R/E/P > Klaus Heyne's Mic Lab Stickies

Mic/Pre impedance "matching"- What Are The Rules?

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From an inquiry by a poster whose name was redacted:

Originally Posted: Fri, 15 September 2006

Much attention has been given to finding the optimum impedance match between a microphone and preamp, especially with the rise of impedance-selectable pres.

Working with a wide range of both mics and pres, I am wondering how one determines - from a technical perspective - which pre would best suit a specific microphone?

The base research that I have done yields guidelines, such as: "the input impedance of the pre should be 10x the source impedance of the mic..."
Sonic factors notwithstanding, what are the "rules" for impedance matching?

Although I'm not an expert, I will tell you one thing Steve Hogan told me.  Steve redesigned much of the current line of Jensen Transformers and works on all of my gear.  Klaus, if you feel it's necessary, I will see if I can get Steve to post this same information (although certainly in a more accurate and concise form).  Steve said:
Any mic preamp that uses a Jensen [input] Transformer, or a similarly-purposed microphone input transformer, will exhibit high frequency ringing when using any of the modern Neumann [transformerless] mics--the ones that have a 50 Ohm output impedance.  That would include TLM170, TLM193, KM140, KM184, M149, TLM103, TLM193, TLM49, TLM67 and TLM50.  These microphones must be loaded differently to avoid the ringing. 
I have had Steve add a Lo-Z switch to all of my preamps that fit this description (14 channels). 

John Roberts {JR}:
The general rule of thumb for modern interchangeable, general purpose mics and preamps, is for an interface termination, 10x the mic's source impedance to deliver a "bridging" termination (for mostly voltage transfer). Mic source impedance is not a single specified number. I generally use 150-200 Ohms nominal for mic source impedance, and 1.5-2k for preamp input terminations.

Straying from a bridging termination can make differences in level, and perhaps subtle frequency response and/or reactive interactions (with dynamic mics). Perhaps of interest to esoteric preamp makers who want to sound "different". 

Mic designers are (or should be) generally aware of the nominal terminations and design to work properly in that context, or not. Terminating a mic differently than the mic designer's intended nominal is operating off the reservation, and your mileage may vary.


Jim Williams:
Mic's output impedance are all over the map, some use iron, some don't and those tend to be lower like 47 ohms for 414TL's and 22 ohms for Schoeps.

Input impedances are also all over the map, some with 1200 ohm iron, some with 2200 ohm loaded transistors, some at 4 k or above. Gordons are at what, 1 meg ohms? So far no one has complained about those, yet.

To optimize input impedance for a mic you will need to set up some test gear. Square waves are used to see tilts in response and ringing. Then you add RC elements to stop it, usually imperically. Then you will need to add switches for every mic preamp to add/create all the permutations of loading, it better be a rotary with lots of steps. Then create a chart to show ideal loading positions for every mic on every mic preamp.

Or, just load lightly, press record and call it a day...

Actually, after speaking with Steve Hogan, I found out I should have said "high frequency overshoot" instead of "ringing"... but it still equates to 'tizzy', accentuated top end, even if only slightly so.  Re: Gordon pre's, their default input impedance position is 1 MegOhm, but they have a lower setting; Grant Carpenter's position is that the wants the mics to be 'unaware' that they are 'looking at' any load.  But it is also my understanding that, because they are not transformer coupled, the overshoot problem with the Neumann 50 Ohm output impedance mics does not occur.  For the record, I will say that the Gordons are very good.  But I will also say that I currently generally don't use my 6 channels of Gordon's any more; rather I use my [990-ish] highly modified Precision Analog (made of 'unobtanium'), Jensen input transformer-coupled (in Lo-Z mode) with my modified TLM170's and TLM193's and am supremely happy.  FWIW. 


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