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

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

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Steve Hogan:
Because this link was sent to me by an inquiring client, and it seems that I might be able to shed further light on the impedance issues regarding microphones, I will offer some basics.

First the theory (which varies in the real world, as I will explain later):

Microphones produce relatively low-level signals that must be amplified in order to be brought up to useable levels. The game plan is to increase the level of the desired signal and minimize any electrical noise coming from the mic.  We also want to interface our microphone to the preamplifier in a way that maintains the best fidelity.   

Exceptions to this generalization abound because some recordists intentionally modify the loading to certain microphones in order to change the way they sound.  That's totally OK in the name of artistic license.  The following discussion, however, relates to those who desire to capture the music with the most faithful fidelity possible without intentional coloration.

   First some definitions:

Load impedance is the complex magnitude and phase presented to the microphone by the preamp circuitry. It can be thought of as a complex impedance placed in parallel with a preamp input with theoretically infinitely high input impedance.  Infinitely high impedance when placed across a signal source will not cause any drop in signal level.

This impedance is not just a resistive load, but contains capacitive and inductive reactance which makes the impedance vary with frequency.  In general, for example, a transformer-based preamp input will have an impedance that is quite flat in the midrange but becomes lower at both low and high frequencies due to the transformer inductance at low frequencies and the input capacitance at high frequencies. Usually the midrange impedance is on the order of 1000 to 2000 Ohms. When you switch in the resistive pad to such a preamp, then the microphone will see a much more resistive load because the pad resistors interface between the microphone and the input transformer so the mic sees the resistors in the pad as the load.

Microphone output source impedance is a little bit more difficult concept to initially grasp. One may think of the microphone's output as a Zero Ohm signal generator in series with an impedance. This generator can be thought of as having infinitely low output (source) impedance -- one that can be loaded with 1 milliOhm and still not drop in voltage level. In series with that zero Ohm generator is a complex impedance (having magnitude and phase) which is the output source impedance of the mic.

 In some very low output impedance transformerless microphone designs, this equivalent series impedance magnitude may be as low as 12 Ohms.  Or it may be greater than 600 Ohms in some ribbon mics.  There are a lot of transformerless mics that have output impedances in the 30 to 50 Ohm range, and most microphones that incorporate an output transformer have output source impedances in the 150 to 250 Ohm range. Depending on the microphone, this output source impedance may be mostly resistive, somewhat capacitive and maybe inductive, depending on the microphone's design.

  Since the microphone's output source impedance lies between the theoretically perfect microphone internal signal generator and the load, there will be some loss across the source impedance caused by the input impedance of the preamp. The source impedance of the microphone and the load impedance of the mic preamp form a voltage divider between the theoretically zero Ohm source microphone and the theoretically infinitely high input impedance (Z) mic preamp.

Matched Impedances:
Although the term "impedance matching" is incorrectly and loosely used to describe any adjustment to the impedances of the microphone/preamp interface, the term "matched impedance" has a very specific meaning which applies to all electronics including mic to preamp interfacing.

The source is considered to be "matched" when loaded by an impedance exactly equal to its output source impedance.  It turns out  that when the load Z and the source Z are exactly the same magnitude, the maximum POWER is transferred from the source device to the load.   The voltage level between the zero Ohm generator and the infinitely high impedance load drops exactly 1/2 or -6dB when the source and load are equal.  Consider the case of a CB radio (Oh, am I old) with a 50 Ohm output Z feeding an antenna with a 50 Ohm input Z in order to transfer maximum power from the transmitter to the antenna.

Because we do not need to transfer power to the mic preamp, but instead we want to maximize the signal Voltage getting to the preamp from the microphone, we generally use the following method to interface mic to preamp:

Bridging Load:
 As a rule of thumb, an output source is considered to have a "bridging load" or to be "bridged" when the load impedance is greater than or equal to ten times the source impedance.  Thus a mic preamp with a 1500 Ohm input impedance will present a bridging load to a microphone with 150 Ohm output source Z.  Such a load will cause the signal voltage level to drop -0.83dB instead of the -6dB that occurs with a matching load.  A bridging load improves the signal level 5.17 dB over the matching configuration. A 5 dB improvement in signal-to-noise ratio is definitely worthwhile.

   If all this seems hopelessly complicated, be of good cheer! One does not have to understand all the engineering details to understand how it all interacts.  If you understand the principles of how the microphone and its output source impedance can interact with a preamp and its complex input impedance, you may be able to make decisions on interfacing which improve your recordings.

Since this post is already rediculously long, I will continue in my next post, unless you have had enough already.

Thanks for the thorough reply.

What needs further mentioning and explanation is the fact that mis-matching impedances between mic and pre increases distortion.

Another aspect: electronically balanced, rather than component or transformer balanced, mic output configurations, regardless of their theoretically superior linearity, are more susceptible to noise.

First I am sorry for my bad English, i dont speak english.

I am designing some mic pre amps and still have many doubts about this.

The transformers that I have to use the microphone preamplifier input has the following characteristics:

- DC resistance in the primary = 200 Ohms

- DC resistance in the secondary = 950 Ohms

- Value = 1: 6

- 20dBm

- Electrostatic shielding

In some tests I did with an attenuator

This association of the resistance R3 with the DC resistance of the transformer primary, thus giving a total resistance of 85.7 ohms

Analyzing a Jensen scheme, I saw that the PAD resistor is 169 Ohms and the DC resistance of the primary trnasformador JT-115K-E is 19.7 ohms, thus having an overall resistance of 17.64 Ohms.

I was wondering if I ignore this DC resistance of the transformer primaries or should take this into account when calculating my circuit.

In my project I have two gain stages and control the gain level with a potentiometer between these two stages, so I do not need PAD, but the tests I did, the PAD is necessary to leave the more linear frequency curve.

These preamps are for SM57 microphones, and I need you guys help me choose the correct values of these resistors to the PAD and that the attenuation is the lowest possible.

tanks !!

Hello George,

My sincere compliments to jump into the deep.

May I suggest for you to get acquainted with electronics before you go on designing circuitry.
This is not meant to discourage you but to open a whole new world of possibilities to you.

The dc resistance of a transformer is not of major importance for the application in audio.


--- Quote from: klaus on July 24, 2012, 12:53:31 PM ---Thanks for the thorough reply.

What needs further mentioning and explanation is the fact that mis-matching impedances between mic and pre increases distortion.

Another aspect: electronically balanced, rather than component or transformer balanced, mic output configurations, regardless of their theoretically superior linearity, are more susceptible to noise.

--- End quote ---

First of all there is no matching of impedances, there is simply the matter of optimum voltage transfer to the input of the preamp.
This is achieved by keeping the source impedance at least ten times lower than the load from the pre amp.
Please explain how distortion can be influencend as long as the rule for optimum output transfer is followed.

The beauty of the European system lies in the universal application.
Regardless of the source as long as its impedance is considerably lower than the load of the pre amp the match is good.

Designs with an input transformer should be suitably damped to avoid overshoot or ringing as a result of ultra low impedance sources.
I am stating European system because it was a deviation from the US maximum power transfer system coming from the telephone industry.
Older American mikes have a 600 Ohm source impedance to give maximum power to the mixer or network.


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