Eletret mic transformer?

[TheBaconWizard]

Hey there...

I am struggling to understand why eletret mics always (so far as I can see) use an amplifier while dynamic mics usually have a transformer.

I do realise that the impedance needs to be matched, but that is no problem these days.

Here is a mic that I am thinking of using. The datasheet gives it an output impedance of 680 ohms @ 1000hz
https://www.mouser.co.uk/datasheet/2/670/cmi_5247tf_k-1777142.pdf

Now, I can find an audio transformer with a primary impedance that in the same ballpark. Why not use it?

[gcewing]

I am struggling to understand why eletret mics always (so far as I can see) use an amplifier while dynamic mics usually have a transformer.

Probably because a dynamic mic can produce an appreciable amount of power on its own, so it's feasible to use a transformer to step up the voltage. But the power available from an electret mic is negligible, so you need an amplifier with its own power source.

Another reason for using a transformer might be to protect the microphone's low-impedance coil from being damaged by DC.

Here is a mic that I am thinking of using. The datasheet gives it an output impedance of 680 ohms @ 1000hz
https://www.mouser.co.uk/datasheet/2/670/cmi_5247tf_k-1777142.pdf

That microphone looks like it already has an amplifier of sorts built into it to lower the output impedance.

Now, I can find an audio transformer with a primary impedance that in the same ballpark. Why not use it?

A couple of things to understand here:

First, you shouldn't be thinking in terms of "matching" impedances here, in the sense of making them equal. That's only important when you're dealing with RF and have to worry about reflections in transmission lines.

When connecting a signal source to an audio amplifier, the aim is to get as much signal voltage at the input as you can. To get that, you want the output impedance of the source to me much lower than the input impedance of the amplifier.

Second, the impedances you see in the specifications of audio output transformers are not the impedance of the transformer! The transformer doesn't have an impedance itself, rather it transforms impedances, in the same way that it transforms voltages and currents -- except that the impedance transformation ratio is the square of the turns ratio.

So if, for example, you see an audio output transformer specified as "800 ohms primary, 8 ohms secondary", all this really means is that it has a turns ratio of 10:1 (i.e. sqrt(800/8)). If you connect a load of 8 ohms to the secondary, you will see an impedance of 800 ohms at the primary. Or if you connect 16 ohms to the secondary, you will see 1600 ohms at the primary, etc.

Now if you want to use this transformer with your mic, you want to step up the voltage, so you'll be using the winding with the smaller number of turns (the "secondary" when used as an audio output transformer) as the primary and connecting it to the microphone. Your 680 ohm mic will then appear as a source of 68k ohms to the amplifier. As long as the amplifier's input impedance is subsantially higher than that, you should get about 10 times more signal than you would from connecting the mic directly to the amp.

How well this would actually work in practice is hard to say, you'd have to try it to find out.

Personally I wouldn't bother trying to use a transfomer; if I needed more signal I'd build a simple preamplifier. It would likely be cheaper and have less detrimental effect on the sound quality.

[bob91343]

I am going through the same thing.  I have a D-104 microphone with a bad element so I tried substituting an electret.  Results were not good.  I added a transformer.  Still not good.

I was using a bare electret and the signal was too small.  A transformer wasn't the answer.

I am reluctant to use an amplifier due to EMI considerations but I may have to.  I ordered a couple of electret elements and will see what happens.

[TheBaconWizard]

That microphone looks like it already has an amplifier of sorts built into it to lower the output impedance.

Yes, they all do, it's a FET. The impedance given on the data sheet is for the entire unit.
But nevertheless, thereafter, a commercial microphone will contain an amplifier in the body of the mic unless it's for an application where there is an amp already provided such as a laptop's mic input.

That all being said, it's good to know that it doesn't need to match, per-se. Just be well under the input. I can work with that and see what happens. The colour added by some transformers is very pleasant. Musician's requirements are about adding character rather than faithful reproduction, hence guitar speakers are awful for hifi and vice-versa.

If it's too expensive though, I can default to received wisdom. Probably there are some nice, quiet amps for less than a good transformer. 

[TheBaconWizard]

I am going through the same thing.  I have a D-104 microphone with a bad element so I tried substituting an electret.  Results were not good.  I added a transformer.  Still not good.

I was using a bare electret and the signal was too small.  A transformer wasn't the answer.

I am reluctant to use an amplifier due to EMI considerations but I may have to.  I ordered a couple of electret elements and will see what happens.

This may interest you; https://astaticd104.blogspot.com/

[Berni]

Transformers just change the impedance by a ratio. If you have a 8 Ohm to 800 Ohm transformer that is a 1:100 winding ratio transformer, so connecting a 50 Ohm source to the input converts it into a 5000Ohm source with 100x more voltage, but 100x less current.

Dynamic microphones have a low output voltage but also low impedance, so a transformer is used to boost the voltage level at the cost of increasing impedance. But since the microphone starts off reasonably low impedance means that the resulting high impedance after the transformer is still plenty low enough to drive the even higher impedance microphone inputs on mixing desks.

Eletret is different. It is a electrostatic/condenser microphone with built in high voltage biasing in the form of eletret material. This gives it really really really high output impedance (These microphones are typically also very small, so this makes it even worse). This is why these types of microphones have an amplifier right next to the microphone, since the impedance is so high that even a piece of long cable can ruin the signal. For this reasons professional microphones have the 48V phantom power 3rd pin (powers the amplifier and biases the condenser mic). In cheep small eletret capsules there is no need for the high voltage biasing, so instead they put a tiny amplifier inside the capsule itself. But consumer microphone inputs on PCs and the like do not have a 3rd phantom power pin, instead they inject a few volts of DC back out on the signal line. The amplifier inside the capsule uses this to power itself. If there is no DC bias supplied to the microphones signal line the amplifier can't work and you get no signal.

[Marco]

For this reasons professional microphones have the 48V phantom power 3rd pin (powers the amplifier and biases the condenser mic).

This sounds wrong, it wouldn't be phantom power if it wasn't on the signal pin. The third pin on XLR is for balanced signalling, not for providing power separately.

[srb1954]

Transformers just change the impedance by a ratio. If you have a 8 Ohm to 800 Ohm transformer that is a 1:100 winding ratio transformer, so connecting a 50 Ohm source to the input converts it into a 5000Ohm source with 100x more voltage, but 100x less current.

An 8  to 800 transformer actually has a winding ratio of 1:10. The impedance transformation is the square of the turns ratio.

[Kleinstein]

Today a transformer is nothing you really want - directly to an amplifier is the preferred way. A transformer does not replace an amplifier, it only supports it to also work best with a different impedance.
So there is nothing wrong / odd in electret microphones going directly to the amplifier. The question is more, why some dynamic microphones still use a transformer. This is because for the low impedance (e.g. 10 Ohms) one can get lower noise with a transformer to set up the voltage and impedance and than use an amplifier with a better noise figure.  Even there it is a balance between more effort with the amplifier or using the transformer - with more parts in parallel on can get the low noise figure also without a transformer, but it costs and needs more power.

The other point with a transformer may be avoiding ground loops. However a transformer can also pick up hum - so it is not the perfect solution.

[Berni]

Ah sorry yes it is 10:1 ratio, forgot about the square.

And yes phantom power is injected into both the differential pairs on a XLR, but this allows there to be power without having the entire signal shifted over in DC. Main point is that microphones having powered amplifiers inside of them is very common. It doesn't cause EMI problems, but actually solves EMI problems because larger stronger signals are sent over the long cable.

[TheBaconWizard]

Thank you all for saving me a little time and money. It is clear to me that I should concentrate on a good pre-amp.