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Author Topic: Frequency Drop Off  (Read 1096 times)

Aaron Baxter

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Frequency Drop Off
« on: April 02, 2021, 12:26:45 am »

I'm trying to isolate a strange problem in our sound system.  First off, the equipment is roughly 10 years old, Yamaha MG166cx board, Sennheiser mics (bout $100 range IIRC), Mackie Thump 15 mains.  We are getting frequency drop off around 1K and up.  It's there but at about 1K the output drops about 5DB on those frequencies and up with a slow downward taper.  What's driving me mad is it doesn't always do it (about 70% of the time) and every time I go to tinker with it it doesn't want to do it lol.  If I use the EQs in the thumps to gain the mids and highs it helps, but the singers still sound muffled.  I'm not sure if it's a fault in the board or the mics or something else.  I replaced the tweeters in the thumps because one wasn't working and the other, well why not keep them the same just like finals in an amp.  Also, sometimes (without adjustment) the singers will be way louder than the instruments or vice versa but it's usually pretty "normal" volume wise which is making me lean toward the board but I without knowing where it is I couldn't begin to repair it.  It's not like electrolytic cap pop or heat fade off in transistors.

I thought I'd get a few opinions on what to try first or if anyone might know what the issue likely is before I start throwing thousands in hardware at it.
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Chris Grimshaw

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Re: Frequency Drop Off
« Reply #1 on: April 02, 2021, 04:43:31 am »

Sounds like some methodical troubleshooting will be required.

First up, find another way of getting signal directly into the speakers (FWIW, having worked with them, I'd advise replacing them asap anyway). Phone/laptop is usually a good place to start. Verify whether or not the speakers themselves are working properly without the mixing desk.

If they're working fine, on to step 2: the mixing desk. If I had to bet, I'd say it's the likely culprit. I don't know what the storage conditions have been, but it sounds to me like dust/moisture in the pots and faders could be the cause of your problems. Play music from the phone/laptop/whatever into the problematic channel(s). Figure out if it's just some of the channels, or all of them. Try the headphone output as well as the main outputs feeding into the Mackies (which you've verified are working fine).

The mics may be the issue. I've heard of saliva build-up seriously degrading a mic's performance, which makes sense: if the internal foam becomes saturated, the HF performance will be very different. You can check easily enough by taking the grille off and poking a finger inside to see how the foam's holding up.
To fix that in the future, external foams swapped out occasionally would likely help.

Let us know how you get on.

Chris
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Steve-White

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Re: Frequency Drop Off
« Reply #2 on: April 02, 2021, 06:42:52 am »

Do some testing.  Run some pink noise through the system and some program music.  You need to determine where it's manifesting from rather than shogtunning all over the place.
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Kevin Maxwell

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Re: Frequency Drop Off
« Reply #3 on: April 02, 2021, 09:04:39 am »

I assume these are powered speakers. It sounds like you have an intermittent section of the internal power amp(s) that feeds the high end of the speakers.
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Jay Marr

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Re: Frequency Drop Off
« Reply #4 on: April 02, 2021, 09:20:04 am »

I assume these are powered speakers. It sounds like you have an intermittent section of the internal power amp(s) that feeds the high end of the speakers.

I had a similar issue with some JBL PRX512s.  I had 4 of them and they all had different amounts of output from the tweeter (so some sounded darker, etc).
They were under warranty....brought them in, and they replaced the amps.  No issues after that.

Different box I know, but just sharing that experience in case it's helpful.
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Mike Caldwell

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Re: Frequency Drop Off
« Reply #5 on: April 02, 2021, 12:08:20 pm »

It sounds like a speaker issue but yet odd they both are doing the same thing, when it does start working do both speakers start working properly at the exact same time?
Are the speakers each directly connected to the mixer or jumping from one speaker to the other.

Don't just start throwing money at a problem that you have not isolated what the cause really is.
Call in someone who knows what they're doing to help go through your system and sort out
the issues.
Where are you located, someone here may be close by.
« Last Edit: April 03, 2021, 05:21:08 pm by Mike Caldwell »
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Steve-White

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Re: Frequency Drop Off
« Reply #6 on: April 02, 2021, 01:29:47 pm »

^^^  Yeah Mike for sure.  First thing that came to mind was horn compression driver diaphragms.  They were replaced.  Could be almost anything or a combination of issues up to and including Operator Error.

