Noisy Octopre

ChuckU

ChuckU

New member
Not sure what's going on, but my Focusrite Octopre is connected to a FW host (RME Fireface 800) via ADAT optical and I'm getting an ungodly hum on the first two channels, which can be connected to via 1/4" in the front or XLR in the rear. The hum has come whether I'm plugged into the front or rear.
I know the unit is the source because the RME can operate without it. Also, I've gotten the noise when my mixer is bypassed. I've tried different cables too...

Sometimes when I turn the unit off and back on, the noise disappears...

I'm not a real gear-head, but I'm wondering if this might be a shielding problem. But could it be digital? Right now, the RME is the Master clock, via BNC cable, despite the fact that Focusrite recommends the Octopre be master since it is sending ADAT clock. I have not done this yet.
 
ChuckU said:
I'm not a real gear-head, but I'm wondering if this might be a shielding problem. But could it be digital? Right now, the RME is the Master clock, via BNC cable, despite the fact that Focusrite recommends the Octopre be master since it is sending ADAT clock. I have not done this yet.
Click to expand...

Clocking errors don't sound like hum. From your description I'd guess an internal problem, possibly in the power supply, but since it only affects two channels, probably more local. Can you post a clip of the noise? Is this a new problem, and have you successfully using the gear together in this manner previously?

Also, you might as well try setting the RME to slave just to eliminate that as a problem.
 
I'm not sure I have server access to link to a clip, but I am leaning toward this being a hardware problem. I was wondering if I should open up the thing, but then what? I have no idea.

I have used these units in tandem this way before without incident, but the Octopre is not brand new and it was opened by the previous owner to install the ADAT I/O board.

I will definitely reverse the clocking though. That's an easy one.
 
Reversed clock, no problem. I have found that the noise problem persists when I plug into the Lo-Z inputs of either the RME or the Octopre if the Octopre is on - no matter which cord I use.
I did consult with the "experts" at Guitar Center and they all believe there is not a problem with the Octo itself, despite the fact that it is without a doubt the source. More likely, proximity of devices to each other. I may try to put the Octo on a different circuit, put it away from other devices, and buy a power conditioner (a good idea, regardless).
 
Sounds like the Octo is giving off interference. If the only connection between the two is digital, it can't be a ground loop. Do you experience the problem with both dynamic and condenser mics plugged into either device?
 
Turns out all 8 channels on the Octo were turned up high (chs 3-8 are xlrs being fed from my Mackie 32 ch mixer via direct outs). When I turned them down, the situation improved greatly. Almost completely eliminated. This is still unacceptible though in the long run because I will need all 8 channels opened up on occasion.

I have not yet put the Octo on a different power circuit than the RME or the PC, but I did change computers and the difference from desktop to laptop was negligible. This is good because it still isolates the interference to the Octo.

FWIW, phantom power was off on the Octo (but on at the Mackie as I was using condensers on 2 of the channels).

Thanks for hanging with me on this
 
ChuckU said:
Turns out all 8 channels on the Octo were turned up high (chs 3-8 are xlrs being fed from my Mackie 32 ch mixer via direct outs). When I turned them down, the situation improved greatly. Almost completely eliminated. This is still unacceptible though in the long run because I will need all 8 channels opened up on occasion.
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Are you feeding mic inputs on the Octo from the Mackie's direct outs? You probably have a problem with gain staging rather than noise in the Octo.
 
mshilarious said:
Are you feeding mic inputs on the Octo from the Mackie's direct outs? You probably have a problem with gain staging rather than noise in the Octo.
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Yes...Mics, via xlr into Mackie inputs. Direct outs (1/4" balanced to xlr) into the back of the Octo.

But I do this with the RME too (except that it's 1/4" trs on both ends).

Do you think I am pushing too much signal from the board?
 
ChuckU said:
Yes...Mics, via xlr into Mackie inputs. Direct outs (1/4" balanced to xlr) into the back of the Octo.

But I do this with the RME too (except that it's 1/4" trs on both ends).

Do you think I am pushing too much signal from the board?
Click to expand...

Yes!
why use 2 micpreamps?
 
ChuckU said:
Reversed clock, no problem. I have found that the noise problem persists when I plug into the Lo-Z inputs of either the RME or the Octopre if the Octopre is on - no matter which cord I use.
Click to expand...

