speaker noise / possible capacitance issue

[julian...@gmail.com]

So I've got a show running, and an issue was reported that I understand as the following:

During a period of relative silence, at least one passive speaker (possibly more, impossible to know without having been there) experienced a gradual and noticeable buildup of white noise. It held steady for a period of time, and was resolved once a louder sound cue was taken.

This seems to me like a power issue. My knowledge of electrical engineering is limited, so I want to call this a buildup of capacitance without really knowing what that necessarily means. Last I've experienced such an issue, it was in a similar situation where lighting and sound shared a common ground, and a lighting instrument was experiencing an issue (note: what I'm describing is not a ground hum). The amps in this instance are on a dedicated breaker on the lighting PD.

My experience tells me that isolating the ground, either by utilizing an isolation transformer or just using an isolated ground (in this case, would be wall power), will resolve my issue - but I wanted to get an opinion of someone well-versed in electrics who can weigh in and explain how this sort of phenomenon works. The key takeaway is that the noise was remediated when the amplifier supplied more wattage when a loud sound cue was played.

[Jonathan Woytek]

Hi! I’m going to talk through a few things that I would check:

- Did an expander or other AGC-type processor get enabled on some channel that is open at the time (and likely not a microphone)? That could bring up the noise floor until audio actually hits the channel. The test here would be to watch the signal meters for potential offenders, or try keeping the potential channels muted until they are needed. 

- I have had a phantom powered condenser do something similar. At low input levels, it would get noisy, but as soon as it had some level coming into it, it quieted right down. The “fix” was to cycle phantom on that channel (muted, of course), with about a 15-second off time to let the caps fully discharge. Again, signal meter showed the culprit here. 

- How are the SFX channels coming into the board? If they’re through a DI or audio interface, check that out for issues. They would show up as signal on the affected channels. Again, could be an AGC type function enabled here somewhere, or could be a failing device, or could be a broken shield or grounding issue somewhere in that chain. 

You could still be onto something with an amp issue. I would also consider checking lines coming into the affected amps for broken shields. I would check that before just trying a ground lift anywhere. Normally (not all the time, of course, as ground potentials can change over time for a lot of reasons), a ground issue will be persistent. 

And, of course, be safe. Lifting mains ground on any equipment with grounded mains is usually a bad idea. 

Jonathan

Sent from my Commodore64

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[Daniel Chin]

Julian,

In my experience, noise issues like this are more common in low voltage signaling devices rather than in power amplifiers and if it is in the amplifier, it's more likely to be an internal fault than something correctable externally. That's just my experience with probabilities though.

Because of that, I'd first make very sure that, as Jonathan recommended, you check your console first. Does the problem still occur with outputs muted? If the sound goes away when you mute outputs, you know it's before them. Divide and conquer. I'm not saying it's not the amplifier, but since you say you weren't there and isolating noise issues can be difficult, especially during a performance and when it's unexpected, I'd spend much more time investigating the source before settling on a diagnosis.

As Jonathan said, grounding issues tend to be persistent, so I don't think that's the most likely cause either. If you do lift a ground, it should be a signal ground NOT a safety ground. If you do lift a signal ground in a balanced circuit (where both output of sending device and input of receiving device are impedance balanced), it's generally best to do it at the input side of the receiving device. The easiest way to do that is to just desolder pin 1 of the XLRs feeding the amps (assuming you're using balanced XLR inputs). The best thing to do if you come back to an electrical interface suspicion is to use an input transformer based box such as Jensen's PI-2XX:
https://www.jensen-transformers.com/product/pi-2xx/

If there is an internal fault that is causing noise, I'd actually expect some partial "open" state on an amplifier (as in type of circuit, not necessarily power amp) input that ends up getting bridged when there's more signal—and therefore voltage—to overcome the open. Finding that can be tricky, but again, divide and conquer. Rule out what you can in terms of devices and channels. Then, once you're sure of where it's happening, hypothesizing about the cause becomes much more productive. If you track it to an amplifier and if lifting the signal ground and/or adding input transformer(s) doesn't help, it's time to try swapping the amplifier with a known good one.

Daniel Chin

------ Original Message ------

From "Jonathan Woytek" <woy...@dryrose.com>

To theatre-s...@googlegroups.com

Date 11/6/2024 7:09:01 AM

Subject Re: [theatre-sound-list] speaker noise / possible capacitance issue

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