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#1
Posted to rec.audio.pro
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Crosstalk in snake cable that include speaker cables and mic cables
I just bought a used 100 foot snake.
I was surprised at the configuration. It has 2 pair of heavy wires designated as speaker 1 and 2. I had always though that running amps of current through a cable parallel to mic cables would magnetically induce unacceptable amounts of crosstalk into the mic cables. So I ran a test. There was some measurable crosstalk but is was surprisingly low. Low enough that if the mic and speaker are in the same room, acoustic feedback by far dominates the electrical feedback caused by the crosstalk. So it seems this is a usable configuration. Does anyone have experience with snakes of this configuration? thanks Mark |
#3
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Crosstalk in snake cable that include speaker cables and mic cables
On Tue, 29 May 2018 10:15:13 -0400, Mike Rivers
wrote: On 5/29/2018 9:49 AM, wrote: I just bought a used 100 foot snake. I was surprised at the configuration. It has 2 pair of heavy wires designated as speaker 1 and 2. I had always though that running amps of current through a cable parallel to mic cables would magnetically induce unacceptable amounts of crosstalk into the mic cables. There was some measurable crosstalk but is was surprisingly low. Low enough that if the mic and speaker are in the same room, acoustic feedback by far dominates the electrical feedback caused by the crosstalk. Does anyone have experience with snakes of this configuration? No problems, as long as it's built correctly. It works because you have balanced connections. Your mic inputs are differential, so that any induced current in the mic pair will be cancelled out at the preamp input. And, to boot, the speaker leads are, in essence, balanced, since one goes positive and the other goes negative at the same time, so the electromagnetic field around the speaker pairs is mostly cancelled, too. It's not perfect, but as you determined, it's good enough for the application for which you intend to use it. Sometimes technology works in your favor. Balanced connections work well, but not when they are very close to a source of interference. They rely for their effect on exactly the same field impinging on both phases. This doesn't apply when the source of interference is close by - whichever wire is locally closer will receive more interference. That's why cancellation is not perfect. I would not run cables this way. d --- This email has been checked for viruses by Avast antivirus software. https://www.avast.com/antivirus |
#4
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Crosstalk in snake cable that include speaker cables and miccables
On 5/29/2018 12:35 PM, Don Pearce wrote:
Balanced connections work well, but not when they are very close to a source of interference. I test common mode rejection by holding The Wall Wart From Hell (I believe Paul Stamler owns a copyright on that name) right next to a cable going to whatever I'm testing. The test fixture that THAT uses to demonstrate the CMRR of their inGenius chip consists of two pieces of cable as close together as they can get, one with the interference, and that's a good enough demo for them. You do need to use good twisted pair for the gozinta in order to get the best results. I would not run cables this way. Well, don't, if you don't have a need to. But it's a convenience for certain setups, for example, when using a powered mixer. And in general, people who use powered mixers don't worry about a barely measurable amount of crosstalk. My preference is to put the power amplifiers near the speakers and run line level from the mixer to the amplifiers. But there's less need to do that in smaller installations now that powered speakers are becoming the norm for a "no engineer" system. -- For a good time, call http://mikeriversaudio.wordpress.com |
#5
Posted to rec.audio.pro
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Crosstalk in snake cable that include speaker cables and mic cables
On Tuesday, May 29, 2018 at 8:49:40 AM UTC-5, wrote:
I just bought a used 100 foot snake. I was surprised at the configuration. It has 2 pair of heavy wires designated as speaker 1 and 2. I had always though that running amps of current through a cable parallel to mic cables would magnetically induce unacceptable amounts of crosstalk into the mic cables. So I ran a test. There was some measurable crosstalk but is was surprisingly low. Low enough that if the mic and speaker are in the same room, acoustic feedback by far dominates the electrical feedback caused by the crosstalk. So it seems this is a usable configuration. Does anyone have experience with snakes of this configuration? thanks Mark I've used snakes like this, admittedly in relatively low-volume applications (folk clubs). Never had any problems. Peace, Paul Stamler |
#6
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Crosstalk in snake cable that include speaker cables and mic cables
wrote:
