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Buzz problem detective wanted...

Postby molecular » Tue Jul 24, 2018 12:07 pm

Hi everyone, haven't been here in a while, but I am having trouble with some new noises in the studio since I introduced one of Focusrite's new X2P Pre/Interface Dante boxes...

I'm still investigating some things, but thought I'd see if anyone here had any advice. I don't *think* it's a Dante issue, but here's the summary...

My X2P generates a horrible buzz which can be heard on the recorded signal, and also in the headphones on the unit. It has a preamp/playback mix knob, and the noise is there in the local mix, so I'm assuming that it is being generated in the pre, before ADC.

The buzz is present when all the following things are true:

1. The flourescent working lights in the live room are on
2. The pre is connected to a router on it's Cat6 cable
3. The mic cable connecting the mic to the X2P is close to, or lying on, the floor.

As of today:

1. if I switch the lights off, the buzz disappears.
2. if I disconnect the Cat6 cable, the buzz disappears. (all audio shuts down when you do this, but you get about 1 second of the "local" signal in the HP mix with no buzz before the audio shuts down).
3. if I lift the mic cable off the floor, the buzz disappears.

So I can use the unit, but within certain limitations which I'd like to get to the bottom of.

Worth saying that I have other Dante equipped pres in the same room which don't have these problems, and other analogue pres which I've never had the issue with.

I replicated the problem in the same room using a single ethernet router which was not connected to the wider network in the building.

As far as I'm aware, the floor is concrete with an industrial lino/marmoleum on it as the base surface.

Any ideas where I should start looking? Just to clarify, at the moment the buzz is only present when the striplights are on, an ethernet cable is present AND the mic cable is on or near the floor. Which seems like a strange selection of conditions to me, but I need to be able to work with all three of these things being true :/

Am also in discussion with focusrite, but maybe I'm just forgetting or overlooking something small or even big and obvious?

Thanks!

Hector
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Re: Buzz problem detective wanted...

Postby Hugh Robjohns » Tue Jul 24, 2018 12:18 pm

I'd start by swapping the mic cable and the mic for known good alternatives as a first step. It would also be worthwhile checking to see if pin 1 in the XLR is linked to the shell at the male (preamp) end, and if so, cut that link (or find a cable that doesn't have the link) to see if that makes a difference.

However, the fact that you have other Dante preamps in the same space that don't exhibit the same problem suggests this is an issue with that specific Focusrite unit. My guess would be either an internal grounding problem in that specific unit or an RFI susceptibility issue (these two possibilities could actually be one and the same, of course!)

For that reason, once you've done those basic mic/cable replacement checks, I think it will need Focusrite's direct involvement. Could they let you borrow a replacement model to check whether it's a fault with that specific unit, or a design issue?

H
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Re: Buzz problem detective wanted...

Postby molecular » Wed Jul 25, 2018 10:45 am

Hi Hugh, thanks for the quick reply.

The buzz is present on a few different cables and all mics with and without phantom. The intermittent crackle I'm less sure about...

I'm waiting on a PoE injector as I haven't tested powering it from that yet, but I know the buzz has been present in other buildings... I'm going to see if I can trigger it using something other than striplights.

I have already sent one back and this is now X2P #2 which is giving me the same issue, so it does seem like they are just sensitive to the by design though :/ which is frustrating as they are otherwise my new favourite thing!

Any idea why the 20cm or so off the floor is causing the havoc, when the offending lights are a good 4m up in the apex of the roof? Is that just the effect of the steel mesh in the concrete floor?
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Re: Buzz problem detective wanted...

Postby ef37a » Wed Jul 25, 2018 11:43 am

If a silly question, tell me to 'od off but. Is the Ethernet cable shielded and if so is the shield continuous all the way, "chassis to chassis"?

I guess the 'floor' issue is a standing RF wave max field strength point.

Just fort. Modern lights should have their casings earthed, do they?
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Re: Buzz problem detective wanted...

Postby molecular » Wed Jul 25, 2018 1:55 pm

Excellent questions, and at present I don't know the answer to either. I'm pretty sure the problem is there when I've isolated it to just one router and the cat6 cable that came with the unit, but don't know about the shielding.

I'll ask about the lights... I am not sure anyone's been up to look at them since the mid 90s...

I was having the same problem in a completely different building but I can't remember if it had striplights or not...I may have been next to a wifi router or something.

I remembered a further oddness: The pres on the X2P are digitally controlled and the buzz is noticeably worse at specific gain levels. E.g. it jumps between 52db and 53db, then at 54db is not quite so bad...!?

I've explained that but to focusrite tech support, hence I got the replacement unit. But they haven't told me about anyone else having similar problems and I would have thought that if they were this sensitive and there's nothing weird at my end, then they would have been getting loads of complaints...
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Re: Buzz problem detective wanted...

Postby ef37a » Wed Jul 25, 2018 3:36 pm

http://www.broadbandbuyer.com/products/ ... eGEALw_wcB

Unshielded plugs do not have the shiny metal bits. I cannot think they would fit them to unshielded CAT cable?

