I'm going through a service manual of a microphone mixer. It mentions on the note that "MIC output loaded with 150ohms". What does loaded mean. and also in another forum I got an answer for one of my question which states that - "try loading the mic with a 100 ohm resistor and see how much the output decreases"

Do they mean that a 150 ohm resistor connected from pin2 to pin3 of an XLR connector or from pin 2 to ground and from pin 3 to ground?..

While i'm on the subject how can I measure an impedance of a microphone. Basically I was measuring the spectrum of couple of identical mics, all of the outputs were same except for one mic - the response was the same but the level/amplitude was different when excited with the same source.

Thanks in advance

 

Most professional low impedance mics are 150 ohm. The mic mixer input is probably designed for these type mics. From the description it would mean that the output impedance of the mic mixer is 150 ohms or the specs on the output levels are based on driving a 150 ohm load. Generally low impedance mic inputs should be 10 times the impedance of the mic. Also dynamic mics can be mid impedance and high impedance. The impedance of the mic should be stated on the mic.
What is the model no. of the mixer?
SG

 

I agree that a 150 ohms mic is never loaded with 150 ohms because that would throw away half of its tiny output level.

 

Most professional low impedance mics are 150 ohm. The mic mixer input is probably designed for these type mics. From the description it would mean that the output impedance of the mic mixer is 150 ohms or the specs on the output levels are based on driving a 150 ohm load. Generally low impedance mic inputs should be 10 times the impedance of the mic. Also dynamic mics can be mid impedance and high impedance. The impedance of the mic should be stated on the mic.
What is the model no. of the mixer?
SG

Hi SG the mixer is a Shure 200M, the note I mentioned is the test condition. The voltages were measured with the output loaded with 150 ohms. I guess my question is how was the resistors connected. I've attached two pictures. How would the resistor be connected as in picture 1 or 2?

 

How do you measure impedance?

I agree that a 150 ohms mic is never loaded with 150 ohms because that would throw away half of its tiny output level.

Load the source with a resistor. When the voltage drops to a half of the no-load signal you have matched the impedance.

For stereo connection, load left channel to ground, and load right channel to ground.

Microphone outputs are rarely stereo.

XLR connection is a different story.
What type of microphone is it? Does the mic need phantom power to be supplied by the mixer XLR connection?
Is the mic a dynamic mic with balanced output?

 

On an XLR plug or jack, pin 1 is the grounded cable shield and is 0V DC if the mic uses phantom power.
Pin 2 and pin 3 are for the balanced audio and carry positive DC for phantom power..

 

How do you measure impedance?


Load the source with a resistor. When the voltage drops to a half of the no-load signal you have matched the impedance.

For stereo connection, load left channel to ground, and load right channel to ground.

Microphone outputs are rarely stereo.

XLR connection is a different story.
What type of microphone is it? Does the mic need phantom power to be supplied by the mixer XLR connection?
Is the mic a dynamic mic with balanced output?

Its a dynamic mic with a balanced output

 

On an XLR plug or jack, pin 1 is the grounded cable shield and is 0V DC if the mic uses phantom power.
Pin 2 and pin 3 are for the balanced audio and carry positive DC for phantom power..

Service manual states that the output was loaded with 150 ohms when they did the voltage measurements on certain parts. Now how can I recreate that scenario.

 

MIC output loaded with 150ohms

You mention mixer, but your talking Mic output. If you were looking at the output of the mic say with a scope (1 meg ohm 1x, 10 meg ohm 10x probe), I would expect that you need to simulate what it's connected to. there is usually a capacitance of 22 pF or so native to the scope.
With a properly compensated 10x probe, that goes away, but at audio frequencies it probably doesn't matter.

For any frequency dependent device, usually some reference frequency is used. I think for speakers, it's 400 Hz.

So, an easy way to measure output impeadance is to measure the output with nothing connected except the meter and a signal source for the MIC. Then add a series potentiometer. Adjust the series potentiometer until the output is 1/2 the original input. Take the potentiometer out of circuit and measure it's resistance. That's the output R of the mic.

 

Service manual states that the output was loaded with 150 ohms when they did the voltage measurements on certain parts. Now how can I recreate that scenario.

In that case, the load resistor goes between pins 2 and 3.

 

Service manual states that the output was loaded with 150 ohms when they did the voltage measurements on certain parts. Now how can I recreate that scenario

Why do you need to create that scenario? Are we still trying to measure the impedance of the mic? If it is not stated on the mic somewhere can you give us a model number?
SG

 

Why do you need to create that scenario? Are we still trying to measure the impedance of the mic? If it is not stated on the mic somewhere can you give us a model number?
SG

Ah I see where you misunderstood me. Basically I was going through this Microphone mixer well just to learn about the design of the circuit, measure voltage at certain stages etc... the service manual mentioned how it was setup when they did the measurement. I wanted to know what they meant about loading the output.

Regarding the mic output impedance.. its completely separate thing I was looking into. Wanted to measure output impedance of exactly identical microphones which were giving different levels of output.

 

I wanted to know what they meant about loading the output

OK, I understand.

I wanted to measure output impedance of exactly identical microphones which were giving different levels of output

If they are the same model mic then their outputs should be appx the same unless one mic is defective, damaged, has a loading resistor inside the mic or connector or who knows why. So I ask you again what is the model no.of the mic?

 

If they are the same model mic then their outputs should be appx the same unless one mic is defective, damaged, has a loading resistor inside the mic or connector or who knows why. So I ask you again what is the model no.of the mic?

The mics were Shure SM57. The odd mic was more sensitive than the others (it was louder than the other mics).
The yellow trace is the "odd" mic.
Correct me if I'm wrong so I could measure the output impedance of the mic by exciting it with a source (sine wave or pink noise?) and connecting a resistor between pin 2 and pin3 and measure the voltage, and also measuring the open-circuit voltage and using the equation R0 = RL((Vopen/VL)-1)

Then I could compare the impedances of the microphone..

Thanks

 

Those old mics have a horrible frequency response don't they? Performers like them for the extra "presence" they produce.
I like modern electret mics that have a flat frequency response like this:

 

Correct me if I'm wrong so I could measure the output impedance of the mic by exciting it with a source (sine wave or pink noise?) and connecting a resistor between pin 2 and pin3 and measure the voltage, and also measuring the open-circuit voltage and using the equation R0 = RL((Vopen/VL)-1)

I don't know if it will make any difference but the procedure is the same as MrChips posted in #5.
Step 1, measure the output of the mic with no resistor connected . Step 2, connect a variable resistor across the mic pins 2 and 3. Step 3, adjust the variable resistor until the output from the mic is 50% of step 1. Step 4, measure the resistance of the variable resistor, this is the impedance of the mic. Which should be somewhere around 150 ohms
SG