I just bought a used 100 foot snake. I was surprised at the configuration. It has 2 pair of heavy wires designated as speaker 1 and 2. This is called a "powered snake" which seems like a bad description to me, but that's what it is. I had always though that running amps of current through a cable parallel to mic cables would magnetically induce unacceptable amounts of crosstalk into the mic cables. It can. You don't have capacitive coupling so much to worry about because the shielding is pretty effective against that, but you do have inductive coupling especially at very low frequencies. One of the tricks to dealing with this is cable twist, and having different twist lengths on different cables (which cat5 cable does). If everything is done just right, the inductive coupling on one twist is opposite polarity from the coupling on the next twist and they all cancel one another out. So I ran a test. There was some measurable crosstalk but is was surprisingly low. Low enough that if the mic and speaker are in the same room, acoustic feedback by far dominates the electrical feedback caused by the crosstalk. So it seems this is a usable configuration. I have seen some very crappy (and possibly damaged) powered snakes where there was enough coupling to cause very low frequency motorboating on the amp. Mike will know what I mean when I mention the old Adventure Theatre. But I have also seen a lot more where everything worked just fine. Does anyone have experience with snakes of this configuration? Yes, they mostly work just fine. But, they are much less popular in the modern age of powered speakers. Also... don't use the speaker returns to run AC power to the board. I saw someone do that once too. Interference resulted, and also the cable is not rated for that. --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
#7
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Crosstalk in snake cable that include speaker cables and miccables
On 30/05/2018 9:48 AM, Scott Dorsey wrote:
Yes, they mostly work just fine. But, they are much less popular in the modern age of powered speakers. Also... don't use the speaker returns to run AC power to the board. I saw someone do that once too. Interference resulted, and also the cable is not rated for that. --scott However in this day of powered speakers we now have combination speaker and mains cable. Official, compliant ones. Just one balanced signal pair with mains included. The ones that I tried (10m, ~33ft) had no obtrusive induced hum. However the signal wires would fracture (2 sets = same result) presumably due to them being stressed when curved along beside the heavier, thicker mains wires/lead. So I gave up. However an acquaintance who uses similar cables of another brand has not had this problem. geoff |
#8
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Crosstalk in snake cable that include speaker cables and mic cables
It can. You don't have capacitive coupling so much to worry about because the shielding is pretty effective yes it appears that way, no crosstalk with the mic unplugged. but you do have inductive coupling especially at very low frequencies. my observation is the opposite, the inductive coupling is greater at the high end. m |
#9
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Crosstalk in snake cable that include speaker cables and mic cables
wrote:
but you do have inductive coupling especially at very low frequencies. my observation is the opposite, the inductive coupling is greater at the high end. ** Do this simple experiment. With a mic plugged in, wrap one turn of balanced line around a transformer wall wart. For a comparison, place the mic head next to the same wall wart. ..... Phil |
#10
Posted to rec.audio.pro
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Crosstalk in snake cable that include speaker cables and mic cables
I can confirm that this type snake CAN create an ultrasonic oscillation that can quickly damage a power amp.
Fortunately this happened during my testing and not during an event and it was a small amp with easy to replace 2N3055's. I WOULD NOT use this configuration with speaker level signals in the snake. If you MUST use it, keep the High end rolled off on the amp. Also I found that the wires intended to be used for the speakers are shielded well enough by all the other cables that the speaker lines can be used to carry line level audio signals. There was some small cross talk between the 2 speaker cables when used as line level but this is of no issue if they are used for 2 monitor mixes for example or for left and right. Bottom line, try very hard to avoid speaker lines and mic lines in the same snake. It definitely can easily damage the amplifier. Mark |
#11
Posted to rec.audio.pro
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Crosstalk in snake cable that include speaker cables and miccables
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#12
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Crosstalk in snake cable that include speaker cables and miccables
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#13
Posted to rec.audio.pro
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Crosstalk in snake cable that include speaker cables and mic cables
wrote:
I can confirm that this type snake CAN create an ultrasonic oscillation that can quickly damage a power amp. Fortunately this happened during my testing and not during an event and it was a small amp with easy to replace 2N3055's. I WOULD NOT use this configuration with speaker level signals in the snake. If you MUST use it, keep the High end rolled off on the amp. ** Not a simple thing to do ... Bottom line, try very hard to avoid speaker lines and mic lines in the same snake. It definitely can easily damage the amplifier. ** Glad to hear you took the advice and tried using the snake in test situation rather than at a gig. I did a bench test with a 1m length of figure 8 speaker cable running alongside a balanced mic line and found that while cross-talk was only minor at audible frequencies it could became severe at 70 kHz or so depending on the mic cable in use. ..... Phil |
#14
Posted to rec.audio.pro
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Crosstalk in snake cable that include speaker cables and mic cables
wrote:
