microphone Input Impedance Settings

[anti--hero]

Hi KVR,

I have a "M-Audio Tampa" pre-amp with a variable input-impedance selector.

the choices are - 2400/1200/600/300 ohms.

The microphone i have is a " studio projects TB1 Tube Mic "
on its spec sheet -

Sensitivity: 27mV/Pa (-32dB re 0dB=1V/Pa)
Output impedance: 250 Ohm.
Load impedance: >1000 Ohm.
Maximum SPL:128 dB SPL(@1000Hz,<1% THD).

What would be the best/correct setting on the input selector ? The mics inpedance is 1000ohm, so logic tells me thats what i should select. Is the 1000ohm the minimum or maximum tht should be selected ?. if its the minimum , would there be any benefits using 2400 setting ? I have always just left on 600 in the past ! yikes !

can anyone pls give me in laymans terms wht all this inpedance is about ?

lastly - would the mic be able to handle close miking a 20wattish tube amp (old piece of shite traynor YGM-3)? its spl is rated as Maximum SPL:128 dB SPL(@1000Hz,<1% THD)

Many thanks in advance and have a great weekend !!

Gef.

[BertKoor]

I'd set the impedance to 1200 or 2400: those are over 1000 (1200 is very common) and ">" means "greater than" in mathematics.

You could set it to 300 and test weather it sounds different, 'cause that's what it should be about. Normally no harm is done with impedance mismatches except compromised quality.

Some articles on the subject: http://sound.westhost.com/impedanc.htm#zmatch
http://en.wikipedia.org/wiki/Impedance_matching

A THD of 1% is rather a lot, although you may not notice it because the amp already delivers a distorted sound. But 128dB indicates it can handle loud sources without getting damaged (some can't take it and WILL be damaged) Often SPL's of amps/speakers are measured at one meter, but 100dB at one meter = 103dB at 50cm, 106dB at 25cm, 109db at 12cm, 112dB at 6cm, 115dB at 3cm. And it's plausible that an amp goes well over 110dB at one meter, so well over 125dB at 3cm (close miked position)

This type of mic (condenser) isn't the best one to mic guitar amps from up close. Most people use dynamic mics (Shure SM57 is rather cheap and popular for this task) because they can handle the extreme volume much better. Your amp may be small, but it can be very loud when miked up close.

[anti--hero]

Thanks bertkoor , perfectly explained.

I'll give the other settings a whirl now that i know its pretty harmless.

the first mic i owned was the SM57, then some bastard stole it and never replaced it. They are proberly pretty cheap now and should buy one back just to be safe with the spl thing with the tube mic.

Thanks again