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

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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

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

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By "polarization", do you mean the fact that electrolytic capacitors are not bi-directional, or that they need to see a DC voltag eacross them?
There are "bipolar" electrolytic capacitors. they are generally considered capable of better performance than polarized ones. However they usually are bigger than polarized ones, because they are constituted of two polarized capacitors head-to-tail.

Polarization of electrolytic capacitors is not a "useful design trait", it's inherent in the formation of the dielectric, and it's only a limitation in the sense that they are not supposed to operate with reverse DC voltage across them, which is extremely simple to ensure in any half-decent design.

A frequent alternative to aluminium electrolytics is tantalum. They offer larger capacitance vs. bulk than AL types, but are not much loved because of their fragility vs. reverse voltage. They also are not considered sonically neutral.

As to using MLCC's for coupling, as Jules mentioned, their value is not particularly stable, but it's also the case with electrolytics.
The usual technique for minimizing the effects of non-linearity is to over-specify the value. The typical audio adage is to make capacitors 10 times bigger than required for LF extension, i.e. if you want to pass 20Hz nicely, you should calculate the -3dB LF turnover frequency at 2Hz. But for distortion reduction, this value should be divided by 5, for 0.4Hz.
Dielectrics used in large value MLCC's are very non-linear (more than electrolytics). The exception would be C0G/NP0 types but they don't offer the same density as the other types. The largest capacitance in C0G/NP0 MLCC's is 0.47 uF. It's a 1206 part (3.2x1.6x1.6mm).

I suggest you check Cyril Bateman's paper