IMHO, the most likely possibility is that your building has a grounding problem. If the Octopre is dumping a ton of LF noise into the ground instead of into the neutral for some reason, and if your building ground sucks, then you're dumping that noise straight into the case ground of the RME, too. If your microphone and/or the RME has a bit of signal leak from the ground into its output (not entirely unlikely), then you're now dumping the noise into the input, regardless of which device the mic is plugged into.

Get your building ground checked, then try to figure out why the Octopre is dumping so much noise into it. My suspicion is a faulty power supply design in the Octopre, but fixing the building ground might make it good enough that it doesn't matter.
 
Are the "Direct Outs" on that Mackie real direct outs or are you referring to the Insert I/O?

If you're referring to the insert I/O, you're probably causing the problem by using a 1/4-in balanced jack. Insert I/O is notoriously unbalanced and uses TRS jacks in a completely different manner than balanced I/O. The same situation will occur if your direct outs are not balanced outs---which is a good bet, BTW.

And then, yeah, you are gain staging in a way that I wouldn't recommend.

I also have had ground loop experiences with a larger Mackie house system when we tied our AH Mixwiz into it. An EbTech HE2 or ATR DTI (preferred the HE2) eliminated the noise.

Paj
8^)
 
The Mackie I use has direct outs on each channel. So yes, they are true direct outs. I think I understand what I was doing by running the signal thru the Mackie, then thru the Octo, adding noise at every step.

The reason I use the Mackie before the Octo is that I like to be able to get my hands on real faders, eq's with knobs, etc. because in the course of tracking, it's the best way I've found of controlling the incoming signal.

I use my aux sends on the Mackie to feed my headphone amp - one buss for each headphone channel, thus enabling custom headphone or monitor mixes while recording. What I guess I really need the Octo for is its converters (and the occasional compressor) because what I really bought it (and the RME) for was to replace my Delta 1010's - strictly converters.
 
It's fine to run your workflow through the Mackie, so long as you pay attention to gain staging and mind the input levels on the Octo. But for the purpose of diagnosing your problem, you gotta take it out of the chain, and try going straight into the Octo with your mic. It could be as simple as an unbalanced line from the Mackie to the Octo picking up interference.

Just because you have a problem with 60Hz hum doesn't automatically mean it's a ground loop. There are lots of household devices that emit interference--dimmer switches are notorious for this, as well as some other things. This is where balanced transmission can be critical.
 
Actually, the problem I am having usually occurs when recording bass into the 1/4" line in of the Octo, So the Mackie is bypassed altogether. I have tried different basses, cords, etc. The same scenario with the 1/4" in on the RME is virtually hum-free.

I have a case going with Focusrite's tech support right now. Will report on their diognosis.
 
Focusrite has been pretty good with helping me narrow down the problem. What I found out (on my own) is that the 8-channel trs x xlr snake, running from the mixer to the Octo, although balanced, appears to be the culprit. I went through each channel, and could hear hum and buzz disappear as I unplugged select connections. Hoping that a snake upgrade will do the trick.
 
ChuckU said:
Focusrite has been pretty good with helping me narrow down the problem. What I found out (on my own) is that the 8-channel trs x xlr snake, running from the mixer to the Octo, although balanced, appears to be the culprit. I went through each channel, and could hear hum and buzz disappear as I unplugged select connections. Hoping that a snake upgrade will do the trick.
Click to expand...

Sounds like the snake is basically acting like an antenna. When it is plugged into some device that not correctly grounded by design, any electrical hum picked up along the cable ground is getting dumped into your audio signal through an input resistor to ground.

My first guess is still a bad building ground, but my second guess is that either the OctoPre has a brain-damaged grounding configuration in which the signal ground isn't really grounded. Try a grounding pigtail.
 
dgatwood said:
Sounds like the snake is basically acting like an antenna. When it is plugged into some device that not correctly grounded by design, any electrical hum picked up along the cable ground is getting dumped into your audio signal through an input resistor to ground.
Click to expand...

That's exactly what happened. As I said, I went through the snake one channel at a time and was really able to isolate it. I had stupidly bought a low-end TRS x XLR snake thinking that if it were balanced and only 8 feet long, it'd be OK. My other equipment is connected to the mixer via $50 Hosa TRS x TRS snakes and I never had a problem.

Anyway, upon confirming the snake to be the problem, I called Sweetwater to have them send me a better one. The guy on the phone there was great. Since I didn't buy the Octopre for the preamps (more the converters and compressors), he suggested I get a TRS x DA25 (parallel connector on the back of the unit).

I tested every channel. Works great. I learned a lot about noise and gain-staging in this exercise.

Thanks to everybody for your feedback.
 
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