Crackling sound system

[kaphc]

As part of our new school, I've now got a sound and lighting system to look after too. All was well with the sound system and I've had some beautiful outputs from it - until one of the teachers has done something - and now there is a "crackle" from the speakers on two channels which have input from a laptop. The microphone channels are fine ... it just seems to be when the laptop is plugged in that the crackle occurs.

 

I've tried all the obvious things like checking the levels and making sure the cables are connected properly but can't seem to eliminate the crackle. Has anyone got any suggestions of any other steps I can take to try and eliminate the crackle? Not worked with sounds systems much before but have been on the other end of them as a keyboard player and I know it can often be a trial and error thing!

[SHimmer45]

blown one of the channels by "over driving it"?

could be worth changing the laptop just in case the sound output on that has been damage?

[DAZZD88]

Have you tried an alternate input source instead of the laptop, maybe a mobile phone or another laptop? It could be that they've damaged the input socket on the laptop or maybe they've damaged the cable itself.

[laserblazer]

Try it with the laptop on battery power.

[Oaktech]

i expect it's the laptop power supply dumping crap onto the audio. does it sound like

 

"did-it did-it did-it bzzzzzzzz" repeatedly?

 

try it just battery only. if it's there with psu and not with battery then you should try using a DI box.

 

a ground loop isolater may improve it, but will probably not fix it.

[teejay]

Is it a Dell laptop? Check if the power supply is a revision A01 as they can cause this problem.

[K.C.Leblanc]

Try it with the laptop on battery power.

 

If that solves it, get one of these.

 

Ground Loop Isolator : InCar Speakers : Maplin

 

EDIT: oops beaten to it.

[kaphc]

Nice one! Just been back to have a look, unplugged the power and the crackling vanished straight away.

 

It's a Toshiba laptop by the way.

 

Interestingly, I plugged the power back in again and there was still no crackle and so am carrying on running with the power plugged in as well for now (teachers unplugging and plugging in laptops could lead to another problems!). But thanks for the links to the isolators too, useful to have as a back-up plan in case it happens again.

[ricki]

If that solves it, get one of these.

 

Ground Loop Isolator : InCar Speakers : Maplin

 

EDIT: oops beaten to it.

 

I agree

[Oaktech]

the crackling is caused by the high frequency switching component in the PSU, when you unplug it the battery discharges, when you plug it back it the psu is working harder to charge the battery and does not drop excess power to earth, so you will find that after a while it will happens again when the battery is full.

 

a ground loop isolater may solve it, but the correct thing to use is this: Samson S Direct Plus Active Stereo Di box

[K.C.Leblanc]

the crackling is caused by the high frequency switching component in the PSU, when you unplug it the battery discharges, when you plug it back it the psu is working harder to charge the battery and does not drop excess power to earth, so you will find that after a while it will happens again when the battery is full.

 

Very good explanation, are there any resources you can recommend for learning more about sound engineering.

[Oaktech]

you can learn more about mixing with a mixing desk from this... Soundcraft - [support]

 

i'll have a look about other things... i had a very good teacher at my previous work place (shame he was a less than ideal manager!)

 

also check out the various parts of this site that are of interest. www.mediacollege.com/audio/microphones/how-to-use.html

 

Remember that i know about this kind of stuff and PM me if you have problems!

Edited October 14, 2011 by Oaktech

[kaphc]

Fantastic - that makes complete sense about why then.

 

Thanks for the links - I'm especially interested in the hand-held microphone techniques on the mediacollege website as I want to teach the kids here good microphone habits for the Christmas productions. I always thought it was better to hold the mic about 6 inches away from your mouth to avoid distortion etc. but then have been puzzled about why singers (singing live) seem to be right on top of the microphone. And this backs it up - though I still don't know why singers eat their mics.

[Oaktech]

depends what mics... things like sennheiser 4xx,6xx,8xx series mics and shure SM and beta series vocal mics are what is known as 'prescency' meaning that they require a lot of air to make the diaphragm move, this is good because they are less likely to pick up stuff you don't want them to, but bad because they are less sensitive. you can make them more sensitive by increasing the gain on the mic channel (or head amp) but this makes them more liable to feed back, it's all a balancing act, it's called mixing for a reason!

 

but the more prescency the mic, the closer you have to get it to source of the air movement - ergo, singers eat the mic!

 

if you have kids with big lungs who are going to belt out a show tune like they don't need a mic then yes, 6" away is perfect!

[kaphc]

I see ... I'll have a look what makes and models the kit is when I'm at work next week.

 

BTW you're talking about a diaphragm inside the mic, not the human diaphragm? (Done a bit of singing myself and the dictat "breathe from the diaphragm is firmly engraved on my mind!)

 

Most of the time the kids won't go anywhere near a hand-held mic and tend to hold it as far away as possible. We've got some clip-on mics (which I don't like cos they pick up every single rustle) and some over-ear mics to deploy too so it'll be interesting to test the distance / gain stuff with them as it gets nearer Christmas.

[mjs_mjs]

also - the more expensive radio gear has input gain that you can adjust on the transmitter to avoid overloading the RF, also just different types of capsules have different responses, a ksm9 and a 87a have very different responses. (fyi - changing the gain of the mic before it's transmitted over RF is called 'head gain' and you should do it on each mic/transmitter combination for each person)

[Oaktech]

I see ... I'll have a look what makes and models the kit is when I'm at work next week.

 

BTW you're talking about a diaphragm inside the mic, not the human diaphragm? (Done a bit of singing myself and the dictat "breathe from the diaphragm is firmly engraved on my mind!)

 

Most of the time the kids won't go anywhere near a hand-held mic and tend to hold it as far away as possible. We've got some clip-on mics (which I don't like cos they pick up every single rustle) and some over-ear mics to deploy too so it'll be interesting to test the distance / gain stuff with them as it gets nearer Christmas.

 

yup, the bit that wobbles in the mic when you push it with air, coil in a magnetic field, movement generates current and all that...

 

if you read the thing about mixing you will find out about EQ ing for feedback reduction which should make your life a bit more predictable with tieclip and over the ear mics.

 

the secret with those mics is to get them as close as possible to the wearers mouth, if this requires using surgical tape to stick the things to the side of their face, or into the hair line then so be it! If you need to use tie clips as tieclips they need to be upright and no more than a fist's width from the underneath of the chin.