Print

Please login or register.
1 Hour 1 Day 1 Week 1 Month Forever
Login with username, password and session length

[Zaharescu Mihai]

Hello.
I hope this is the place to post this question:
The sound system in our church has a buzzing sound that follows the singer's voice and instantly gets louder or softer from time to time.
I uploaded a recording in which for the first 5 seconds the buzzing is very evident, than a clicking sound is heard and the buzzing almost disappears. It is still there and it is rather annoying especially at certain frequencies and volumes. When it get's louder again it could be said that it would be better without a sound system because it is very distracting.
We cave 11 condenser microphones, 2 amplifiers (TOA A-1706 and TOA A-1724), 3 processors (1 equlizer - dbx 131 s, 1 feedback destroyer behringer fbq1000, 1 compresser behringer mdx1600), 1x6 volume controller (APART 19-VOL660).
The connections are as follows: 6 mics in the low power TOA. Exit on REC-OUT and enter in AUX of the high power TOA (with RCA cables). The aditional 5 mics enter in this TOA. Exit from LINE-OUT and enter in the EQ (RCA-XLR). than EQ - Feedback (XLR-XLR) than Feedback - Compresser (XLR-XLR). Than Compresser - POWER-AMP-IN of the low power TOA (XLR - RCA) (when this problem is fixed we will have a split here and we will enter in both amplifiers, to have one for the exterior and one for the interior). The output is split in parallel to the APART to control the volumes of 6 pairs of monitors.
All volume leds from all devices seam to be at about the same level, except the forst TOA which shows a much lower volume (probably because the signal is incomplete here - only from 6 mics and they are also at lower lever because they are closer to the monitors). Clipping seams to occur at much greater volume for any mic than the one at which this recording is made.

And here is the recording: problem.wav

Thank you

[Mac Kerr]

Hello.
I hope this is the place to post this question:

Please go to your profile and change your name to your real name to comply with the posting rules.

Mac

[Ken Cross]

I would begin by checking that the trimmer on the mixer isn't set too high. It sounds like that may be right.
Also, listen with headphones plugged into the mixer to determine if the distortion is in the microphone/mixer side of things.
If the sound is clean there, then progress toward the audio processor (if you have a separate one), amplifier, and speaker(s).
It's possible that it's a bad speaker.

With that much distortion, you may be able to easily see it on an oscilloscope to chase it down.

Ken

[Zaharescu Mihai]

I would begin by checking that the trimmer on the mixer isn't set too high. It sounds like that may be right.
Also, listen with headphones plugged into the mixer to determine if the distortion is in the microphone/mixer side of things.
If the sound is clean there, then progress toward the audio processor (if you have a separate one), amplifier, and speaker(s).
It's possible that it's a bad speaker.

With that much distortion, you may be able to easily see it on an oscilloscope to chase it down.

Ken

Thank you for your time.
Unfortunately we don't have a mixer, the amplifiers themselves (TOA) have 6 microphone input channels each.
It's not from the monitors because the buzzing comes from all of them no mater if the main volume is very low or very high.
It doesn't seam to be from the microphones because it's the same result no mater in which mic we speak into.
The gain/trim could be a problem and I don't understand the diference between dBu/dBv/dBm/db.
TOA has  0dB output = 1V.
The feedback has two operation modes:
- for +4dBu: max in +16 dBu / max out +16 dBu
- for -10dBv: max in +2 dBv / max out +2dBv
The compressor has two options +4dBu and -10dBv and they don't make a difference in the manual for the max values: max in is +22 dBu and max out +21 dBu
The equalizer hasonly one option: +4 dBv and max in is 21 dBu and max out 21 dBu.
As for the impedances there doesn't seam to be any problem putting the devices in any order because the inputs are tens of K ohms and outputs are tens (except for the power amp in from the TOA that has 600 ohms, but entering from the feedback that has 30 ohms it's ok- we interchanged the devices today with no difference).
I tried with a single microphone, microphone boost at 12 'o clock, all the levels are at about the same dB through all the devices and with a medium input volume that buzzing is evident.
Writing this I realized that I don't know what volume is displayed on the TOA that outputs the signal to the processors, from what I understand from the block diagram it should be the power sent to the 100V speakers, but it doesn't have any connected yet (only the other TOA has monitors attached to it).
2 days from now I will be caught with some school assignment but after that I will try removing all processors and seing if the amplifiers alone work on.
If you can help me with information of what operating modes I should choose and how to connect them together it would be helpful because I can't imagine what could be wrong (I have a feeling that the signal coming out of the TOA line out preamplifier is already corrupted). I don't know what I did today, bust I selected the switches and gains in order for all the volumes to be identical (I didn't overdo the gains).
Thank you

[Zaharescu Mihai]

