[BC] Mic impedance question...

[Glen Kippel]

On Mon, Feb 6, 2012 at 11:58 PM, Milton Holladay <miltron at att.net> wrote:
>Nahhh.....
>Generally, the higher the input Z, the better, but usage has been all
>over the board.
>E.g., early preamps used a step up xfmr into the grid of the tube (6J7,
>5879, 6267, etc.), usually with a resistor in series as a spurious
>oscillation suppressor, and only the interelectrode capacitance of the
>tube was any sort of termination.
>Yet, there were passive mic mixers using 150 / 250 Ohm ladder or T
>attenuators and a combining network into a single preamp.
>There are excellent input xfmrs by Jensen and others for use with 5534
>or 5532 chips as well as published circuits for use with these same
>chips with no xfmr  (at a cost of more noise ).
>I can't imagine what you have in mind to use a pot for, as you need
>higher Z, not lower, IMHO.
>Just connect it and see how it sounds; if not good, get a good preamp..
----------------------
 
I don't intend to hook my mic up to a 6J7, nor can I afford to buy a bunch of preamps to see which one I like best.  As an example of what I am talking aboout, the Neumann TLM-102 shows an output impedance of 50 ohms but specifies a load of 1000 ohms; the Joemeek JM47a specifies >2000 ohms.  Not 600.  The idea of a dual 1-K pot was to have each side in series so as to increase the load impedance.  Thus, with maximum resistance, there would be a 2.6-K load.  I suppose a couple of fixed 220-ohm resistors would hit the 1-K mark close enough. The pot would enable me to dial in the precise load.   
 
BTW, after looking at specs for a couple of dozen mics, I narrowed my list down to 2 -- the E/V RE-320 and the Oktava MK-319.  The `320 doesn't have a HPF filter and I thought that might be useful, as my RTA shows some LF stuff going on when the A/C is running.  I found a barely used Oktava on Ebay with a shockmount for a little more than half of the street price of a `320 (with no shockmount) and that arrived yesterday.  I have some experience with the Oktava and know that it works well in a difficult acoustic environment, while the AKG C-4500B-BC (for example) does not.  I had one microphone manufacturer ridicule me for asking what the f/b ratio of their mic is, and wouldn't provide the data.  I don't need that kind of attitude.  So I didn't buy his mic, though it otherwise had good specs, came with a shockmount and looked nice.
 
Yes, I would like to have a nice mic processor, mainly for the expander to suck out any residual background noise and room reflections, but don't have the money for that.