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#1
Posted to rec.audio.pro
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Mic Splitter Curiousity
I'm rebuilding my splitter (new connectors this time; it's 40 years
old), and noted 150 ohm resistors in series with the direct outs, both legs. There are also 150 ohm resistors in series on each leg of each iso out (two iso outs per channel). I'm wondering what these series resistors are for -- only thing I can think of is that any output -- direct or iso -- can go into a dead short and all the other outputs will continue to work (though it seems a little odd that the isos have these). I've looked online and see many different variants in the design of transformer iso'd splitters. Some have these series resistors and some don't; only have them on the direct out; use values other than 150 ohms, etc. I'd perhaps like to remove those resistors from the direct outs, but don't want to mess up anything. Comments as to the purpose of these resistors? Thanks in advance, Frank Mobile Audio -- |
#2
Posted to rec.audio.pro
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Mic Splitter Curiousity
Frank Stearns wrote:
I'm rebuilding my splitter (new connectors this time; it's 40 years old), and noted 150 ohm resistors in series with the direct outs, both legs. There are also 150 ohm resistors in series on each leg of each iso out (two iso outs per channel). I'm wondering what these series resistors are for -- only thing I can think of is that any output -- direct or iso -- can go into a dead short and all the other outputs will continue to work (though it seems a little odd that the isos have these). ** I think you are dead right. The isolated windings need them because a short across one would be reflected to the other windings on the same tranny and drop the level 26dB or more. The "isolation" is really only at DC voltages, for phantom power and not the actual signal. Achieving both requires an active splitter and multiple transformers. ..... Phil |
#3
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Mic Splitter Curiousity
On Thursday, December 3, 2015 at 2:14:36 AM UTC-5, Phil Allison wrote:
Frank Stearns wrote: I'm rebuilding my splitter (new connectors this time; it's 40 years old), and noted 150 ohm resistors in series with the direct outs, both legs. There are also 150 ohm resistors in series on each leg of each iso out (two iso outs per channel). I'm wondering what these series resistors are for -- only thing I can think of is that any output -- direct or iso -- can go into a dead short and all the other outputs will continue to work (though it seems a little odd that the isos have these). ** I think you are dead right. The isolated windings need them because a short across one would be reflected to the other windings on the same tranny and drop the level 26dB or more. The "isolation" is really only at DC voltages, for phantom power and not the actual signal. Achieving both requires an active splitter and multiple transformers. .... Phil doesn't the multiple winding audio transformer isolation also prevent ground loops. If the device was just for isolation of DC phantom power, that could be accomplished simply using DC blocking caps and no need for an expensive transfomer Mark |
#4
Posted to rec.audio.pro
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Mic Splitter Curiousity
Frank Stearns wrote:
I'm rebuilding my splitter (new connectors this time; it's 40 years old), and noted 150 ohm resistors in series with the direct outs, both legs. There are also 150 ohm resistors in series on each leg of each iso out (two iso outs per channel). I'm wondering what these series resistors are for -- only thing I can think of is that any output -- direct or iso -- can go into a dead short and all the other outputs will continue to work (though it seems a little odd that the isos have these). What transformers are in use? These may have been added so that the microphone sees about the same impedance through the splitter as without the splitter, because the transformers are too low ratio to use properly for the application. They might also have been added because the transformers are touchy about loading. If you have high ratio transformers that are comparatively immune to loading, there's no reason to have any of this. Also, as you point out, it means when something goes horribly wrong at the truck and everything is shorted, the FOH guys don't lose their feed. For a general discussion, permit me to recommend Jensen application note #5. Before removing them, apply a 1kc square wave through a 500 ohm resistor to simulate an SM-57, plug preamps into the direct and indirect outputs, and look at the signals on each output with a scope. If you short the resistors does it ring more or less? --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
#5
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Mic Splitter Curiousity
wrote:
The "isolation" is really only at DC voltages, for phantom power and not the actual signal. Achieving both requires an active splitter and multiple transformers. doesn't the multiple winding audio transformer isolation also prevent ground loops. ** Nope. Many splitter boxes have ground lift switches on XLR pin 1s for that. If the device was just for isolation of DC phantom power, that could be accomplished simply using DC blocking caps and no need for an expensive transformer ** Correct, but using DC blocking caps creates possible surge current issues that transformers do not. .... Phil |
#6
Posted to rec.audio.pro
