Should I mod my FireBox?

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BirdFLU
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Should I mod my FireBox?

Post by BirdFLU » Thu Aug 13, 2009 1:27 am

I don't use the audio outs on my Presonus FireBox very much. If I open it up and there is a capacitor on each output, would disconnecting each cap enable the FireBox to pass CV? If so would I still be able to use them as audio outs? Is there a downside to disconnecting the caps?

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Kent
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Post by Kent » Thu Aug 13, 2009 3:00 am

Try it with just one channel. If it works, mod the rest.

There is a good chance that removing the capacitor will do what you wish, but there is more to it than just the cap. You can always put the cap back. Also, you can simply solder a wire across the + & - connections on the PCB and lift one of the legs of the cap. Unless it is SMD, then just pull it.

Please report back with success or failure.

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1nput0utput
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Post by 1nput0utput » Thu Aug 13, 2009 8:47 am

I considered doing this with an old original Mbox that doesn't get much use any more, but I couldn't figure out which capacitors to remove. My circuit analysis skills were not quite strong enough. If anyone has any advice, it would be much appreciated.

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Post by doctorvague » Thu Aug 13, 2009 9:20 am

I might be mistaken about this, but I thought you'd want to short across the cap rather than remove it (or replace it with a jumper - same thing). These caps are in series and therefore removing them would give no output at all, yes-no?

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Kent
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Post by Kent » Thu Aug 13, 2009 12:28 pm

doctorvague wrote:I might be mistaken about this, but I thought you'd want to short across the cap rather than remove it (or replace it with a jumper - same thing). These caps are in series and therefore removing them would give no output at all, yes-no?
Well, yeah.. you gotta remove 'em and then put a bridge (piece o' wire) across where the cap was. You don't really have to remove the cap in order to test the theory, just bridge it in parallel. However, it is best to remove it. Sometimes the type of construction will make this decision for you. Such as, is it all SMD?

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Post by doctorvague » Thu Aug 13, 2009 12:53 pm

Kent wrote:
doctorvague wrote:I might be mistaken about this, but I thought you'd want to short across the cap rather than remove it (or replace it with a jumper - same thing). These caps are in series and therefore removing them would give no output at all, yes-no?
Well, yeah.. you gotta remove 'em and then put a bridge (piece o' wire) across where the cap was. You don't really have to remove the cap in order to test the theory, just bridge it in parallel. However, it is best to remove it. Sometimes the type of construction will make this decision for you. Such as, is it all SMD?
OK, I get it now. If you get lucky and can physically get to it, you might even be able to bridge a cap with a small alligator clip (especially the tiny flat-jawed ones) to short out the cap, just as a test before going deeper in the rabbit hole.

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Post by 1nput0utput » Thu Aug 13, 2009 2:24 pm

doctorvague wrote:… you might even be able to bridge a cap with a small alligator clip (especially the tiny flat-jawed ones) to short out the cap, just as a test before going deeper in the rabbit hole.
That's what I tried with the Mbox. There are a bunch of caps that look like they're in the signal path to the output jacks. I just shorted one at a time while generating a steady DC signal with Volta and measuring it with a voltmeter. I couldn't figure it out, though.

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BirdFLU
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Post by BirdFLU » Thu Aug 13, 2009 6:43 pm

I'll tinker around and report back.

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Post by johnnymad » Thu Aug 13, 2009 7:25 pm

please let us know. i have a firebox, and would consider trying the mod if it works.

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BirdFLU
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Post by BirdFLU » Thu Aug 13, 2009 8:30 pm

Image

So the outputs 2,4, and 6 are labeled here in orange. Are the tiny things marked by the pink pointers the capacitors for those outputs? There isn't one on the input, that's why I ask. Or, hopefully, are the row of 6 capacitors I circled in yellow the ones I should try?

I'm not touching those rice grain sized SMDs, I'll never get them back on.[/img]

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Post by johnnymad » Thu Aug 13, 2009 8:33 pm

presonus is located in my town, and i know a bunch of people that work there. i'll see if they know anything about the capacitors.

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BirdFLU
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Post by BirdFLU » Thu Aug 13, 2009 10:03 pm

Cool! Thanks! They replied promptly to my e-mail asking if it was DC-coupled (answer: no).

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Post by doctorvague » Fri Aug 14, 2009 6:47 am

BirdFLU wrote:Image

So the outputs 2,4, and 6 are labeled here in orange. Are the tiny things marked by the pink pointers the capacitors for those outputs? There isn't one on the input, that's why I ask. Or, hopefully, are the row of 6 capacitors I circled in yellow the ones I should try?

