Noticed a reference to the "Mackie Onyx phantom power whine" thing over in the classified section, and since I recently dealt with this I thought I'd (start to) post how to fix that.
I bought a second-hand Satellite recently (no hate to the seller - it sounds good without the phantom and it was a fun challenge to figure out how to fix the whine), and noticed a really annoying high frequency whining noise when the phantom was engaged. A few emails with tech support actually got me all the service info, including schematics. They've kind of abandoned the satellite at this point. Anyway, the problem (which is apparently shared by the 400f rackmount thing) is that they designed the DC-DC converter to get 48vdc (phantom power voltage) from the lower Firewire voltage by running at audio frequency. A DC-DC converter works by changing DC to AC, pushing the voltage up either by a transformer or a doubler-type circuit, and then converting back to DC. Best if you run that at a very high frequency, so it's kind of stupid for Mackie to have done this at 6KHz.
So, how do you fix this? The service docs told all, though not in a very easy form. The DC-DC circuitry is pretty crude, a 555 timer being used as an oscillator to push AC into a sort of voltage doubler/tripler/something stack - I think this may be what's referred to as a "charge pump". The 555 frequency is set by a capacitor and resistor, and the last revision (according to the docs) changed the critical resistor from 10K (might have been some 1K versions) all the way down to some 301 ohms, changing the frequency from 6K to 160K. That's a big change, but a lucky one for us solder-savvy end users. Because all you have to do is replace one resistor, and the problem is pretty much gone. The downside is that you have to deal with surface-mount, but that's the worst of it. |
