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Buzzing mic conundrum

Postby snipedog » Thu Dec 14, 2017 5:23 pm

Here's a mic connection problem I could use some help with. I have a couple of Rode M5's through a Thomann snake connected to the mic inputs of an RME Babyface and there is a substantial amount of hum (50Hz plus harmonics) in the signal. I checked the wiring of the snake and all XLRs and all looks good, all cores are balanced, individually screened, (no overall screen) all earths connected and the soldering looks good, though the chassis of the stagebox is not connected to the signal earths. I connected the M5's directly to the Babyface and there's very little buzz, quite a clean signal.
I then connected a Rode NT5 and a Rode NT3 (both also phantom powered) to the same Thomas multicore, and there's virtually no buzz !

The output levels of the three mics are very similar, so there's the same amount of gain on the Babyface with each connection. Is there something about the Rode M5 that means it doesn't like long leads ?
It has a balanced 200 ohm output, like the other two, and a JFET impedance converter, so shouldn't have any special requirements in that respect. Anybody have any clues or suggestions what might be happening here? Thanks Steve http://www.redtapemusic.biz
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Re: Buzzing mic conundrum

Postby Wonks » Thu Dec 14, 2017 5:44 pm

Hi and welcome to the forum.

The use of the snake infers to me that the place you tested the M5s directly with the Babyface is a different location to when you first plugged in the M5s with the snake.

Is this correct? If so, could it then be that there is a lot more electrical noise in the snake location compared to the Babyface location, and that the M5s are just more prone to picking up noise compared to an NT5 and NT3? is it possible to bring all the snake to the Babyface location and see if the M5s still produce a similar amount of noise when connected through it?

Also, it's worth checking at the end of the snake plus any local XLR/XLR lead that you are getting +48V between both pins 2 and 3 and pin 1 (ground). If there is a high resistance somewhere and the phantom power voltage is significantly less, then some mics may become noisier as a result (they will almost certainly have less dynamic range before distortion).
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Re: Buzzing mic conundrum

Postby snipedog » Thu Dec 14, 2017 8:21 pm

Hello Wonks, Thank you very much for your suggestions I appreciate you taking the time to help. To take your suggestions in turn; All the tests with all the mics were carried out in the same room, with the Babyface and the mics in the same room together.

I initially suspected the room was what we used to call an "EMI hostile space', however there's nothing around that would account for this and we tried turning off all fluorescent strip lights and moving the snake around the room whilst listening to the buzz etc. with no audible improvement in the levels of buzz at all.

If the extra length of the snake was picking up the EMI, then I'm still not sure why it would do so at a much lower level when a different mic was connected with pretty much the same output level and impedance!
I did also check the phantom supply, and it was reading reliably 47.7V both at the Babyface and the other end of the snake. Interestingly the Rode M5s are stated to be happy with 24V phantom, whilst the NT5s, (which were not noisy,) need 48V. The M5s are electret (permanently polarised) whilst the NT5s are true condensers, which I suspected might be relevant BUT both have very similar output levels, and the same 200 ohm output impedance. I'm still completely stumped by this one and not sure what to check next, apart from a different snake ! S
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Re: Buzzing mic conundrum

Postby James Perrett » Thu Dec 14, 2017 9:03 pm

Just checking - were you using the same channel in the snake for all the tests? Are the symptoms the same for all the channels in the snake? How about the mic cables? Could it be one particular connector not making good contact with the M5?
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Re: Buzzing mic conundrum

Postby Hugh Robjohns » Thu Dec 14, 2017 9:29 pm

Buzzing suggests a grounding problem to me.

Can I ask how you've checked the integrity of the snake wiring?

