postie1717
Member
I guess Ill be attempting to get someone to fork over the cash to get it professionally looked at, or Ill be bribing people at the theater I sometimes work with to see if they can fix it. Gotta love low budget theaters.
Loudspeakers Hum issues in main speakers
- Thread starterpostie1717
- Start date
I guess Ill be attempting to get someone to fork over the cash to get it professionally looked at, or Ill be bribing people at the theater I sometimes work with to see if they can fix it. Gotta love low budget theaters.
It's not entirely accurate as it is measuring only the resistance and not the complex impedance, but disconnecting the problem line at the amp and making a resistance measurement across the line may give some indication of any major problems (e.g. something shorted or open) without having to take down the speaker. If the measurement reflects a short or some very high resistance value then you that's s good indication that you have something wrong.
The other aspect of this is that if you do have a blown speaker then the question arises of whether it is the result of anything in particular. Failures do just happen for no specific reason but they can also happen for a reason and if that is the case it makes little sense to repair the speaker without also addressing the cause of the failure.
The other aspect of this is that if you do have a blown speaker then the question arises of whether it is the result of anything in particular. Failures do just happen for no specific reason but they can also happen for a reason and if that is the case it makes little sense to repair the speaker without also addressing the cause of the failure.
bishopthomas
Well-Known Member
ArthurRiot
Member
Heh, I had a similar problem in my theatre at one point. Randomly it seemed, my one side of speakers would hum.
I started the process of isolating the issue, and I was caught off guard where it was; the cable that patched it in the patch bay. It was the right-most wire, and was closest to people bumping into it. It wouldn't stay locked into the patch port.
I'm probably not helping. But you reminded me. Frustrated the crap out of me, testing my amps and going through all the wiring, switching everything and researching...
I laugh about it now.
I started the process of isolating the issue, and I was caught off guard where it was; the cable that patched it in the patch bay. It was the right-most wire, and was closest to people bumping into it. It wouldn't stay locked into the patch port.
I'm probably not helping. But you reminded me. Frustrated the crap out of me, testing my amps and going through all the wiring, switching everything and researching...
I laugh about it now.
David Ashton
Well-Known Member
It is common and very bad practice to run the amps at full, and the major cause of hum and dynamic range problems, but don't believe me, go to Rane or Yamaha or Mackie or any other source you trust and read how to set your gain structure, this is the best explanation, http://www.live-audio.com/studyhall/gain.pdf others are simpler, but understanding it will transform your sound.
Amplifiers by themselves rarely are a noise problem even when run wide open, the cause of noise and dynamic range problems is usually not with the amplifier itself but rather with the system gain structure prior to the amplifier. So it's not that running the amp wide open is a poor practice or inherently problematic but rather that it is often unnecessary may be indicative of being used to compensate for poor gain structure.
What I have seen most people recommend is to optimize the system gain structure without considering the actual system output, in fact this is often done with the speakers disconnected. If that results in more output than desired then you can adjust the amp level down to be whatever provides the appropriate output levels. However, it is a completely acceptable practice to run the amplifiers wide open and install fixed pads of an appropriate value directly in front of the amps. That makes it easier to maintain the proper levels and prevents people from easily adjusting the levels. I also know many people that apply the same concept by making use of the analog attenuation available on the inputs and outputs of many DSP devices installed immediately before the amps, applying any attenuation at the DSP outputs rather than at the amp input.
So running amps at full is a very common and acceptable practice provided that it is associated with good gain structure and not with compensating for poor gain structure.
David Ashton
Well-Known Member
That is totally dead wrong, because lots of people do it makes it no less wrong, you cannot show me any source by any competent authority to back up this false assumption.Just as you adjust the input gain on your mixer to match the input of the mixer, you need to adjust the input of your amp to accomodate the input level coming into the amp.If you run your amp at full then you are driving something back in the chain at lower than optimal level.This reduces your signal/noise ratio and dynamic range.You can of course run your amps at "full", for convenience but only if you fit a pad in the system to match the input to the signal level.If you have an attenuation device on the preceding unit, that is a pad, but it is fairly unusual in lower end systems.In rock and roll with a huge system, no one cares about a bit of noise, but in a quiet theatre situation gain structure is crucial.Again don't believe me, Google gain structure at the authority of your choice and read, or better still just try it.
David, I don't want to get into an argument over this. I won't go though all my background on the subject or other resources and will just focus on the article you referenced. The point Chuck makes in that paper is that it is the amplifier's input sensitivity that is important in reference to gain structure. Because that level is typically much lower than the maximum output levels of the devices earlier in the signal chain he notes that this typically requires having to attenuate the signal right before the amplifier. I think we agree up to this point. However, his comment is that "...the obvious step is to put a pad (usually the amplifier input attenuator) so the amplifier will clip at about the same point as the next least capable device." Note the "usually the amplifier input attenuator" qualification as whether you attenuate at the output of the prior device, insert a dedicated pad or attenuate at the amplifier input is the same thing as terms of the gain structure. Using the amp level controls may indeed be the most common approach, however there is more than one way to achieve the same result and it is possible for the amp to be run wide open and still have completely proper gain structure. It is not simply a matter of the amplifier input levels but of how they relate to the overall system gain structure.
Now we can look at why you might want to not rely on the amplifier's input attenuators. If you look at some amplifier manufacturers you often find that one difference in the 'contractor' oriented models is that the level controls are on the rear panel or have security provisions. This is to minimize people inadvertently or intentionally changing the settings. For installed system and/or when dealing with inexperienced operators, if you use an amplifier that does not have such provisions or if you simply want an additional level of protection then it is indeed common and completely proper to attenuate the signal into the amp via the prior device's output or a dedicated attenuator so that the amp levels can be left at their maximum setting. This allows the amp controls to easily be returned to the proper settings while also preventing someone from trying to get more out of the system than it can give. In fact, in some situations running the amp level controls at their highest setting and attenuating the signal prior to the amplifier would probably be considered the best practice.