I'm not touching those rice grain sized SMDs, I'll never get them back on.[/img]
The electrolytics circled in yellow are not it.
The other ones you marked probably are. Set up your test and then just short across the cap with a wire, alligator clip, etc.

Are we looking at unbalanced or balanced outs here?
I assumed balanced outs would have 2 caps per channel - one for the hot and one for the cold, but that's just conjecture on my part.

I might post in the DIY section about that question.
EDIT: I did - here's the thread
viewtopic.php?p=92347#92347
Last edited by doctorvague on Fri Aug 14, 2009 6:57 am, edited 1 time in total.

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Kent
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Post by Kent » Fri Aug 14, 2009 6:50 am

Do you have a continuity tester? If so, probe from one side of the cap to either the HOT or COLD of a cable stuck in the jack.

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Post by doctorvague » Fri Aug 14, 2009 7:01 am

Kent wrote:Do you have a continuity tester? If so, probe from one side of the cap to either the HOT or COLD of a cable stuck in the jack.
You beat me to it again Kent!
One side of the cap should be a dead short to the output (zero ohms) and the other side of the cap should be an open circuit (infinite ohms). Which means it is blocking the meter's DC, as intended.

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BirdFLU
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Post by BirdFLU » Fri Aug 14, 2009 6:21 pm

I put the negative probe in the tiny hole next to the SMD by jack 6 and the positive probe registered "closed" on the metal tab furthest away from the chip on jack 6. So should I remove the SMD or bridge across it?

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BirdFLU
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Post by BirdFLU » Fri Aug 14, 2009 10:23 pm

With the power on, I put a probe on either side of the SMD chip and got no voltage. Shouldn't I have gotten something? Also with the power on, I used an alligator clip to bridge the SMD and it had no effect on the voltage at the output jack.

Any suggestions?

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Tim Stinchcombe
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Post by Tim Stinchcombe » Sat Aug 15, 2009 11:36 am

BirdFLU wrote:Are the tiny things marked by the pink pointers the capacitors for those outputs?
No, those are not capacitors, they are resistors!

Tim

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Post by Tim Stinchcombe » Sat Aug 15, 2009 12:07 pm

I've circled what look to be caps. That MC33079D chip is a low-noise quad op amp, so the caps in the red circles look like normal bypassing to the rails (I'm guessing it is [edit: at least] a four-layer board, so the vias either end are taken to internal power and ground planes).

The 4 orange-circled ones look to be stability caps for the op amps, being in parallel to the feedback resistors. The 4 blue-circled ones, since they have a via at one end which might be going to an internal ground plane, are likely to be performing some sort of filtering for high frequency noise.

In short, none of these look like DC-blocking caps in the signal path!

Tim
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Post by doctorvague » Sat Aug 15, 2009 4:00 pm

Thank you Tim!

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BirdFLU
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Post by BirdFLU » Sat Aug 15, 2009 5:34 pm

Tim, thanks. Yes it appears to be a multi-layered board. I gave up and put it back together. Unless someone has a good suggestion. I'll just use it for MIDI and Audio.

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Post by Tim Stinchcombe » Sat Aug 15, 2009 6:00 pm

Having looked at the spec sheet for the box, I can kind of get the gist of part of the circuit: the 'bottom half' (i.e. two op amps-worth) of the chip is turning the differential (i.e. balanced) parts of both inputs to a single-ended voltage (I'm reasonably sure the op amps are configured as subtractors) - one of the inputs must be coming from the upper board through that multi-pin header. The 'TRS outputs' are spec'd as being '50 ohms', so I think the pink-labelled resistors are probably marked '50xx' or somesuch, being 50ohm or thereabouts ('500' would be good for a 3-digit code, but I can't quite make out what that last character is likely to be). It looks feasible that the large electrolytics (circled in yellow) are in fact the DC blocking caps, but you'd probably need to trace the circuit out a bit more to try and understand what each of the major 'blocks' of circuit actually are. Not impossible, but would require a modest investment of time...
Tim

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Post by BirdFLU » Sun Aug 16, 2009 12:31 am

It would also require some skills I don't have. I'm afraid I would do some damage I couldn't undo. Thanks for your help though.

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Post by Sound » Sat Sep 26, 2009 4:37 am

Thanks for share this experience BirdFLU, here gzifcak did it:
viewtopic.php?p=108441#108441

Regards.

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