H
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Re: Buzzing mic conundrum

Postby snipedog » Fri Dec 15, 2017 1:09 pm

Hello James,
Thanks very much for your input and ideas.
The snake is a Thomann one, four XLR to XLR and four TRS jack to jack. Not the best quality, and the XLR connectors look like Chinese Neutrik copies, but it tests out OK. We checked all four XLR channels, and got the same result with each one. I also checked the XLR lead from the M5 to the snake, and it was fine too. The puzzling aspect was if I set up the wring and connect the M5 there's an audible buzz, if I remove the M5 and replace it with an NT5, (or an NT3 or a Pure LDC) and change nothing else at all (same gain on the Babyface too) the buzz disappears. The buzz comes from BOTH the M5s too, so if they are faulty or damaged, then they are both faulty in the same way!
It all points to an interaction between the M5s and long cables, as when they are plugged directly into the Babyface, (with the same cable I used to plug them into the snake) there is no buzz ! S
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Re: Buzzing mic conundrum

Postby Mike Stranks » Fri Dec 15, 2017 1:17 pm

I've just bought a set of M5s and shall be using them tonight... I'll report back. However, my initial testing on short cables has not revealed any issues...
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Re: Buzzing mic conundrum

Postby snipedog » Fri Dec 15, 2017 1:20 pm

Hello Hugh,
thanks very much for your input. I used a combination of a Digital Multimeter to check the connection between pin 1 in the XLR and the chassis of the stagebox, (and to measure the resistance of the solder joints to make sure they were not dry joints, and measure the phantom voltage) I also used a Behringer CT100 cable tester to check the snake itself, which checks out ok, apart from intermittency in the XLR sockets if I waggle the XLR plug hard.

The puzzling bit is if I remove the M5 from the connection through the snake, and replace it with an NT5 or an NT3 (but change nothing else at all, even the cable remain in the same place!) the buzz level drops to an acceptable level (almost inaudible)The output levels of the NT3 and the M5 are almost the same, and the gain on the Babyface is unchanged.

If the snake is connected directly to the Babyface, and the Babyface is powered over USB, Im assuming the connection to ground must be via the USB lead into the PC. Is that likely to be adequate? Is it worth running a wire from pin 1 of the XLR to a cold water pipe!

I recorded in Paul's place when he had a TEAC 3340 and a Great British Spring about 40 years ago, and never had problems like this!
He might remember me as Badger !

Thanks again Hugh.
Steve
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Re: Buzzing mic conundrum

Postby ef37a » Fri Dec 15, 2017 1:25 pm

One thing occurs to me and schematics for confirmation would be hard to find, are the M5s 'impedance' balanced and the other mics using a differential amplifier drive or even a transformer?

Or, hum on the 48V supply that the M5s don't reject so well? Not likely since power supply hum would start at 100Hz.

If you don't want to take the mics apart you could make up XLR-XLR 'slugs' of some 100mm and try various pin 1&2 combinations? With pin 2 O/C there would be no signal at all if Z balanced.

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Re: Buzzing mic conundrum

Postby Hugh Robjohns » Fri Dec 15, 2017 2:02 pm

Intriguing!

I think I'd start a conversation with Rode...

Can you run enough ordinary XLR cables to approximate the added length of the snake? That might give some clue about whether it's a cable driving issue.

Trying an external earth to the babyface might also be instructive and well worth doing.

I can see what Dave is thinking with the impedance balanced output idea -- and a lot of mics are impedance balanced these days -- but that shouldn't be an issue.

The way that the mic is grounded internally can cause problems with RF pickup, and that can vary with the impedance of the ground path through the cable -- hence getting different results with long and short cables and different mics. That's also why trying to replicate the cable length with single XLR cables might be informative.

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Re: Buzzing mic conundrum

Postby snipedog » Fri Dec 15, 2017 2:05 pm

Thanks very much for your help Mike Stranks.
We are not getting problems with short cables with the M5s either, just some strange interaction with the Thomas Snake. I have another better quality multicore I will try the next time I'm in, and report back.
S
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Re: Buzzing mic conundrum

Postby snipedog » Fri Dec 15, 2017 2:19 pm

Hugh Robjohns wrote:Intriguing!

I think I'd start a conversation with Rode...

Can you run enough ordinary XLR cables to approximate the added length of the snake? That might give some clue about whether it's a cable driving issue.

Trying an external earth to the babyface might also be instructive and well worth doing.

