Mackie Onyx Satellite - phantom power whine

Scodiddly

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I just recently bought one of those Mackie "Onyx Satellite" things pretty cheap second hand, but it turns out to have a nasty high frequency whine when phantom power is switched on.

So it's a fun challenge! I'm still wrangling with tech support over this, but I was able to find some schematics too, and I'm figuring that I'll probably end up doing some soldering rather than getting it fixed by Mackie, assuming the latter is even an option since they seem to be abandoning this product.

Here's the phantom power DC-DC oscillator:
555.jpg


According to the notes, last revision was "R5 change from 10k to 301, phantom Fsw from 6k to 160k". I did a little poking around for sample 555 timer circuits and didn't see one that looked like the above, so I'm wondering if the resistor change from 10k to 301 ohms makes sense for that frequency change. Obviously taking it out of the audible range seems like a no-brainer, especially if a single resistor can do it.

Plausible?

I haven't cracked the case open yet to see which revision I have.
 

Scodiddly

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I put in a 332 ohm resistor (closest thing I had), and I had a nice quiet output for a few minutes. Then I started hearing the damn whine again. Thinking it over, I decided to try the 1K resistor that's shown on the schematic. The whine is a bit higher, but still audible.

I need to grab my scopemeter from work so I can see what's really going on. I'm thinking this wasn't the greatest design by a long shot, though maybe it's possible to salvage it still. Just by the sloppy schematic (many different font sizes over revisions and whatnot) I'm not very impressed.
 

JohnRoberts

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That schematic is truncated so it;s hard to see in context but looks like a cap doubler or tripler to make 48V from some lower voltage. If that frequency is too low it could be audible. Or it could be beating with some other high frequency noise.

555 is an old school bistable, so this is a pretty simple circuit. Frequency too low is from too large resistor.

I'd look up 555 app notes to make sense of it all..

JR
 

Scodiddly

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Yeah, I did look up the app notes, but hadn't seen a circuit exactly like that. The output goes back into the threshold input?

Anyway, lower values of resistor raise the frequency. It's still audible, though I need to find an actual 301R resistor since the closest I had was 332R. Maybe I have one at work...

The other funky thing in this design is that this circuit is very close to the multipin connector for the dock, so there's an easy path for the noise to radiate into the inputs. This evening I used copper foil tape to cover the oscillator and doubler/tripler/quadrupler? circuitry to help shield that. I'm getting very close - most of the time the whine is lost in the noise floor. With the official resistor value I'm thinking maybe I'll be at a frequency that doesn't happen to have some harmonic relationship with the digital stuff.

All that aside, I did some recording with my original Prodigy-Pro fum group buy ribbon mic, and it sounds quite good. I'm not much on the whole "mic pre" hype, but without the phantom problem this unit sounds quite good with that mic and plenty of gain. Better monitoring would be nice; near as I can figure if you want to hear yourself live you're stuck with some significant (30-40mS???) latency in the headphones while recording.
 

Scodiddly

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OK, we have success!

Changing to a 300 ohm resistor pushed the frequency up to around 154KHz, as measured by my Fluke scopemeter. Not the 160KHz mentioned in the revision, but apparently close enough. Between that and the copper tape shielding it seems like the whine is completely gone. I might do some testing to see if the noise floor has increased, but so far no complaints.
 

tk@halmi

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YO' DA ACE!!! Nice fix.
All the Onyx based gear has this whining problem going on. Sales staff at Guitar Satan can't hear it of course. I have returned three 400Fs then gave up.
 

Scodiddly

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Maybe the smaller units have the problem. We've got a couple of the Onyx 1640 mixers at work and the have a real phantom supply.

While researching this I found that there's a different sort of whine in some of the units, related to the Firewire implementation. "Mac Firewire whine" or something to that effect. No problems with that, anyway.
 
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