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Mic Splitter Curiousity
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#7
Posted to rec.audio.pro
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Mic Splitter Curiousity
wrote:
The "isolation" is really only at DC voltages, for phantom power and not the actual signal. Achieving both requires an active splitter and multiple transformers. doesn't the multiple winding audio transformer isolation also prevent ground loops. ** Nope. Many splitter boxes have ground lift switches on XLR pin 1s for that. But they wouldn't be able to provide that feature if it were not for the transformer providing isolation for pins 2 and 3 ???? ** The signal wires from a low Z, balanced mic are already isolated from ground - normally coming direct from the voice coil or in some cases via an impedance matching transformer. Pin 1 merely provides a ground for the metalwork. Perfectly possible to connect one mic to several balanced inputs in parallel. Ground lift may be needed to break hum loops between various units. Using using a splitter transformer ensures that phantom supply voltages will not transfer between outputs and input nor any common mode hum voltage. .... Phil |
#8
Posted to rec.audio.pro
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Mic Splitter Curiousity
Many splitter boxes have ground lift switches on XLR pin 1s for that. But they wouldn't be able to provide that feature if it were not for the transformer providing isolation for pins 2 and 3 ???? The "ground lift" switch only isolates pin 1 on a connector from pin 1 on another connector. Since in most devices enclosed in a metal box (including mic splitters), all pins 1 are connected together through the box unless they are "ground lifted." Pin 1 is normally connected to the cable shield. When you "lift" a ground, what you're doing is preventing what's connected to pin 1 on the other end of the cable from being connected to whatever's on the other end of a cable plugged into another connector on the splitter. If the cable goes to a microphone, pin 1 won't go to the brown dirty stuff under the house. But if it goes to a mic preamp, most likely it will end up being connected to earth, or at least, to system ground. That's what makes (or breaks) ground loops. On 12/3/2015 10:40 PM, Phil Allison wrote: ** The signal wires from a low Z, balanced mic are already isolated from ground - normally coming direct from the voice coil or in some cases via an impedance matching transformer. Pin 1 merely provides a ground for the metalwork. In the case of a singled ended balanced output (and some microphones have this configuration), pin 3 (unless you're arguing with Scott Dorsey who believes that Pin 3 is hot) is connected to ground (the same reference point as the output voltage) through a resistor equal to the source impedance feeding Pin 2. So this configuration may not be completely isolated from ground. -- For a good time, visit http://mikeriversaudio.wordpress.com |
#9
Posted to rec.audio.pro
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Mic Splitter Curiousity
On Thursday, December 3, 2015 at 10:40:29 PM UTC-5, Phil Allison wrote:
wrote: The "isolation" is really only at DC voltages, for phantom power and not the actual signal. Achieving both requires an active splitter and multiple transformers. doesn't the multiple winding audio transformer isolation also prevent ground loops. ** Nope. Many splitter boxes have ground lift switches on XLR pin 1s for that. But they wouldn't be able to provide that feature if it were not for the transformer providing isolation for pins 2 and 3 ???? ** The signal wires from a low Z, balanced mic are already isolated from ground - normally coming direct from the voice coil or in some cases via an impedance matching transformer. Pin 1 merely provides a ground for the metalwork. Perfectly possible to connect one mic to several balanced inputs in parallel. Ground lift may be needed to break hum loops between various units. Using using a splitter transformer ensures that phantom supply voltages will not transfer between outputs and input nor any common mode hum voltage. ... Phil I agree with you that the mic source is inherently balanced and isolated. And there is no issue when one mic is connected to one console. But when you connect one mic to two consoles without an isolator, the two grounds of the two __consoles___ can get tied together and form a loop. It can be a big loop if one console is in a remote truck for example. I thought that was the reason for using an isolated mic splitter and why a multi winding transfomer is used to provide galvanic isolation between the two consoles. It also isolates the phantom. Mark |
#11
Posted to rec.audio.pro
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Mic Splitter Curiousity
On Friday, December 4, 2015 at 8:57:18 AM UTC-5, wrote:
On Thursday, December 3, 2015 at 10:40:29 PM UTC-5, Phil Allison wrote: wrote: The "isolation" is really only at DC voltages, for phantom power and not the actual signal. Achieving both requires an active splitter and multiple transformers. doesn't the multiple winding audio transformer isolation also prevent ground loops. ** Nope. Many splitter boxes have ground lift switches on XLR pin 1s for that. But they wouldn't be able to provide that feature if it were not for the transformer providing isolation for pins 2 and 3 ???? ** The signal wires from a low Z, balanced mic are already isolated from ground - normally coming direct from the voice coil or in some cases via an impedance matching transformer. Pin 1 merely provides a ground for the metalwork. Perfectly possible to connect one mic to several balanced inputs in parallel. Ground lift may be needed to break hum loops between various units. Using using a splitter transformer ensures that phantom supply voltages will not transfer between outputs and input nor any common mode hum voltage. ... Phil I agree with you that the mic source is inherently balanced and isolated. And there is no issue when one mic is connected to one console. But when you connect one mic to two consoles without an isolator, the two grounds of the two __consoles___ can get tied together and form a loop. It can be a big loop if one console is in a remote truck for example. I thought that was the reason for using an isolated mic splitter and why a multi winding transfomer is used to provide galvanic isolation between the two consoles. It also isolates the phantom. Mark GALVANIC? Jack |
#12
Posted to rec.audio.pro
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Mic Splitter Curiousity
wrote:
I agree with you that the mic source is inherently balanced and isolated. And there is no issue when one mic is connected to one console. But when you connect one mic to two consoles without an isolator, the two grounds of the two __consoles___ can get tied together and form a loop. Yes, this is bad news. It can be a big loop if one console is in a remote truck for example. I thought that was the reason for using an isolated mic splitter and why a multi winding transfomer is used to provide galvanic isolation between the two consoles. The most common way this is done is with a high-Z to low-Z transformer bridged across the microphone... so the microphone ground is connected to the ground of the direct output... and the isolated output that comes off the transformer has its own ground. The guy with the direct output supplies the phantom. The problem with this is that people fight over who gets the direct and who gets the iso. It is a political issue. One way around that is to have a 1:1:1 transformer with two secondaries so _everybody_ gets an isolated output, but that then means a phantom supply has to be integrated into the splitter. It also isolates the phantom. This is both a good and a bad thing, politically. I have had the PA guys throw a fit at me because I had phantom turned on in the truck for a given mike or didn't have it turned on for a given mike. If I am recording fewer channels than the PA guy has, I may be repatching between sets and so my numbering changes and the channels I need to have phantom on change too. --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
#13
Posted to rec.audio.pro
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Mic Splitter Curiousity
wrote:
doesn't the multiple winding audio transformer isolation also prevent ground loops. ** Nope. Many splitter boxes have ground lift switches on XLR pin 1s for that. But they wouldn't be able to provide that feature if it were not for the transformer providing isolation for pins 2 and 3 ???? ** The signal wires from a low Z, balanced mic are already isolated from ground - normally coming direct from the voice coil or in some cases via an impedance matching transformer. Pin 1 merely provides a ground for the metalwork. Perfectly possible to connect one mic to several balanced inputs in parallel. Ground lift may be needed to break hum loops between various units. Using using a splitter transformer ensures that phantom supply voltages will not transfer between outputs and input nor any common mode hum voltage. I agree with you that the mic source is inherently balanced and isolated. And there is no issue when one mic is connected to one console. But when you connect one mic to two consoles without an isolator, the two grounds of the two __consoles___ can get tied together and form a loop. ** A balanced mic can feed two consoles, be grounded at the first one and link only pins 2 & 3 to the other. Usually works just fine. It can be a big loop if one console is in a remote truck for example. ** In a case like that, the use of splitting transformers is probably essential. The two consoles may be on different AC power systems and operated by separate organisations so the chances of a problem are much higher. .... Phil |
#14
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Mic Splitter Curiousity
Phil Allison wrote:
** In a case like that, the use of splitting transformers is probably essential. The two consoles may be on different AC power systems and operated by separate organisations so the chances of a problem are much higher. 65 VAC measured between the truck ground and the stage ground at the 9:30 club at one gig back in the nineties, using a Simpson 260. Needless to say the building has since been torn down for numerous fire code violations. --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
#15
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Mic Splitter Curiousity
On 12/4/2015 7:36 PM, Phil Allison wrote:
** A balanced mic can feed two consoles, be grounded at the first one and link only pins 2 & 3 to the other. Usually works just fine. That's the classic "ground lift." Of course if neither console had a "Pin 1 Problem" you wouldn't have to lift grounds at all, because there wouldn't be any signal on the ground of the source end, and if there was, it wouldn't go anywhere harmful on the destination end. But nothing's perfeck. -- For a good time, visit http://mikeriversaudio.wordpress.com |
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