I can see what Dave is thinking with the impedance balanced output idea -- and a lot of mics are impedance balanced these days -- but that shouldn't be an issue.

The way that the mic is grounded internally can cause problems with RF pickup, and that can vary with the impedance of the ground path through the cable -- hence getting different results with long and short cables and different mics. That's also why trying to replicate the cable length with single XLR cables might be informative.

H
Hello Hugh, I did try connecting the mic through a similar length of known good Sommer balanced cable with Neutriks on and there was slightly less buzz on the signal, but still significantly more than the NT3 and NT5.

I had wondered whether the earth arrangements inside the mics were different. In the NT3 & NT5 pin 1 is connected to the mic body, (as I think is normal?) the M5 has a ceramic coating all over the body, so its a bit harder to test, but theres a fixing nut visible in the XLR connector on the mic that is connected to pin1 so Im guessing the casing probably is too.

I will check Dave's suggestion that the M5 output is impedance balanced rather than "full" balanced output. If I remember rightly, impedance balanced should still allow Common Mode Rejection and noise cancellation to work as normal in the Babyface, just with a 3dB lower signal?

The M5 user handbook states the output has a "JFET impedance converter with a bipolar output buffer, so Im guessing that is non-transformer but true balanced output? Im pretty sure the NT3 and NT5 I'm comparing with don't have output transformers fitted.

I also have another slightly better quality multicore i can try, and I'll follow up your suggestion of contacting Rode themselves.
Thanks

Steve
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Re: Buzzing mic conundrum

Postby ef37a » Fri Dec 15, 2017 2:25 pm

"Bipolar output buffer" shouts Emitter Follower (yuk!) to me and then a resistor in the pin 3 circuit to Z balance it.

Yes, impedance balance should be as good as two drive amps AFAIK (Hugh?) but then the CMRR is proportional to the ratio of source and sink impedances and with mic circuits that is never going to be great.

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Re: Buzzing mic conundrum

Postby snipedog » Fri Dec 15, 2017 2:28 pm

The manuals state the Babyface mic input impedance is 2k ohms, and the M5 output impedance is 200 ohms.
S
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Re: Buzzing mic conundrum

Postby Hugh Robjohns » Fri Dec 15, 2017 2:45 pm

snipedog wrote:Hello Hugh, I did try connecting the mic through a similar length of known good Sommer balanced cable with Neutriks on and there was slightly less buzz on the signal, but still significantly more than the NT3 and NT5.

Okay. So it is probably linked to cable length then.

I had wondered whether the earth arrangements inside the mics were different.

Very possibly. I have come across similar problems to this in a mic where the XLR pin 1 was connected to the mic case via a short length of wire. That wire was, at radio frequencies, sufficient to form an inductor, raising the ground impedance substantially and turning the while mic into a radio receiver. In that specific case, the manufacturer had to redesign the construction so that pin 1 was essentially soldered directly onto the mic case.

This is why a direct conversation with Rode might prove useful, as it could be a design/manufacturing issue that they are not yet aware of.

I will check Dave's suggestion that the M5 output is impedance balanced rather than "full" balanced output.

Impedance balancing is full balancing! A balanced line is all about the having matched (or balanced) impedances to ground from both signal wires. Whether the signals being conveyed are symmetrical or not is irrelevant to the way the interface works.

If I remember rightly, impedance balanced should still allow Common Mode Rejection and noise cancellation to work as normal in the Babyface, just with a 3dB lower signal?

Yes, the common mode rejection should still work -- although that actually requires the cold-leg impedance to match that of the hot leg across the entire audio bandwidth. If the active output's output impedance varies with frequency (as it might well) that must be modelled accurately in the cold-leg's impedance to ground (which might be difficult), other wise the common mode rejection will be greatly reduced at some frequencies (potentially making the system more prone to external RF interface, for example).

There is no issue with 3dB less signal. In a symmetrically balanced system each leg carries identical signals, both 6dB lower than the source, and with the cold side polarity inverted. In an impedance balanced system all of the signal is carried on the hot leg, and there is no signal at all on the cold leg, just a constant reference voltage (normally 0V).

