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Balanced is the term. Symmetrical usually refers to multiphase power systems ?

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Hello, everyone.

I am someone who has been receiving a lot of help from this community.
Recently, I have been studying microphone output impedance.

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I noticed that there are various terms commonly used, such as "output impedance" and "symmetrical impedance."First of all, I am confused about the definitions of these terms.

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Can I assume that the two terms mean the same thing?

Not really. Symmetrical impedance is not a concept.
A connection? (input or output) can be symmetrical. An impedance is a ratio between voltage and current. It does pot pertain to the type of connection.

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From what I understand:

• "Output Impedance" refers to the impedance between Pin 2 and ground on the microphone.
• "symmetrical Impedance" refers to the impedance between Pin 2 and Pin 3 on the microphone.

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If that is correct,

It s not correct.

I would like to know whether the impedance values provided by manufacturers refer to output impedance or symmetrical impedance.

The impedance value the manufacturer specifies is the output (source) impedance between teh points that are used to drive the subsequent stage (usually a mic preamp).
Since the input of the preamp can use any type of connectors, numbers are meaningless.

The only thing that matters is the points that are used for connection. E.g. for a 1/4" jack, these points would be Tip (hot) and Ring for a balanced connection, but would be Tip and Sleeve for an unbalanced connection.

The relevant impedance is across the balanced connection (across pins 2 and 3 for XLR connections).

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Secondly, I have a question about the method for measuring impedance.

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From what I have studied, among the XLR pins (GND-1, HOT(+)-2, COLD(-)-3):

In the case of measuring condenser microphone impedance using voltage drop, should I connect the load RL between Pin 2 and Pin 3 to calculate the impedance?
Or should I connect separate loads RL between Pin 2 and ground, and Pin 3 and ground, to calculate the impedance? Which method is correct?

The first one.

In order to measure the voltage drop, you need a constant acoustic source. Do you have it?
Beware that some mics have a very low output impedance (e.g. Schoeps).
Loading them for 6dB attenuation may very well make them clip, which would give a false reading.

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I learned about Symmetrical Impedance from the DPA website below.

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I hate to say this, but te dpa site's technical readings are sometimes inaccurate or simply confusing. I know that they are redacted by junior engineers who are not very scientifically exact.

So, generally, the Output Impedance provided refers to the impedance between Pin 2 and ground, a

No. The impedance between either pin 2 or 3 is somewhat parasitic. It's almost never specified.

nd when a microphone is described as 'balanced,' it means that the impedance between Pin 3 and ground is identical to that of Pin 2 and ground, correct?

Although they should, not always. For example a Schoeps mic has different impedances. The difference is minor but it exists. It's an example of an imperfect balanced connection.

In that respect, transformer-balanced mics are different than transformerless.

Typically, dynamic (moving-coil or ribbon)mics have nominally infinite Z between pins and ground (actually several 100 Megohms).
Phantom-powered mics have about 1-10kilohms bertween pins and ground, when they have about 50-200 ohms between pins 2 & 3.

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hello Jerry,

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thanks to your reply.

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There seems to be a difference between Jules' response and yours.

Could you explain it in a way that is easier to understand?

I believe Jules mad esome kind of mental typo.

Jules explained that the impedance is between Pin 2 (or Pin 3) and ground.

On the other hand, you mentioned that it is "parasitic" and almost never specified.

I maintain this.

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In that case, does the impedance value specified by microphone manufacturers refer to the impedance between Pin 2 and Pin 3?

In all cases, yes.

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Additionally, assuming I have all the necessary equipment for measurement, could you explain how to measure the impedance of a condenser microphone?

There are several ways.

The best is using an impedance analyzer, but I doubt you have one handy, so you can resort to the -6dB loading.

You need to generate a constant acoustic level and measure the output level of the microphone (via a suitable preamp),? then load the mic with a resistor. When the level drops by 6dB, the resistor is equal to the combined impedance of the mic and the preamp.

