Can a microphone preamplifier circuit be converted for electret capsule?

[szegedin]

...I am talking about one of the Bruel and Kjaer 0v prepolarlized capsules.

Can some modification be made to a preamplifier circuit (designed for an externally polarized capsule) to make it work for a 0v prepolarized capsule?

It would be really nice to have the option to use a tube preamp circuit. Can someone help me understand how this works.

 

[rock soderstrom]

Can some modification be made to a preamplifier circuit (designed for an externally polarized capsule) to make it work for a 0v prepolarized capsule?

I'm not sure if I understand you correctly. If by preamplifier circuit you mean the head amplifier in the microphone, the answer is pretty simple.

I think most (if not all) will also work with an electret capsule by simply leaving out the polarisation voltage!

Here as an example, an EF800 U47 circuit. Simply omit the polarisation voltage for the capsule and connect your Electret capsule. (R5/6/7 and C2)

 

[szegedin]

I'm not sure if I understand you correctly. If by preamplifier circuit you mean the head amplifier in the microphone, the answer is pretty simple.

I think most (if not all) will also work with an electret capsule by simply leaving out the polarisation voltage!

Here as an example, an EF800 U47 circuit. Simply omit the polarisation voltage for the capsule and connect your Electret capsule. (R5/6/7 and C2)

Thanks very much, yes that's what I mean.

Can you be more specific about "leave out the polarization voltage" on this circuit, for example?

 

[rock soderstrom]

Can you be more specific about "leave out the polarization voltage" on this circuit, for example?

As mentioned before, just omit R5/6/7 and bridge C2 with a piece wire, so that the backplate (the middle part of the drawn capsule here) is directly connected to ground.

Another example is this circuit, it is super easy to build an works perfect with electret capsules.


Same thing, cut the polarisations voltage wire and connect the backplate to the ground.

 

[rock soderstrom]

Here an cheap chinese electret capsule connected to the SELA T12 headamp.

 

[szegedin]

As mentioned before, just omit R5/6/7 and bridge C2 with a piece wire, so that the backplate (the middle part of the drawn capsule here) is directly connected to ground.

Another example is this circuit, it is super easy to build an works perfect with electret capsules.

View attachment 120585
Same thing, cut the polarisations voltage wire and connect the backplate to the ground.

Wow great, thanks so much for that (didn't see your prior instruction on R5/6/7 etc).

I will start looking into what circuit to use for this. Will check back when I pick something. I want something low noise for subtle acoustic instrument micing. The B&K nickel capsule is great but it's currently paired with a very mediocre pcb preamp head (older DPA 2015).

Do you know of any where I can buy just the pcb? I have a lot of the other components laying around.

 

[rock soderstrom]

I will start looking into what circuit to use for this. Will check back when I pick something. I want something low noise for subtle acoustic instrument micing. The B&K nickel capsule is great but it's currently paired with a very mediocre pcb preamp head (older DPA 2015).

Do you know of any where I can buy just the pcb? I have a lot of the other components laying around.

If you want to go the solid state route than this small kit is the right thing for you. MicUlli´s super low noise mic circuit, ideal for electret capsule. (and very cheap..)

https://groupdiy.com/threads/a-very-versatile-input-stage-for-sdc-mics.80172/post-1084357

 

[szegedin]

If you want to go the solid state route than this small kit is the right thing for you. MicUlli´s super low noise mic circuit, ideal for electret capsule. (and very cheap..)

https://groupdiy.com/threads/a-very-versatile-input-stage-for-sdc-mics.80172/post-1084357

Cool - bookmarked that. Although I'm of a mind to go big or go home, and use a tube circuit.

What about the other B&K capsules that are "externally polarized (200v) electrets." I don't get how they are electrets when they take an external polarization voltage. Could one of these tube circuits get 200v for this purpose with a 48v phantom source?

 

[rock soderstrom]

What about the other B&K capsules that are "externally polarized (200v) electrets." I don't get how they are electrets when they take an external polarization voltage.

This actually sounds like a contradiction, but you can operate prepolarised elctrets with external voltage. Why do this? I don't know, I'm not familiar with the other Bruel&Kjaer mics either.

I think someone else will have to say something about that...

Although I'm of a mind to go big or go home, and use a tube circuit.

Yeah, let it glow!

Could one of these tube circuits get 200v for this purpose with a 48v phantom source?

Well, maybe it is somehow possible, IDK, but in such a case you would rather build a dedicated power supply for the microphone.

 

[k brown]

The absolute simplest would be Royer's legendary "Country Boy's Capacitor Microphone", which I'm pretty sure used the Primo EM21 electret capsule. In a later article he uses essentially the same circuit with an externally polarized capsule, using grid leakage (which also occurs with the 'CountryBoy).

Apparently most electrets are not bothered by external polarization voltage, though I have no experience with doing so on a DPA capsule. One could simply put a DC cap after the mic capsule to be safe.

 

[szegedin]

The absolute simplest would be Royer's legendary "Country Boy's Capacitor Microphone", which I'm pretty sure used the Primo EM21 electret capsule. In a later article he uses essentially the same circuit with an externally polarized capsule, using grid leakage (which also occurs with the 'CountryBoy).

Apparently most electrets are not bothered by external polarization voltage, though I have no experience with doing so on a DPA capsule. One could simply put a DC cap after the mic capsule to be safe.

Thanks that's interesting.

I have a functioning preamplifier for the capsule, but it's just not a very good one - and I know the capsule is good. Next to a Schoeps CMC5-MK41 it is lacking a softness and sweetness to it. Heck I might even be better off with the microphone-parts Schoeps copy preamp if I can modify it to remove that polarization voltage? I don't want to "experiment" with zapping this capsule - it's not a Primo, it's worth some hundreds...

 

[rock soderstrom]

The absolute simplest would be Royer's legendary "Country Boy's Capacitor Microphone", which I'm pretty sure used the Primo EM21 electret capsule. In a later article he uses essentially the same circuit with an externally polarized capsule, using grid leakage (which also occurs with the 'CountryBoy).

That is also a very good suggestion. I've built both the Sela T12 and the CountryBoy, and I really liked them both.

To my ears, the SELA T12 circuit is rather smooth and "vintage", the circuit from David Royer is more modern, real and fast.

 

[rock soderstrom]

Heck I might even be better off with the microphone-parts Schoeps copy preamp if I can modify it to remove that polarization voltage?

It can't be that difficult. Schematic?

 

[szegedin]

It can't be that difficult. Schematic?

I'm not sure if the microphone-parts copy is an exact replica, but I think it's similar to my CMC5....
https://microphone-parts.com/products/transformerless-sdc-circuit-kit

 

[rock soderstrom]

It's quite late here but at first glance I would connect the electret capsule to 1 and 2 instead of 3. That way the backplate would be on ground and not on the polarization voltage.

 

[zvukofor]

Re Schoeps schematic above:
You can connect a capsule instead of T1 if capsule already has FET onboard.
Otherwise - to points 1 and 2.
Lower part of scheme is for polarisation voltage, so omit it.

 

[Mr. Moscode]

Thanks very much, yes that's what I mean.

Can you be more specific about "leave out the polarization voltage" on this circuit, for example?

In the AMI circuit leave out R5 and C2 and replace C2 with a wire to ground IF the capsule does not have a built in FET.