Time to systematically sift through the chain to find the squeak.
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Nathan Vanderslice

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Re: Frequency Drop Off
« Reply #7 on: April 03, 2021, 03:44:42 pm »

I'm trying to isolate a strange problem in our sound system.  First off, the equipment is roughly 10 years old, Yamaha MG166cx board, Sennheiser mics (bout $100 range IIRC), Mackie Thump 15 mains.  We are getting frequency drop off around 1K and up.  It's there but at about 1K the output drops about 5DB on those frequencies and up with a slow downward taper.  What's driving me mad is it doesn't always do it (about 70% of the time) and every time I go to tinker with it it doesn't want to do it lol.  If I use the EQs in the thumps to gain the mids and highs it helps, but the singers still sound muffled.  I'm not sure if it's a fault in the board or the mics or something else.  I replaced the tweeters in the thumps because one wasn't working and the other, well why not keep them the same just like finals in an amp.  Also, sometimes (without adjustment) the singers will be way louder than the instruments or vice versa but it's usually pretty "normal" volume wise which is making me lean toward the board but I without knowing where it is I couldn't begin to repair it.  It's not like electrolytic cap pop or heat fade off in transistors.

I thought I'd get a few opinions on what to try first or if anyone might know what the issue likely is before I start throwing thousands in hardware at it.
  The one thing I haven't heard yet here is what about connections? I don't know anything about these speakers, but what about problems with various connectors that may be in the boxes? Push ons or screw held connections that are loose. Is it possible that the trouble is coming from connections that are just loose enough that they are making a partial connection that is affecting the performance of the speaker?  Could this include some solder connections?
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Tim McCulloch

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Re: Frequency Drop Off
« Reply #8 on: April 03, 2021, 07:01:20 pm »

If it's the Thuds« that are lacking HF, it's their way of telling you it's long past time to buy better speakers.

No, I'm not kidding.  The "Thud"« is the satisfying sound they make (the only one) when they hit the bottom of the dumpster.

Ask for more if you want to know how I really feel.
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"Practicing an art, no matter how well or badly, is a way to make your soul grow, for heaven's sake. Sing in the shower. Dance to the radio. Tell stories. Write a poem to a friend, even a lousy poem. Do it as well as you possible can. You will get an enormous reward. You will have created something."  - Kurt Vonnegut

Steve-White

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Re: Frequency Drop Off
« Reply #9 on: April 03, 2021, 09:33:03 pm »

^^^ ROFL - I tend to feed the recycle bin much sooner in the "turd polishing" cycle these days as well.
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Paul G. OBrien

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Re: Frequency Drop Off
« Reply #10 on: April 04, 2021, 04:08:55 pm »

Sounds like some methodical troubleshooting will be required.

That is an understatement, virtually every component in your setup may be a contributing factor including your mixing technique.

If this problem manifest and then corrects itself like a switch being thrown in and out but without anybody touching anything PA related then this is most definitely a hardware problem, but it could be anything from anywhere in the signal chain from a mic to the mic cable to a dodgy connector or a loose ribbon cable or dirty pot in the mixer to a cold solder joint on a circuit board in the powered speakers.

As for how your mixing technique plays into this, my first hand experience with these MG series mixers indicates they don't have much headroom, mixing a full band on one of these requires all inputs remain solidly in the green because the combined signal level will easily overload the output bus. Things just start to sound congested and brittle when the board is overloaded, I can't say I hear it as a drop in level it just gets dirty sounding a bit too easily.
As a former Markie Thump owner I can also say these are definitely a potential source of your problem. The boxes I had suffered blown drivers as well which interestingly presented early on with random drops in output level. In my case it was the woofers that died but with a 2khz crossover they also contribute quite a lot of mid output.
IMO these boxes are marginal for recorded music playback at moderate levels and would be quite unsuitable for live sound applications, I remember being quite underwhelmed the first time I heard 4 of these being used for a DJ event in a high school gym(with subs), they just didn't produce much output and even the DJ wanted more mids. They are processed with a music playback friendly scooped mid response(even when set flat), and the 1000w rating is a hilarious over exaggeration.. I doubt there is really more than 250w available so it's all too easy to push them into limiting.

So bottom line is you have some work to do, you need to borrow or rent another board and speakers and either buy or borrow a cable tester to check the health of your cable inventory, setup your rig and try to replicate the problem and then start swapping things. You have to be methodical and go though the signal chain from mic to speaker one channel at a time, wiggle connections, exercise controls, leave the equipment on for a day or two to see if heat is a contributing factor. If you can setup a second PA.. even just a small mixer and a single powered speaker you can quickly check to see if your mics or cables behave the same on that system.

If you can isolate the problem and say for example it turns out to nobody's surprise be the speakers, then I'm sure the consensus would be to cut your losses and upgrade to something better. There is a whole world of better out there in terms of powered speakers but that will require investing some good money, you won't get appreciably better results with entry level speakers from any manufacturer.
« Last Edit: April 04, 2021, 04:13:18 pm by Paul G. OBrien »
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Aaron Baxter

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Re: Frequency Drop Off
« Reply #11 on: April 06, 2021, 12:09:27 pm »

Sounds like some methodical troubleshooting will be required.