Since a good preamp is supposed to have a larger impedance than the mic, we usually make the simplification of neglecting it.
So the generic method is to consider the mic impedance is eaqual to the resistor that produces 6dB attenuation.
Neglecting the role of the preamp impedance results in a 10-20% error.
Anyway, the actual impedance of a mic varies with frequency. If you want to measure that, you need to measure at several discrete frequencies, or use an impedance analyzer.
The problem is that impedance analyzers do not provide phantom power, so a separate phantom power supply must be added to the set-up, and it's parasitic impedance (typically 13.6 kohms) must be taken into account.

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Let me know if you need further assistance!

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Best Regards,

kandoit7

Discussing how to load a mic output to measure the -6dB point to determine output impedance...

On 12/18/24 20:12, kandoit7 via groups.io wrote:

If we consider it as a load on the output, I think it would be sufficient to connect a resistor between Pin 2 and ground to measure the output.

The difficulty is that there are several (at least three) types of microphone outputs to be aware of.

1. Classic dynamic mic.? No connection at all to ground (other than for shielding), the output is between pins 2 and 3.? In this case you would put the resistor between pins 2 and 3.

2.? Balanced active mic.? This is where an active output stage feeds pins 2 and 3 separately but equally (other than the two pins have opposite polarity).? You would probably put the resistor between pin 2 and pin 1.

3.? Unbalanced active mic.? This is where one pin gets all the audio output, the other is often "impedance balanced" so that a balanced input would get equal induced noise (noise picked up by the cable in between mic and preamp).? You'd have to figure out which pin has the audio, and connect the resistor between that pin and pin 1.? As an alternative, you could try to measure the resistance between the other pin and pin 1 to see what the mic designer thought would be the correct output impedance.

Sorry for making this even more complicated. That's a part of the process, I guess. :)

-Scott

--
---- Scott Helmke ---- scott@... ---- (734) 604-9340 ----
"I have ceased distinguishing between the religious and the secular,
for everything is holy" - Joe Henry

Le 19/12/2024 à 03:12, kandoit7 via groups.io a écrit?:

Thank you, Jerry.
I have one question:
When connecting the load resistor here, should it be connected between Pin 2 and Pin 3 of the XLR?

Yes. The output is between pins 2 an 3. that's where it should be loaded.

Or should two resistors be connected separately, one between Pin 2 and ground, and the other between Pin 3 and ground?

No.

From what I’ve thought, it seems like the latter is correct, but I wanted to ask if my understanding is wrong.
If we consider it as a load on the output, I think it would be sufficient to connect a resistor between Pin 2 and ground to measure the output.

Then you'd be wrong.

Le 19/12/2024 à 03:27, Scott Helmke a écrit?:

Discussing how to load a mic output to measure the -6dB point to determine output impedance...

On 12/18/24 20:12, kandoit7 via groups.io wrote:

If we consider it as a load on the output, I think it would be sufficient to connect a resistor between Pin 2 and ground to measure the output.

The difficulty is that there are several (at least three) types of microphone outputs to be aware of.

1. Classic dynamic mic.? No connection at all to ground (other than for shielding), the output is between pins 2 and 3.? In this case you would put the resistor between pins 2 and 3.

2.? Balanced active mic.? This is where an active output stage feeds pins 2 and 3 separately but equally (other than the two pins have opposite polarity).? You would probably put the resistor between pin 2 and pin 1.

It would not make sense. The output of an active balanced mic is between pins 2 and 3, as almostany other XLR-connected mic.

3.? Unbalanced active mic.? This is where one pin gets all the audio output, the other is often "impedance balanced" so that a balanced input would get equal induced noise (noise picked up by the cable in between mic and preamp).? You'd have to figure out which pin has the audio, and connect the resistor between that pin and pin 1.

No.

As an alternative, you could try to measure the resistance between the other pin and pin 1 to see what the mic designer thought would be the correct output impedance.

Measuring resistance is nonsense, because when the mic is not powered, the output impedance is very different than when it's powered. What's more is that there are often capacitors in the outputs, that are not taken into account when making resistance measurements.

Le 19/12/2024 à 03:48, kandoit7 via groups.io a écrit?:

Your response is very helpful to me.
Complexity is not an issue when it comes to learning.

