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Phantom Power and Y-Splitters
Old 6th March 2014
  #1
Gear Head
 
Rob23xyz's Avatar
 
🎧 10 years
Phantom Power and Y-Splitters

I've always used a Y -Connector instead of a pre-mixer to split the feed from a boom condenser microphone to two input channels on my location recorder so I could record it both normal and cut a bit as a safety track in case an actor unexpectedly shouts a line (and I cannot ride the faders fast enough). So far I've had no problem with doing that on an HD-P2, or later on an R44Pro, but I just started using an HS-P82 and have Phantom power on on channel 1 and off on channel 2 (where I'm patching my Y-Connector into the Tascam). I kept getting level fluctuations on track 1 but not on track 2, and really couldn't figure out what the problem was. The editor said he was able to use my safety track 2 after boosting it a bit, but the primary track 1 was pretty much unusable without a tedious dialog editing effort.

Could the weird level fluctuations on channel 1 but not on the same mic on channel 2 via the Y-Connector be due to running phantom power out through channel 1 but not channel 2? Or is this probably something else? Is it a bad idea to do what I'm doing without using a phantom power blocking mic splitter? Am I risking damaging something? BTW, the HD-P2 and R44Pro both had to have phantom power to both boom input jacks or neither, while the HS-P82,allows me to select for each channel and thus I opted to just send it to channel 1 only.
Old 6th March 2014
  #2
Lives for gear
 
Richard Crowley's Avatar
 
🎧 10 years
Certainly try it with phantom power turned ON for both channels! And with the Y-connector outputs swapped.

Using a Y-cable for two inputs ON THE SAME DEVICE is virtually always "safe" IMHO. HOWEVER is it extremely risky and NOT RECOMMENDED when you connect to mic inputs on DIFFERENT pieces of equipment.

It might be possible that the HS-P82 is using some weird method of switching phantom that could cause those symptoms, but I seriously doubt it. Furthermore, I would expect that you would experience the anomalies on the channel WITHOUT the phantom power enabled.

Now, there are people who will complain about using a simple Y-cable to feed an ordinary mic into TWO "paralleled" input channels. They will complain that the load impedance is dramatically lower, etc. etc. While that is true, I can't recall anyone actually reporting audible trauma to the signal as a result of this practice.
Old 6th March 2014 | Show parent
  #3
Gear Head
 
Rob23xyz's Avatar
 
🎧 10 years
Quote:
Originally Posted by Richard Crowley ➑️

Now, there are people who will complain about using a simple Y-cable to feed an ordinary mic into TWO "paralleled" input channels. They will complain that the load impedance is dramatically lower, etc. etc. While that is true, I can't recall anyone actually reporting audible trauma to the signal as a result of this practice.
Thanks for the fast reply Richard. I will do a test with limiter only on channel 2, as well as swapping the Y ends (I have a spare Y cable and will try that one as well).

What do you mean by "paralleled" input channels? I presume you mean linked? I wasn't explicitly linking the two channels because I wanted to be able to ride the gain separately and only send the primary boom to the mix. But the HS-P82 channel linked functions are:


2MIX volume

SOLO

LOW CUT FILTER

LIMITER

REC FUNCTION

INPUT SOURCE (analog/digital selection)

Phantom power ON/OFF


Now I did try to rule out a flaky limiter action and tried it both with the limiter on and off for both channels, with no fix to the level fluctuation. I only used the limiter because I knew there might be shouting at this press conference.

I wondered about possible dirty A/C power, except that the shoot took place in a real press conference room for a pro sports team, and I would find it extremely unbelievable that the power in the room's outlets could be that dirty.

My takeaway (aside from trying to isolate this problem and fix it) is that I am very glad I had enough tracks to record safety tracks for the key speakers, with two different mics on the podium: each going to an ISO track and one going to the right mix track, and the boom being split into normal and attenuated: each recording to an ISO track, with normal being sent to the left mix track. Aside from the editor having to pull the levels up in post on the safety track, and dealing with a little higher relative noise floor (he'll fix that with room tone), it wasn't a total (read: we'll never hire you again) fiasco.
Old 9th March 2014 | Show parent
  #4
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Richard Crowley's Avatar
 
🎧 10 years
Quote:
Originally Posted by Rob23xyz ➑️
What do you mean by "paralleled" input channels? I presume you mean linked?
No. I meant "parallel" in the electrical sense. All mic inputs have some sort of "load impedance" or "input impedance". And all microphones have some "source impedance". Typically we design mic inputs with input impedance of 10x the microphone's source impedance in order to effectively eliminate any effect of signal loading.

Using a Y-cable cuts the load/input impedance in half. If the input impedance is 2.5K, then using a Y-cable decreases it to 1.25K ohms. Now, for some very low impedance microphones (typically condenser mics with built-in buffer amplifiers) this makes no practical difference.

