox FM background noise [solved] kinda digital origin Rate Topic: 1 Votes

#1 Schrabikus

MIDIbox Newbie

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Posts: 37
Joined: 20-April 11
LocationSiberia, Russia

Posted 15 January 2012 - 11:20

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.

Attached File(s)

MBFM_noise.mp3 (620.8K)
Number of downloads: 22

This post has been edited by Schrabikus: 03 March 2012 - 08:22

#2 Shuriken

MIDIbox Tweaker

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Posts: 359
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LocationAlkmaar, the Netherlands

Posted 15 January 2012 - 12:43

Although not easy, i would try a different PSU. The PSU is probably a switcher, which can cause noise if the switching frequency is low.

This post has been edited by Shuriken: 15 January 2012 - 12:44

#3 TK.

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LocationGermany

Posted 15 January 2012 - 14:17

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.

Buy TK a Beer Disclaimer: buying TK a beer gets you absolutely nothing in return likesuchas firmware enhancements, technical advices and MIDIbox troubleshooting assistance.

#4 Schrabikus

MIDIbox Newbie

Group: Members
Posts: 37
Joined: 20-April 11
LocationSiberia, Russia

Posted 15 January 2012 - 15:29

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!

Attached File(s)

MBFM_noise_capacitors.mp3 (474.55K)
Number of downloads: 14
MBFM_noise_capacitors_resistors.mp3 (290.33K)
Number of downloads: 18

This post has been edited by Schrabikus: 15 January 2012 - 15:32

#5 Schrabikus

MIDIbox Newbie

Group: Members
Posts: 37
Joined: 20-April 11
LocationSiberia, Russia

Posted 15 January 2012 - 15:43

I've removed additional caps from OPL3 board and voila!!! No noise at all!
So increasing capacity of PSU output caps + adding 10 ohm resistors totally eliminates the noise.

#6 TK.

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Joined: 01-January 01
LocationGermany

Posted 16 January 2012 - 00:04

Very cool!
Great to know that this simple method works!

Best Regards, Thorsten.

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