Mic Switcher w/48V

Hi fellas,

I was thinking about building a mic switcher with 48V for measurement purposes. I'm running Smaart and want to have the ability to have more mics in different places in the venue.
Here is the general idea:
- 8 Inputs
- 48V to all 8 (individual switch ???)

I'm not sure wheather I should go with a intricate relay circuit or simple rotary switches.

Here is a poorly done diagram.. (sorry, Paint is not a very complex softwater)




Please tell me if there is something terribly wrong with this.

thanks
Gil

Well, first of all, you have all the mics connected together in parallel--which not only defeats the purpose of the switch, but guarantees a low, noisy output from the mics. Use individual P48 feed resistors for each mic.

To avoid or at least minimize thumps when switching, I recommend capacitor-coupling from the mics to the switch with tie-down resistors on the "cold" side of the caps.

Makes total sense..
I never claimed to be smart...

I'll work on it some more and post again.

Hey Dave. Could you help out understanding what you wrote about the cap-coupling and tie down resistor? I'm having a hard time visualizing this.
This may be a little more complicated than I thought.
Sorry for the stupid questions.
peace
Gil

[quote author="NewYorkDave"]guarantees a low, noisy output from the mics. Use individual P48 feed resistors for each mic.[/quote]
..also each mic will pull down the voltage until there's insufficient to do anything with... Yes, the first thing that I noticed was the switch is completely bypassed as drawn...

[quote author="NewYorkDave"]capacitor-coupling from the mics to the switch with tie-down resistors on the "cold" side of the caps.[/quote]
I may not be picturing what you're describing correctly, but the cap/tie-down R arrangement may still pop like a mad thing when the switch is made...

Patchbay/unplugging is easiest. There's no thoroughly good/elegant/reliable way to swap before a pre, in my opinion...

Keith

Would relays work "better"?

Also, I'm not too concerned about thumping the system, since this signal will not be going thru a speaker system. Just into Smaart for measurements.

I'm just trying to do something so I dont' have to buy/build 8 pres.

thanks for all the input so far.

Just redrew the diagram a little..

and one more question:

how horrible would it be to have 48V on the other end of the switch??

Yes, individual 6K8 PP resistors for each mic input. If I were you, I wouldn't capacitor couple - you're better off running the transformer primary at the 48V level just as in your second diagram. If your primary winding has a centre-tap, take that via a large-ish resistor (47K) to 48V to keep the winding up there while switching between sources. This will minimise transients into the preamp when you switch.

why not just use the primary centertap for 48v supply?

[quote author="Svart"]why not just use the primary centertap for 48v supply?[/quote]
There isn't one shown... perhaps there isn't one?

keith

Why is there a transformer there in the first place? Is this going to connect to a DIY preamp or a commercial one? In either case, wouldn't it be easier to use the phantom supplied by the preamp and switch between the mics with relays?

/Anders

There isn't one shown... perhaps there isn't one?

Click to expand...

spec the transformer with one! :green:

aha!

http://www.jlmaudio.com/JLM1272simplemod.pdf

http://www.jlmaudio.com/JLM1272hotrodmod.pdf

48v to the centertap.

Good Luck!

So this is basically what you are talking about:




Hi Anders,
I'm trying not to rely on 48V from some of these firewire interfaces. I really just wanted to all Mic inputs to already have 48V and have my switching not pop as much. As well as have a reliable 48V source.
Basically what you are saying is that I could just use a 2pole rotary in a box and switch away. I just think it can get really messy that way.

Another idea was to have a somewhat simple/clean preamp in the same box and just have an line level signal going to my A/D. I have a bunch of 312 boards and may just try to build one of those with a switcher in the front end. what do you think??

thanks for the inputs so far guys..

Ok. I see. Just be sure to use a really good transformer. You don't want any transformer coloration in your measurements.

/Anders

Would something like this work better??

the DP switches are relays.



now I got the pics..

Looks way more expensive, but better I think..
The traffo after the switch can just be the input traffo of a preamp. I was actually thinking about doing that and have it all in the same box and feed line level signal to my interface.
any thoughts?

thanks
Gil

bump.. picture is working now.