Puzzling spikes and crackles on the church sound system the last couple of weeks, much worst this morning, seems to be made up of individual spikes lasting less than a mS, expecially on sound peaks, also background noise around 13.8kHz which only the teenagers could hear. Any suggestions?

System schematic attached. We have an Australian Monitor MX883 mixer with 8 inputs (2 UHF radio mics, 3 wired mics, digital piano, PC headphones output and Bluuetooth receiver). The main output goes through a dbx 2-series graphic equaliser to a QSC-GX3 power amp feeding 4 ceiling speakers. An aux output from the mixer provides phono output to a home-build op amp pre-amp and hence PC mic input, and XLR output split between a PDA500-2 induction loop and and a home-build class D amp for sound relay to the creche room.

Take a look at the attached sound clip with Audacity. You'll see several spikes, the largest at 10.853 secs, and crackles throughout the speech, and the high frequency throughout.

The interference appears on both the mixer main and aux outputs and is provoked by peaks on any (at least 4 tested) of the 8 inputs. The only common factor seems to be the mixer PSU, but then why should it be provoked by higher sound levels? Are the individual spikes what you might expect from a Class X capacitor breaking down and self-healing? Do any of these symptoms mean anything to anybody?

 

Puzzling spikes and crackles on the church sound system the last couple of weeks, much worst this morning, seems to be made up of individual spikes lasting less than a mS, expecially on sound peaks, also background noise around 13.8kHz which only the teenagers could hear. Any suggestions?

System schematic attached. We have an Australian Monitor MX883 mixer with 8 inputs (2 UHF radio mics, 3 wired mics, digital piano, PC headphones output and Bluuetooth receiver). The main output goes through a dbx 2-series graphic equaliser to a QSC-GX3 power amp feeding 4 ceiling speakers. An aux output from the mixer provides phono output to a home-build op amp pre-amp and hence PC mic input, and XLR output split between a PDA500-2 induction loop and and a home-build class D amp for sound relay to the creche room.

Take a look at the attached sound clip with Audacity. You'll see several spikes, the largest at 10.853 secs, and crackles throughout the speech, and the high frequency throughout.

The interference appears on both the mixer main and aux outputs and is provoked by peaks on any (at least 4 tested) of the 8 inputs. The only common factor seems to be the mixer PSU, but then why should it be provoked by higher sound levels? Are the individual spikes what you might expect from a Class X capacitor breaking down and self-healing? Do any of these symptoms mean anything to anybody?
View attachment 317322

Sounds like a bad mic or mic cord. But you will have to see if it is associated with one mic channel.

 

Am I correct that this happens no matter the source connected to the mixer’s four tested channels?

 

The interference appears on both the mixer main and aux outputs and is provoked by peaks on any (at least 4 tested) of the 8 inputs. The only common factor seems to be the mixer PSU, but then why should it be provoked by higher sound levels? Are the individual spikes what you might expect from a Class X capacitor breaking down and self-healing? Do any of these symptoms mean anything to anybody?

I didn't see this when I first posted, but if it worked fine for a few years then malfunctioned later it could be a dirty or bad potentiometers in the mixer or a bad cord in the signal chain.

 

Electrolytic caps starting to go bad?
Had that experience with a combined radio/TV set which finally died. Caps were the culprit.

 

Am I correct that this happens no matter the source connected to the mixer’s four tested channels?

May thanks for replying. We upgraded the system 3 or 4 years ago and it's been fine since, until very recently. It's possible the spikes have only appeared (or only been noticeable) since we had the installation engineers in a few weeks ago to adjust the levels - the loop amp wasn't getting the drive it needed.

I went down there this morning to try and pin down the cause. I tried all 8 inputs to the mixer but there wasno sign of either the spikes or the high pitched sound. So I'm more puzzled than ever.

 

Electrolytic caps starting to go bad?
Had that experience with a combined radio/TV set which finally died. Caps were the culprit.

I did wonder about that. Do they produce narrow spikes <1mS wide as the dielectric breaks down and self-heals?

 

I didn't see this when I first posted, but if it worked fine for a few years then malfunctioned later it could be a dirty or bad potentiometers in the mixer or a bad cord in the signal chain.

Thank you for taking the trouble to reply, but as I said, it doesn't seem to be associated with any of the 8 mixer inputs in particular, and affects both the mixer main and aux outputs. A glance at the scematic will show you that it's hard to imagine where a bad connection or noisy pot could be located.