Mic pre front end attenuator and filter design?

[JohnRoberts]

While there is no standard, the common practice it to terminate a microphone with a "bridging" load, which means approximately 10x the source impedance, for good voltage transfer. Since microphones vary in source impedance over the general range of 150-200 ohms the mic preamp termination maps out to 1.5-2k. It can't be both so landing somewhere in that range is IMO adequate.

Likewise the preamp will be designed expecting that 150-200 ohm range source impedance and will be dialed in accordingly for good noise and/or frequency response.

For a little trivia they used to use 50 ohm nominal microphones back on movie sets to mitigate against high frequency losses due to the capacitance of thousands of feet long cable runs. These are obsoleted today by wireless microphones.

JR

 

[JohnRoberts]

[quote author="guavatone"]

BTW, why doesn't Batman or any superheroes ever carry a gun?[/quote]

Because it is fiction?
===============
Why did Superman smile while the bad guys bullets bounced off his chest, but always duck when the bad guy ran out of ammo and threw the gun at him?

JR

 

[Ilya]

I'm working on racking Spectra Sonics 101 cards (with input and output txs) and I have exactly the same problem.
After some googling and searching this forum I decided that an U-Pad before the input tx is the way to go if you don't want to overload your tx. But it's a nightmare to build anything that has more than 5-6 steps. First, you need a 3-deck switch (4-deck if you need an O-pad) which are not cheap as you might know. Second, it's better to gradually increase resistors value and put them in series instead so they sum up to the total resistance you need. This is to avoid weird level drops when you change switch setting (assuming you have a shorting switch, you'll have 2 resistors switched in momentarily).
So... In the end I decided that something like 20 or 30 dB pad at the input and (probably) some level control on the output is the way to go.
One question that bugs me is this though. If I change the input primary impedance and the pad is engaged the trafo will still see 200R (assuming that it's 1500/200 pad). From the other side, I don't need to change pad output impedance because the output impedance of the mic didn't change. Are these assumptions right?
Well, it's a mess to explain it properly. I hope all this made sense.

 

[guavatone]

Ilya, If one needs an O-Pad it is cheaper to add a single terminating resistor to a single pole switch for the higher attenuation values. That's my plan for the tube pre variable pads I plan to make.

-Are you planing on having an i/p transformer with a ratio switch? f so there is math involved to figure what will appear on the other side.

If you know the turns ratio is say 1:10
and the primary is terminated at 200

then the secondary would see
(10(squared)) * 200
=100*200=20,000

You find Turns ratio by taking the square root of Z1/Z2

Well, it's a mess to explain it properly. I hope all this made sense.

-that's the thing about trying to understand something, it's hard to ask a question sometimes without thinking you are sounding, um, off or somthing. But you raise good and valid points in your question.

With impedance a receiving side wants to see a lower Z than itself so the 200 Ohm should keep it in the clear range of that depending on your circuit. (maybe by an order of 10:1)

-I am trying to learn too so if I am wrong someone correct me please