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Noisy Mic Issue

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Re: Noisy Mic Issue

Postby ef37a » Tue Aug 04, 2020 7:02 am

blinddrew wrote:No files because I forgot to export them, but I have narrowed it down to a) it definitely needed a new valve last time, but also b) the power supply box is humming at about 150Hz (with a little resonance at 225).
If you stick your ear up against it you can pretty clearly hear it but it's then getting into the recorded signal as well even if the microphone is a long way away. Putting a couple of notches on the eq at those frequencies can remove it but unfortunately that's in a pretty critical area for my voice and leaves it sounding ugly.
Which is a bit vexing.

Don't like the sound of that. Both ways Drew! Acoustically bad of course but a transformer putting out much third harmonic indicates it is being run close to or even beyond its rating.

Someone hasn't modded it perchance and fitted BIGGER filter caps? Counter-intuitive I know but more capacitance can upset transformers. Can you SAFELY check your actual mains voltage? Does the transformer get hot? AGAIN, safety first!

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Re: Noisy Mic Issue

Postby blinddrew » Tue Aug 04, 2020 9:30 am

No mods, I'm the only owner and I know enough not to play around with high voltages. Which also answers your question about safely testing the actual mains voltage! ;)
Despite this being a new-ish house (less than 20 years old) I'm not entirely convinced by the electrics here. We've had them checked of course, with nothing found amiss, but we go through a crazy amount of lightbulbs and have quite a lot of power outs. Maybe it's not the house but an underpowered sub-station? Maybe I have no idea what I'm talking about! ;)
Anyway, getting back to the mic, if I was recording anything loud it really wouldn't be a problem, it's a low enough level to get lost in the signal to noise. But I've recently been recording some quiet stuff and really not been happy with the end result.
I assume your take on it would be to get it checked out by a proper tech?
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Re: Noisy Mic Issue

Postby Hugh Robjohns » Tue Aug 04, 2020 10:34 am

blinddrew wrote:...we go through a crazy amount of lightbulbs and have quite a lot of power outs. Maybe it's not the house but an underpowered sub-station?

Unlikely to be the house, but your local substation could be set to a higher tapping -- if you go through a lot of bulbs you're either buying cheap nasty bulbs, or the mains voltage is on the high side of standard. Buy yourself a plug-in mains power meter like one of these (there are lots of similar devices around) that can measure the mains voltage and frequency, as well as the current and power consumption of whatever you plug into it. Very useful -- and entirely safe -- device to have kicking around.

Image
https://www.amazon.co.uk/gp/product/B07FZZ17ZY?pf_rd_r=1SQXQVNDDVZ62Y9QXBBS&pf_rd_p=e632fea2-678f-4848-9a97-bcecda59cb4e

The voltage range that the UK power providers are legally allowed to supply is 216 volts to 253.0 volts, but the nominal voltage is 230 and all new build (in the last 25 years or so) and refurbished substations are usually configured for something very close to that. Older systems are more likely to be up around 240V, and if you're particularly close to the substation is could even be closer to 250...

I assume your take on it would be to get it checked out by a proper tech?

A few quick checks before going any further...

1. To make absolutely sure the 150Hz hum isn't coming from the mic preamp, make up a 'shorting plug' by finding a spare XLR and wiring pins 2 and 3 together. (You can bodge a temporary one by opening the female end of an XLR cable and shorting between pins 2/3 with a paperclip. That will be sufficient to prove the point. Connect in place of the mic (with the same gain selected) and listen for hum. If no hum then that's good!

2. Make absolutely sure there's no hum being induced into the XLR cable from the mic's PSU to the preamp. Again, you can use the shorting plug at the end o the usual mic cable... but it's often easier just to move the cable around a bit -- lift it well off the floor, pull it well away from the walls... etc. Do the same with the 7-pin cable between mic and PSU, too. You're trying to make sure the cables are no where near mains cables or other mains-powered equipment. Listen for changes in the level of hum. If there is no variation, the hum is not being induced in the cables...

3. Many valve-mic PSU's have a ground-lift switch, and if present try flipping it because it's possible you have a ground loop between preamp and PSU. If you don't have a ground-lift switch, then you could either use a line-isolating transformer box for the experiment, or butcher a spare XLR cable by cutting and isolating the cable screen at the male XLR end.

4. However... I suspect the real problem is a failing or under-performing mic power supply unit. An audible 150Hz (third harmonic) present on the mic's output suggests the power supply isn't regulating the DC power rails as well as it should, so you're getting some mains-related AC on what should be pure DC, and that AC is inevitably getting into the audio circuitry.

It should be fairly easy for an experienced technician to track down and fix... but it will need a trip to the mic doctors surgery...
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Re: Noisy Mic Issue

Postby blinddrew » Tue Aug 04, 2020 11:07 am

Thanks for all this Hugh, will work through the list. :thumbup:
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Re: Noisy Mic Issue

Postby ef37a » Tue Aug 04, 2020 11:45 am

Regarding mains voltage variations, an awful lot of heavy engineering has been lost in UK in the last 20 years.

Just my town and in a radius of only a click or so of me NN5 5P* we have lost Timkin bearings, Express Lifts, Standard Valves, and Pollard Bearings. All these pulled a lot of juice and one wonders if all the surrounding subs have been reset? My mains is a consistent 245V but at a factory complex about 4km away it was 258 at times!

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Re: Noisy Mic Issue

Postby blinddrew » Thu Oct 15, 2020 3:54 pm

Finally getting time to follow up on this a bit this weekend, but it has just occured to me to wonder if having my cables (mic to power supply and power supply to interface) coiled could be contibuting to the hum? Some kind of induction loop?
I shall perform this trivial test first.

In the meantime I can confirm that the voltage round here is averaging around 239 and peaking at about 245; so no concerns there.
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Re: Noisy Mic Issue

Postby Hugh Robjohns » Thu Oct 15, 2020 4:15 pm

blinddrew wrote:it has just occured to me to wonder if having my cables (mic to power supply and power supply to interface) coiled could be contibuting to the hum? Some kind of induction loop?

It's easy to test, as you say, but with both being wired for balanced audio there shouldn't be any issues with induced hum pickup -- not unless something generating a strong AC magnetic field was placed right next to the audio cables -- in which case the extreme proximity would bugger up the common mode working!
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Re: Noisy Mic Issue

Postby ef37a » Thu Oct 15, 2020 4:18 pm

blinddrew wrote:Finally getting time to follow up on this a bit this weekend, but it has just occured to me to wonder if having my cables (mic to power supply and power supply to interface) coiled could be contibuting to the hum? Some kind of induction loop?
I shall perform this trivial test first.

In the meantime I can confirm that the voltage round here is averaging around 239 and peaking at about 245; so no concerns there.

I have read many times of people thinking coiled cables are 'induction loops' not so. Any cable that has juice 'coming and going' is by nature non-inductive.

Do note however that extension mains cables must always be fully unwound before use, keeping them on the drum DRASTICALLY reduces their thermal capacity. Seen more than one a molten mass with even quite low loads.

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Re: Noisy Mic Issue

Postby blinddrew » Thu Oct 15, 2020 5:22 pm

Thank you both. :)
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