But some dynamic microphones may have a much higher output impedance, and that decreased load impedance may affect the signal output or even the performance/tone of the microphone.

But, of course, you are always monitoring the audio on good headphones, so if there were an issue with this, it would be audible during pre-production testing. Right?
Old 9th March 2014
  #5
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huub's Avatar
 
🎧 15 years
In my experience:

When recording a live show, very often the signals, including phantom powered mics, are split to at least two sources
Very rarely the age-old technique of first patching into an active splitter and then using the splitters outputs to feed FOH, monitors and broadcast is used anymore.

This works totally fine. I'm sure there's potential issues with impedance and what not but I have not heard a touring act complain that on recording night sources sound different to other shows.

EDIT: I have no experience splitting sources using battery powered devices.
Old 10th March 2014
  #6
Gear Head
 
Rob23xyz's Avatar
 
🎧 10 years
Having used a simple Y cable so many times in the past on phantom-powered condenser mics on several different recorders, I believed this should not cause an issue. I wondered though if this could could potentially cause damage to the mic preamp that might manifest as fluctuating levels.

I had used the HS-P82 four times without incident prior to this news conference, so I hadn't suspected a problem. I was on A/C power for this press conference. Thinking back, I remember the lightly padded bag in which the RC-F82 remote fader controller was in was dropped to concrete from about a foot off the ground. So I suspected a possible damage to the controller. I used the recorder and mixing controller this weekend for a short narrative film, and only used channels/tracks 2-8 just to avoid any nasty surprises, and all went well. I again used the Y connector to create a safety track for the boom mic. I will do some more intensive troubleshooting this week.

So I was pretty much asking here if anyone had experienced similar problems or having more solid knowledge of electronics than I, could tell me if I was being foolish to use a Y cable to split a mic signal in which phantom power was being supplied out. If I had to take an uneducated case though, I would logically have thought if anything, the phantom power going out on the channel input 1 would have come back and damaged channel 2 rather than the same channel (1) it was going out. But again, I reiterate that I am not an electronics expert.

I had hoped tech support at Tascam could answer this, but so far they haven't responded to my question on their support website. And BTW, on several occasions, I have seen some flaky things (like no sound on the headphones though the meters were dancing) and a simple reboot (off/on) cleared the problem. I guess this is one reason Hollywood production mixers with unconstrained budgets use Sound Devices and Zaxcoms for higher reliability.
Old 21st August 2014
  #7
Gear Head
 
Rob23xyz's Avatar
 
🎧 10 years
So now I've experienced the gain drop off on channel two. I have no choice but to ship the unit to Tascam for repair as it is not reliable anymore. No Y connector this time.
Old 21st August 2014
  #8
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Richard Crowley's Avatar
 
🎧 10 years
Without knowing the exact circuit configuration for the mic preamps in your gear, it is impossible to offer an authoritative response.
However, if I had to guess, it would be safer to turn ON the phantom power on BOTH of the inputs that you are connecting together with the Y-adapter.
That doesn't provide any additional voltage to the microphone, so there is no danger to the mic.
Since both inputs are in the same gear, they are providing phantom voltage from the same source at the same voltage.
Of course you would never use a simple Y-adapter to do this with with mic inputs on TWO DIFFERENT pieces of gear.
And turning ON the phantom power on the second input would at least engage any DC-blocking circuit and protect the second input.
This assumes that the phantom power DC blocking may be switched in/out. But we don't know that without better info about the circuits.
It doesn't seem likely to me that your symptoms are caused by use of the Y-adapter.
Old 22nd August 2014
  #9
Gear Addict
 
OzGizmo's Avatar
 
🎧 10 years
I have had a Mackie 1604 mixer die when phantom was fed into the inputs from another mixer via a Y cord.

To solve the problem of phantom leakage easily just snip the earth off pin 1 inside one of XLR connectors of Y leg .....(and mark it as 'NO pin 1')

I have done this to feed inputs into a main and standby mixers of an OB truck... Works well.
Old 26th October 2014
  #10
Gear Nut
 
Ukiah Bass's Avatar
 
🎧 5 years
I'm also curious about this topic but am not sure I'm following the logic above - at least in my circumstance.

I want to use three instrument sources for two purposes during a gig performance - record each channel separately, and send the mix to a powered speaker (it's a small restaurant). These are a DI from an acoustic guitar, a large diaphragm mic'd signal of acoustic bass, and a Shure Beta91a boundary mic inside a cajon. The latter two need phantom power. I want to connect their XLR cables to a Mackie 802VLZ4 mixer, into which are plugged three Y connectors. The other ends of the Y cables are to be directly plugged into separate channels in a Zoom H6 recorder. Meanwhile, the Mackie is sending the blended signal out to a powered speaker.

