# Microphone Impedance Matching



## fractile (Mar 15, 2009)

*Problem:* ...Also, terminate the SM-58 with 600 ohms, as it improves the overall sound of the microphone, particularly if you are using a preamp with a higher input impedance.

If you are also looking at micing other sources, you need to consider the best choice of mic for each source, however the vocal sound will be the most important aspect to the overall feel of rap/hip-hop etc.

*Q:* How do you terminate for 600 ohms between the mic and preamp?


*A:* First you need to find out what the input impedance is for your mic preamp or mixer. Then you calculate the parallel resitor value that will give you 600 ohms when attached to this input impedance. Now you can wire a resistor in a special cable across the XLR pins 2 and 3 so the mic will see 600 ohms when plugged into the preamp using this cable.

For example, if the input impedance of your preamp was 1K5, you'd need a 1K resistor across the mic pins 2 and 3 so the mic sees 600 ohms when plugged in.


*Comment:* Ok, great, thanks.
Each different mic has a different sound for different preamp impedances. I've heard preamps with variable input impedance are getting more popular. I picked up a Summit Audio 2BA-221 [100-10k] and a Focusrite [60-3k3] that have this.


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## Equilibrium8 (Sep 11, 2008)

fractile said:


> *Problem:* ...Also, terminate the SM-58 with 600 ohms, as it improves the overall sound of the microphone, particularly if you are using a preamp with a higher input impedance


This is quite a subjective statement. A lot of the sonic differences between mics and preamps come from the impedance interaction. Almost every engineer will have his own favorite combination of mic and preamp for different purposes. There is no perfect matching for this. Although a basic rule of thumb is that the preamps input impedance should be about 10 times higher than the mic's output.


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## fractile (Mar 15, 2009)

Equilibrium8 said:


> This is quite a subjective statement. A lot of the sonic differences between mics and preamps come from the impedance interaction. Almost every engineer will have his own favorite combination of mic and preamp for different purposes. There is no perfect matching for this. Although a basic rule of thumb is that the preamps input impedance should be about 10 times higher than the mic's output.


Well this was specified for a Shure SM57. He may have derived some optimal impedance match for this mic. [Shure may have adhered to the 600 Ohm standard in the design of the mic?]
The 10x ratio is for power transfer?:gulp:; in a microphone I think we want voltage [transient] transfer?
There are also comments about power transfer being related to vacuum tube interface while voltage is toward solid-state... :gulp:


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## Equilibrium8 (Sep 11, 2008)

You mean it was in the documentation? 



> The 10x ratio is for power transfer?; in a microphone I think we want voltage [transient] transfer?


 Matching the impedance exactly would result in a 6dB drop in level, resulting from the voltage decrease, caused by output loading.


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## DrGeoff (Dec 27, 2007)

Remember the SM-58 (and SM-57) were originally made back in the days when analogue consoles all had 600 ohm or 150 ohm input impedances for microphones, and were optimised for this load. These days the input impedances of mic channels seems to be a lot higher, so the mics behave a bit differently. Try it out and see for yourself, let your ears be your guide


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## fractile (Mar 15, 2009)

It could be called impedance interfacing, rather than matching equally. The SM58 is listed as having 150 ohm impedance (300 ohm actual). This means that the DC impedance (resistance) of the mic is 300, while the characteristic (at 1kHz?) impedance is 150.

So if the preamp input is 600, this is a 1:4 ratio; not 1:10, but with mics we're more concerned with sound quality than efficiency of power transfer. And how a mic is loaded by the preamp will affect the frequency characteristic of the sound that comes out. Lowering the ratio increases low frequency response, raising it increases high frequencies. So, at 1:4 it would be slightly below the middle.


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## Equilibrium8 (Sep 11, 2008)

gswan said:


> Try it out and see for yourself, let your ears be your guide


Sure, that was what I meant by my original reply, I got distracted by numbers there for a bit. But every engineer will have a difference preference or a different goal for an application. I will try it out sometime, but right now I don't have any pre-amps with variable impedance.


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## fractile (Mar 15, 2009)

Equilibrium8 said:


> Sure, that was what I meant by my original reply, I got distracted by numbers there for a bit. But every engineer will have a difference preference or a different goal for an application. I will try it out sometime, but right now I don't have any pre-amps with variable impedance.


As an elaboration of the 1k resistor across pins 2 and 3 solution, put a 10k pot wired across pins 2 and 3 of some connectors in a small case. Technically it would be wired as a variable resistor (rheostat), using only terminal 1 and the wiper of the pot. Then you could dial in about any mic/preamp combination.


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## Luke3 (Jul 14, 2011)

This discussion is valid for a connection between a A/D converter (A16 in my case) and an analog mixer (TAC Scorpion)?

Briefly, I would know if I've signal problems, connecting the A16 outputs at 600 ohm to the input line of the Scorpion that has an impedence of 10k ohm.

Thank you..


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