There are also potential reasons to deviate from the ideal gain structure settings in terms of the amplifier input levels. One that Chuck notes in that article is to reduce the level into the amp in order to allow operators to think they are closer to the limits of the system than they actually are, however the same concept also applies if you want to reduce the system output levels while maintaining optimal gain structure through the rest of the system. The risk with that, as Chuck notes, is that you could have other devices clip before the amplifier, but that is simply the kind of issue that must be assessed in terms of what provides the best overall result.
A couple of details to note, one of which is also addressed in the referenced paper. When I mention attenuating a device's output that is referencing an attenuator on the device output, not reducing the gain of an internal output device. It is sometimes difficult to establish whether the output level controls on a device are actually controlling an attenuator between the internal electronic circuits and the output connector or if they are varying something in an internal circuit, but this can be an important difference in terms of gain structure. The other point is that this discussion is referencing analog gain structure, gain structure for a digital signal path avoids some of the gain structure issues present in analog systems but can also introduce other issues.
Now we can look at why you might want to not rely on the amplifier's input attenuators. If you look at some amplifier manufacturers you often find that one difference in the 'contractor' oriented models is that the level controls are on the rear panel or have security provisions. This is to minimize people inadvertently or intentionally changing the settings. For installed system and/or when dealing with inexperienced operators, if you use an amplifier that does not have such provisions or if you simply want an additional level of protection then it is indeed common and completely proper to attenuate the signal into the amp via the prior device's output or a dedicated attenuator so that the amp levels can be left at their maximum setting. This allows the amp controls to easily be returned to the proper settings while also preventing someone from trying to get more out of the system than it can give. In fact, in some situations running the amp level controls at their highest setting and attenuating the signal prior to the amplifier would probably be considered the best practice.
There are also potential reasons to deviate from the ideal gain structure settings in terms of the amplifier input levels. One that Chuck notes in that article is to reduce the level into the amp in order to allow operators to think they are closer to the limits of the system than they actually are, however the same concept also applies if you want to reduce the system output levels while maintaining optimal gain structure through the rest of the system. The risk with that, as Chuck notes, is that you could have other devices clip before the amplifier, but that is simply the kind of issue that must be assessed in terms of what provides the best overall result.
A couple of details to note, one of which is also addressed in the referenced paper. When I mention attenuating a device's output that is referencing an attenuator on the device output, not reducing the gain of an internal output device. It is sometimes difficult to establish whether the output level controls on a device are actually controlling an attenuator between the internal electronic circuits and the output connector or if they are varying something in an internal circuit, but this can be an important difference in terms of gain structure. The other point is that this discussion is referencing analog gain structure, gain structure for a digital signal path avoids some of the gain structure issues present in analog systems but can also introduce other issues.
David Ashton
Well-Known Member
"So running amps at full is a very common and acceptable practice ----provided that it is associated with good gain structure and not with compensating for poor gain structure."
In the US it may be common practice to fit pads to adjust the input gain to amps, unfortunately it is not a practice I have ever seen in thousands of installs here in Australia, where it is universal to wind the amps up to "full" and just run the mixer and/or graphics at low level.You are very lucky if that is the situation, I am fighting a losing battle at this end.
In the US it may be common practice to fit pads to adjust the input gain to amps, unfortunately it is not a practice I have ever seen in thousands of installs here in Australia, where it is universal to wind the amps up to "full" and just run the mixer and/or graphics at low level.You are very lucky if that is the situation, I am fighting a losing battle at this end.
I don't think we really disagree at all and there are indeed many systems run with the amps wide open and resulting poor gain structure. All I'm saying is that the problem is not really that the amps are at "full" but rather the resulting gain structure. An amp being run wide open is not inherently a problem and if approached correctly can be advantageous in some applications, however you are correct that it is probably much more often not approached correctly.
The reason I made such a point of this is because I spend a lot of time on projects dispelling misunderstandings and misconceptions that my clients have. I see things like this and immediately envision setting up a system with proper gain structure with the amps wide open, maybe attenuating the signal at the processor output rather than the amp input, and then having someone assume that I don't know what I'm doing and 'fixing' it.
Anonymous067
Active Member
Back on subject...I've dealt with this situation in multiple venues. I always find myself doing the same thing...(perhaps someone else already posted this, if so sorry. I skimmed the earlier posts)...start bypassing until the problem goes away. Even if your cables run through a patchbay, you can still unplug the connector that feeds the patchbay and plug a good ole XLR into it. It won't hurt anything.
Try bypassing the whole rack scene and run a cable from the output of the console to the amp. If it's still there, then the hum isn't coming from anything you bypassed, obviously. Pretty standard troubleshooting advice...
Try bypassing the whole rack scene and run a cable from the output of the console to the amp. If it's still there, then the hum isn't coming from anything you bypassed, obviously. Pretty standard troubleshooting advice...
You could still have a ground loop created between the devices on a circuit at FOH and devices on a circuit at the amp rack. What often makes more sense than bypassing is breaking the signal paths. Disconnect the output of the console, if the noise goes away then it is probably before that or in the run between the console and the next device. If it is still there then disconnect the other end of that cable, if the noise goes away then it is related to that cable run, if not then it is further downstream in the signal path.
In this particular case, the swapping of cables at the amps with the result that the problems follows the one speaker and speaker cable seems to indicate that the problem is related to that cable or speaker.
Anonymous067
Active Member
That's what I meant. ;]
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