PA buzzing problem!

[MadisonSH]

Yesterday during rehearsal, we started experiencing a very bad buzzing sound coming from our PA system (Yamaha stagepas 500) when we would play our backing tracks from my Roland SPD-SX performance pad. The PAs would buzz the most on lower frequency notes of the keyboard track. We've played the same tracks many times before with no problems until yesterday and everything was hooked up the same as always.

At first we thought there might be something wrong with the speakers so we plugged in an iPod and played a song and it sounded fine. We even boosted the bass and it still sounded fine. Vocals, bass guitar and acoustic guitar also sounded fine. So it seems that there is nothing wrong with the speakers. Next we plugged the pad back in and checked and replaced all chords going from the pad to the mixer and the speakers still buzzed.

After ruling out the possibility of there being a problem with the speakers, chords or mixer, we automatically assumed that there was a problem with the pad outputs. We then ran the pad straight into a powered monitor and there was NO BUZZ!

It seems like we've ruled out everything, but still haven't found the source of the problem. We have a gig next week so we need to try to resolve this problem quickly. Any thoughts or suggestions on what the problem might be would be greatly appreciated!

-Madison

[The Red Bladder]

Either something isn't earthed properly, or something is not using balanced cable.

Make sure that all the connections are balanced and actually have connection on all three pins and everything is properly earthed.

[Mike Stranks]

I agree with Bladder... your testing is indicating it could well be a ground-loop problem.

If the problem is still there after you've checked that the cables (cords) are all good you probably need one of THESE (or something similar) between your pad output and mixer input.

If you're in the UK then check-out THESE. Like others here I'm happy to recommend Orchid's products unequivocally.

[MadisonSH]

Thank you for the input!

If it is a ground-loop problem why do you suppose the problem only started the other day? And why would there be buzzing only on lower notes or more bassy parts of the tracks?

-Madison

[The Red Bladder]

Define 'buzzing' please!

[MadisonSH]

Buzzing meaning that it sounds like the speakers can't handle the amount of bass I'm putting through it and they're being overloaded with certain low frequency notes. It basically sounds like the speakers are damaged. However it sounds fine when I crank up the bass on anything else. It's JUST the tracks on the pad that I'm having the problem with. It doesn't make sense because we've been playing the exact same tracks; same volume, eq, etc. for months with no problem.

-Madison

[The Red Bladder]

It sounds as if you have some infra-sound coming from the pad.

[MadisonSH]

If that is the problem how do you suggest I fix it? And why do you think it started all of a sudden?

-Madison

[Exalted Wombat]

Perhaps the pad puts out a lot of particularly low bass. The PA bass units have finally been damaged by it and are complaining. Your monitor speakers (a) don't have that low range and (b) haven't been damaged.

Can you synthesise something with lots of content below 50Hz and test it through the PA?

[MadisonSH]

Yes I can try that in the morning. Thank you for the suggestion!

-Madison

[MadisonSH]

If it helps, I've also tried playing the same tracks using an iPod instead of the pad and it sounds fine that way.

-Madison

[grab]

Have you definitely pinned down the source of the buzzing to the speaker cone itself? It's quite possible that some bit of hardware in/on the speaker cab has worked its way loose. Resonance being what it is, it might not show up on other tracks because they don't do sustained notes at the same frequency.

Also check for clipping. Speakers don't like it much, and even assuming you've not killed the speaker cone, the result is going to be nasty. Pull the level down a bit and see what happens.

Ground loops. If it buzzes when plugged into one bit of kit but not another, check grounding. If your keyboard isn't currently going into a DI box, then get one and use it.

And remember that trying to get any significant low end from a Stagepas, especially sustained notes from a keyboard, is never going to be very productive. The speakers simply can't do it, and if you hit them with too much of it then they're effectively just going to flap around. It's quite possible that you've pushed levels up a bit (whether on the keyboard or the PA itself) to try and compensate for the lack of low end, and the speaker is objecting.

[mpvano]

Here's a thought!

The apple IOS devices all contain graphic equalizers that affects sound playback considerably. (The also contain a compressor setting called "sound check")

A lot of people are unaware of these settings. They're controlled from the "Music" application settings in the "Settings" App. The entries to check iare marked "EQ" and "Sound Check" respectively.

Perhaps someone has changed these settings on the device in question to enormously boost some frequencies the stagepas can't handle. (Or perhaps your stagepas relied on special EQ settings someone has turned OFF!).

I've seen this sort of thing happen here before - I have a 13 year old car speaker with a buzz that gets driven crazy by any EQ settings that boost the bass beyond normal (on the car radio/cd player). When I'm checking masters in the car, I sometimes boost the bass to inspect it. If I forget to restore it to a neutral setting, my wife complains about the broken speaker cone until I reset it.

Just a long shot, but it COULD explain the confusing behavior.

(Also, the EQ could probably be used to fix the problem if it wasn't the original cause).

M

[MadisonSH]

I've been unable to pin the source on the speaker cones by the fact that the speakers only buzz or get distorted when the tracks are played through the pad. But NOT when the SAME tracks are played through an iPod. So that would lead me to believe that the speakers weren't the problem.