First up, find another way of getting signal directly into the speakers (FWIW, having worked with them, I'd advise replacing them asap anyway). Phone/laptop is usually a good place to start. Verify whether or not the speakers themselves are working properly without the mixing desk.

If they're working fine, on to step 2: the mixing desk. If I had to bet, I'd say it's the likely culprit. I don't know what the storage conditions have been, but it sounds to me like dust/moisture in the pots and faders could be the cause of your problems. Play music from the phone/laptop/whatever into the problematic channel(s). Figure out if it's just some of the channels, or all of them. Try the headphone output as well as the main outputs feeding into the Mackies (which you've verified are working fine).

The mics may be the issue. I've heard of saliva build-up seriously degrading a mic's performance, which makes sense: if the internal foam becomes saturated, the HF performance will be very different. You can check easily enough by taking the grille off and poking a finger inside to see how the foam's holding up.
To fix that in the future, external foams swapped out occasionally would likely help.

Let us know how you get on.

Chris

Chris/all,

Thanks for the troubleshooting ideas. 

Turns out it was indeed the mics.  I ran music through one of the channels and it sounded pretty ok (mind you the EQs were set for specific singers vocals) so I replaced them with SM58s and that solved most of the issue.  The rest I was able to EQ out on the board/speakers.  Granted the board still has other issues like inconsistent volume levels depending on what mood it's in, and apparently the Thumps are not popular on the forum, but for now it's tolerable.
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Mike Caldwell

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Re: Frequency Drop Off
« Reply #12 on: April 06, 2021, 11:52:16 pm »

Chris/all,

Thanks for the troubleshooting ideas. 

Turns out it was indeed the mics.  I ran music through one of the channels and it sounded pretty ok (mind you the EQs were set for specific singers vocals) so I replaced them with SM58s and that solved most of the issue.  The rest I was able to EQ out on the board/speakers.  Granted the board still has other issues like inconsistent volume levels depending on what mood it's in, and apparently the Thumps are not popular on the forum, but for now it's tolerable.

How many mics are you using?

You said about 70% time things sounded muffled. When the system would start working was it just all at once it popped on so to speak and all sounded good instantly for sounding bad or was it one day it sounded bad and the next day everything was working?

Did you ever play recorded music back through the system when the vocal mics were sounding bad?

Aaron Baxter

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Re: Frequency Drop Off
« Reply #13 on: April 09, 2021, 07:11:02 pm »

How many mics are you using?

You said about 70% time things sounded muffled. When the system would start working was it just all at once it popped on so to speak and all sounded good instantly for sounding bad or was it one day it sounded bad and the next day everything was working?

Did you ever play recorded music back through the system when the vocal mics were sounding bad?

It would generally be when you turned the board on.  It seems to be moody but with the new mics it's providing a much cleaner input so it sounds better.  The sound most all of the time was either all live or all recorded and the rare times it was mixed it wasn't acting up (that I can recall that is).  It seems to be bad mics exasperated by the board because it's still not as clean and clear on the 4 mic channels.  I could play musical channels, but IMHO the game isn't worth the candle since I pray in a few months I can replace it with an X32.  I've had all the channels sound muted/muffled at one time or another so I'm leaning toward the preamps going out in the channels.  I'll rebuild the board, if possible, as a spare and for fun (lol) after it's replaced. 
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Mike Caldwell

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Re: Frequency Drop Off
« Reply #14 on: April 09, 2021, 07:57:26 pm »

I'm guessing the board is the main problem. The Yamaha MG series is know for bad switches and becoming intermittent over time.

As for a digital replacement there are other options that work great and maybe faster to get up to speed on for a volunteer sound team (assuming this is for a church) than an X32.

Aaron Baxter

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Re: Frequency Drop Off
« Reply #15 on: April 12, 2021, 08:46:04 pm »

I'm guessing the board is the main problem. The Yamaha MG series is know for bad switches and becoming intermittent over time.

As for a digital replacement there are other options that work great and maybe faster to get up to speed on for a volunteer sound team (assuming this is for a church) than an X32.

Good to know about the sticky switches.  Whenever I get around to rebuilding it, I'll know where to focus.  Tonight wouldbhave been horrid with the old mics as the board was super driving the bass but at least we could hear the highs.  It is a church, and well, they were getting by with limited knowledge.  Thankfully I do have more experience than others here but am by no means an expert.

What I like about the X32 is I can setup some automatic presets that should work 96% of the time for most places we'll travel but gives me and whoever I can train a crazy amount of options.  It'll be a large curve for me too, but I'm computer systems engineer so it's a matter of merging my physical rack knowledge and my software knowledge.  Thankfully there are a tons of great YouTube vidoes, even one guy that's from a HoW and lots of other how tos on the x32/m32.
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ProSoundWeb Community

Re: Frequency Drop Off
┬ź Reply #15 on: April 12, 2021, 08:46:04 pm ┬╗


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