The M5 user handbook states the output has a "JFET impedance converter with a bipolar output buffer, so Im guessing that is non-transformer but true balanced output?

Yes, There probably isn't an output transformer, but the bipolar reference is vague. It could relate to a symmetrical output format, or it could just mean the output circuitry uses a bipolar junction transistor drive circuit...

H
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Re: Buzzing mic conundrum

Postby snipedog » Fri Dec 15, 2017 3:04 pm

Thanks Hugh,
That gives me a few more things to check. I've flagged a query with Rode via their website.

If there is indeed a problem with the M5 driving longer cables I would expect other users to have come across it. The environment we are using the mics in is not particularly EMI or RF "hostile".

The M5s are electrets rather than "true" condensers, but their output is around the same as the NT3 so I can't see that being a relevant factor.

I have my suspicions about the earthing of the screening in the snake as Ive never thought USB leads made a very convincing connection. If the only route for the screen to earth is via the Babyface USB lead and into the computer to the mains earth that seems a bit fragile to me, so I'll start there with the next fault finding session. That still doesn't explain why the NT3 and NT5 are not affected by that, but I need to start somewhere !
Thanks again
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Re: Buzzing mic conundrum

Postby John Willett » Sat Dec 16, 2017 12:29 pm

Talking of the Babyface ...

Synthax UK are doing a special deal at the moment for all those who have a Babyface to upgrade to the Babyface PRO at a special price.

See HERE :thumbup:
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Re: Buzzing mic conundrum

Postby snipedog » Sat Dec 16, 2017 2:28 pm

John Willett wrote:Talking of the Babyface ...

Synthax UK are doing a special deal at the moment for all those who have a Babyface to upgrade to the Babyface PRO at a special price.

Hello John,
Thanks for the pointer, it is actually the Babyface Pro we are using! Maybe I should have specified that, but I didn't realise there was an older version. S
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Re: Buzzing mic conundrum

Postby James Perrett » Sat Dec 16, 2017 5:42 pm

snipedog wrote:The puzzling bit is if I remove the M5 from the connection through the snake, and replace it with an NT5 or an NT3 (but change nothing else at all, even the cable remain in the same place!) the buzz level drops to an acceptable level (almost inaudible)

I must admit that I'm a little worried by this comment. I wouldn't expect to be able to hear any buzz at all in a properly working system - just hiss from the microphone or preamp. Is this an area of high interference or is there a grounding problem somewhere?
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Re: Buzzing mic conundrum

Postby snipedog » Sat Dec 16, 2017 5:54 pm

James Perrett wrote:
snipedog wrote:The puzzling bit is if I remove the M5 from the connection through the snake, and replace it with an NT5 or an NT3 (but change nothing else at all, even the cable remain in the same place!) the buzz level drops to an acceptable level (almost inaudible)

I must admit that I'm a little worried by this comment. I wouldn't expect to be able to hear any buzz at all in a properly working system - just hiss from the microphone or preamp. Is this an area of high interference or is there a grounding problem somewhere?

Hello James,
Thanks for your input. I agree entirely there shouldn't be any audible buzz in the system, but it becomes clearly audible with the M5's connected. Wiith the NT3's or 5's the monitoring volume has to turned up very high (well beyond normal monitoring volumes) in order for any 50 Hz buzz to be audible at all. (This is what I meant by "an acceptable level"). I suspect you are right, and there is a grounding problem somewhere, but I can't identify it in the tests on the wiring Ive done so far. If there is one its having such an effect on the M5s as to make them unusable for recording solo piano, but having very little effect if any on the NT3s and the NT5s ? As I mentioned in the earlier posts, this is not an RF or EMI "hostile" space, there are no phone masts nearby, the mains wiring in the building looks to be less than 25 years old, and nobody is using arc welding kit nearby! The interference is a steady 50Hz (plus harmonics) with no audible spikes or "digital fizz". Did you have a grounding problem in mind that might account for this anomaly James?
Thanks
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