Rode cable... noise with a dynamic mic

[RionZ]

Hey folks... made interesting discovery this evening.

I do a lot of spoken word recording, which ends up getting anywhere from 6 to 12db added in post production, and so am acutely aware of any background hiss or line noise now matter how small. Just recently I switched from a Rode NT1a condenser to a Beyerdynamic M99 dynamic mic. Obviously, I had to increase my gain to compensate for the dynamic and in doing so noticed an audible hum in my signal. So I started investigating, and everything checked out in general terms.. so just out of interest I thought I'd swap the cable...

Here's the interesting part... I was still using the cable that came with the Rode mic, i.e. a brand new cable, which has never done anything except stay plugged in to the mic and interface. The cable I swapped it for is an old one I've had for donkey's years.. it's done literally 1000s of gigs.. the cable says img "stageline".
Point is... the noise is gone. So clearly this cable is better shielded somehow.

I guess it's horses-for-courses since the rode cable had no noticeable hum with the condenser due to its higher signal level.

I'd just never considered the idea that some cables, even brand new "low noise" cables supplied with decent audio gear do in fact have worse shielding than others. I'm surprised at Rode for not supplying a better cable with their product really.

I'd happily buy stageline cables any day, even though they're not considered high-end like van dam/neutrik, etc (and priced accordingly!)

[RionZ]

Actually I'm going to qualify what I said about Stageline cables... the ones I have were bought about 15 - 20 years ago as far as I can remember!

Whether or not they're still decent... I guess I'll find out next time I need to buy some cables

[Hugh Robjohns]

So clearly this cable is better shielded somehow.

It might not be as simple as that!

Hum is normally introduced through magnetic induction, and the cable's screen plays almost no part in reducing that. The screen is to minimise electrostatically coupled interference (radio frequency interference). Instead, it is the way the two core wires are twisted together that counteracts magnetic induction.

The number of twists per unit of distance is called the lay length, and the shorter the lay length, the better the rejection of magnetically induced interference:



So it is true that different cable constructions will have differing performance when it comes to minimising electrostatic and electromagnetic interference. But a short lay-length also makes the cable stiffer and less flexible, so there is a compromise to be found... hence the variation in different brands of cable.

But then it also matters where the cable is placed in relation to sources of interference. On one of our Studio SOS visit we had to help someone eliminate hum from his mic lines which turned out to be caused by his depositing a wall-wart power unit and mains plugboard on top of the neatly coiled spare multicore cable from his studio wall box! So keep those mic cables away from mains power supplies and amplifiers -- anything with a mains transformer in, basically!

There is another common issue with mic cables, too, and that is the way the thing is wired internally. The current AES recommendation is that the XLR shell should be linked internally to pin-1 -- the cable shield connection -- in order to maintain a fill Faraday shield around the signal wires. And consequently, a lot of commercial XLR cables are built this way.

If the equipment you're plugging that XLR cable into is engineered correctly, pin-1 will be connected directly to the equipment's chassis (and nowhere else) so that any noise currents in the cable shield (or the chassis itself) are taken away to the mains safety ground via the equipment's mains cable and kept well away from the audio electronics.

However, in poorly designed equipment -- including an awful lot of legacy gear -- pin-1 is actually carried across to the audio circuit board's ground, where it dumps any noise currents straight into the sensitive audio reference ground. Moreover, if the XLR cable links the shell to pin-1, any noise currents on the equipment's chassis are also coupled into pin-1 via the XLR plug, and from there into the audio reference ground as well. And that's often where the unwanted hum (or other noises) comes from!

Often breaking the link between pin 1 and the chassis in the XLR plug resolves the problem completely... as does substituting a hummy cable with another which doesn't have that link...

I don't know whether Rode's cable have the shell linked to pin-1 or not -- it's easy enough to check if you have a multimeter, but I suspect it does, and probably your old Stageline cable doesn't.

I'm surprised at Rode for not supplying a better cable with their product really.

That could well be a presumption too far... It may well be that the cable is actually engineered perfectly well, and it is the equipment it is being connected to that is at fault!

[RionZ]

Thanks Hugh for a very informative reply,

Not too sure about the internal "lay length" of each cable - hard to tell without dissecting them.

As for chassis ground to pin 1, I can confirm both cables are wired that way. The equipment is a Focusrite Forte. Again, I haven't dismantled the interface to check if it's chassis grounded, but i suspect it should be.

