Interesting gain story...

I was just doing FOH at the X-games in LA for a band and from my understanding, these shows were the first ever to use the new Yamaha PM5. I got the band up and saved the mix. When the band hit after 2 other bands, the gain structure was WHACKED...and it was my recalled mix. My question is this... if you use a y-cable to split a mic signal (which is what the band uses) to 2 mic amps, will one affect the other?

Thanks-

With modern mic pre's it shouldn't make enough difference to make the gain structure whacked. Still, you're not really meant to y-split, but everybody does it...

Bjorn

Yes absolutely it will. The source impedance of the two mics will shunt each other.

No modern gear trickery will have any effect on this, it's like trying to repeal a law of physics! :wink: a 600Ω source paralleled with a 150Ω source will suffer something in the order of 15dB gain-drain, unless it was loaded by too low an impedance to begin with.

'Y' cables are the Devil's handiwork. Summing outputs causes much grief. I -*-never-*- do it.

Keith

I think he means a Y the other way - into two channels of a board, or into two boards.

Everyone does it but a mic splitter transformer is a better way of doing it. A good mic amp should pose a constant impedance regardless of setting, so it shouldn't - it was added between soundcheck and performance. Another possibility is a phase invert in one of the Y cables, or an incorrectly wired one. Or maybe if the monitor system is pretty hot (and if that changed between soundcheck and show) that could cause issues - and make it look like the FOH is whacked when it was the monitor system that caused the grief.

I like to use the inserts to Y a mic to two channels, if they're on the same board.

In the case of this event, I split 3 ways...kick drum to a y-cable to a bedwringer mixer and a splitter to the Midas 3k and the house. I'll bet the split line to the Midas was thru. Not a transformer in sight. I didn't have a monitor guy to fight for channels, so I know that the gain was changed...Isolation transformers would would be bulletproof, no?

Isolation transformers would would be bulletproof, no?

Click to expand...

Isolation transformers wouldn't help you here. If your problem was earth loops and hum, then yes, but as described here, no. Impedence effects on transformers get reflected input to output, and output to input.

A decent mic amp should have a reasonably consistent input impedance as the gain is changed. If it hasn't, then changing the gain on one desk could affect the level received by the other desks.

However, if you are saying that a third desk was introduced somewhere between your sound check, and the performance - that would certainly affect the level received by the existing desks. Eack new desk loads down the mic signal. It may not be a lot (depends upon the desk), but it likely to be noticeable. A mic is (say) a 200 ohm source, and you are adding three (say) 5k loads to it - these loads appear in parallel (a classic potential divider) - the more desks you add, the more it loads the mic. If any particular load (desk) is less than the others, it will have more effect (so the desk is a variable), if the mic is more than 200R, it will have more effect (so the mic will be a variable).

It might also change the sound of the mic (recall the current fashion for putting variable impedance impedence on up-market mic pres, for the 'subtle' effect it has on the sound?).

Q. Were all these desks supplying phantom? (Ok, don't answer that!)

Alan

[quote author="dale116dot7"]I think he means a Y the other way - into two channels of a board, or into two boards.[/quote]
...of course he did... Now that I re-read it properly.. -my mistake!

-Having said that, I have seen a number of people Y-split mics into a single input and mic congas etc. this way... they should be shot! :twisted:

Okay, now that I understand correctly, all that's been said makes perfect sense. you are now free to move about the forum! :wink:

Keith

"Q. Were all these desks supplying phantom?"

I wasn't the system tech, so I'm not sure...

So what is the answer? How do I mult a microphone to multiple sources without them interacting?

A good way of doing this is using an active splitter (eg the BSS MSR 604; used a lot for live performances).

This machine splits every input into four outputs: one is called main, another mon and the last two are named feed A and feed B.


The FOH desk is connected to the main out, the mon desk to the mon out. The main and mon out are not transformer separated from the input but are buffered using an opamp.

The two feeds A and B are transformer separated and lifted and can be used for multitrack recording etc...


Since this splitter is active and has opamps inside, the first gain applied to the mic is the gain of the splitter. This means this gain must be set accurately. Phantom power is also supplied from the splitter.

So actually the splitter gains the mic signal up to line level and the FOH and MON desk input gains are set to handle line level signals (PAD in, low gain).


The setup described above is not handy on live festivals because for every change in the input patch the gain of the corresponding input needs to be optimised on the splitter, and not on the FOH and MON desk.


I work for a PA company for years and our FOH and MON desks are just run in parallel, but one of the two is lifted to avoid ground loops.


greetz,


Rogy