Originally Posted By: rharv
Originally Posted By: Mike Halloran
Quote:
As for microphones...eh, they all record...you can work out the foibles of any mic to your advantage


Yes. Even the SM57/58 — both sound less than ideal into a modern solid state interface due to mismatched high impedance (these were designed to see the 600Ω Bell Telephone spec found in nearly everything in the '50s–'60s). A little tweak to a barrel connector or a mic cable fixes that and turns these into the excellent mics that they were designed to be. I recommend reading this article on the problem, then the sidebar on what you can do inexpensively to fix it.

Recording Mag article on SM57


Wouldn't something with switchable impedance like a VTB1 preamp do the same thing?
Seems to here.
I don't trust my soldering .. <grin>


Perhaps and perhaps not. Here is why

Certain things need to be known. What is the impedance of the input device?

If you want to calculate a load resistor that’s tailored to your preamp/board’s actual resistance, it’s easy to do. The formula is:

1/Rg = 1/Zd – 1/Za

where Rg is the resistor to be used in the Gizmo, Zd is the desired total load impedance, and Za is the actual load impedance of the input.

A Mackie board with XDR Pro preamps has an input impedance of 1300 ohms; plugging the numbers into the equation, we get:

1/Rg = 1/500 – 1/1300 = 0.002 – 0.000769 = 0.00123

Pushing the “1/x” button on the calculator, the answer is:

Rg = 812.5 ohms

The nearest value in the 1% tolerance series of resistors is 806 ohms, so that would be the one to use.

I did not download a schematic for the VTB1 but Sweetwates shows specifications as Switchable Impedance between 50/200 Ohm which would not work for a 600-ohm mic.

I will try to find a variable impedance device or schematic.

A little electrical stuff. A microphone produces an alternating current (AC). So...things get a little more complex as opposed to direct current (DC) like coming out of your flashlight. Impedance is not really easy to understand.

Take a simple circuit, battery connected to a resistor.


The battery pushes current through the resistor, which "resists" this. Thus, the higher the resistance offered, the lower the current through the circuit.

But there are also AC circuits that involve capacitors (C) and inductors (L).


Each of these also contributes to the overall "resistance" offered by the circuit, except this combined "resistance" is called impedance for the simple reason that it involves not just the resistors, but also the capacitors and inductors in the circuit. It "impedes" the flow of current through the circuit. The term reactance is used to refer to the non-resistive component of the capacitors and inductors.

Yes, as I said it is not exactly simple.

If I knew the impedance of your input device and knew the impedance of your mic I could add a resistor into the circuit in five minutes.

It is also possible to change the impedance of you input device but that can cause other issues.

Billy

EDIT: I found this video pretty well explains this subject in an easy-to-understand way. It also shows how to build a variable impedance device for an SM57. I will go build one and check it out.


Last edited by Planobilly; 02/17/22 03:55 PM.
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