If I use the Mackie to send a phantom signal to the condenser mics, will this damage the preamps on the Mackie or the Zoom? I've had a Zoom die on me before so I'm gun shy about this. I can't imagine turning phantom power on both the Mackie and the Zoom.

Any words of advice? Don't want to do anything stupid here! Thank you.
Old 26th October 2014
  #11
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Richard Crowley's Avatar
 
🎧 10 years
I would NOT recommend hard-wiring "splits" (aka "Y-cables") to two different pieces of gear like that with phantom power present.
My advice would be to use a proper splitter like this: MST-412
4 channels of properly-isolated split for only $115. You can't hardly beat that.

Old 26th October 2014 | Show parent
  #12
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Richard Crowley's Avatar
 
🎧 10 years
Quote:
Originally Posted by OzGizmo ➑️
I have had a Mackie 1604 mixer die when phantom was fed into the inputs from another mixer via a Y cord.

To solve the problem of phantom leakage easily just snip the earth off pin 1 inside one of XLR connectors of Y leg .....(and mark it as 'NO pin 1')

I have done this to feed inputs into a main and standby mixers of an OB truck... Works well.
Since the Mackie mic input circuit is designed to handle phantom power, it is not at all clear how phantom power from an external souuce could have "killed" it??

And, by breaking the ground, you aren't really eliminating the 48V, you are simply adding the ground-loop on to the 48V. I would be very wary of that method. Norhing like a proper magnetically-coupled transformer for isolation between two systems.
Old 27th October 2014
  #13
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Lotus 7's Avatar
There are two reasons why phantom power mic inputs should never be connected directly together with a "Y" cable or adapter. Such adapters or "Y" cables are fine for use with unpowered dynamic mic inputs, but not for phantom powered inputs. That includes inputs on the same device like (2) side by side channels on a mixer.

1. Potential mic damage:

Modern phantom powered microphones use the phantom power to power the impedance converter and the output line driver stages, and to supply the capsule bias (on non-electret mics). By placing two powered inputs in parallel the source impedance of the phantom power source is halved. On a P-48 circuit, instead of have a single 6810 ohm resistor on each input line feeding the mic you now have an effective power source impedence of 3405 ohms on each line. The pp circuit can now supply twice the current. The lowered PP resistance changes the operating voltage at the mic. That can change the operating point of the output stage drivers and the intended impedance converter operating point, and possibly the capsule bias.

Some mics (not the majority), use a voltage regulator (usually a single Zener diode) to keep the mic active-electronics operating at a constant voltage. On those mics, using paralleled powered inputs will greatly increase the power dissipation in the Zener regulator. The Zener current can more than double.

A modern phantom powered mic is designed to work with a known source impedance for the phantom power current. Halving that impedance is risky. Additionally paralling the inputs drops the load impedance the mic "sees" to half of a single input. That's not a mic safety issue like lower the PP current source impedance, but may affect the sound of some mics.

Incidentally, cutting the pin-1 return for one of the inputs may not change the current supplied from the input. there will be a good chance that there will be another ground path like a power line common connection or another audio cable between the two pieces of gear which will still provide a return path for the phantom power current.

2. Input circuit damage:

If phantom powered inputs on different pieces of gear have their inputs paralleled using a simple direct "Y" cable it's possible to damage the mic input stage of one when it's operating power (not phantom Power) is off and the phantom power on the other input is switched on or off. Most modern op amp input can be damaged if either of the differential input terminals is allowed to reach a voltage that exceeds the power supply rail voltage. When one input amp is powered down, it's rails are at or very near "0" voltage. Even the application of a few volts coupled in from an external power source (the PP on the "other" mic input) may produce a input voltage transient that exceeds the (now "0" ) power rail voltage. Some input's have voltage "clamps" to prevent that but many do not. Remember, the designers did not design the inputs to withstand [externally-applied] phantom power when the device was switched "off".

That is not an issue if you are running (2) mixer inputs in parallel (same power supplies), but can be if you are using separately powered devices.

Bottom line: If you care about protecting your powered mics and your mic pre inputs, don't ever use a directly coupled parallel "Y" cable on a phantom powered input no matter what anyone tells you. An isolated transformer splitter, or capacitively-coupled "splitter" is the only wise solution.
Old 27th October 2014
  #14
Gear Nut
 
Ukiah Bass's Avatar
 
🎧 5 years
Thanks for all the detailed feedback! Very helpful!
Old 28th October 2014
  #15
Gear Nut
 
Ukiah Bass's Avatar
 
🎧 5 years
Just received the MST-412 and it works like a champ! Thanks again for the recommendation.
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