I just bought a used 100 foot snake. I was surprised at the configuration. It has 2 pair of heavy wires designated as speaker 1 and 2. I had always though that running amps of current through a cable parallel to mic cables would magnetically induce unacceptable amounts of crosstalk into the mic cables. So I ran a test. ** Can you detail how you did that? Were mics connected to the balanced lines ? There was some measurable crosstalk but is was surprisingly low. Low enough that if the mic and speaker are in the same room, acoustic feedback by far dominates the electrical feedback caused by the crosstalk.. So it seems this is a usable configuration. Does anyone have experience with snakes of this configuration? ** In a portable PA system, this can be risky when you allow for simple mistakes made during set up. The risk is supersonic oscillation, induced by crosstalk. Running balanced and speaker lines tied together is commonly done with amp racks. It works fine until someone powers the rack up without connections being made at the far end of the cables and advances the amp gain controls. The unterminated balanced line couples capacitively to the hot speaker wire and oscillation is possible. Usually at a frequency like 50 or 100kHz. The risk is higher if you are using a powered desk since so much more gain is available. ..... Phil |
#15
Posted to rec.audio.pro
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Crosstalk in snake cable that include speaker cables and mic cables
Phil Allison wrote:
** In a portable PA system, this can be risky when you allow for simple mistakes made during set up. The risk is supersonic oscillation, induced by crosstalk. Running balanced and speaker lines tied together is commonly done with amp racks. It works fine until someone powers the rack up without connections being made at the far end of the cables and advances the amp gain controls.. The unterminated balanced line couples capacitively to the hot speaker wire and oscillation is possible. Usually at a frequency like 50 or 100kHz. The risk is higher if you are using a powered desk since so much more gain is available. ** I should point out that supersonic oscillations, while inaudible, will destroy high frequency drivers and seriously damage most power amplifiers in a few seconds. ..... Phil |
#16
Posted to rec.audio.pro
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Crosstalk in snake cable that include speaker cables and mic cables
The risk is higher if you are using a powered desk since so much more gain is available. ** I should point out that supersonic oscillations, while inaudible, will destroy high frequency drivers and seriously damage most power amplifiers in a few seconds. .... Phil Yep, thats what I was worried about, the magnetic coupling between the balanced lines has a high pass characteristic. For the tests I ran, yes I had a mic connected and the other end connected to a mixer. The crosstalk was not present with the mic disconnected but was present with the mic connected. That makes sense that it is magnetic coupling that needs a closed circuit. The mic pairs are electrostatic shielded with foil so that should do a good job to drastically reduce capacitive coupling, but no so effective for magnetic coupling. But still the coupling is more effective at the higher frequencies so I was concerned about ultrasonic oscillations. I plan to use these only for the stage monitors and will keep the high end EQd down in the amp. I get all the theory but practice and theory don't always agree, that's why I am asking about real world experiences with these types of snakes. thanks Mark |
#17
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Crosstalk in snake cable that include speaker cables and mic cables
wrote:
** I should point out that supersonic oscillations, while inaudible, will destroy high frequency drivers and seriously damage most power amplifiers in a few seconds. .... Phil Yep, thats what I was worried about, the magnetic coupling between the balanced lines has a high pass characteristic. For the tests I ran, yes I had a mic connected and the other end connected to a mixer. The crosstalk was not present with the mic disconnected but was present with the mic connected. That makes sense that it is magnetic coupling that needs a closed circuit. The mic pairs are electrostatic shielded with foil so that should do a good job to drastically reduce capacitive coupling, but no so effective for magnetic coupling. But still the coupling is more effective at the higher frequencies so I was concerned about ultrasonic oscillations. ** A lot depends on the mic pre-amps, usually they have good common mode rejection of low and mid frequencies diminishing at high and supersonic frequencies. They deal with supply frequency hum and harmonics well - but not so well with supersonics. I get all the theory but practice and theory don't always agree, that's why I am asking about real world experiences with these types of snakes. ** Other folks experience may have no relevance to to yours. You need to be sure that YOUR set up is not susceptible to supersonic oscillation - or somehow, one day it will happen. ..... Phil |
#18
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Crosstalk in snake cable that include speaker cables and mic cables
I recall an event at Peabody Institute in Baltimore. They ran long lines from a concert hall to their recording studio. How long I don't recall, but long. They had a Sony MXP3000 console. The long lines were ringing so much from local EMI that it was blowing out the preamps in the console. They had to modify the console inputs with transformers. That solved the problem.
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