You might also try foil shielded mic cable? Not as flexible but it does give 100% screening. This also might be a case FOR grounding XLR shells to pin one?

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Re: Buzz problem detective wanted...

Postby molecular » Wed Jul 25, 2018 6:43 pm

Thanks both!

The problem is definitely there when using only shielded Cat6 (with shiny bits).

I don't think any of the XLRs have pin 1 tied to the shell, but I'll see if I can sort that to test it.

I'm doing a big swap around of various mics, cables and locations and am discovering some very odd results that disappear immediately when not using the X2P, so will report back here and to focusrite :/
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Re: Buzz problem detective wanted...

Postby molecular » Wed Jul 25, 2018 9:55 pm

OK, well the good/bad news is that I've managed to replicate the issue at home by setting the X2P up in the proximity of a wifi booster (crackle every 1-3 secs), a macbook charging (continuous hum), and a fridge (popping sounds when the light comes on and off).

The initial response from Focusrite was that they tested the one I sent back and couldn't find anything wrong, but I'll press them to consider looking into some more real world situations. I've never been in a studio that didn't have a wifi router, a macbook AND a fridge.

I'm probably in the wrong bit of the forum now, as I was initially thinking someone might point me in the direction of a suspected dodgy wiring issue or something, but I'll let you know how I get on.
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Re: Buzz problem detective wanted...

Postby James Perrett » Wed Jul 25, 2018 10:16 pm

One thought - is there a ground in the system? I'm just wondering whether everything is floating which can make things more susceptible to interference. What happens if you connect an input or output to a piece of properly grounded gear?
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Re: Buzz problem detective wanted...

Postby molecular » Fri Jul 27, 2018 6:55 am

Not sure actually - I would have thought that if I was connected to the building's ethernet infrastructure there would be a ground somewhere (it's a college building) but perhaps not.

I'll see what happens if I connect it up to some outboard or monitors...
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Re: Buzz problem detective wanted...

Postby James Perrett » Fri Jul 27, 2018 6:18 pm

molecular wrote:Not sure actually - I would have thought that if I was connected to the building's ethernet infrastructure there would be a ground somewhere (it's a college building) but perhaps not.

If they're using unshielded twisted pairs then the ethernet won't be earthed. Even if they are using shielded cables, the path to earth will probably be fairly convoluted and could easily be broken if there is an unshielded switch or connector in the chain.
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Re: Buzz problem detective wanted...

Postby molecular » Thu Aug 09, 2018 10:57 am

update...

So I've been chatting away with Focusrite support, who all credit have been very attentive, but have not got to the bottom of this and thought someone here might have some more advice, If they can face sitting through 15 mins of video content :/

I was asked to make a video demonstrating the problem I'm having and did that, and tried to include some of the weird idiosyncrasies of the noise.



password is rednet

They suggested the same thing - that the unit wasn't earthing properly and it needs to earth through the ethernet connection. (they raised the fact that my comparison dante unit was earthed through it's PSU).

So I made a second video in which the other Dante unit isn't earthed and still works fine.

Also - removing the ethernet cable altogether from the X2P gets rid of the problem, which doesn't make sense to me, as then it definitely isn't earthed any way at all.

second video here:

password is also rednet

I'm really stumped, as the requirement to earth it through the network in order to avoid interference seems weird and is not mentioned anywhere in the manual - and rules out, for example, setting up something on the fly with a small router and a laptop.

Also, obviously a whole host of small pres and interfaces work fine with no earth anywhere.

So I don't know, it's unusable in the studio, but also at home or anywhere else I've tried it, and Focusrite say they couldn't find anything wrong with the one I sent back. So if anyone has any time to sit through the two vids (13 mins and 7 mins) and see if they have anything to add I'd be really grateful.

It's otherwise a great concept and I'm reluctant to write it off and ask for a refund.

Thanks!

H.
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Re: Buzz problem detective wanted...

Postby blinddrew » Fri Aug 10, 2018 7:52 pm

That's weird. In a number of ways. What's the peak-to-peak time for that interference noise?
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Re: Buzz problem detective wanted...

Postby molecular » Fri Sep 07, 2018 8:39 am

Sorry just noticed there was a new reply, Drew!

It is, and remains, weird. I don't have the session in front of me right now, but the interference picked up is completely different in different places at different times. The one in that video is caused by the lights, but I have also had a buzz which grew over the course of about an hour, then vanished, then slowly appeared again. Also intermittent crackles, pops, all kinds. I can pick up the fridge door light going on and off at my parents house, and their WiFi booster. On two location records I picked up awful buzzes coming from god knows what.

I am in touch with someone at Focusrite who is being very helpful and responsive, but insists no-one else has registered any similar problems globally with the X2P, and they can't replicate my results, and the design team seem stumped.

But I cannot replicated the results using ANY combination of other Dante devices, or any devices, in the same venues. I've had all manner of desks, pres, interfaces, mics, hardware and whatever else through a couple of the "offending locations" over the last few years and not had this problem with a single one of them.

So I am really confused and frustrated.
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