Both cables were routed the same way, away from sources of Electromagnetic interference... however, looking at it now, there's a small possibility the rode cable may have slipped down and ended up near a DC transformer... I'll have to check later and do a comparison with both cables in the exact same position.

[MOF]

recently I switched from a Rode NT1a condenser to a Beyerdynamic M99 dynamic mic

Why have you changed microphones, has the NT1a gone to microphone heaven?

[jacobvosmaer]

As for chassis ground to pin 1, I can confirm both cables are wired that way.

Pedantically, it's not about chassis ground and pin 1, but the XLR shell and pin 1. And you should measure this with both ends of the cable disconnected.

Inside the device, it's not uncommon for audio ground and chassis ground to be connected somewhere. So if you unplug one end of an XLR cable and measure continuity between its pin 1 and the chassis of the device of the other end, you would usually see continuity, regardless of whether the cable has the "pin 1 to XLR shell" property Hugh talks about.

[RionZ]

recently I switched from a Rode NT1a condenser to a Beyerdynamic M99 dynamic mic

Why have you changed microphones, has the NT1a gone to microphone heaven?

No the NT1a is still good!

I decided to change to a dynamic specifically for audio voiceover narration work - the dynamic picks up less extraneous noise outside my booth and is also much more tame in the high end - condensers generally and in particular the Rode are prone to picking up every little sparkle and squelch from my mouth!

Have to say I'm extremely happy with the M99 as I deliberated for quite some time between that and the Sure SM7 and the Electrovoice range. It's a nice bonus that the M99 makes a fantastic kick drum mic too!

[Saxum esse et non provolvere]

May I just ask in what sense this is a “Rode cable” ? I hadn’t previously been aware of Rode supplying standard XLR cables with their mics (non-standard ones, yes). Might this just have been a cable bundled in by the retailer ? If so, it’s hardly fair to attribute the problems to Rode …

[Wonks]

May I just ask in what sense this is a “Rode cable” ? I hadn’t previously been aware of Rode supplying standard XLR cables with their mics (non-standard ones, yes). Might this just have been a cable bundled in by the retailer ? If so, it’s hardly fair to attribute the problems to Rode …

Rode do a few of their own bundled packs which include an XLR cable. The NT1 and NT1a certainly have bundles like this. Whilst I doubt Rode make up the cable, it's certainly one they supply.

https://www.gak.co.uk/en/rode-nt1a-micr ... pack/31723
https://www.gak.co.uk/en/rode-nt-2a/5744

[RionZ]

I didn't really mean to malign Rode as a company! As far as the Rode NT1a goes, it's a fantastic mic at that price point. And, given the price, I wouldn't expect a top-notch cable to be thrown in to the bundle.

Furthermore, I should point out that as far as the NT1a mic is concerned, there was no audible hum with that mic...

Whether or not the cable is of an inferior quality for general use... I have yet to be sure. As I'm sure everyone can understand, I just don't feel comfortable with idea of having "suspect" cables in my setup and would rather everything is of at least a base-line quality.

I probably should have titled the thread "suspect cable" rather than use a company name, generally a good name at that.

[MOF]

The reason I asked why you’d changed microphones is because Dynamic v Condenser for voiceovers was discussed in this thread

https://www.soundonsound.com/forum/view ... hp?t=79653

[RionZ]

The reason I asked why you’d changed microphones is because Dynamic v Condenser for voiceovers was discussed in this thread

https://www.soundonsound.com/forum/view ... hp?t=79653

Thanks for the link.. interesting thread. Have added my 2 cents' worth

[RionZ]

Ok so here goes... gulp...

It turns out my "rode" cable had fallen onto a small DC power supply behind my desk!

My apologies to all...

However... I did learn something interesting about shielding/grounding, my thanks for that.

It turns out both cables are electrically continuous between pin 1 and the chassis of the plug at both ends. It was not possible to test for grounding to the chassis of the Focusrite Forte (without dismantling) since it turns out the casing, although metallic in appearance, is not in fact conductive!

So, happy to know I can trust all my cables again and lesson learnt to be a bit more careful/observant of my cable routing

[Saxum esse et non provolvere]

Hah ! Even Homer nods … Glad you got it sorted out.

[Hugh Robjohns]