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napierzaza

Audio Output Issue

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Hey,

I got a mixer and every-time I plug the audio cables into the mixer my MIDIBOX FM shuts off. If I pull them out again it turns back on. Is this an electrical issue? I'm not sure what kind of problem there might be.

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Is this an electrical issue?

Well, I assume the audio cables that you're plugging into the mixer, are also connected to the MBFM? If so, it's an electrical issue; otherwise you have poltergeists.

I guess you made a soldering error on the MBFM, if this issue doesn't effect other synths. Time to grab your multimeter and a copy of the schematic....

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But strangly it works when I plug it directly into small speakers (except that there isn't much audio mind you).

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reversed polarity on audio outs?

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No, that' can't be. My outputs are RCA and they're very straight forward in regards to the audio pinning.

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Ground Loop Issues?

Are they both using the same power supply?

Just guessing. :-)

LyleHaze

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They're not using the same power supply, but I don't really think there's anything I can do about that. My FM does not supply 120V AC, and so I'm pretty stuck on that part.

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You could use a DI box between your MB & Desk.. it sounds like some grounding issue to me.. like there is a voltage potential between your MB's ground, and your desks ground.. (For the hell of it, measure it with your multimeter)

http://www.wikirecording.org/DI_box

It will need to be fixed, but a DI or even an audio transormer could help for the mean time.

Regards

Mike

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How could such a large ground difference happen though? I have them plugged into the exact same outlet. Would it be better if I connected my GND to my metal case? This in turn is connected to the GND of the outlet.

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How could such a large ground difference happen though? I have them plugged into the exact same outlet. Would it be better if I connected my GND to my metal case? This in turn is connected to the GND of the outlet.

That could help, but because your MB is crashing when you connect it to something that is earthed, doing the above might just stop it from working..?

did you try, as smash said, reversing the audio outputs?

I'm still thinking it's something to do with the way you have wired up your power transformer.. triple check it's how it should be.. (Or are you running of a wall wart?)

Regards

Mike

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Hmm something occurred to me - have you, anywhere in your studio, done the 'i have an earth loop, so I'll cut the earth pin on this thing' trick?

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No I have not tried that. Is this a good thing or a bad thing?

Also I am worried about grounding my +-12v power supply to my case, it's a Canakit where the middle gnd appears to be one of the AC lines. This does not seem kosher to me, but I have no idea about such a thing.

I can't find a picture, but will take on this evening, someone on this page recommended it. It doesn't require a center tap and it uses a 7912 and a 7812.

I can make one of my own if I have to, does anyone have a schematic? I have a transformer with two separate 15vAC and I have a single 24vAC I can use.

I want to get this thing working finally, I feel very lazy for not playing with the actual synth yet. Too much troubleshooting.

Also, would it be bad news if I applied power to the amps without the 5v being on?

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...it's a Canakit where the middle gnd appears to be one of the AC lines. ...

i have little doubt that this is the source of your troubles.  it must be that you have the neutral ac line connected to ground in your circuit, which is in turn connected to the shield on your cables.  this is no good in general, and very bad if the outlet wiring has hot and neutral reversed (a common error)

if so, you should be able to break the connection between the ac side and your circuit ground at the psu...

and only the earth ac line (not neutral) should ever be connected to the chassis

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Here is the schematic of this particular circuit. It does not include the capacitors to keep it more simple. Though the pointless fork may confuse you, it's just there for the sake of the capacitors which aren't included.

So you can see that on AC line is directly taken as GND. Is that kosher?

3963_Untitled-1_png19a6a9085bc66dde26a87

3963_Untitled-1_png19a6a9085bc66dde26a87

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7912 +12V / 7812 -12V ? Is it wired that way too?

Edit: maybe a version with the caps too would be good...

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Whoops, I may have mislabeled.

I can assure you that it is producing the +-12 though.

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make sure that neutral on the primary side of your transformer is not connected to gnd.  if it's not, then try swapping the ac inputs on the secondary side, i think it might be a phase issue...

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Okay, thanks. I will give that try. But I guess I still can't ground the common to the case?

Also, if anyone does have a schematic for a bipolar circuit using either two transformers or a center-tap, I'm all ears.

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...But I guess I still can't ground the common to the case?...

i wouldn't, but it might be difficult to avoid if you use metal jacks...

Also, if anyone does have a schematic for a bipolar circuit using either two transformers or a center-tap, I'm all ears.

they're all over the net.

here's a good one:

http://www.cgs.synth.net/modules/cgs14_psu.html

here's one in our own wiki:

http://www.midibox.org/dokuwiki/doku.php?id=bipolar_12v_psu

here's a better dual wave circuit:

http://www.musicfromouterspace.com/analogsynth/WALLWARTSUPPLY/WALLWARTSUPPLY.php

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ACtually now that you mention it, I guess it is already more or less grounded to the case. Hrrmm, anyhow, will switch around the PSU inputs today just to see.

Much appreciated.

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Wait,

am I changing the ac lines on the primary 120vAC side or the secondary side? I was just thinking about it and realized that I'm not sure if it would matter if I switched the secondary connections.

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either way, though the hot and neutral on the primary usually are color coded...

phase usually doesn't matter until you connect it to another device on the same ac circuit.  with a full wave rectifier it still doesn't matter, but in this case...

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