[MadisonSH]

Thanks for the input, but my brother and I are the only ones who touch the mixer or any other sound equipment and I can assure you that everything is exactly the same as of just a few days before the problem arose when we last rehearsed. I can also assure you that none of our tracks are peaking. We can go a hell of a lot louder and bassier with any other audio we put through them.

I can cut bass on the mixer's eq but then I loose some volume, and when I turn the volume up a bit to compensate, the buzzing on the lower frequencies is back.

[MadisonSH]

We are also not clipping.

[Sam Spoons]

If all you say is correct this one is bizarre in the extreme. You need to systematically change every component, lead, amp, speaker, etc one at a time. It's unlikely that a combination of components is causing the problem rather than one (though it sounds as if that one is interacting with another to cause the issue) and the only way to isolate the problem is by systematic substitution.

Good Luck

[MadisonSH]

We've changed every component as best we can. However we do not have a second PA system, but I'll do my best. Thanks for tip!

[MadisonSH]

Fixed the problem. We had the pad plugged into an input on the mixer that we don't normally plug it in to. For some reason those inputs produce more power which distorted the sound.

[Sam Spoons]

Aah, the thorny subject of gain structure. Glad to hear you've sorted it.

[Hugh Robjohns]

(The also contain a compressor setting called "sound check")

Sorry for the detour, but a lot of people have the wrong idea about SoundCheck...

SoundCheck is NOT a compressor. When activated on iTunes, the SoundCheck system evaluates the average 'loudness' across the duration of every individual track in your music library. It then stores a value that represents that loudness for each track either into the header of the track file itself (in the case of MP3s, ALAC, AAC files etc), or into the library database (in the case of wav files and others).

If you have the SoundCheck function turned on in your replay device, it will use the loudness value found in the file or database to adjust the replay volume automatically to the appropriate static level for that track, to ensure that the average loudness at the output is around -16dBFS.

There is no compression going on; it is a simple replay volume adjustment which is made before the start of the track and remains static throughout. it will then change for the next track (if necessary). The aim is to maintain a constant perceived loudness between tracks, and itworks very well.

It is similar to the ITU-R BS.1770 loudness standard system which is now being adopted around the world for TV broadcasters (it's already standard practice in much of the US broadcast chain and in the UK's HDTV networks). It will start being applied to radio within the next couple of years.

By setting levels according to perceived loudness, the so-called 'loudness wars' becomes self-destructive, and hat's a good thing! If a you compress a track to squeeze the energy as close as possible to the maximum peak level -- which is what happens now -- and then measure its loudness and match it to other tracks, the hypercompressed track will sound pathetically weak and feeble, and extremely uninteresting, compared to something recorded with more natural dynamics.

Loudness normalisation (as this arrangement is called) encourages dynamic range and light-and shade in the music, and penalises the hyper-compression that has become the norm with the current 'peak normalisation' culture.

I urge people to use SoundCheck on their iOS devices because loudness normalisation is the way of the future!

H

[Hugh Robjohns]

my brother and I are the only ones who touch the mixer or any other sound equipment and I can assure you that everything is exactly the same as of just a few days before the problem arose when we last rehearsed.

Oh no it wasn't!

It's very easy to overlook the obvious and make assumptions in this kind of situation. using different channels configured with different gain structures is a classic.

Glad you've resolved it now, though.

H

[tacitus]

And don't forget that channel swapping on an older desk might lead you to use one that's not at its best - one channel of my rather elderly folio 10/2 occasionally has a 'hissy fit' (almost literally) so I normal don't use it, except accidentally, and that's when you're in a hurry and it takes a stupid length of time to realise what I've done.

[shufflebeat]

Sorry for the detour...

SoundCheck is NOT a compressor. When activated on iTunes, the SoundCheck system evaluates the average 'loudness' across the duration of every individual track in your music library.

Now that the issue is resolved:

Not quite with you yet, H. Is the playback volume setting:

1) a single constant for each track in which case it will presumably match the highest peak to the headroom, we'll still lose the quiet bits in heavy traffic, or

2) variable across the track, in which case it's compression.

[Hugh Robjohns]

1) a single constant for each track in which case it will presumably match the highest peak to the headroom, we'll still lose the quiet bits in heavy traffic, or

2) variable across the track, in which case it's compression.

As I said previously, it is a single constant replay level for each track. It is not a compressor, and the replay level does not change during a track.

There is no matching of 'peaks' to headroom. It determines the perceived loudness of the track with complete disregard for peak level, and then stores a replay level setting that ensures each track is replayed with the same average loudness as every other track if the SoundCheck mode is engaged in the replay device.

In order to accommodate a sensible dynamic range and transient peaks, the average loudness is currently set to -16dBFS, which means that hyper-compressed tracks will sound much quieter than they would without SoundCheck engaged. But then they would, because they have no dynamic range! Tracks recorded with more natural dynamics sound fine, and you don't get your ears blown off when shuffle mode follows a gentle ballad from the 1970s with the latest totally-buggered metallica track.

H