If we can derive the correct results, we should endure it. Haha
1. In the case of dynamic microphones, I think impedance measurement will be easier than other types of microphones due to the connection between pin 2 and pin 3.
2. Balanced active microphone - This is the result I'm currently trying to achieve.
In a situation where two pins have opposite polarity but supply the signal equally,

This is not mandatory for a connection to qualify as balanced.
The condition for being balanced is that both in-phase (hot) and out-of-phase (cold) pins present the same Common Mode impedance, i.e. teh impedance measured from pin 2 to pin 1 is equal to the impedance measured between pin 3 and Pin 1.
This common-mode impedance is almost never specified by manufacturers. It is wildly variable, nominally infinite for dynamic/ribbon mics, a few kiloohms for phantom powered mics. It is not specified because it's not a performance indicator.

I'm wondering whether the output impedance should be defined as the impedance between pin 2 and pin 3,
or the impedance between the output going out of pin 2 and ground.

The former.

From my personal perspective, when thinking about output, I believe the latter is correct.

Wrong.

However, as Jerry mentioned, I can't rule out the possibility that it might be the impedance between pin 2 and pin 3.

You's better.

But then again, I think there might be about a 2-fold difference between the former and the latter (in the case of balanced active microphones).

Yes, one is the output impedance (what you're endevouring to assess), the other is teh common-mode impedance, which you are probably not interested in.

There's a lot of information about sensitivity, frequency response, and polar patterns when it comes to microphone measurements.
However, I couldn't find definitive information about impedance measurement.

Impedance measurement is well documented in signal processing. Microphones are not an exception to teh general case.

I'm really curious about what method microphone manufacturers are using to measure the impedance values they provide.

There are several ways of measuring impedance at the design stage, either an impedance analyzer or the quite common method of injecting a known AC current and measuring the resulting voltage. Once the assessment has been made, the designers assign a nominal impedance, which is what the mktg dept uses for publication.

There is no need for QC'ing the output impedance, since it it is governed by design.
Note that impedance varies significantly with frequency. The published value is a mix between the average measured value and whatever the mktg dept deems acceptable.

I have measured a number of dynamic mics, uwing an impedance bridge, which is easy because they don't need to be applied phantom power.
For example, the Shure SM58 measures at 300 ohms at 1kHz and 470 ohms at 140 Hz, although it's specified as 150 ohms!
Conversely, the SM7B, rated at 150 ohms, measures 140 ohms at 1kHz and 240 ohms at 100 Hz.

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Thank you for your response, Jerry.

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Based on the methods you provided, I will proceed to experiment with impedance measurement.

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The experimental setup is as follows:Speaker (Sine wave Signal Source)→ Mic → DI Box (high-precision potentiometer) → Audio AnalyzerThe DI box is equipped with a high-precision potentiometer, which will be connected as a variable resistor

between pins 2 and 3 of the XLR connection from the microphone.

Subsequently, it will be connected to the audio analyzer via an XLR cable.

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The speaker emits sine wave signals at frequencies such as 100Hz, 200Hz, 400Hz, 1000Hz, 2000Hz, and 10000Hz.Initially, the sound pressure level (SPL) at each frequency is measured without connecting the DI box.

You can't do that, because you don't know the gain (or attenuation) of the DI box.

What value is this potentiometer? Ideally it should be about 500 ohms.

You must do a measurement without a load, then another with the potentiometer attached. These measurements don't need a DI box. In addition, most DI boxes are unbalanced.

Another method is to rent the MR-PRO device from NTi Audio.
According to the NTi Audio website, this device is also used for measuring speaker impedance. (However, I anticipate that renting the device might be somewhat difficult...)

The MR-PRO does not provide phantom power. You would have to add phantom power separately.
OTOH it would give more accurate results than the -6dB method, and allow meaurements at different frequencies.

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I would suggest there is a better method, that requires a balanced signal generator.
What "Audio Analyzer" do you intend to use?

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OK, that's the problem with using a commonly accepted name for a different contraption.
So you should consider using a switch to connect/disconnect the potentiometer.

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I have uploaded the file Microphone impedance test jig, which uses the signal injection model (IMO a less brutal method than the added load method).
You calibrate the unit by making R1 and R2=zero (you could use a switch for that), then you measure with the resistors in circuit. When the reading at the preamp output drops by 6dB, R1+R2=microphone impedance.
With this method you're not limited in terms of frequency (microphone impedance often vary very much over the 20-20k range).