I found this page where under the title "Hang on" there is a mention that a 0 dBV device will probably not be able to drive 600 ohms and sound distorted, a solution being an amplifier (what kind of amplifier?) between the processor and power amplifier of the TOA. What do you think, i'm just a beginner in this domain and it's to much to understand at once.
This will also mean that removing the processors from the line will work undistorted. I will have to try that when I finish the school assignments.
[edit:] I just looked on the internet and I have no idea what I should put between the processors and the two 600 ohm amplifiers (actually 300 ohms because the signal will be split between them) if this is the problem.
[edit2:] Thinking again, the problem can also be elsewhere because the same noise is evident even when the volume that exits from the preamp (line out) is very low. You can hear a very low distorted sound. I think the processors can cope with that level even with this impedance problem. I will have to remove the processors and see what happens...

[Andrew Hollis]

Thanks for describing the signal flow and including a sample, that helps a lot!

What kind of speakers are here? Is this a 100 volt high impedance speaker system?

The main issue is this is a hodgepodge of gear and connectors that are not fit for purpose, looping around back into each other!

Some of this gear is for 100 volt 'conference' type systems, like for airports, not for live music production.

You are mixing consumer unbalanced RCA levels with pro balanced XLR line level devices. That often leads to exactly these kinds of noise issues. Your processors should be set to -10 dBV.

Regardless, it can probably be improved:

First you must make sure your amps are set to the correct speaker type: 100 volt vs low impedence 'normal' speakers.

Normally one does not put a volume control between a power amp and a speaker... but I don't know enough about 100V. Get rid of that.

I suggest you remove all of the 'downstream' processors as a test.

I suggest you break it all down and test with just this:

1 mic > 1 amp > speaker

Get rid of everything else, the volume control, processors, etc, and do not loop back into the power amp in.

Also, why would you use the low power amp as the main drive amp ? It would make more sense to use the higher powered amp as the 'last amp in the chain.'

[Ken Cross]

It sounds to me like what you are describing is what we would call a powered mixer. If you just have microphones connected to one box and speakers on the output of the box, (usually many controls on the box itself) that is what you have.  Some powered mixers are much more complicated than others. A make and model number might help. If your system has more components, let us know what is there.

Ken

[Zaharescu Mihai]

Thank you for the replies.
Unfortunately I removed the processors, leaving just the two amplifiers the mics and speakers and the problem persists. I am sure that they used to work in this simple configuration. We are in the process of adding the exterior monitors so I can test each amplifier individually like you said.
This problem also doesn't seam to be impedance related because we just split the output from the processors to enter in both power amplifiers, reducing the impedance by half and no gain in noise levels can be noticed. Also we added two monitor volume controllers between the processor and poweramps that can increase the impedance to infinity and still no difference in noise levels.
This looping hybrid system emerged because we used to have only the two amplifiers, but they were unusable in the church because there was so much echo that feedback emerged even at unusable low volumes. That is why we also bought the processors and 100 v monitor controllers. For about half a year everything worked perfectly, until someone decided to push buttons. For three months it worked at very low volume because of the bad configuration and when we tried to redo everything a few weeks back (including cables and adding external volume controllers in order to lock the devices) thus we noticed this problem trying to get back to normal volume level.
The loop is as follows: because we don't have a mixer, the amplifiers are used as mixers, we need to construct the final signal by connecting them together (because we have 11 mics and 6 channels per station). Than the complete signal is feed through the processors and the processed signal is sent to the amplifiers that have an input that connects directly to the power amplifier, disconnecting the internal signal (thus it actually doesn't create a loop, just splits the two devices in preamp and poweramp with no connection between them). Thus the TOA stations are split in half: two mixers connected together that output signal in the processors and the power amplifiers that are completely disconnected from the mixers and get their signal from the processors.
If I am not clear enough please tell me so I can try to explain better. I'll get back to you as soon as I test each TOA individually.
Somehow, temporarily, we managed to avoid the distortion by setting very low microphone sensitivity (about 9 'o clock) and very high volume, but I know that it used to work with sensitivity to the max without problems (except for the more noticeable mic noise) as the mics are ambient mics, above and rather far away from the singers.
About the speakers, they are 100v. The power amp is the high power amplifier and it is the last in the chain (before the monitor controllers that lower the volume for the closest monitors to the mics).
We also tried to keep everything balanced, the problem being that the outputs of the TOA are only simple RCA, I don't know why...
Thank you for your time

[Ken Cross]

So are you saying you have 2 amplifiers ganged together with different inputs? Model numbers might help.

It seems in this case, the amps need to be separated to identify the one that is misbehaving.
Ken

[Ray Aberle]

So are you saying you have 2 amplifiers ganged together with different inputs? Model numbers might help.

Model numbers are posted in the initial post of the thread.

-Ray

[ProSoundWeb Community]